Myotonic Dystrophy (DM1 & DM2)

1. Clinical Overview

Myotonic dystrophy (DM) is the most common inherited muscular dystrophy in adults, with a prevalence of approximately 1 in 8,000 for DM type 1 (DM1). [1] It represents a multisystem disorder characterized by progressive muscle weakness and wasting, myotonia (impaired muscle relaxation after contraction), and involvement of multiple organ systems including the heart, eyes, endocrine system, and central nervous system. [2]

There are two genetically distinct forms: myotonic dystrophy type 1 (DM1, also known as Steinert disease) and myotonic dystrophy type 2 (DM2). DM1 is caused by an expansion of CTG trinucleotide repeats in the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene on chromosome 19q13.3. [3] DM2 results from a CCTG tetranucleotide repeat expansion in intron 1 of the cellular nucleic acid-binding protein (CNBP) gene on chromosome 3q21. [4] Both conditions follow an autosomal dominant inheritance pattern with the phenomenon of anticipation, where disease severity increases and age of onset decreases in successive generations.

The clinical significance extends beyond neuromuscular manifestations. Cardiac involvement, particularly conduction abnormalities, represents the leading cause of sudden death in DM1 patients, with CTG repeat length correlating with cardiac event risk. [5] Anesthetic complications pose substantial risk, with respiratory depression and prolonged neuromuscular blockade being major concerns. [6] The congenital form of DM1, occurring when mothers with large CTG expansions transmit the mutation, presents with severe hypotonia, respiratory failure, and intellectual disability. [7]

Management remains primarily supportive, focusing on multidisciplinary surveillance and intervention for cardiac, respiratory, ophthalmologic, and endocrine complications. Annual ECG screening is critical, with pacemaker or implantable cardioverter-defibrillator (ICD) insertion recommended for significant conduction delays. [8]

2. Epidemiology

Prevalence and Incidence

The prevalence of DM1 exhibits marked geographic variation due to founder effects. [1] The Saguenay-Lac-Saint-Jean region of Quebec demonstrates the world's highest prevalence (approximately 1 in 500) due to a founder mutation. [2] DM2 prevalence is less well-established but appears more common in populations of Northern European descent. [4]

Demographic Features

Age Distribution:

• Congenital DM1: Present at birth with severe hypotonia and respiratory insufficiency
• Childhood-onset DM1: Typically 1-10 years with cognitive and behavioral difficulties
• Adult-onset DM1 (Classic): Usually 20-40 years, most common presentation
• Late-onset DM1 (Mild): > 40 years, often limited to cataracts and mild myotonia
• DM2: No congenital form; typical onset 30-60 years

The phenomenon of anticipation means that successive generations experience earlier onset and more severe disease, particularly through maternal transmission in DM1. [7]

3. Aetiology and Pathophysiology

Genetic Basis

DM1 (Chromosome 19q13.3):

• Gene: DMPK (dystrophia myotonica protein kinase)
• Mutation: CTG trinucleotide repeat expansion in 3' UTR
• Normal: 5-34 CTG repeats
• Premutation: 35-49 repeats (unstable, may expand in subsequent generations)
• Mild DM1: 50-150 repeats
• Classic DM1: 100-1,000 repeats
• Congenital DM1: > 1,000 repeats (typically > 1,500) [3]

DM2 (Chromosome 3q21):

• Gene: CNBP (cellular nucleic acid-binding protein, formerly ZNF9)
• Mutation: CCTG tetranucleotide repeat expansion in intron 1
• Normal: less than 26 CCTG repeats
• DM2: 75-11,000 repeats (mean ~5,000) [4]

Exam Detail: ### Molecular Pathogenesis: RNA Toxicity Mechanism

DM1 and DM2 exemplify RNA gain-of-function disorders rather than loss of protein function. The expanded repeats in the mutant RNA transcripts cause disease through a toxic RNA mechanism. [9]

The "Toxic RNA" Hypothesis:

1. Nuclear Accumulation: Expanded CUG (in DM1) or CCUG (in DM2) repeat-containing RNA forms stable hairpin structures that accumulate in discrete nuclear foci (RNA "foci") visible on fluorescence in situ hybridization (FISH). [9]

2. Sequestration of Splicing Regulators: These toxic RNA hairpins act as molecular "sponges," sequestering essential RNA-binding proteins, particularly members of the Muscleblind-like (MBNL) family (MBNL1, MBNL2, MBNL3). [10]

