Neuroanatomy - Brain & Cerebral Circulation

1. Quick Answer

Brain anatomy encompasses the cerebral hemispheres, brainstem (midbrain, pons, medulla), cerebellum, and diencephalon (thalamus, hypothalamus), all protected by the meninges and skull, and supplied by the anterior (carotid) and posterior (vertebrobasilar) circulations.

Key Concepts:

• The brain weighs approximately 1400g (2% body weight) but receives 15-20% of cardiac output
• Cerebral blood flow (CBF) is maintained at ~50 mL/100g/min through autoregulation (MAP 60-150 mmHg)
• The Circle of Willis provides potential collateral circulation but is complete in only 50% of individuals
• The blood-brain barrier (BBB) tightly regulates molecular transport into the CNS

ICU Relevance:

• Understanding herniation syndromes is critical for recognizing impending brainstem compression
• Stroke territory anatomy guides thrombolysis eligibility and prognosis
• Ventricular anatomy is essential for EVD insertion and CSF drainage
• Cerebral autoregulation guides blood pressure targets in TBI and stroke

Exam Focus:

• CICM First Part examiners commonly ask about Circle of Willis anatomy, herniation syndromes, brainstem nuclei, and CSF circulation

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2. CICM First Part Exam Focus

What Examiners Expect

Written SAQ:

Common question stems:

• "Draw and label the Circle of Willis. Describe the clinical consequences of anterior communicating artery aneurysm rupture"
• "Describe the blood supply to the brainstem"
• "Outline the anatomy relevant to external ventricular drain placement"
• "Describe the pathophysiology and clinical features of uncal herniation"
• "Draw and label a cross-section of the brainstem at the level of the mid-pons"
• "Describe the anatomy of the meninges with particular reference to the dural venous sinuses"

Expected depth:

• Detailed arterial anatomy with named branches and territories
• Clear understanding of functional localization (motor strip, speech areas, visual cortex)
• Ventricular system anatomy and CSF circulation pathway
• Brainstem organization with cranial nerve nuclei levels
• Clinical application to stroke syndromes and herniation
• Accurate, well-labeled diagrams

Written MCQ:

Common topics tested:

• Circle of Willis components and common variations
• Arterial territories (MCA vs ACA vs PCA)
• Cranial nerve nuclei locations within brainstem
• Dural venous sinus anatomy and drainage
• CSF production and absorption
• Blood-brain barrier structure
• Herniation types and clinical features

Difficulty level:

• Applied anatomical scenarios (e.g., "A patient with MCA territory stroke would most likely have which deficit?")
• Identification of structures from descriptions
• Clinical correlations of lesion locations

Oral Viva:

Expected discussion flow:

1. Overview - Major brain divisions and their functions
2. Arterial Supply - Anterior vs posterior circulation, Circle of Willis, territories
3. Venous Drainage - Superficial and deep venous systems, dural sinuses
4. CSF Circulation - Production, flow pathway, absorption
5. Meninges - Layers, spaces, dural reflections
6. Applied Anatomy - Herniation syndromes, EVD placement, craniotomy landmarks
7. ICU Relevance - CBF autoregulation, ICP monitoring, stroke management

Common viva scenarios:

• "A patient presents with fixed dilated left pupil and right hemiparesis. Describe the anatomical basis"
• "Describe the anatomy relevant to inserting an EVD at Kocher's point"
• "A patient has bitemporal hemianopia. What is the anatomical explanation?"

Pass vs Fail Performance

Pass Standard:

• Accurate naming of Circle of Willis components
• Correct description of MCA, ACA, and PCA territories
• Clear understanding of herniation types and clinical features
• Knowledge of brainstem levels and key nuclei
• Ability to describe EVD insertion landmarks
• Draws clear diagrams of arterial supply and ventricular system

Common Reasons for Failure:

• Confusing anterior and posterior communicating arteries
• Inability to describe herniation syndromes and their progression
• Not knowing which cranial nerve nuclei are in which brainstem level
• Cannot describe dural sinus drainage pathway
• Poor understanding of CSF circulation
• Failure to relate anatomy to clinical presentation

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3. Key Points

Must-Know Facts

1. Brain Weight and Blood Flow: The brain weighs ~1400g (2% body weight) but receives 15-20% of cardiac output (~750 mL/min). Cerebral blood flow (CBF) is approximately 50 mL/100g/min in grey matter (higher: 80 mL/100g/min) and 20 mL/100g/min in white matter (PMID: 9134637).

2. Circle of Willis: Formed by internal carotid arteries, anterior cerebral arteries, anterior communicating artery, posterior cerebral arteries, and posterior communicating arteries. Complete circle present in only 50% of individuals; most common variant is hypoplastic posterior communicating artery (PMID: 16323865).

3. Cerebral Autoregulation: CBF maintained constant across MAP 60-150 mmHg in healthy individuals through myogenic, metabolic, and neurogenic mechanisms. Autoregulation is impaired in TBI, acute stroke, and severe sepsis - CBF becomes pressure-passive (PMID: 27832955).

4. Arterial Territories: MCA supplies lateral hemisphere (motor/sensory strip for face and upper limb, Broca's and Wernicke's areas in dominant hemisphere). ACA supplies medial hemisphere (motor/sensory for lower limb). PCA supplies occipital lobe (visual cortex) and inferomedial temporal lobe (PMID: 22710164).

5. Ventricular System: CSF is produced by choroid plexus (500 mL/day; total volume 150 mL) → lateral ventricles → interventricular foramen (of Monro) → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space via foramina of Luschka and Magendie → absorbed by arachnoid granulations into superior sagittal sinus (PMID: 26514579).

6. Uncal Herniation: Mass effect causes medial temporal lobe (uncus) to herniate through tentorial hiatus → compresses CN III (ipsilateral fixed dilated pupil) → compresses cerebral peduncle (contralateral hemiparesis, but ipsilateral if Kernohan's notch phenomenon) → compresses PCA (occipital infarction) → progressive brainstem compression → death (PMID: 22621955).

7. Brainstem Organization: Midbrain contains CN III/IV nuclei, red nucleus, substantia nigra, cerebral peduncles. Pons contains CN V/VI/VII nuclei, respiratory pneumotaxic centre. Medulla contains CN IX/X/XI/XII nuclei, cardiovascular and respiratory centres, pyramidal decussation (PMID: 29261972).

8. Blood-Brain Barrier: Formed by tight junctions between cerebral endothelial cells, astrocyte end-feet, and pericytes. Disrupted in TBI, stroke, infection, and tumours → vasogenic oedema. BBB breakdown contributes to secondary brain injury (PMID: 29724657).

9. Dural Venous Sinuses: Superior sagittal sinus receives superficial cerebral veins → confluence of sinuses (torcular Herophili) → transverse sinuses → sigmoid sinuses → internal jugular veins. Cavernous sinus receives ophthalmic veins and contains CN III, IV, V1, V2, VI and internal carotid artery (PMID: 24684791).

10. EVD Landmarks (Kocher's Point): 11 cm posterior to nasion (or 1 cm anterior to coronal suture), 3 cm lateral to midline (at mid-pupillary line). Trajectory: perpendicular to skull, aimed toward medial canthus of ipsilateral eye in coronal plane, and toward external auditory meatus in sagittal plane. Target: ipsilateral frontal horn of lateral ventricle (PMID: 26553433).

Essential Anatomical Relationships

Tentorium Cerebelli:

• Separates cerebral hemispheres from cerebellum
• Tentorial hiatus (incisura) allows passage of midbrain, PCA, SCA, CN III, CN IV
• Key landmark in transtentorial (uncal) herniation
• Attached to posterior clinoid processes, petrous temporal ridges, and transverse sinuses

Foramen Magnum:

• Contains medulla, vertebral arteries, spinal accessory nerve (CN XI), anterior/posterior spinal arteries
• Site of tonsillar herniation (cerebellar tonsils descend >5mm)
• Crowded in Chiari malformation

Normal Values Table

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4. Cerebral Hemispheres

4.1 Overview and Surface Anatomy

The cerebral hemispheres constitute the largest portion of the brain, divided into left and right hemispheres by the longitudinal fissure. The surface is highly convoluted with gyri (ridges) and sulci (grooves) that increase cortical surface area to approximately 2500 cm² (PMID: 23853097).

Major Sulci (Landmarks)

Central Sulcus (of Rolando):

• Separates frontal lobe from parietal lobe
• Runs from superior margin of hemisphere inferolaterally toward lateral sulcus
• Precentral gyrus (motor) lies anterior; postcentral gyrus (sensory) lies posterior
• Identification: "omega" or "inverted omega" (hand knob) on axial MRI

Lateral Sulcus (Sylvian Fissure):

• Separates temporal lobe from frontal and parietal lobes
• Contains middle cerebral artery branches (insular cistern)
• Deep within: insula (hidden fifth lobe)

Parieto-occipital Sulcus:

• Visible on medial surface
• Separates parietal lobe from occipital lobe
• Landmark for PCA territory

Calcarine Sulcus:

• Primary visual cortex (V1) located along its banks
• Macular representation at posterior pole; peripheral vision anteriorly

4.2 Lobes and Functional Areas

Frontal Lobe

Boundaries:

• Anterior: frontal pole
• Posterior: central sulcus
• Inferior: lateral sulcus
• Medial: cingulate sulcus

Key Gyri:

• Precentral gyrus: Primary motor cortex (M1, Brodmann area 4)
• Superior frontal gyrus: Premotor and supplementary motor areas
• Middle frontal gyrus: Dorsolateral prefrontal cortex (executive function)
• Inferior frontal gyrus: Broca's area (posterior part - pars opercularis and triangularis) in dominant hemisphere

Functional Areas:

Motor Homunculus: The precentral gyrus has somatotopic organization (Penfield's homunculus):

• Face and tongue: lateral (inferior) - near lateral sulcus
• Upper limb and hand: middle portion (large representation)
• Lower limb: medial surface (extends into longitudinal fissure)
• Note: MCA supplies face/upper limb; ACA supplies lower limb

Clinical Correlation:

• MCA stroke: Face and arm weakness greater than leg (faciobrachial pattern)
• ACA stroke: Leg weakness greater than arm
• Pure motor stroke: Lacunar infarct in internal capsule

