Cranial Nerve Examination Viva

Short Case Viva Defence — Case A: Pupil-Involving Third Nerve Palsy

Candidate's opening statement (model answer)

"I examined Mrs Walsh, a 58-year-old woman with hypertension, who presents with a sudden severe right periorbital headache and double vision. On examination of the cranial nerves, the right eye is deviated down and out at rest with a complete ptosis. The right eye fails to adduct, elevate and depress, while abduction is preserved. Critically, the right pupil is 6 millimetres and unresponsive to light, while the left pupil is 3 millimetres and reactive. The swinging-flashlight test shows no relative afferent pupillary defect. The remaining cranial nerves, including the facial nerve and the tongue, are intact. [1]

In summary, this patient has a complete right third nerve palsy with the pupil involved. A pupil-involving complete palsy is compressive until proven otherwise; the classical cause is a posterior communicating artery aneurysm, and the sudden severe headache raises the possibility of a sentinel leak. This is a neurosurgical emergency, and my immediate step is urgent CT angiography of the intracranial circulation." [1]

Examiner probing questions and model answers

Q1: "Why does pupil involvement change your management?" [1]

"Because the parasympathetic fibres that constrict the pupil travel on the surface of the third nerve, while the motor fibres to the extraocular muscles travel in its centre. A compressive lesion, such as an expanding aneurysm at the posterior communicating artery to internal carotid junction, presses on the nerve from outside and picks off the surface pupillary fibres first, producing a dilated unresponsive pupil before or alongside the motor palsy. A microvascular, ischaemic palsy, by contrast, infarcts the centre of the nerve in a patient with diabetes or hypertension and spares the pupil. So a pupil-involving palsy points to a compressive, surgical lesion, and a pupil-sparing palsy points to a medical, microvascular one. The reason pupil involvement is so urgent is that an expanding aneurysm may rupture into a subarachnoid haemorrhage, which carries a high mortality, so I cannot afford to observe — I must image and, if confirmed, secure the aneurysm." [1]

Q2: "How sensitive is the rule of the pupil, and how do you apply it safely?" [1]

"It is a guide, not a law. The classic teaching — pupil-sparing and complete is microvascular, pupil-involving is compressive — holds in most cases, but there are two important exceptions. First, a pupil-involving palsy can rarely be microvascular, particularly in poorly controlled diabetes, where the ischaemia can involve the pupillary fibres. Second, and more dangerously, a compressive lesion can present with a pupil-sparing palsy, especially early, when it is partial or evolving; the aneurysm may not yet have compressed the surface fibres. So the safe application is this: a pupil-involving palsy is always imaged urgently. A pupil-sparing palsy that is complete and fits the microvascular pattern, in a patient with vascular risk factors, may be observed — but I re-examine the pupil every few hours for the first day or two, and any pupillary change, any incomplete palsy, or any severe pain prompts urgent imaging. In practice, many neuro-ophthalmologists image any new third nerve palsy that does not perfectly fit the microvascular pattern." [1]

Q3: "What would you do if the CT angiogram is negative?" [1]

"If the CT angiogram is negative but my clinical suspicion remains high — a painful, pupil-involving palsy — I proceed to digital subtraction angiography, which is the gold standard and can detect smaller aneurysms that CTA misses. I would also reconsider the differential: a cavernous sinus lesion such as a tumour, thrombosis or carotid-cavernous fistula can cause a painful third nerve palsy and may involve the neighbouring fourth, sixth and the first two divisions of the trigeminal nerve, so I would examine specifically for those. If the angiogram and imaging are negative and the picture is one of a painful pupil-involving palsy, I would involve neuro-ophthalmology and consider rare causes such as an inflammatory or infiltrative process. A microvascular cause becomes more likely if the vasculopathic risk factors are strong and the palsy resolves over the expected three months." [1]

Short Case Viva Defence — Case B: Horner Syndrome

Candidate's opening statement (model answer)

"I examined Mr Chen, a 47-year-old man, who presents with a droopy right eyelid noticed by his wife. On examination there is a partial ptosis on the right of a few millimetres, the right pupil is 2 millimetres and the left is 4 millimetres, and both pupils constrict briskly to light and on accommodation. The right side of his face is noticeably drier than the left. Visual acuity, eye movements and the rest of the cranial nerves are normal. There are no other focal signs. [1]

In summary, this patient has a right Horner syndrome — the triad of partial ptosis, miosis and ipsilateral facial anhidrosis, from interruption of the sympathetic chain to the eye. The most urgent cause I must exclude is a carotid artery dissection, which is a treatable cause of ischaemic stroke, so I would ask specifically about neck pain and take a careful history, and arrange urgent imaging of the carotid and the sympathetic pathway." [1]

Examiner: "How do you confirm and localise a Horner syndrome?" [1]

"Pharmacologically, I can confirm it with apraclonidine drops, which have largely replaced cocaine. Apraclonidine is a weak alpha-1 agonist; in a Horner pupil, which has lost its sympathetic innervation and developed denervation supersensitivity at the alpha-1 receptor, apraclonidine causes the pupil to dilate, whereas in a normal pupil it causes mild constriction. So reversal of the anisocoria after apraclonidine confirms Horner syndrome. Localising the level uses both the clinical pattern and imaging. The three-neuron chain runs from the hypothalamus (first-order), through the spinal cord to the ciliospinal centre at T1 and up over the lung apex (second-order, preganglionic), and with the carotid into the cavernous sinus (third-order, postganglionic). The distribution of anhidrosis helps: central and preganglionic lesions cause hemifacial anhidrosis, because the sweat fibres to the face travel with the external carotid, whereas a postganglionic lesion causes anhidrosis confined to the forehead. In the modern era I rely on MRI of the sympathetic pathway rather than stepwise hydroxyamphetamine drop testing, because the imaging can identify a carotid dissection, a Pancoast tumour, or a brainstem stroke in a single study [3]."