3. Aberrant Alternative Splicing: Loss of functional MBNL proteins leads to mis-splicing of numerous pre-mRNAs, causing a reversion to fetal splice patterns in adult tissues. [10] Key mis-splicing events include:

   • CLCN1 (Chloride channel 1): Mis-splicing reduces chloride conductance in muscle membranes → membrane hyperexcitability → myotonia [11]
   • INS-R (Insulin receptor): Inclusion of fetal exon 11 → reduced insulin sensitivity → insulin resistance and diabetes [11]
   • SCN5A (Cardiac sodium channel): Aberrant splicing → cardiac conduction defects [12]
   • RYR1 (Ryanodine receptor 1): Mis-splicing contributes to muscle weakness [10]
   • TNNT2 (Cardiac troponin T): Fetal isoform expression → cardiomyopathy [12]
   • MAPT (Tau protein): Altered splicing → CNS involvement [13]

4. CELF1 Upregulation: In addition to MBNL sequestration, there is compensatory upregulation and hyperphosphorylation of CUGBP-Elav-like family member 1 (CELF1), another splicing regulator, which further dysregulates splicing. [10]

CTG Repeat Instability and Somatic Mosaicism:

CTG repeats exhibit marked instability, particularly during maternal meiosis and early embryonic development. [7] This accounts for:

• Anticipation: Progressive expansion through generations
• Somatic mosaicism: Different repeat lengths in different tissues
• Tissue-specific severity: Longer repeats in more severely affected tissues

DM1 vs DM2 Pathogenic Differences:

While both involve RNA toxicity through MBNL sequestration, DM2 differs in:

• No congenital form (larger fetal brain expression differences)
• Milder phenotype overall
• More proximal muscle involvement
• Greater repeat size variability
• Different tissue-specific expression patterns of CNBP vs DMPK [4]

4. Clinical Presentation

DM1 Clinical Phenotypes

DM1 demonstrates remarkable phenotypic heterogeneity based on CTG repeat length and age of onset:

Congenital Myotonic Dystrophy (CDM)

CDM represents the most severe form, occurring exclusively through maternal transmission with repeat sizes typically > 1,500 CTG. [7]

Presentation:

• Severe generalized hypotonia ("floppy infant")
• Respiratory failure requiring ventilatory support at birth
• Facial diplegia with "tented" or "carp" mouth appearance
• Feeding difficulties with poor suck and swallow
• Talipes equinovarus (club feet)
• Polyhydramnios (in utero due to impaired fetal swallowing)
• Delayed motor milestones
• Intellectual disability (mean IQ 70-80)

Outcome: Mortality in neonatal period is 25-50% due to respiratory complications. [7] Survivors have significant developmental delays and lifelong disability.

Childhood-Onset DM1

Typically presents at ages 1-10 years with:

• Learning difficulties and poor school performance
• Behavioral problems (ADHD-like symptoms, autism spectrum features)
• Facial weakness with myopathic facies
• Speech difficulties (dysarthria)
• Delayed motor development
• Mild limb weakness

Myotonia is often clinically absent or mild in childhood.

Classic Adult-Onset DM1 (Most Common)

Onset typically 20-40 years with 100-1,000 CTG repeats.

Neuromuscular Manifestations:

Myotonia:

• Grip myotonia: Difficulty releasing handshake; improved with repeated contractions ("warm-up phenomenon")
• Percussion myotonia: Sustained muscle contraction after percussion, particularly thenar eminence (classic bedside test)
• Eyelid myotonia: Slow eyelid opening after forced eye closure
• Tongue myotonia: Longitudinal furrow after percussion
• Worsened by cold temperatures [2]

Weakness Distribution (Distal-Predominant):

• Facial muscles: Bilateral facial weakness, ptosis, temporal wasting
• Sternocleidomastoid weakness: "Swan neck" appearance
• Forearm and hand muscles: Wrist extensors, finger extensors, intrinsic hand muscles
• Ankle dorsiflexors: Foot drop, steppage gait
• Bulbar muscles: Dysphagia, dysarthria, nasal speech

Characteristic Physical Signs:

• "Hatchet face": Temporal and masseter muscle wasting creating narrow, elongated facial appearance
• Frontal balding (males) – highly characteristic
• Drooping eyelids without compensatory frontalis contraction
• Expressionless "myopathic" facies

Late-Onset (Mild) DM1

Onset > 40 years with 50-100 CTG repeats.