Parietal Lobe

Boundaries:

• Anterior: central sulcus
• Posterior: parieto-occipital sulcus (medial), imaginary line to preoccipital notch (lateral)
• Inferior: lateral sulcus and arbitrary line to temporal lobe

Key Gyri:

• Postcentral gyrus: Primary somatosensory cortex (S1, Brodmann areas 1, 2, 3)
• Superior parietal lobule: Somatosensory association, spatial orientation
• Inferior parietal lobule: Contains supramarginal gyrus and angular gyrus

Functional Areas:

Sensory Homunculus:

• Similar somatotopic organization to motor cortex
• Lips, tongue, and hands have disproportionately large representation

Clinical Correlation - Gerstmann Syndrome (dominant parietal lesion):

1. Finger agnosia
2. Left-right confusion
3. Agraphia
4. Acalculia

Clinical Correlation - Non-dominant Parietal:

• Hemispatial neglect (patient ignores left side of space)
• Anosognosia (unawareness of deficit)
• Constructional apraxia

Temporal Lobe

Boundaries:

• Superior: lateral sulcus
• Posterior: arbitrary line from parieto-occipital sulcus to preoccipital notch
• Medial: contains hippocampus, parahippocampal gyrus, uncus

Key Gyri:

• Superior temporal gyrus: Primary auditory cortex; Wernicke's area (posterior portion in dominant hemisphere)
• Middle temporal gyrus: Language processing
• Inferior temporal gyrus: Visual object recognition
• Parahippocampal gyrus: Memory consolidation
• Uncus: Olfactory processing; site of uncal herniation

Functional Areas:

Clinical Correlation - Wernicke's Aphasia:

• Fluent speech with neologisms and word-finding difficulty
• Impaired comprehension
• Poor repetition
• Patient unaware of deficit

Clinical Correlation - Hippocampal/Medial Temporal Damage:

• Herpes simplex encephalitis has predilection for medial temporal lobe
• Bilateral damage: profound anterograde amnesia (HM case study)
• Temporal lobe epilepsy: complex partial seizures with automatisms

Occipital Lobe

Boundaries:

• Anterior: parieto-occipital sulcus (medial), arbitrary line to preoccipital notch (lateral)
• Contains: primary and association visual cortex

Key Gyri:

• Cuneus: Superior to calcarine sulcus; receives contralateral inferior visual field
• Lingual gyrus: Inferior to calcarine sulcus; receives contralateral superior visual field

Functional Areas:

Retinotopic Organization:

• Macula (central vision): Posterior occipital pole (large representation)
• Peripheral vision: Anterior calcarine cortex
• Superior visual field: Inferior bank (lingual gyrus)
• Inferior visual field: Superior bank (cuneus)

Clinical Correlation - Visual Field Defects:

• Homonymous hemianopia: Contralateral to occipital lesion, with macular sparing (dual blood supply from MCA and PCA)
• Cortical blindness: Bilateral PCA infarction; Anton syndrome (denial of blindness)
• PCA stroke: Homonymous hemianopia + memory impairment (if medial temporal involved)

Insula (Island of Reil)

Hidden Fifth Lobe:

• Located deep within lateral sulcus, covered by frontal, parietal, and temporal opercula
• Functions: Taste, visceral sensation, autonomic control, emotion, interoception
• Clinical: Insular strokes may cause autonomic instability (cardiac arrhythmias)
• PMID: 24637357

4.3 White Matter Pathways

Association Fibres (Intrahemispheric)

Commissural Fibres (Interhemispheric)

Corpus Callosum Divisions:

• Rostrum: Most anterior
• Genu: Anterior bend; connects prefrontal regions
• Body: Middle; connects motor, sensory, auditory cortex
• Splenium: Posterior; connects occipital and posterior parietal

Projection Fibres

Internal Capsule: Most critical white matter structure for ICU - lacunar strokes cause pure motor hemiparesis.

Corona Radiata:

• Fan-shaped projection fibres between cortex and internal capsule
• Vulnerable to hypertensive haemorrhage from lenticulostriate arteries

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5. Brainstem

5.1 Overview

The brainstem connects the cerebral hemispheres to the spinal cord and comprises three regions: midbrain (mesencephalon), pons, and medulla oblongata. It contains critical autonomic centres, ascending and descending tracts, and cranial nerve nuclei III-XII.

Key Functions:

• Cardiovascular and respiratory regulation
• Consciousness (reticular activating system)
• Cranial nerve function
• Motor and sensory pathway relay

Clinical Relevance:

• Brainstem death testing assesses brainstem reflexes
• Locked-in syndrome: Ventral pontine lesion sparing reticular formation
• Central herniation progresses rostrocaudally through brainstem

5.2 Midbrain (Mesencephalon)

Level: Superior to pons; surrounds cerebral aqueduct

Dorsal Surface (Tectum - "Roof"):

Superior Colliculi (paired):

• Visual reflex centre
• Receives input from optic tract
• Controls saccadic eye movements and pupillary reflexes

Inferior Colliculi (paired):

• Auditory relay centre
• Part of auditory pathway to medial geniculate nucleus

Ventral Surface:

Cerebral Peduncles:

• Contain corticospinal and corticobulbar tracts
• Crus cerebri: Corticospinal (middle 3/5), frontopontine (medial 1/5), parietotemporo-pontine (lateral 1/5)
• Substantia nigra: Dopaminergic neurons; degeneration causes Parkinson's disease

Red Nucleus:

• Motor control centre
• Receives input from cerebellum
• Origin of rubrospinal tract

Cranial Nerve Nuclei:

• CN III (Oculomotor): Edinger-Westphal (parasympathetic) and motor nuclei at level of superior colliculus
• CN IV (Trochlear): Only CN to exit dorsally; at level of inferior colliculus

Periaqueductal Grey (PAG):

• Surrounds cerebral aqueduct
• Descending pain modulation (endogenous opioid system)
• Autonomic responses
• PMID: 29763992

Clinical Correlations:

5.3 Pons

Level: Between midbrain and medulla; forms ventral bulge of brainstem

Ventral Pons (Basal Pons):

• Pontine nuclei: Relay station from cortex to cerebellum (corticopontocerebellar pathway)
• Middle cerebellar peduncle: Connects pons to cerebellum
• Contains descending corticospinal tracts

Dorsal Pons (Tegmentum):

Cranial Nerve Nuclei:

• CN V (Trigeminal): Motor nucleus (mastication), principal sensory nucleus, mesencephalic nucleus (proprioception), spinal nucleus (extends into medulla)
• CN VI (Abducens): Near midline at pontomedullary junction
• CN VII (Facial): Motor nucleus, superior salivatory nucleus (parasympathetic)
• CN VIII (Vestibulocochlear): Cochlear and vestibular nuclei (at pontomedullary junction)

Key Structures:

Medial Longitudinal Fasciculus (MLF):

• Connects vestibular nuclei, CN III, IV, VI nuclei
• Coordinates conjugate eye movements
• MLF lesion: Internuclear ophthalmoplegia (INO) - impaired adduction on attempted lateral gaze

Pontine Respiratory Centre:

• Pneumotaxic centre (upper pons): Limits inspiration, fine-tunes respiratory rhythm
• Apneustic centre (lower pons): Promotes inspiration
• PMID: 25392349

Locus Coeruleus:

• Major noradrenergic nucleus
• Regulates arousal, attention, stress response
• Near floor of fourth ventricle

Clinical Correlations:

Locked-In Syndrome:

• Devastating pontine stroke, often from basilar artery occlusion
• Reticular activating system (dorsal) preserved → patient conscious
• Corticospinal/corticobulbar tracts (ventral) destroyed → paralysis
• Only vertical eye movements spared (CN III/IV preserved)
• PMID: 22305244

5.4 Medulla Oblongata

Level: Extends from pons to foramen magnum; continuous with spinal cord

Key External Features:

• Pyramids: Ventral surface; contain corticospinal tracts
• Pyramidal decussation: 85-90% of corticospinal fibres cross here
• Olives (inferior olivary nucleus): Lateral; relay to cerebellum
• Gracile and cuneate tubercles: Dorsal; nuclei for posterior column sensation

Cranial Nerve Nuclei:

• CN IX (Glossopharyngeal): Nucleus ambiguus (motor), nucleus solitarius (taste, visceral sensation), inferior salivatory nucleus
• CN X (Vagus): Nucleus ambiguus (larynx, pharynx), dorsal motor nucleus (parasympathetic), nucleus solitarius
• CN XI (Spinal Accessory): Spinal nucleus in upper cervical cord
• CN XII (Hypoglossal): Near midline, floor of fourth ventricle

Autonomic Centres:

Cardiovascular Centre:

• Nucleus solitarius: Receives baroreceptor and chemoreceptor afferents
• Dorsal motor nucleus of vagus: Parasympathetic output to heart
• Rostral ventrolateral medulla (RVLM): Sympathetic output via spinal cord
• Regulates heart rate and blood pressure
• PMID: 20380820

Respiratory Centre:

• Dorsal respiratory group (DRG): Inspiratory neurons; nucleus solitarius
• Ventral respiratory group (VRG): Expiratory neurons; nucleus ambiguus region
• Pre-Bötzinger complex: Respiratory rhythm generator
• PMID: 28774678

Area Postrema:

• Chemoreceptor trigger zone (CTZ)
• Outside blood-brain barrier
• Senses toxins in blood → triggers vomiting
• Clinical: Target of antiemetic drugs

Clinical Correlations:

Wallenberg Syndrome (Lateral Medullary) - PMID: 23059476: Most common brainstem stroke syndrome; classic PICA territory infarct.

Structures affected:

• Lateral spinothalamic tract → Contralateral body pain/temperature loss
• Spinal trigeminal nucleus → Ipsilateral facial pain/temperature loss
• Nucleus ambiguus → Dysphagia, hoarseness, vocal cord paralysis
• Vestibular nuclei → Vertigo, nystagmus, nausea
• Inferior cerebellar peduncle → Ipsilateral ataxia
• Descending sympathetic tract → Ipsilateral Horner's syndrome

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6. Cerebellum

6.1 Anatomy and Divisions

The cerebellum ("little brain") occupies the posterior cranial fossa, posterior to the pons and medulla, separated from cerebral hemispheres by the tentorium cerebelli.