Examiner: "Which Horner syndrome most concerns you, and why?" [1]

"Any acute or painful Horner syndrome concerns me, because the two treatable, dangerous causes are a carotid artery dissection and a preganglionic lesion from a Pancoast tumour. A carotid dissection classically presents with ipsilateral neck or face pain, a Horner syndrome, and a risk of ischaemic stroke from thromboembolism or distal hypoperfusion; it needs urgent MR or CT angiography of the carotid and, in many cases, antithrombotic therapy. A Pancoast tumour at the lung apex presents with arm pain from brachial plexus involvement, a preganglionic Horner, and sometimes hand muscle wasting; it needs a chest X-ray then CT and an urgent oncology referral. A central Horner from a lateral medullary stroke presents with other brainstem signs such as ataxia, crossed sensory loss and dysphagia. So a painful Horner syndrome is a carotid dissection until I have excluded it, and a Horner with arm pain is a Pancoast until I have excluded it." [1]

Short Case Viva Defence — Case C: Lower Motor Neuron Facial Palsy

Candidate's opening statement (model answer)

"I examined Mr Davies, a 33-year-old man, who woke with painless drooping of the left side of his face. On examination the entire left hemiface is weak: the left forehead cannot be wrinkled, the left eye cannot be closed and the globe rolls upward on the attempt, and the left mouth angle droops with escape of air on puffing the cheek. Taste on the anterior left tongue is reduced. The ear canal, the tympanic membrane, the parotid gland and the limbs are normal. There are no vesicles. [1]

In summary, this is a lower motor neuron facial nerve palsy involving the entire left hemiface, with the forehead paralysed. With no ear, parotid or middle-ear signs, this is consistent with Bell palsy. My management is oral corticosteroids started within 72 hours of onset, together with rigorous eye protection." [1]

Examiner: "How do you know this is lower motor neuron and not upper motor neuron?" [1]

"Because the forehead is involved. The upper facial muscles, frontalis and orbicularis oculi, receive bilateral cortical innervation from both motor cortices. An upper motor neuron lesion, such as a hemisphere stroke, spares the forehead because the intact ipsilateral cortical input keeps it working, even though the lower face on the opposite side is paralysed. A lower motor neuron lesion of the facial nerve itself abolishes the entire hemiface, including the forehead, because there is no cortical input at all downstream of the lesion. Mr Davies cannot wrinkle his forehead or close his eye, so his lesion is in the nerve, distal to the nucleus — a lower motor neuron palsy. The same principle applies to the tongue: an upper motor neuron hypoglossal lesion deviates the tongue away from the cortical lesion without wasting, while a lower motor neuron lesion deviates it toward the lesion with wasting and fasciculation." [1]

Examiner: "What is the evidence for corticosteroids, and what about antivirals?" [1]

"The landmark evidence is the Sullivan 2007 New England Journal factorial trial, which randomised patients with Bell palsy within 72 hours of onset to prednisolone, aciclovir, both, or placebo. Prednisolone significantly improved complete recovery — 83 per cent versus 64 per cent at 3 months and 94 per cent versus 82 per cent at 9 months — while aciclovir conferred no benefit, alone or in combination [1]. The Cochrane review of corticosteroids confirms that they improve the rate of complete recovery, and the Cochrane review of antivirals shows no, or at most a marginal, added benefit [2]. So my treatment is prednisolone — typically 60 milligrams daily for five days then tapering — and I do not routinely add antivirals. I would add them if I suspected Ramsey Hunt syndrome, where herpes zoster vesicles are present in the ear and the prognosis is worse. The third essential element is eye protection: because the eye cannot close, I prescribe preservative-free lubricants by day and ointment and lid taping by night to prevent exposure keratopathy, and I review the eye."

Examiner: "Why is the relative afferent pupillary defect so important in cranial nerve examination?" [1]

"Because it is the most sensitive clinical sign of an optic neuropathy, more reliable than acuity or the appearance of the optic disc in early disease [4]. The swinging-flashlight test compares the afferent input of the two optic nerves: when the light swings into an eye with an optic nerve lesion, the reduced afferent drive causes both pupils to dilate — a paradoxical dilation that signals a Marcus Gunn pupil. The defect is relative, so bilateral symmetric optic nerve disease produces no RAPD. It localises the problem to the retina or optic nerve, which is exactly the information I need before reaching for a scan, and it is the test most often omitted by candidates in the short case. I test it on every patient whose acuity is reduced or whose disc looks abnormal."