• Cataracts may be the only manifestation
• Mild myotonia detected only on examination or EMG
• Minimal weakness
• Often diagnosed only after family member receives diagnosis

DM2 Clinical Phenotype

DM2 (also called PROMM: Proximal Myotonic Myopathy) differs significantly from DM1:

Key Features:

• Proximal-predominant weakness: Neck flexors, hip flexors, hip extensors, finger flexors
• Proximal muscle pain and stiffness (often the presenting symptom)
• Milder myotonia (clinically less apparent than DM1)
• Later onset: Typically 30-60 years
• No congenital form
• Milder cardiac involvement (though still clinically significant)
• Similar multisystem involvement: Cataracts, insulin resistance, CNS changes [4]

Multisystem Involvement

Exam Detail: #### Cardiac Manifestations

Cardiac involvement is the leading cause of death in DM1, accounting for up to 30% of mortality. [5]

Conduction System Disease:

• Progressive conduction delays: First-degree AV block (PR > 200ms) → second-degree → complete heart block
• Bundle branch blocks: RBBB, LBBB, bifascicular block
• Intraventricular conduction delay: QRS > 120ms
• Atrial arrhythmias: Atrial flutter, atrial fibrillation
• Ventricular arrhythmias: Ventricular tachycardia (risk of sudden cardiac death)

CTG repeat length correlates with cardiac event risk. Patients with PR > 240ms or QRS duration > 120ms have significantly increased risk of high-degree AV block and sudden death. [5,8]

Cardiomyopathy:

• Less common than conduction disease
• Dilated cardiomyopathy may develop
• Systolic dysfunction typically occurs late

Management Implications:

• Annual ECG mandatory for all DM patients
• Holter monitoring if symptoms or ECG abnormalities
• Pacemaker indications: PR ≥240ms, any degree of AV block, symptomatic bradycardia [8]
• ICD consideration: For ventricular arrhythmias or severely reduced ejection fraction

Ophthalmologic Manifestations

Posterior Subcapsular Cataracts:

• May be earliest manifestation (even in asymptomatic "premutation" carriers)
• "Christmas tree" or "polychromatic" appearance: Iridescent, multicolored refractile deposits in lens cortex on slit-lamp examination (pathognomonic)
• Occur earlier than age-related cataracts (30s-40s)
• Require surgical extraction when symptomatic [2]

Additional Ocular Features:

• Ptosis (part of facial weakness)
• External ophthalmoplegia (rare)
• Retinal abnormalities (rare)

Endocrine and Metabolic Manifestations

Insulin Resistance and Diabetes Mellitus:

• Prevalence 10-40% depending on cohort
• Mechanism: Mis-splicing of insulin receptor leads to reduced insulin sensitivity [11]
• Type 2 diabetes phenotype

Gonadal Dysfunction:

• Males: Testicular atrophy, reduced testosterone, oligospermia/azoospermia, erectile dysfunction
• Females: Irregular menses, increased risk of obstetric complications (see below)
• Mechanism involves both hypothalamic-pituitary-gonadal axis dysfunction and primary gonadal failure

Thyroid Dysfunction:

• Hypothyroidism reported in some patients
• May contribute to fatigue and weakness

Growth Hormone Deficiency:

• Occasional finding, may contribute to muscle wasting

Gastrointestinal Manifestations

Smooth muscle involvement causes:

• Dysphagia: Both pharyngeal and esophageal phases affected; aspiration risk
• Delayed gastric emptying: Nausea, early satiety
• Intestinal pseudo-obstruction: Abdominal pain, constipation, diarrhea
• Gallstones: Increased incidence
• Fecal incontinence: Anal sphincter dysfunction

Respiratory Manifestations

Progressive respiratory insufficiency is major cause of morbidity and mortality:

Mechanisms:

• Respiratory muscle weakness: Diaphragm, intercostals, accessory muscles
• Central hypoventilation: Reduced chemoreceptor sensitivity to CO₂ and hypoxia
• Obstructive sleep apnea: Due to pharyngeal muscle weakness and obesity
• Aspiration pneumonia: Secondary to dysphagia

Clinical Features:

• Nocturnal hypoventilation (earliest sign): Morning headaches, daytime somnolence
• Exertional dyspnea
• Orthopnea
• Recurrent chest infections
• Type 2 respiratory failure in advanced disease

Assessment and Management:

• Pulmonary function tests (FVC, MIP/MEP)
• Overnight oximetry or polysomnography
• Arterial blood gases if symptomatic
• Non-invasive ventilation (NIV) for nocturnal hypoventilation [2]
• Aggressive treatment of respiratory infections
• Vaccination (influenza, pneumococcal)