General Structure:

• Surface: Folia (parallel folds), more compact than cerebral gyri
• Weight: ~10% of brain weight
• Neurons: Contains 50% of all brain neurons
• PMID: 20392275

Anatomical Divisions

Vermis (midline):

• Connects two cerebellar hemispheres
• Related to axial and proximal limb control
• Lesions cause truncal ataxia (wide-based gait)

Hemispheres (lateral):

• Control ipsilateral limb coordination
• Lesions cause ipsilateral limb ataxia, dysmetria

Flocculonodular Lobe (vestibulocerebellum):

• Connects to vestibular nuclei
• Controls balance and eye movements
• Lesions cause vertigo, nystagmus, gait ataxia

Functional Divisions

6.2 Cerebellar Peduncles

Three paired fibre bundles connect cerebellum to brainstem:

6.3 Cerebellar Nuclei

Deep nuclei receive input from cerebellar cortex and send output via peduncles.

From medial to lateral: "Don't Eat Greasy Food"

• Fastigial: Vermis output → vestibular nuclei, posture
• Globose and Emboliform (interposed nuclei): Paravermis output → red nucleus
• Dentate: Hemisphere output → thalamus (VL) → motor cortex (largest nucleus)

6.4 Cerebellar Signs

Clinical Features of Cerebellar Lesions (all ipsilateral to lesion):

DANISH Mnemonic: Dysdiadochokinesia, Ataxia, Nystagmus, Intention tremor, Scanning speech, Hypotonia

6.5 Blood Supply

Superior Cerebellar Artery (SCA):

• From basilar artery just before bifurcation
• Supplies: Superior surface of cerebellum, superior cerebellar peduncle, part of midbrain
• Infarct: Dysarthria, limb ataxia, ipsilateral Horner's syndrome

Anterior Inferior Cerebellar Artery (AICA):

• From lower basilar artery
• Supplies: Anterolateral cerebellum, middle cerebellar peduncle, CN VII/VIII
• Infarct: Hearing loss, facial weakness, vertigo, ataxia (AICA syndrome)

Posterior Inferior Cerebellar Artery (PICA):

• From vertebral artery
• Supplies: Posteroinferior cerebellum, inferior cerebellar peduncle, lateral medulla
• Infarct: Wallenberg syndrome (if lateral medulla involved), cerebellar signs

Clinical Relevance:

• Cerebellar infarcts/haemorrhages can cause acute hydrocephalus (fourth ventricle compression)
• Space-occupying cerebellar lesions may require urgent posterior fossa decompression
• Signs of tonsillar herniation: neck stiffness, respiratory arrest
• PMID: 18025032

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7. Diencephalon

7.1 Thalamus

The thalamus is the major relay station for sensory, motor, and limbic information to the cerebral cortex. It forms the lateral walls of the third ventricle.

Gross Anatomy:

• Two ovoid masses joined by interthalamic adhesion (massa intermedia) in 70% of individuals
• Located deep to lateral ventricles, superior to hypothalamus

Thalamic Nuclei and Functions

Specific Relay Nuclei (project to defined cortical areas):

Association Nuclei (integrative functions):

Non-specific Nuclei:

Clinical Correlations:

• Thalamic pain syndrome (Dejerine-Roussy): VPL lesion → contralateral burning pain (central post-stroke pain)
• Thalamic dementia: MD lesion → memory impairment, personality changes
• Arousal disorders: Bilateral intralaminar lesions → decreased consciousness
• PMID: 25547124

7.2 Hypothalamus

The hypothalamus lies below the thalamus, forming the floor and inferior lateral walls of the third ventricle. It is the master regulator of autonomic, endocrine, and homeostatic functions.

Boundaries:

• Superior: Hypothalamic sulcus (separates from thalamus)
• Anterior: Lamina terminalis, optic chiasm
• Posterior: Mammillary bodies
• Inferior: Pituitary stalk (infundibulum), tuber cinereum

Hypothalamic Nuclei and Functions

Anterior (Supraoptic) Region:

Middle (Tuberal) Region:

Posterior (Mammillary) Region:

Hypothalamic Control Summary:

Clinical Correlations:

• Central diabetes insipidus: SON/PVN damage → loss of ADH → polyuria, polydipsia
• SIADH: Excessive ADH release → hyponatraemia
• Wernicke's encephalopathy: Thiamine deficiency → mammillary body damage → amnesia, ataxia, ophthalmoplegia
• Poikilothermia: Loss of temperature regulation → patient adopts environmental temperature
• PMID: 27891260

7.3 Epithalamus

Pineal Gland (Pineal Body):

• Midline structure at posterior third ventricle
• Produces melatonin (circadian rhythm regulation)
• Calcifies with age (visible on X-ray/CT as landmark)
• Pineal tumours can cause Parinaud syndrome (compression of superior colliculi)

Habenula:

• Part of limbic system
• Connects basal ganglia to brainstem nuclei
• Involved in reward, aversion, decision-making

Posterior Commissure:

• White matter crossing posterior to pineal
• Contains fibres for pupillary light reflex pathway
• Lesion: Light-near dissociation

7.4 Subthalamus

Subthalamic Nucleus (STN):

• Located below thalamus, adjacent to internal capsule
• Part of basal ganglia circuitry (indirect pathway)
• Lesion causes hemiballismus (violent flinging movements)
• Target for deep brain stimulation in Parkinson's disease
• PMID: 24486356

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8. Ventricular System and CSF

8.1 Ventricular Anatomy

The ventricular system is a continuous series of fluid-filled cavities within the brain, lined by ependymal cells.

Lateral Ventricles (2)

Location: Within cerebral hemispheres, one in each

Parts:

Boundaries:

• Roof: Corpus callosum
• Floor: Caudate nucleus, thalamus
• Medial wall: Septum pellucidum

Clinical Significance:

• EVD tip placed in frontal horn
• Lateral ventricle enlargement indicates hydrocephalus
• Temporal horn dilation: Early sign of hydrocephalus

Third Ventricle

Location: Midline, between two thalami

Boundaries:

• Lateral walls: Thalamus (connected by interthalamic adhesion)
• Anterior: Lamina terminalis, foramen of Monro
• Posterior: Pineal recess, posterior commissure, cerebral aqueduct
• Floor: Hypothalamus, optic chiasm, infundibulum, mammillary bodies
• Roof: Choroid plexus, tela choroidea

Recesses:

• Suprapineal recess
• Pineal recess
• Infundibular recess
• Optic recess
• Supraoptic recess

Cerebral Aqueduct (of Sylvius)

Location: Through midbrain, connecting third and fourth ventricles

Characteristics:

• Narrowest part of ventricular system (1-2 mm diameter)
• Surrounded by periaqueductal grey
• Common site of obstruction (aqueductal stenosis → obstructive hydrocephalus)
• PMID: 17702540

Fourth Ventricle

Location: Between pons/medulla (anteriorly) and cerebellum (posteriorly)

Boundaries:

• Anterior (floor): Pons and medulla (rhomboid fossa)
• Posterior (roof): Cerebellum (vermis, superior and inferior medullary vela)
• Lateral: Cerebellar peduncles

Outlets (CSF escapes to subarachnoid space):

• Median aperture (foramen of Magendie): Posterior midline → cisterna magna
• Lateral apertures (foramina of Luschka): Lateral recesses → cerebellopontine angle cisterns

Clinical Significance:

• Cerebellar haemorrhage can compress fourth ventricle → obstructive hydrocephalus
• Ependymoma: Common fourth ventricle tumour in children
• Floor contains cranial nerve nuclei (facial colliculus, hypoglossal triangle)

8.2 Choroid Plexus

Structure:

• Vascular fringes of pia mater + ependymal cells
• Located in lateral ventricles (temporal horn and body), third ventricle (roof), fourth ventricle (roof)
• NOT present in frontal or occipital horns, or cerebral aqueduct

Function:

• Produces CSF (70-80% of total); remainder from ependyma and blood-brain barrier
• Rate: ~0.35-0.40 mL/min = ~500 mL/day
• Active secretion of Na+, Cl-, HCO3- with passive water movement
• PMID: 24429455

Blood Supply:

• Lateral ventricle: Anterior choroidal artery (from ICA), posterior lateral choroidal artery (from PCA)
• Third ventricle: Posterior medial choroidal artery (from PCA)
• Fourth ventricle: PICA, AICA

Clinical Significance:

• Choroid plexus papilloma: Overproduction of CSF → hydrocephalus
• Choroid plexus carcinoma: Aggressive tumour, usually in children
• Calcification: Normal finding on CT in adults

8.3 CSF Circulation

Pathway:

1. Produced by choroid plexus in lateral ventricles
2. Flows through interventricular foramina (of Monro) to third ventricle
3. Through cerebral aqueduct (of Sylvius) to fourth ventricle
4. Exits via median aperture (Magendie) and lateral apertures (Luschka) to subarachnoid space
5. Circulates around brain and spinal cord
6. Absorbed by arachnoid granulations into superior sagittal sinus

CSF Characteristics:

Arachnoid Granulations (Pacchionian Bodies):

• One-way valves projecting into dural venous sinuses (mainly superior sagittal sinus)
• Absorb CSF when CSF pressure exceeds venous pressure (5-7 cm H2O differential)
• Increase in size and number with age
• May cause skull indentations visible on imaging

8.4 Hydrocephalus

Definition: Abnormal accumulation of CSF within ventricular system

Classification:

Common Sites of Obstruction:

1. Foramen of Monro (tumour, colloid cyst) → unilateral lateral ventricle enlargement
2. Cerebral aqueduct (stenosis, tumour) → lateral and third ventricle enlargement
3. Fourth ventricle outlets (ependymoma, Chiari malformation) → all ventricle enlargement
4. Arachnoid granulations (post-SAH, meningitis) → communicating hydrocephalus

Clinical Features:

• Acute: Headache, vomiting, decreased consciousness, Cushing's response
• Chronic/NPH: Wacky (dementia), Wobbly (gait ataxia), Wet (urinary incontinence)
• Infants: Bulging fontanelle, sun-setting eyes, increasing head circumference

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9. Meninges

9.1 Overview

The meninges are three connective tissue membranes surrounding the brain and spinal cord. From superficial to deep: dura mater, arachnoid mater, pia mater.