Central Nervous System Manifestations

Cognitive and Behavioral Features:

• Executive dysfunction: Planning, organization, decision-making deficits
• Apathy and lack of initiative ("avoidant personality")
• Excessive daytime somnolence: Central origin (not purely due to sleep apnea)
• Social cognition deficits: Difficulty recognizing emotions
• Reduced IQ (particularly in congenital and childhood-onset forms)
• Visuospatial difficulties

Neuroimaging shows:

• White matter hyperintensities on MRI
• Cerebral atrophy (frontal and temporal lobes)
• Ventricular enlargement [13]

Management:

• Modafinil: Effective for excessive daytime sleepiness (100-400mg/day) [2]
• Neuropsychological support
• Occupational therapy

Brain Involvement in DM2: DM2 also shows CNS involvement with white matter changes, though cognitive deficits may be less severe than DM1. [4]

5. Investigations

Genetic Testing (Definitive Diagnosis)

Molecular Genetic Testing:

For DM1:

• PCR-based assays: Detect small CTG repeat expansions (less than 100 repeats)
• Southern blot analysis: Required for large expansions (> 100 repeats), which are common in classic and congenital DM1 [3]
• Repeat-primed PCR: Can detect expansions and estimate size range

For DM2:

• Repeat-primed PCR: Preferred method for CCTG repeat detection
• Southern blot: May be needed for sizing large expansions [4]

Important Considerations:

• Somatic mosaicism: Repeat length varies among tissues; blood DNA may underestimate muscle tissue repeat length
• Genetic counseling: Essential before testing due to:
  • Anticipation and reproductive implications
  • Predictive testing considerations
  • Psychosocial impact
  • No disease-modifying treatment currently available

Electrophysiology

Electromyography (EMG):

Classic Myotonic Discharges:

• "Dive-bomber" or "motorcycle" sound: High-frequency discharges (20-80 Hz) that wax and wane in both amplitude and frequency
• Triggered by needle insertion or muscle percussion
• Persist after movement stops
• Present in clinically unaffected muscles

Myopathic Motor Unit Potentials:

• Short duration
• Low amplitude
• Polyphasic
• Early recruitment

Nerve Conduction Studies:

• Usually normal motor and sensory conduction
• May show reduced compound muscle action potential (CMAP) amplitudes in weak muscles

Cardiac Investigations

Electrocardiogram (ECG) - Annual Mandatory:

• PR interval: Prolongation is earliest and most common finding
• QRS duration: > 120ms associated with increased risk [5,8]
• QT interval: May be prolonged
• Conduction blocks: First-degree, second-degree, third-degree AV block; bundle branch blocks
• Arrhythmias: Atrial flutter/fibrillation, ventricular ectopy

24-Hour Holter Monitoring:

• Indicated for:
  • Symptoms (palpitations, presyncope, syncope)
  • ECG abnormalities (PR > 200ms, QRS > 120ms)
  • Family history of sudden cardiac death
• Detects: Asymptomatic arrhythmias, conduction abnormalities, heart rate variability

Echocardiography:

• Baseline and if symptoms develop
• Assess for:
  • Left ventricular systolic dysfunction
  • Diastolic dysfunction
  • Valvular abnormalities
  • Chamber enlargement

Electrophysiological Study (EPS):

• May be indicated for:
  • Unexplained syncope
  • Severe conduction disease
  • Risk stratification for ICD placement

Respiratory Assessment

Pulmonary Function Tests:

• Forced Vital Capacity (FVC): Reduced in respiratory muscle weakness
• Maximum Inspiratory Pressure (MIP): Marker of diaphragm strength
• Maximum Expiratory Pressure (MEP): Assesses expiratory muscle strength
• Serial monitoring (annually or more frequently if symptomatic)

Sleep Studies:

• Overnight oximetry: Screening for nocturnal hypoxemia
• Polysomnography: Comprehensive assessment if oximetry abnormal or symptoms suggestive of sleep-disordered breathing
• Identifies: Obstructive sleep apnea, central hypoventilation, REM-related hypoventilation

Arterial Blood Gas:

• If daytime symptoms or reduced FVC
• May show chronic hypercapnia (elevated PaCO₂) indicating ventilatory failure

Ophthalmologic Assessment

Slit-Lamp Examination:

• All patients at diagnosis and periodically
• Detects characteristic posterior subcapsular cataracts
• "Christmas tree" cataracts pathognomonic even in early stages [2]