9.2 Dura Mater

Structure:

• Thickest and most superficial meningeal layer
• Two layers in cranium: periosteal (outer, attached to skull) and meningeal (inner)
• Single layer in spinal cord (epidural space contains fat and venous plexus)

Dural Folds (Formed by Meningeal Layer):

Falx Cerebri

Location: Vertical partition in longitudinal fissure, between cerebral hemispheres

Attachments:

• Anterior: Crista galli of ethmoid bone
• Superior: Inner table of skull (sagittal suture line)
• Posterior: Tentorium cerebelli
• Inferior (free edge): Corpus callosum

Associated Sinuses:

• Superior sagittal sinus: Within superior attached edge
• Inferior sagittal sinus: Within inferior free edge
• Straight sinus: At junction with tentorium

Clinical Significance:

• Subfalcine herniation: Cingulate gyrus pushed under falx
• Falcine meningioma: Common site

Tentorium Cerebelli

Location: Horizontal partition between cerebral hemispheres and cerebellum

Attachments:

• Anterior (free edge): Posterior clinoid processes
• Posterior: Occipital bone, transverse sinuses
• Lateral: Petrous temporal bone

Tentorial Incisura (Hiatus):

• Anterior opening around midbrain
• Contents: Midbrain, PCA, SCA, CN III, CN IV
• Critical for understanding uncal herniation

Supratentorial Compartment: Cerebral hemispheres, diencephalon Infratentorial Compartment (Posterior Fossa): Brainstem, cerebellum

Clinical Significance:

• Transtentorial (uncal) herniation through tentorial hiatus
• Pressure gradient between compartments causes herniation
• PMID: 22621955

Falx Cerebelli

Location: Small vertical partition between cerebellar hemispheres Association: Contains occipital sinus

Diaphragma Sellae

Location: Horizontal partition over pituitary fossa Function: Forms roof of pituitary fossa with central aperture for pituitary stalk Clinical: Limits pituitary tumour expansion; "snowman" configuration on MRI

9.3 Dural Innervation

Sensory Supply:

• Anterior cranial fossa: Anterior and posterior ethmoidal nerves (V1)
• Middle cranial fossa: Meningeal branches of V2, V3
• Posterior cranial fossa: C1-C3 (upper cervical nerves), meningeal branch of vagus

Clinical Significance:

• Dura is pain-sensitive; headache from dural irritation (meningitis, SAH, tension headache)
• Referred pain patterns based on innervation territory

9.4 Arachnoid Mater

Structure:

• Thin, avascular membrane
• Attached to overlying dura (subdural space is potential space)
• Separated from pia by subarachnoid space containing CSF, blood vessels, cranial nerves

Arachnoid Granulations:

• Project into dural venous sinuses (mainly superior sagittal sinus)
• Absorb CSF into venous circulation
• Pressure-dependent one-way valves

Subarachnoid Cisterns: Major CSF-filled expansions of subarachnoid space:

Clinical Significance:

• Subarachnoid haemorrhage: Blood fills cisterns (star-shaped on CT)
• Lumbar puncture accesses subarachnoid space
• Cisterns are surgical landmarks and pathways for CSF drainage

9.5 Pia Mater

Structure:

• Thinnest, innermost membrane
• Highly vascular, closely adherent to brain surface
• Follows all sulci and gyri
• Forms tela choroidea and contributes to choroid plexus

Clinical Significance:

• Leptomeninges = arachnoid + pia (involved in bacterial meningitis)
• Pia carries blood vessels into brain substance

9.6 Meningeal Spaces

Clinical Correlations:

• Epidural haematoma: Usually arterial (middle meningeal artery), lens-shaped, does not cross suture lines
• Subdural haematoma: Usually venous (bridging veins), crescent-shaped, crosses suture lines
• Subarachnoid haemorrhage: Fills cisterns and sulci, "star sign" around Circle of Willis
• PMID: 30085601

---

10. Cerebral Circulation

10.1 Arterial Supply Overview

The brain receives dual arterial supply from anterior (carotid) and posterior (vertebrobasilar) systems.

Blood Flow Statistics:

• Total CBF: 750-800 mL/min (15-20% of cardiac output)
• Grey matter: 80 mL/100g/min
• White matter: 20 mL/100g/min
• Ischaemic penumbra: 18-22 mL/100g/min (potentially salvageable)
• Infarction threshold: <10-12 mL/100g/min
• PMID: 21147005

10.2 Internal Carotid Artery

Course:

1. Common carotid bifurcates at C4 level (upper border of thyroid cartilage)
2. Internal carotid ascends without branches in neck
3. Enters skull through carotid canal in petrous temporal bone
4. Traverses cavernous sinus (S-shaped course = carotid siphon)
5. Exits cavernous sinus, pierces dura, enters subarachnoid space
6. Terminates by dividing into ACA and MCA

Segments (Bouthillier classification):

Key Branches:

Ophthalmic Artery:

• First major intradural branch
• Enters orbit through optic canal
• Supplies eye and orbit
• Anastomoses with external carotid branches (important collateral)
• Ophthalmic artery occlusion: Monocular blindness

Posterior Communicating Artery (PCoA):

• Connects ICA to PCA
• Passes above CN III
• Common site of aneurysm (25-30% of intracranial aneurysms)
• Ruptured PCoA aneurysm: CN III palsy + SAH
• PMID: 20824718

Anterior Choroidal Artery:

• Arises just before ICA bifurcation
• Supplies choroid plexus (lateral ventricle), hippocampus, posterior limb of internal capsule, optic tract
• Occlusion: Contralateral hemiplegia, hemianesthesia, homonymous hemianopia (rare complete syndrome)

10.3 Anterior Cerebral Artery

Origin: Terminal branch of ICA (medial)

Course:

1. Passes anteromedially above optic nerve
2. Connected to opposite ACA by anterior communicating artery
3. Curves around genu of corpus callosum
4. Runs in callosal sulcus on medial hemisphere surface

Segments:

Territory:

• Medial surface of frontal and parietal lobes
• Superior portion of lateral surface (1-2 cm strip along superior margin)
• Corpus callosum (anterior portion)

Clinical Correlation - ACA Stroke:

• Contralateral leg weakness >> arm/face (motor homunculus: leg on medial surface)
• Leg sensory loss
• Urinary incontinence
• Transcortical motor aphasia (dominant)
• Abulia, apathy (bilateral or dominant ACA)
• Alien hand syndrome (callosal lesion)
• PMID: 17346253

Anterior Communicating Artery (ACoA):

• Short artery connecting two ACAs
• Most common site of intracranial aneurysm (30-35%)
• Rupture: SAH + frontal lobe dysfunction (confabulation, amnesia if perforators damaged)

10.4 Middle Cerebral Artery

Origin: Terminal branch of ICA (lateral continuation)

Course:

1. Passes laterally into lateral (Sylvian) fissure
2. Courses over insula
3. Divides into superior and inferior divisions

Segments:

Lenticulostriate Arteries:

• "Arteries of stroke"
• end arteries supplying basal ganglia, internal capsule
• Small caliber, susceptible to hypertensive damage → lacunar infarcts, hypertensive ICH

Territory (Largest of any cerebral artery):

• Lateral surface of hemisphere (majority)
• Insular cortex
• Motor and sensory cortex for face and upper limb
• Broca's area (dominant hemisphere)
• Wernicke's area (dominant hemisphere)
• Basal ganglia (via lenticulostriate)
• Internal capsule (posterior limb)

Clinical Correlation - MCA Stroke:

• Contralateral hemiparesis: Face and arm >> leg
• Contralateral hemisensory loss: Face and arm
• Homonymous hemianopia (if optic radiation involved)
• Dominant hemisphere: Aphasia (global, Broca's, or Wernicke's depending on territory)
• Non-dominant hemisphere: Neglect, anosognosia
• Gaze deviation toward side of lesion (frontal eye field damage)
• Most common stroke syndrome
• PMID: 22710164

10.5 Vertebrobasilar System

Vertebral Artery

Origin: First branch of subclavian artery

Course:

1. Ascends through transverse foramina of C6-C1
2. Exits atlas, curves posteriorly behind C1 lateral mass
3. Enters cranium through foramen magnum
4. Joins contralateral vertebral to form basilar artery at pontomedullary junction

Segments:

Branches:

• Posterior inferior cerebellar artery (PICA) - largest branch
• Anterior spinal artery
• Posterior spinal artery
• Medullary perforators

Clinical Significance:

• Vertebral artery dissection: Neck trauma, chiropractic manipulation
• Bow hunter's syndrome: Vertebral compression with head rotation
• PICA occlusion → Wallenberg syndrome

Basilar Artery

Course: Formed by union of vertebral arteries at pontomedullary junction; ascends in pontine cistern to bifurcate into PCAs.