Muscle Biopsy (Rarely Needed)

Diagnosis is typically made genetically. Biopsy findings if performed:

• Increased fiber size variability
• Increased central nuclei
• Type 1 fiber predominance and atrophy
• Ring fibers (peripheral myofibrils surround muscle fiber core)
• Sarcoplasmic masses
• Nonspecific changes; not diagnostic

Endocrine and Metabolic Screening

• HbA1c / Fasting glucose: Screen for diabetes mellitus
• Lipid profile: Assess cardiovascular risk
• Thyroid function tests: TSH, free T4
• Testosterone (males with symptoms of hypogonadism)
• Calcium, vitamin D: Assess bone health

6. Differential Diagnosis

7. Management

General Principles

There is no disease-modifying treatment for myotonic dystrophy. Management focuses on:

1. Surveillance for complications
2. Symptomatic treatment
3. Preventive interventions (particularly cardiac)
4. Multidisciplinary coordination
5. Genetic counseling and family planning support

Cardiac Management

Surveillance Protocol:

• Annual ECG for all patients (mandatory) [8]
• Holter monitoring: If symptomatic, ECG abnormalities, or high-risk features
• Echocardiography: Baseline and if cardiac symptoms
• Cardiology referral: For PR ≥240ms, QRS ≥120ms, any arrhythmia, or symptoms

Pacemaker Indications (Per ACC/AHA/HRS Guidelines): [8]

• PR interval ≥240ms (even if asymptomatic)
• Any degree of second-degree or third-degree AV block
• Alternating bundle branch block
• Symptomatic bradycardia
• Syncope with conduction abnormalities

ICD Considerations:

• Sustained ventricular tachycardia
• Left ventricular ejection fraction less than 35%
• Family history of sudden cardiac death with high-risk features
• Syncope with inducible VT on EPS

Anesthesia Precautions: Cardiac assessment is mandatory before any general anesthesia. DM patients have increased perioperative mortality up to 8-10 times normal. [6]

Myotonia Management

Clinical Approach:

• Many patients adapt to myotonia and do not require pharmacological treatment
• Reassurance that "warm-up" reduces myotonia
• Avoid cold exposure
• Pharmacotherapy only if myotonia significantly impacts function

Pharmacological Options (If Needed):

Mexiletine:

• Mechanism: Class Ib antiarrhythmic; blocks sodium channels, reducing membrane hyperexcitability
• Dosing: 150-200mg two to three times daily
• Efficacy: Most evidence-based antimyotonic agent [2]
• Caution: Pro-arrhythmic in structural heart disease; cardiology consultation essential before initiation
• Monitoring: ECG before and during treatment

Alternatives (Less Evidence):

• Phenytoin: 100mg twice daily
• Lamotrigine: 25-100mg twice daily
• Generally mexiletine preferred when treatment needed

Contraindicated Medications:

• Suxamethonium (succinylcholine): Can trigger life-threatening generalized myotonic contracture; absolutely contraindicated [6]
• Statins: May exacerbate weakness and myotonia in some patients; use with caution

Respiratory Management

Surveillance:

• Annual pulmonary function tests (FVC, MIP, MEP)
• Overnight oximetry if symptoms or FVC less than 60% predicted
• Polysomnography if oximetry abnormal or excessive daytime sleepiness

Interventions:

• Non-invasive ventilation (NIV): For nocturnal hypoventilation; improves daytime function and quality of life [2]
• Cough assist devices: If weak cough (MEP less than 60 cmH₂O)
• Aggressive management of respiratory infections: Low threshold for antibiotics
• Vaccinations: Annual influenza; pneumococcal (PCV13 + PPSV23)
• Chest physiotherapy: For secretion clearance
• Tracheostomy: May be necessary in advanced disease or congenital DM

Anesthetic Considerations:

• Extreme sensitivity to sedatives, opioids, and anesthetic agents
• Risk of prolonged post-operative respiratory depression
• Post-operative ventilation may be required
• Regional anesthesia preferred when possible [6]

Musculoskeletal Management

Physical and Occupational Therapy:

• Moderate aerobic exercise: Improves conditioning without worsening weakness
• Resistance training: Light to moderate intensity; avoid overwork
• Stretching: Maintain range of motion
• Orthotics: Ankle-foot orthoses (AFOs) for foot drop
• Assistive devices: Canes, walkers, wheelchairs as needed
• Fall prevention strategies
• Adaptive equipment: For activities of daily living