Branches:

Clinical Correlation - Basilar Artery Occlusion:

• "Top of the basilar" syndrome: PCA territory + midbrain signs
• Complete basilar occlusion: Locked-in syndrome, coma, death
• Thrombectomy critical: Basilar occlusion has 80-90% mortality without treatment
• PMID: 31780438

Posterior Cerebral Artery

Origin: Terminal bifurcation of basilar artery (embryologically from ICA, but definitive supply from basilar in 70%)

Segments:

Territory:

• Occipital lobe (visual cortex)
• Inferomedial temporal lobe (hippocampus)
• Posterior thalamus
• Midbrain (perforators)

Clinical Correlation - PCA Stroke:

• Contralateral homonymous hemianopia (with macular sparing)
• Memory impairment (hippocampus)
• Visual agnosia, alexia without agraphia (dominant)
• Thalamic pain syndrome (if thalamoperforators involved)
• Weber/Benedikt syndrome (if midbrain perforators)
• PMID: 26037433

10.6 Circle of Willis

Location: Interpeduncular cistern at base of brain, surrounds optic chiasm and pituitary stalk

Components (anterior to posterior):

1. Anterior communicating artery (ACoA) - connects two ACAs
2. Anterior cerebral arteries (ACA) - A1 segments
3. Internal carotid arteries (ICA) - terminal portions
4. Posterior communicating arteries (PCoA) - connect ICA to PCA
5. Posterior cerebral arteries (PCA) - P1 segments
6. Basilar artery (contributes via PCA origins)

Function:

• Provides potential collateral circulation
• Allows redistribution of blood flow if one feeding vessel is occluded

Anatomical Variations (Circle complete in only 50%):

Clinical Significance:

• Incomplete circle reduces collateral potential → increased stroke risk with carotid stenosis
• Aneurysm sites: ACoA (30%), PCoA (25%), MCA bifurcation (20%), ICA bifurcation (5%), basilar apex (5%)
• PMID: 16323865

10.7 Cerebral Autoregulation

Definition: Intrinsic ability of cerebral vessels to maintain constant CBF despite changes in perfusion pressure

Mechanism:

1. Myogenic response: Vascular smooth muscle constricts with increased pressure (Bayliss effect)
2. Metabolic regulation: CO2, H+, adenosine, K+ cause vasodilation
3. Neurogenic regulation: Sympathetic and parasympathetic innervation

Autoregulation Range:

• Normal: MAP 60-150 mmHg (CPP 50-150 mmHg)
• Chronic hypertension: Curve shifts rightward
• TBI/acute stroke: Curve impaired → CBF becomes pressure-passive

CPP Formula: CPP = MAP - ICP (or MAP - CVP, whichever is higher)

Pressure-Flow Relationship:

CO2 Reactivity:

• CBF increases 3-4% for each mmHg rise in PaCO2 (range 20-80 mmHg)
• Hyperventilation (↓PaCO2) causes vasoconstriction → reduced CBF, reduced ICP
• Clinical use: Temporary ICP control in emergency
• Prolonged hyperventilation: Avoided due to risk of ischaemia

Clinical Applications (PMID: 27832955):

• TBI: Target CPP 60-70 mmHg, avoid hypotension
• Acute ischaemic stroke: Permissive hypertension initially
• SAH: Triple-H therapy (controversial) or euvolaemic management
• Post-carotid endarterectomy: Risk of hyperperfusion syndrome

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11. Venous Drainage

11.1 Overview

Cerebral venous drainage is a valveless system that drains into dural venous sinuses, which ultimately empty into the internal jugular veins.

Key Features:

• No valves → bidirectional flow possible (infection spread risk)
• Dural sinuses are endothelium-lined spaces between dural layers
• Sinuses receive cerebral veins, arachnoid granulations, and diploic veins

11.2 Superficial (Cortical) Veins

Superior Cerebral Veins:

• Drain superior portions of hemispheres
• Empty into superior sagittal sinus
• Cross subdural space as "bridging veins" → vulnerable in head trauma

Superficial Middle Cerebral Vein (Sylvian Vein):

• Runs along lateral sulcus
• Drains into cavernous sinus or sphenoparietal sinus

Inferior Cerebral Veins:

• Drain inferior surface of hemispheres
• Empty into transverse sinus, cavernous sinus

Clinical Significance:

• Bridging veins torn in acceleration-deceleration injuries → subdural haematoma
• Risk factors: Brain atrophy (elderly, alcoholism) increases bridging vein stretch

11.3 Deep Venous System

Internal Cerebral Veins (paired):

• Formed at interventricular foramen by union of:
  • Thalamostriate vein
  • Septal vein
  • Choroidal vein
• Run posteriorly in velum interpositum (above third ventricle)

Great Cerebral Vein (of Galen):

• Short midline vein formed by union of internal cerebral veins
• Receives basal veins (of Rosenthal)
• Joins inferior sagittal sinus to form straight sinus

Basal Veins (of Rosenthal):

• Drain basal surface of hemispheres, midbrain
• Course around midbrain in ambient cistern
• Empty into great vein of Galen

Clinical Significance:

• Deep venous thrombosis: Bilateral thalamic infarcts, impaired consciousness
• Vein of Galen malformation: Arteriovenous fistula, high-output cardiac failure in neonates

11.4 Dural Venous Sinuses

Superior Sagittal Sinus

Location: Upper attached border of falx cerebri

Course: From crista galli posteriorly to confluence of sinuses

Receives:

• Superior cerebral veins
• Arachnoid granulations (CSF absorption)
• Diploic and emissary veins

Clinical Significance:

• Superior sagittal sinus thrombosis: Headache, papilloedema, parasagittal infarcts (leg weakness bilateral), seizures
• Lacunae laterales (venous lakes): Lateral expansions containing arachnoid granulations

Inferior Sagittal Sinus

Location: Lower free edge of falx cerebri

Course: Runs posteriorly to join great vein of Galen → straight sinus

Straight Sinus

Location: Junction of falx cerebri and tentorium cerebelli

Formation: Inferior sagittal sinus + great vein of Galen

Termination: Confluence of sinuses

Confluence of Sinuses (Torcular Herophili)

Location: Internal occipital protuberance

Receives: Superior sagittal sinus, straight sinus, occipital sinus

Drains to: Transverse sinuses (usually asymmetric - right often larger)

Transverse Sinuses

Location: Along attached edge of tentorium cerebelli

Course: From confluence laterally along posterior cranial fossa

Continues as: Sigmoid sinus

Clinical Significance:

• Transverse sinus thrombosis: Papilloedema, headache, otitic hydrocephalus (associated with otitis media)
• Asymmetry common: May mimic pathology on imaging

Sigmoid Sinus

Location: S-shaped, in sigmoid sulcus of temporal bone

Course: Continuation of transverse sinus → internal jugular vein at jugular foramen

Relations: Mastoid air cells (risk of septic thrombosis with mastoiditis)

Cavernous Sinus

Location: Lateral to sella turcica, surrounding pituitary gland

Structure: Trabeculated venous space (not a simple tube)

Contents:

Receives:

• Ophthalmic veins (superior and inferior)
• Sphenoparietal sinus
• Superficial middle cerebral vein

Drains to:

• Superior petrosal sinus → transverse sinus
• Inferior petrosal sinus → internal jugular vein
• Pterygoid plexus (via emissary veins)

Clinical Significance:

• Cavernous sinus thrombosis: Proptosis, chemosis, painful ophthalmoplegia, fever (often from facial/orbital infection)
• Carotid-cavernous fistula: Pulsatile proptosis, bruit, chemosis
• Pituitary adenoma: May invade cavernous sinus, affect CN function
• PMID: 24684791

11.5 Cerebral Venous Thrombosis

Risk Factors: Prothrombotic states, dehydration, pregnancy, oral contraceptives, infection, trauma

Clinical Features:

• Headache (90%)
• Papilloedema (50%)
• Focal neurological deficits
• Seizures (40%)
• Impaired consciousness

Imaging: CT/MR venography - absent flow signal, "empty delta sign" on contrast CT

Treatment: Anticoagulation (even with haemorrhagic infarction), ICP management

Prognosis: 80-90% good recovery with treatment

• PMID: 28124668

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12. Blood-Brain Barrier

12.1 Structure

The blood-brain barrier (BBB) is a selective permeability barrier that separates circulating blood from the CNS extracellular fluid.

Structural Components:

Neurovascular Unit:

• BBB + neurons + microglia = neurovascular unit
• Integrated functional unit that couples neuronal activity to blood flow
• PMID: 29724657

12.2 Transport Mechanisms

Key Concepts:

• Glucose transport: GLUT1 transporter; insulin-independent
• Lipophilic drugs cross easily; hydrophilic drugs poorly penetrate
• P-glycoprotein efflux: Major cause of limited CNS drug penetration
• Tight junctions have electrical resistance 50-100× higher than peripheral capillaries

12.3 Circumventricular Organs (CVO)

Definition: Brain regions lacking typical BBB, allowing direct blood-brain communication

Locations and Functions:

Clinical Significance:

• Area postrema: Target for antiemetics, also senses toxins
• Subfornical organ/OVLT: Involved in osmoregulation, hypertension
• Infections/tumours may preferentially affect CVOs

12.4 BBB Disruption

Causes of BBB Breakdown:

Vasogenic Oedema:

• Results from BBB breakdown
• Fluid accumulates in extracellular space (white matter)
• Responds to corticosteroids (reduce permeability)
• Contrast enhancement on MRI indicates BBB disruption

Cytotoxic Oedema:

• Intracellular swelling (cell energy failure)
• BBB intact initially
• Does not respond to steroids
• Occurs in ischaemic stroke, hypoxia, hyponatraemia

Clinical Implications:

• Drug penetration increases with BBB disruption (antibiotics in meningitis)
• Vasogenic oedema: Consider dexamethasone for tumour/abscess
• Cytotoxic oedema: Osmotherapy (mannitol, hypertonic saline)
• PMID: 29724657

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13. Applied Anatomy

13.1 Herniation Syndromes

Definition: Displacement of brain tissue from one intracranial compartment to another due to pressure gradients

Types of Herniation:

Uncal (Transtentorial Lateral) Herniation

Anatomy: Medial temporal lobe (uncus) herniates through tentorial hiatus

Cause: Unilateral supratentorial mass (haematoma, tumour, swelling)

Progressive Clinical Signs:

Key Points:

• Ipsilateral pupil dilates FIRST (parasympathetic fibres on outside of CN III)
• Cushing's response (hypertension, bradycardia) is LATE sign
• "Coning" refers to irreversible brainstem compression
• PMID: 22621955

Central (Transtentorial Downward) Herniation

Anatomy: Bilateral descent of diencephalon and midbrain through tentorial hiatus

Cause: Bilateral supratentorial mass or diffuse swelling

Progressive Signs (Rostral-Caudal Deterioration):

Clinical Correlation:

• Posturing indicates level of dysfunction
• Decorticate (flexion): Above red nucleus
• Decerebrate (extension): Below red nucleus
• Flaccid: Medulla/cervical cord

Subfalcine (Cingulate) Herniation

Anatomy: Cingulate gyrus displaced under falx cerebri to opposite side

Cause: Unilateral frontal mass effect

Clinical Features:

• Often asymptomatic initially
• ACA compression → contralateral leg weakness
• May progress to uncal herniation

Tonsillar Herniation

Anatomy: Cerebellar tonsils descend through foramen magnum

Cause: Posterior fossa mass, or terminal event of supratentorial herniation

Clinical Features:

• Neck stiffness (meningeal irritation)
• Lower cranial nerve palsies
• Respiratory arrest (compression of medullary respiratory centre)
• Cardiovascular collapse

Warning: Lumbar puncture contraindicated if raised ICP suspected → may precipitate tonsillar herniation

Upward (Ascending) Transtentorial Herniation

Anatomy: Cerebellum herniates upward through tentorial hiatus

Cause: Posterior fossa mass

Clinical Features:

• Midbrain compression (Parinaud syndrome)
• Obstructive hydrocephalus (aqueduct compression)
• Posterior fossa surgery positioning may worsen

13.2 Stroke Territory Anatomy

Territory Summary Table:

Lacunar Syndromes (small vessel disease, basal ganglia/internal capsule/pons):

Clinical Relevance:

• Territory identification guides acute treatment decisions
• Large vessel occlusion (LVO): Consider thrombectomy
• Lacunar stroke: Small vessel disease, less amenable to intervention
• PMID: 22710164

13.3 EVD Insertion Anatomy

External Ventricular Drain (EVD): Catheter placed in lateral ventricle for CSF drainage, ICP monitoring, and drug administration

Kocher's Point (Standard Entry Site):

Surface Landmarks:

• 11 cm posterior to nasion (OR 1 cm anterior to coronal suture)
• 3 cm lateral to midline (at mid-pupillary line)
• Usually right side preferred (non-dominant hemisphere)

Trajectory:

• Coronal plane: Aim toward medial canthus of ipsilateral eye
• Sagittal plane: Aim toward external auditory meatus
• Perpendicular to skull surface

Target: Frontal horn of lateral ventricle

Depth: Typically 6-7 cm (catheter tip should be at level of foramen of Monro)

Structures Traversed:

1. Scalp
2. Galea aponeurotica
3. Pericranium
4. Skull (burr hole)
5. Dura mater
6. Subdural space
7. Cortex (superior frontal gyrus)
8. White matter (corona radiata)
9. Ependyma
10. Ventricular cavity

Anatomical Hazards:

• Motor strip: Posterior to insertion point (safe if anterior to coronal suture)
• Superior sagittal sinus: Midline (stay ≥2.5 cm lateral)
• Cortical bridging veins: Cross subdural space

Freehand vs Image-Guided:

• Accuracy of freehand: 60-80% first-pass success
• Image guidance: Higher accuracy, especially in small/displaced ventricles
• PMID: 26553433

13.4 Decompressive Craniectomy Anatomy

Indications: Malignant MCA infarction, severe TBI with refractory ICP, aneurysmal SAH

Standard Hemicraniectomy (Malignant MCA Infarction):

Size: Minimum 12 cm × 15 cm (or 12 cm diameter)

• Smaller bone flaps associated with worse outcomes (external cerebral herniation, laceration)

Boundaries:

• Anterior: Frontal bone (1 cm from midline to avoid sagittal sinus)
• Posterior: Parietal bone
• Lateral: Squamous temporal bone (flush with middle fossa floor)
• Superior: 1-2 cm from sagittal suture

Key Anatomy:

• Temporal squama: Thin bone, remove flush with floor to allow temporal lobe expansion
• Middle meningeal artery: Runs beneath pterion - control bleeding
• Superior sagittal sinus: Stay 1-2 cm lateral to avoid injury
• Pterion: Weakest point of skull, junction of frontal, parietal, temporal, sphenoid bones

Duroplasty:

• Dura opened widely (cruciate or stellate)
• Augmented with dural substitute or pericranium
• Allows brain expansion

Evidence:

• DECIMAL, DESTINY, HAMLET trials: Reduced mortality but increased disability
• DESTINY II: Benefit extends to >60 years, but quality of life considerations
• PMID: 17259658, 24670893

13.5 Craniometric Points

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14. Australian/NZ Context

14.1 Indigenous Health Considerations

Aboriginal and Torres Strait Islander Stroke Disparities:

• Stroke incidence 2-3× higher than non-Indigenous population
• Younger age at presentation (mean 10-15 years younger)
• Higher proportion of haemorrhagic stroke
• Higher prevalence of risk factors: Hypertension (40-50%), diabetes (30-40%), smoking, obesity
• Remote communities: Limited access to acute stroke care, thrombolysis, thrombectomy
• PMID: 27641247

Cultural Safety in Neurological Assessment:

• Involve Aboriginal Health Workers/Liaison Officers (AHWs/ALOs)
• Family/community involvement in decision-making
• Traditional healing practices may complement Western medicine
• Clear, jargon-free communication
• Awareness of "shame" around incontinence/dependence
• Spiritual connection to Country - consider repatriation requests

Māori Health Considerations:

• Stroke incidence elevated compared to non-Māori New Zealanders
• Cultural concepts: Whānau (family) involvement, Tikanga (customs), Wairua (spiritual wellbeing)
• Māori Health Workers assist with communication
• Respect for tapu (sacredness) of the head in examinations
• Te Tiriti o Waitangi obligations in healthcare

14.2 Remote and Retrieval Medicine

Challenges in Remote Australian/NZ Settings:

• Distance to definitive neurosurgical care (often >1000 km)
• Limited CT availability (some communities have no CT)
• Time-critical conditions (stroke thrombolysis window, neurosurgical emergencies)
• Aeromedical retrieval times: 4-12+ hours

RFDS/Retrieval Considerations:

• Early consultation with retrieval service for suspected neurological emergencies
• Telemedicine for remote stroke assessment (Telestroke)
• Airway management before retrieval (RSI if GCS ≤8)
• ICP management during transport (head elevation, sedation, normocapnia)
• Altitude considerations: Cabin pressurization to sea level for raised ICP
• PMID: 28885871

Telestroke Networks:

• Hub-and-spoke model for remote stroke assessment
• Enables thrombolysis decision at remote sites
• Video assessment of NIHSS, CT interpretation
• Victorian Stroke Telemedicine (VST), NSW Telestroke

14.3 Acute Stroke Pathways

Australian Stroke Guidelines (Stroke Foundation Clinical Guidelines):

• Door-to-needle time target: <60 minutes for thrombolysis
• Door-to-puncture time target: <90 minutes for thrombectomy
• Thrombectomy: Available at Comprehensive Stroke Centres
• Transfer protocols: Direct to thrombectomy-capable centre if LVO suspected

Thrombolysis Eligibility:

• Time window: 0-4.5 hours (extended to 9 hours with perfusion imaging in selected patients - EXTEND trial)
• CT: Exclude haemorrhage
• BP: <185/110 mmHg

Thrombectomy Eligibility:

• Large vessel occlusion (ICA, M1, proximal M2, basilar)
• Extended time window: Up to 24 hours with favourable perfusion imaging (DAWN, DEFUSE-3)
• PMID: 31780438

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15. SAQ Practice

SAQ 1: Circle of Willis and Aneurysm

Question (15 marks):

A 52-year-old woman presents with sudden severe headache ("worst headache of my life") and photophobia. GCS is 14 (E4V4M6) with a left partial third nerve palsy (ptosis, dilated pupil, preserved abduction).

a) Draw and label the Circle of Willis (5 marks)

b) Which artery is most likely affected and why? Describe the anatomical relationship explaining her clinical findings (4 marks)

c) Describe the blood supply to the cerebral hemispheres, indicating which regions are supplied by each major cerebral artery (4 marks)

d) What is the significance of anatomical variations in the Circle of Willis? (2 marks)

---

Model Answer:

a) Circle of Willis Diagram (5 marks)

[Diagram should include]:

• Anterior communicating artery (ACoA) connecting both ACAs
• Anterior cerebral arteries (ACA) - A1 segments
• Internal carotid arteries (ICA) - terminal portions
• Posterior communicating arteries (PCoA) - connecting ICA to PCA
• Posterior cerebral arteries (PCA) - P1 segments
• Basilar artery contributing to PCAs
• Clear labeling of all components
• Correct spatial relationships (anterior ACoA, posterior basilar)

Marking: 1 mark for correct overall structure, 1 mark each for anterior circulation (ACoA, ACAs, ICAs) and posterior circulation (PCAs, PCoAs, basilar) correctly drawn and labeled

b) Posterior Communicating Artery Aneurysm (4 marks)

The most likely affected artery is the posterior communicating artery (PCoA). (1 mark)

Anatomical explanation:

• The PCoA arises from the ICA and passes posteriorly to join the PCA (0.5 mark)
• The oculomotor nerve (CN III) runs in the interpeduncular cistern, passing between the PCA and SCA, and lies in close proximity to the junction of the ICA and PCoA (0.5 mark)
• The parasympathetic fibres for pupillary constriction run on the external surface of CN III, making them vulnerable to external compression (1 mark)
• PCoA aneurysm expansion or rupture compresses CN III externally, affecting parasympathetic fibres first → ipsilateral pupil dilation (unopposed sympathetic action) with preserved or impaired eye movements (1 mark)

The clinical triad of:

• Sudden severe headache (SAH)
• Third nerve palsy with pupil involvement (compression)
• Preserved lateral rectus function (CN VI intact)

...is highly suggestive of ruptured or expanding PCoA aneurysm.

c) Cerebral Artery Territories (4 marks)

Anterior Cerebral Artery (ACA) (1 mark):

• Medial surface of frontal and parietal lobes
• Motor and sensory cortex for lower limb
• Anterior corpus callosum
• Orbitofrontal cortex

Middle Cerebral Artery (MCA) (1.5 marks):

• Lateral surface of hemisphere (largest territory)
• Motor and sensory cortex for face and upper limb
• Broca's area (dominant inferior frontal gyrus)
• Wernicke's area (dominant posterior superior temporal)
• Insular cortex
• Basal ganglia and internal capsule (via lenticulostriate branches)

Posterior Cerebral Artery (PCA) (1.5 marks):

• Occipital lobe (primary visual cortex)
• Inferomedial temporal lobe (hippocampus)
• Posterior thalamus (via thalamoperforating and thalamogeniculate branches)
• Splenium of corpus callosum

d) Circle of Willis Variations (2 marks)

Significance:

• Complete circle present in only ~50% of individuals (0.5 mark)
• Common variations include hypoplastic/absent PCoA (30-40%) and hypoplastic P1 (fetal PCA configuration) (0.5 mark)
• Incomplete circle reduces collateral capacity:
  • Increased stroke risk with carotid stenosis/occlusion (0.5 mark)
  • Reduced tolerance for temporary vessel occlusion during surgery (0.5 mark)

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SAQ 2: Herniation Syndromes

Question (15 marks):

A 45-year-old male is intubated and ventilated following a fall from a ladder. CT brain shows a large right-sided acute subdural haematoma with significant midline shift. His right pupil is now 6mm and unreactive.

a) Describe the pathophysiology of uncal herniation, including the structures compressed and their anatomical relationships (5 marks)

b) Explain the clinical features expected with progressive uncal herniation, correlating each feature with the underlying anatomical structures (5 marks)

c) Describe the anatomy relevant to placement of an external ventricular drain (EVD), including surface landmarks, trajectory, and structures traversed (5 marks)

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Model Answer:

a) Pathophysiology of Uncal Herniation (5 marks)

Definition and Mechanism (1 mark): Uncal herniation occurs when a unilateral supratentorial mass (in this case, acute subdural haematoma) creates a pressure gradient that forces the medial temporal lobe (specifically the uncus) to herniate medially and inferiorly through the tentorial incisura (hiatus).