Pain Management (Particularly DM2):

• Acetaminophen (paracetamol)
• NSAIDs (with caution if cardiac/renal involvement)
• Neuropathic pain agents: Gabapentin, pregabalin, duloxetine
• Avoid opioids if possible due to respiratory risks

Ophthalmologic Management

• Slit-lamp examination: At diagnosis and periodically
• Cataract surgery: When visually significant; standard phacoemulsification
• Anesthetic caution: For intraocular surgery; monitor for respiratory depression with sedation

Gastrointestinal Management

• Dysphagia: Speech and language therapy assessment; modified diet consistency; consider videofluoroscopy
• Gastroparesis: Small frequent meals; metoclopramide or domperidone (caution with QT prolongation)
• Constipation: Adequate hydration, fiber, laxatives as needed
• Pseudo-obstruction: Usually managed conservatively; surgical intervention risky

Endocrine Management

• Diabetes mellitus: Standard glycemic control; metformin first-line
• Hypogonadism: Testosterone replacement in symptomatic males with confirmed deficiency (monitor for cardiac effects)
• Thyroid dysfunction: Levothyroxine for hypothyroidism

CNS and Behavioral Management

Excessive Daytime Sleepiness:

• Modafinil: 100-400mg daily in divided doses; most effective intervention [2]
• Exclude/treat obstructive sleep apnea and nocturnal hypoventilation first

Cognitive and Behavioral Support:

• Neuropsychological assessment
• Cognitive rehabilitation
• Educational support for children
• Occupational therapy for executive function strategies
• Psychosocial support and counseling

Obstetric Management

Pregnancy in DM1 women is high-risk:

Maternal Risks:

• Worsening weakness during pregnancy
• Increased risk of preterm labor
• Uterine atony and postpartum hemorrhage
• Anesthetic complications during delivery

Fetal Risks:

• 50% risk of inheriting mutation (autosomal dominant)
• Risk of congenital DM if large maternal CTG expansion
• Polyhydramnios
• Reduced fetal movements

Management:

• High-risk obstetric care
• Genetic counseling: Offer prenatal diagnosis (CVS or amniocentesis)
• Fetal monitoring: Ultrasound for polyhydramnios, reduced movements
• Delivery planning: Multidisciplinary team; anesthetic assessment; neonatal team present for delivery
• Postpartum: Monitor for hemorrhage; observe neonate for hypotonia, respiratory distress

Genetic Counseling and Family Planning

Key Points:

• Autosomal dominant inheritance: 50% risk to offspring
• Anticipation: Offspring may have earlier onset and more severe disease
• Maternal transmission: Higher risk of large expansions and congenital DM1
• Prenatal testing: Available via CVS (10-12 weeks) or amniocentesis (15-18 weeks)
• Preimplantation genetic diagnosis (PGD): Option for IVF with genetic screening
• Predictive testing: Available for at-risk relatives; requires careful counseling

Ethical Considerations:

• Genetic testing in minors generally deferred until decision-making capacity
• Psychological impact of diagnosis
• Employment and insurance implications (varies by jurisdiction)

8. Anesthetic Risks and Perioperative Management

Exam Detail: ### The High-Risk Anesthetic Patient

Patients with myotonic dystrophy have 8-10 times higher perioperative mortality than the general population. [6] Anesthetic complications are a leading cause of death.

Key Risks:

1. Extreme Sensitivity to Anesthetic Agents:

   • Opioids: Profound respiratory depression at normal doses
   • Benzodiazepines: Marked sedation and respiratory depression
   • Propofol and volatile anesthetics: Excessive myocardial depression
   • Thiopental: Prolonged apnea

2. Myotonic Crisis:

   • Succinylcholine (Suxamethonium): ABSOLUTELY CONTRAINDICATED [6]
   • Causes sustained generalized myotonic contracture → impossible to ventilate → cardiac arrest
   • Non-depolarizing neuromuscular blockers (rocuronium, vecuronium): Safe but may have prolonged effect

3. Cardiac Complications:

   • Arrhythmias during anesthesia
   • Heart block
   • Acute hemodynamic compromise

4. Respiratory Complications:

   • Postoperative respiratory failure (most common serious complication)
   • Inability to extubate
   • Aspiration due to bulbar weakness and delayed gastric emptying
   • Prolonged mechanical ventilation

5. Malignant Hyperthermia:

   • NO increased risk (common misconception)
   • Volatile anesthetics can be used if needed (though sensitivity to myocardial depression)