Structures Compressed in Sequence (4 marks):

1. Oculomotor nerve (CN III) (1 mark):

   • CN III exits the midbrain between the PCA and SCA
   • Runs along the edge of the tentorial incisura before entering the cavernous sinus
   • Parasympathetic fibres run on the external surface of the nerve, making them vulnerable to external compression before somatic motor fibres

2. Ipsilateral cerebral peduncle (1 mark):

   • Contains corticospinal tract
   • Compressed against the tentorial edge
   • Causes contralateral hemiparesis (fibres decussate at medullary pyramids)

3. Contralateral cerebral peduncle (Kernohan's notch phenomenon) (0.5 mark):

   • Midbrain pushed against contralateral tentorial edge
   • Creates a "notch" in the peduncle
   • Causes false localizing sign: ipsilateral hemiparesis

4. Posterior cerebral artery (PCA) (0.5 mark):

   • Compressed against tentorial edge
   • Causes occipital lobe infarction → homonymous hemianopia

5. Midbrain and reticular activating system (1 mark):

   • Progressive compression leads to:
     • Impaired consciousness
     • Duret haemorrhages (midbrain and pontine)
     • Irreversible brainstem injury → death

b) Clinical Features of Progressive Uncal Herniation (5 marks)

Key Point (0 marks but important for viva): The progression is rostral to caudal - diencephalon → midbrain → pons → medulla. Cushing's response is a LATE sign indicating brainstem compression.

c) EVD Placement Anatomy (5 marks)

Surface Landmarks (Kocher's Point) (2 marks):

• Distance from nasion: 11 cm posterior (OR 1 cm anterior to coronal suture)
• Distance from midline: 3 cm lateral (at mid-pupillary line)
• Side: Usually right (non-dominant hemisphere) unless pathology dictates otherwise

Trajectory (1.5 marks):

• Coronal plane: Aim toward medial canthus of ipsilateral eye
• Sagittal plane: Aim toward external auditory meatus (or tragus)
• Perpendicular to skull surface initially

Depth: 6-7 cm (catheter tip at foramen of Monro level)

Structures Traversed (1.5 marks):

1. Scalp (skin, subcutaneous tissue, galea aponeurotica)
2. Pericranium (periosteum)
3. Skull (frontal bone - burr hole created)
4. Dura mater
5. Subdural space (potential space)
6. Arachnoid and subarachnoid space
7. Cerebral cortex (superior frontal gyrus)
8. Subcortical white matter (corona radiata)
9. Ependyma (ventricular lining)
10. Lateral ventricle (frontal horn) - TARGET

Anatomical Hazards:

• Motor cortex: Lies posterior to insertion point (safe if anterior to coronal suture)
• Superior sagittal sinus: At midline - stay ≥2.5-3 cm lateral
• Bridging veins: Cross subdural space near midline

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16. Viva Scenarios

Viva 1: Circle of Willis and Stroke

Scenario: A 68-year-old right-handed man presents with sudden onset left-sided weakness and difficulty speaking. CT angiography shows right MCA occlusion.

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Examiner: Describe the arterial blood supply to the brain.

Candidate: The brain receives dual arterial supply from the anterior and posterior circulations:

Anterior (Carotid) Circulation:

• The common carotid artery bifurcates at C4 level into internal and external carotid arteries
• The internal carotid artery (ICA) has no branches in the neck, enters the skull through the carotid canal, traverses the cavernous sinus, and terminates by dividing into the anterior cerebral artery (ACA) and middle cerebral artery (MCA)
• Key branches include the ophthalmic artery, posterior communicating artery, and anterior choroidal artery

Posterior (Vertebrobasilar) Circulation:

• The vertebral arteries arise from the subclavian arteries
• They ascend through the transverse foramina of C6-C1, enter the skull through the foramen magnum, and unite to form the basilar artery
• The basilar artery gives off AICA, pontine branches, and SCA, then bifurcates into the posterior cerebral arteries

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Examiner: Draw and describe the Circle of Willis.

Candidate: [Draws circle] The Circle of Willis is an arterial anastomosis at the base of the brain in the interpeduncular cistern, surrounding the optic chiasm and pituitary stalk.

Components from anterior to posterior:

• Anterior communicating artery connecting the two ACAs
• A1 segments of both anterior cerebral arteries
• Terminal portions of both internal carotid arteries
• Posterior communicating arteries connecting each ICA to the ipsilateral PCA
• P1 segments of both posterior cerebral arteries

The circle provides potential collateral circulation, but is anatomically complete in only about 50% of individuals. Common variations include hypoplastic or absent posterior communicating arteries and fetal PCA configuration where the PCA arises predominantly from the ICA rather than the basilar artery.

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Examiner: This patient has a right MCA occlusion. What territory does the MCA supply?

Candidate: The MCA has the largest territory of any cerebral artery:

Cortical Territory:

• Lateral surface of the hemisphere, including:
  • Motor cortex for face and upper limb (precentral gyrus, lateral portion)
  • Somatosensory cortex for face and upper limb (postcentral gyrus, lateral portion)
  • Broca's area (inferior frontal gyrus, dominant hemisphere)
  • Wernicke's area (posterior superior temporal gyrus, dominant hemisphere)
  • Insula
  • Frontal, parietal, and temporal operculum

Deep Territory (via lenticulostriate arteries):

• Basal ganglia (putamen, globus pallidus, caudate head)
• Internal capsule (posterior limb)

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Examiner: What clinical features would you expect with a complete right MCA stroke?

Candidate: In a right-handed patient with right MCA stroke (non-dominant hemisphere):

Motor Deficits:

• Left hemiparesis, affecting face and arm more than leg (leg is ACA territory on medial surface)
• The pattern is typically dense face and arm weakness

Sensory Deficits:

• Left hemisensory loss (face and arm > leg)

Visual Field:

• Left homonymous hemianopia (if optic radiation involved)

Gaze Abnormality:

• Gaze deviation toward the right (toward the lesion) due to frontal eye field damage - the intact left FEF pushes eyes to the right

Non-Dominant Hemisphere Signs:

• Left hemispatial neglect (ignores left side of space, may not recognise left arm)
• Anosognosia (unawareness of or denial of deficits)
• Constructional apraxia

Since this patient has "difficulty speaking" and a right MCA stroke, this could represent dysarthria rather than aphasia, as the right hemisphere is typically non-dominant for language in right-handed individuals.

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Examiner: Why does the patient have face and arm weakness greater than leg?

Candidate: This relates to the motor homunculus and arterial territories:

The precentral gyrus (primary motor cortex) has somatotopic organization - the motor homunculus:

• Face and tongue: Represented laterally, near the lateral sulcus
• Upper limb and hand: Represented on the lateral convexity (large representation for fine motor control)
• Lower limb: Represented on the medial surface of the hemisphere, extending into the longitudinal fissure

The MCA supplies the lateral surface of the hemisphere, including the motor cortex for face and upper limb.

The ACA supplies the medial surface, including the motor cortex for the lower limb.

Therefore, an MCA stroke spares the leg motor representation, resulting in face and arm weakness much greater than leg weakness. This "faciobrachial" pattern is characteristic of MCA territory stroke.

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Examiner: What are the lenticulostriate arteries and why are they important?

Candidate: The lenticulostriate arteries are small perforating branches that arise from the M1 segment of the MCA. They are sometimes called the "arteries of stroke" because of their clinical importance.

Anatomy:

• They penetrate the anterior perforated substance
• Supply the basal ganglia (putamen, globus pallidus, caudate) and internal capsule

Clinical Significance:

• They are end arteries with minimal collateral supply
• Small caliber makes them susceptible to:
  • Lipohyalinosis (chronic hypertension)
  • Thrombosis → lacunar infarcts
  • Rupture → hypertensive intracerebral haemorrhage

Lacunar Syndromes:

• Pure motor hemiparesis: Internal capsule lacune (face, arm, and leg equal weakness)
• Ataxic hemiparesis: Posterior internal capsule

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Examiner: What is cerebral autoregulation and how is it affected by stroke?

Candidate: Cerebral autoregulation is the intrinsic ability of the cerebral vasculature to maintain constant cerebral blood flow despite changes in perfusion pressure.

Normal Autoregulation:

• CBF remains constant at approximately 50 mL/100g/min
• Effective across MAP range of 60-150 mmHg
• Mechanisms include:
  • Myogenic response (Bayliss effect - vessels constrict with increased pressure)
  • Metabolic regulation (CO2, H+, adenosine)
  • Neurogenic regulation

Autoregulation in Stroke: In acute ischaemic stroke, autoregulation is impaired or absent in the penumbral zone:

• CBF becomes pressure-passive (directly proportional to MAP)
• This is why we allow permissive hypertension initially (up to 220/120 if not for thrombolysis)
• Lowering BP may reduce perfusion to penumbra → increase infarct size

Clinical Implications:

• Avoid aggressive BP lowering in acute stroke (unless thrombolysis planned: target <185/110)
• Post-thrombolysis: Target <180/105
• Haemorrhagic stroke: Different targets (intensive BP lowering)

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Viva 2: Herniation and ICP

Scenario: A 32-year-old motorcyclist is brought to ICU following a high-speed crash. CT shows an acute right-sided extradural haematoma with 12mm midline shift. His GCS has deteriorated from 12 to 8, and his right pupil is now dilated and fixed.