Perioperative Management Strategy

Preoperative Assessment:

• Cardiology clearance: ECG, consider Holter, echocardiography
• Pulmonary function tests: Baseline FVC, MIP, MEP
• Anesthesia consultation: Discuss risks, optimize patient
• Informed consent: Document increased risks

Anesthetic Technique:

• Regional/local anesthesia preferred when feasible (spinal, epidural, peripheral nerve blocks)
• If general anesthesia required:
  • Avoid succinylcholine (absolutely contraindicated)
  • Use minimal doses of all agents
  • Total intravenous anesthesia (TIVA) with propofol/remifentanil often preferred
  • "Non-depolarizing muscle relaxants: Use if necessary but anticipate prolonged effect; monitor with nerve stimulator"
  • Sugammadex available for rocuronium/vecuronium reversal (safe in DM)

Intraoperative Monitoring:

• Standard ASA monitors
• Neuromuscular monitoring if relaxants used
• Continuous ECG monitoring

Postoperative Care:

• High-dependency unit or ICU monitoring
• Prolonged observation (minimum 24 hours)
• Monitor for:
  • Respiratory depression (may be delayed onset)
  • Cardiac arrhythmias
  • Inadequate reversal of neuromuscular blockade
• Low threshold for postoperative ventilation
• Aggressive pulmonary toilet, incentive spirometry
• DVT prophylaxis (reduced mobility)

Day Surgery:

• Generally not recommended for patients with significant DM
• If minor procedure with local anesthesia only: Possible with extended observation

9. Prognosis and Natural History

Life Expectancy

DM1:

• Congenital: High neonatal mortality (25-50%); survivors have severe disability and reduced lifespan
• Classic adult-onset: Mean age of death 48-55 years (versus ~75 in general population)
• Late-onset/mild: Near-normal life expectancy

DM2:

• Generally milder with near-normal or slightly reduced life expectancy [4]

Causes of Death

1. Sudden cardiac death: 30% of DM1 deaths; arrhythmias or complete heart block [5]
2. Respiratory failure: Progressive weakness, pneumonia, aspiration
3. Cardiovascular disease: Heart failure, myocardial infarction
4. Perioperative complications: Anesthetic-related deaths

Disease Progression

DM1:

• Progressive weakness: Continues throughout life; rate varies
• Cardiac conduction: PR interval prolongs ~3ms/year; QRS duration increases ~1ms/year
• Wheelchair dependence: Variable; 10-20 years after symptom onset in classic form
• Functional decline: Gradual loss of independence

DM2:

• Slower progression than DM1
• Many patients remain ambulatory throughout life
• Pain may be the most disabling symptom

Quality of Life

Significantly impacted by:

• Physical disability and weakness
• Chronic pain (especially DM2)
• Excessive daytime sleepiness
• Cognitive and behavioral changes
• Social isolation
• Employment difficulties
• Psychosocial burden of genetic disease

Multidisciplinary support and symptomatic management improve quality of life.

10. Emerging Therapies and Future Directions

Exam Detail: While no disease-modifying treatment currently exists, several therapeutic strategies are in development:

RNA-Targeted Approaches

Antisense Oligonucleotides (ASOs):

• Designed to bind and degrade mutant DMPK or CNBP transcripts
• Reduces toxic RNA levels
• Preclinical studies show improvement in myotonia and splicing defects
• Clinical trials ongoing

Small Molecules Targeting RNA:

• Furamidine and derivatives: Bind to CUG repeats, disrupt MBNL sequestration [14]
• Restore normal splicing patterns in cell models
• Increase MBNL protein levels

AntimiR Therapy:

• MicroRNA-based approaches to reduce toxic RNA
• Shown efficacy in patient-derived primary cell models across various CTG repeat sizes [15]

Splicing Modulation

MBNL Protein Replacement:

• Adeno-associated virus (AAV)-mediated MBNL1 overexpression
• Rescues splicing defects in animal models
• Challenges: CNS penetration, immune responses

CELF1 Inhibition:

• Reduce hyperactive CELF1 to rebalance splicing
• Small molecule inhibitors in development

Genome Editing

CRISPR-Cas9 Approaches:

• Excision of CTG repeat expansions
• Disruption of DMPK transcription
• Proof-of-concept in cell and animal models
• Delivery and safety challenges for clinical translation

Symptomatic Treatments

Mexiletine:

• Currently available; most evidence for antimyotonic effect [2]
• Ongoing trials for optimization

Modafinil:

• Established for excessive daytime sleepiness [2]

11. Examination Focus: High-Yield Topics for MRCP/Neurology Exams

Viva Voce Scenarios

Scenario 1: The Diagnosis

Examiner: "A 35-year-old man presents with difficulty releasing his grip when shaking hands. What is your differential diagnosis?"