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Examiner: What is happening to this patient neurologically?

Candidate: This patient is demonstrating clinical signs of uncal (transtentorial) herniation secondary to the right-sided extradural haematoma causing mass effect.

The key findings are:

• Deteriorating GCS (12→8): Indicates progressive brainstem compression affecting the reticular activating system
• Ipsilateral (right) fixed dilated pupil: Classic sign of CN III compression on the same side as the lesion
• 12mm midline shift: Significant subfalcine herniation likely present as well

This is a neurosurgical emergency requiring immediate decompression.

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Examiner: Describe the anatomy of uncal herniation and explain the pupil findings.

Candidate: Anatomical Basis:

The uncus is the most medial portion of the temporal lobe, part of the parahippocampal gyrus. It lies adjacent to the tentorial incisura (hiatus).

When a supratentorial mass causes raised ICP, the pressure gradient forces the uncus to herniate medially and inferiorly through the tentorial incisura.

Structures Compressed:

1. Oculomotor nerve (CN III):

   • CN III exits the midbrain in the interpeduncular cistern
   • Runs between the PCA and SCA
   • Passes along the edge of the tentorium before entering the cavernous sinus
   • The hernating uncus compresses CN III against the free edge of the tentorium

2. Why does the pupil dilate?:

   • Parasympathetic fibres for pupillary constriction run on the external (peripheral) surface of CN III
   • These fibres are compressed first by external pressure
   • Loss of parasympathetic tone → pupil dilation (mydriasis)
   • Unopposed sympathetic activity dilates the pupil
   • This occurs before significant motor involvement (eye movement abnormalities)

Progression:

• Initially the pupil becomes sluggishly reactive
• Then fixed and dilated
• Then oculomotor palsy (ptosis, "down and out" eye position)

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Examiner: What other structures may be compressed in herniation?

Candidate: As herniation progresses:

Cerebral Peduncle (ipsilateral):

• Contains corticospinal tract
• Compression causes contralateral hemiparesis (fibres decussate at pyramids)

Kernohan's Notch Phenomenon:

• The midbrain can be pushed against the contralateral tentorial edge
• This compresses the contralateral cerebral peduncle
• Results in ipsilateral hemiparesis (false localizing sign)

Posterior Cerebral Artery:

• Compressed against the tentorium
• Can cause occipital infarction → homonymous hemianopia

Midbrain/ARAS:

• Progressive compression causes decreased consciousness
• Duret haemorrhages (secondary brainstem haemorrhage) indicate irreversible injury

Aqueduct of Sylvius:

• May be compressed, causing obstructive hydrocephalus
• Further increases ICP

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Examiner: What are the different types of brain herniation?

Candidate: There are five main types:

1. Subfalcine (Cingulate) Herniation:

   • Cingulate gyrus pushed under the falx cerebri
   • Often asymptomatic or causes ACA compression (leg weakness)
   • May precede or accompany uncal herniation

2. Uncal (Lateral Transtentorial) Herniation:

   • Uncus herniates through tentorial hiatus
   • Unilateral supratentorial mass
   • CN III palsy → ipsilateral peduncle compression → contralateral hemiparesis

3. Central (Downward Transtentorial) Herniation:

   • Bilateral descent of diencephalon and midbrain
   • Diffuse cerebral oedema or bilateral masses
   • Rostral-caudal progression: Small reactive pupils → midposition fixed → dilated fixed

4. Tonsillar Herniation:

   • Cerebellar tonsils descend through foramen magnum
   • Posterior fossa masses or terminal event
   • Neck stiffness, respiratory arrest

5. Upward (Ascending) Transtentorial Herniation:

   • Cerebellum herniates upward through tentorial hiatus
   • Posterior fossa masses
   • Parinaud syndrome, obstructive hydrocephalus

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Examiner: The neurosurgeon is performing a decompressive craniectomy. Describe the relevant surgical anatomy.

Candidate: For a right-sided decompressive hemicraniectomy:

Size:

• Minimum 12cm diameter or 12cm × 15cm
• Smaller flaps associated with worse outcomes due to brain herniation through the defect

Boundaries:

• Anterior: Frontal bone, staying 1-2cm from midline
• Posterior: Parietal bone
• Inferior: Temporal squama (thin bone), taken flush with middle fossa floor
• Superior: 1-2cm from sagittal suture to avoid sagittal sinus

Key Anatomical Structures:

1. Superior Sagittal Sinus:

   • Runs in the midline within the falx cerebri attachment
   • Major venous structure - injury causes significant haemorrhage
   • Stay at least 1.5-2cm lateral

2. Middle Meningeal Artery:

   • Runs beneath the pterion
   • Often damaged in extradural haematoma (this patient)
   • Must be controlled during surgery

3. Pterion:

   • Junction of frontal, parietal, temporal, and sphenoid bones
   • Thinnest part of skull
   • Overlies middle meningeal artery

4. Temporal Squama:

   • Thin temporal bone
   • Must be removed flush to allow temporal lobe decompression

5. Dura:

   • Opened widely (cruciate or stellate incision)
   • Duroplasty performed to allow brain expansion

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Examiner: How would you manage this patient's ICP if surgery were delayed?

Candidate: This is an emergency requiring immediate evacuation. However, if there were any delay:

Immediate Measures:

1. Airway: Already intubated - ensure tube secure
2. Head position: 30° elevation, head midline, avoid neck rotation (impedes jugular drainage)
3. Ventilation: Target normocapnia (PaCO2 35-40 mmHg)
   • Brief hyperventilation to 30-35 mmHg only as temporary bridge to surgery
4. Osmotherapy:
   • Mannitol 0.5-1 g/kg IV bolus
   • OR Hypertonic saline 3% (150-250mL) or 23.4% (30mL via central line)
5. Sedation: Propofol or midazolam to reduce metabolic demand
6. Blood pressure: Maintain CPP >60-70 mmHg (CPP = MAP - ICP)
7. Temperature: Avoid hyperthermia (increases metabolic demand)
8. Glucose: Avoid hyperglycaemia
9. Seizure prophylaxis: Consider levetiracetam

Targets:

• ICP <22 mmHg (BTF guidelines)
• CPP 60-70 mmHg
• PaCO2 35-40 mmHg
• Temperature 36-37°C

The priority is surgical decompression - medical management is temporizing only.

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17. MCQ Practice Questions

Question 1

A 65-year-old man presents with sudden onset of right arm and face weakness with preserved leg strength, and difficulty producing speech. He is right-handed. Which artery territory is most likely affected?

A. Left anterior cerebral artery B. Left middle cerebral artery C. Right middle cerebral artery D. Left posterior cerebral artery E. Basilar artery

Answer: B

Explanation: The presentation of right face and arm weakness with preserved leg strength and expressive aphasia (difficulty producing speech in a right-handed person) indicates a left MCA territory stroke. The MCA supplies the lateral hemisphere including motor cortex for face and arm (leg is on the medial surface, supplied by ACA) and Broca's area (speech production) in the dominant (left) hemisphere. Right MCA stroke would cause left-sided weakness and typically neglect rather than aphasia.

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Question 2

Which of the following structures passes through the tentorial incisura?

A. Superior sagittal sinus B. Internal carotid artery C. Oculomotor nerve (CN III) D. Facial nerve (CN VII) E. Transverse sinus

Answer: C

Explanation: The tentorial incisura (hiatus) is the opening in the tentorium cerebelli through which the midbrain passes. Contents include: the midbrain, posterior cerebral arteries, superior cerebellar arteries, CN III, and CN IV. The oculomotor nerve (CN III) runs along the edge of the incisura, making it vulnerable to compression in uncal herniation. The superior sagittal sinus and transverse sinus are within the dura at the falx and tentorium attachments. The internal carotid artery passes through the cavernous sinus, not the tentorial incisura.

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Question 3

In a patient with uncal herniation from a right-sided mass, which pupil dilates first?

A. Left pupil (contralateral) B. Right pupil (ipsilateral) C. Both pupils simultaneously D. Neither - pupils remain normal E. Depends on site of lesion

Answer: B

Explanation: In uncal herniation, the ipsilateral pupil dilates first. This is because the uncus (medial temporal lobe) on the side of the mass herniates through the tentorial incisura and compresses the ipsilateral CN III. The parasympathetic fibres for pupil constriction run on the external surface of CN III and are affected first by external compression, causing loss of parasympathetic tone and pupil dilation. The contralateral pupil may dilate later as the brainstem is compressed and the contralateral CN III becomes involved.

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Question 4

A patient has a stroke with complete homonymous hemianopia but intact motor function and language. Which artery territory is affected?

A. Anterior cerebral artery B. Middle cerebral artery C. Posterior cerebral artery D. Anterior choroidal artery E. Posterior communicating artery

Answer: C

Explanation: The posterior cerebral artery (PCA) supplies the occipital lobe, including the primary visual cortex along the calcarine sulcus. PCA territory stroke classically causes contralateral homonymous hemianopia without motor or language deficits (which are MCA territory). The PCA also supplies the inferomedial temporal lobe (memory) and posterior thalamus. MCA stroke would typically cause hemiparesis and aphasia/neglect. ACA stroke causes leg weakness. Anterior choroidal artery can cause hemianopia but typically with hemiplegia and hemisensory loss.

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Question 5

What is the rate of CSF production by the choroid plexus?

A. 50 mL/day B. 150 mL/day C. 500 mL/day D. 1000 mL/day E. 2000 mL/day

Answer: C

Explanation: The choroid plexus produces approximately 500 mL of CSF per day (about 0.35-0.4 mL/min). The total CSF volume is approximately 150 mL, meaning CSF turns over 3-4 times per day. CSF is absorbed primarily by arachnoid granulations into the superior sagittal sinus. This production rate is relatively constant but absorption is pressure-dependent. In hydrocephalus, either production exceeds absorption (communicating) or flow is obstructed (non-communicating).