Model Answer:

• Primary consideration: Myotonic disorders
  • Myotonic dystrophy (DM1/DM2) - most common
  • Myotonia congenita (Thomsen/Becker)
  • Paramyotonia congenita
• Key distinguishing features:
  • "DM: Progressive weakness, multisystem involvement, autosomal dominant"
  • "Myotonia congenita: Pure myotonia without weakness, chloride channel mutation"
  • "Paramyotonia: Cold-induced, sodium channel mutation"
• Initial assessment:
  • "Detailed history: Family history, distribution of weakness, systemic symptoms"
  • "Examination: Percussion myotonia (thenar eminence), distribution of weakness (distal in DM1, proximal in DM2), facial appearance, cataracts"
  • EMG: Myotonic discharges ("dive-bomber")
  • "Genetic testing: CTG repeat analysis for DM1"

Follow-up: "How would you demonstrate myotonia clinically?"

Model Answer:

• Grip myotonia: Ask patient to make tight fist then quickly release; observe slow finger extension
• Percussion myotonia: Percuss thenar eminence with tendon hammer; observe sustained thumb adduction/flexion with slow relaxation (5-10 seconds)
• Eyelid myotonia: Forceful eye closure for 10 seconds then rapid opening; observe delayed eyelid elevation
• Tongue myotonia: Percussion of tongue creates longitudinal furrow
• Warm-up phenomenon: Repeated contractions reduce myotonia

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Scenario 2: The Cardiac Risk

Examiner: "You diagnose myotonic dystrophy in a 40-year-old woman. Her ECG shows PR interval of 250ms. What is your management?"

Model Answer:

• Cardiac conduction disease is leading cause of sudden death in DM1
• PR ≥240ms is an indication for pacemaker insertion (even if asymptomatic) per ACC/AHA guidelines [8]
• Additional assessment:
  • 24-hour Holter monitoring: Assess for higher-degree AV block, arrhythmias
  • "Echocardiography: Baseline left ventricular function"
  • "Cardiology referral: Urgent"
• Pacemaker indications in DM:
  • PR ≥240ms
  • Any second-degree or third-degree AV block
  • Alternating bundle branch block
  • QRS ≥120ms with symptoms
  • Syncope with conduction abnormalities
• ICD consideration if:
  • Ventricular arrhythmias
  • Severely reduced ejection fraction
  • Family history of sudden cardiac death
• Annual ECG mandatory for all DM patients thereafter

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Scenario 3: The Anesthetic Risk

Examiner: "A patient with myotonic dystrophy requires cholecystectomy. What are your concerns regarding anesthesia?"

Model Answer:

• DM patients have 8-10 times higher perioperative mortality [6]
• Major risks:
  1. Respiratory complications (most common):
     • Extreme sensitivity to opioids, benzodiazepines, anesthetics
     • Postoperative respiratory failure
     • Prolonged ventilation requirements
  2. Myotonic crisis:
     • Succinylcholine ABSOLUTELY CONTRAINDICATED
     • Can cause generalized myotonic contracture → impossible to ventilate → cardiac arrest
  3. Cardiac complications:
     • Arrhythmias
     • Conduction blocks
  4. Aspiration risk:
     • Bulbar weakness
     • Delayed gastric emptying

Preoperative optimization:

• Cardiology assessment: Recent ECG, consider Holter
• Pulmonary function tests: Baseline respiratory status
• Anesthesia consultation: High-risk consent
• Consider regional technique if possible (laparoscopic cholecystectomy may be feasible under spinal/epidural)

Anesthetic plan:

• Regional anesthesia preferred
• If GA required:
  • AVOID succinylcholine
  • Minimal doses of all agents
  • Non-depolarizing muscle relaxants acceptable (rocuronium with sugammadex reversal)
  • TIVA or volatile (both acceptable; no MH risk)

Postoperative:

• HDU/ICU monitoring (minimum 24 hours)
• Low threshold for continued ventilation
• Aggressive pulmonary toilet

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Scenario 4: Genetic Counseling

Examiner: "A 28-year-old woman with DM1 (CTG repeat ~800) is planning pregnancy. What counseling would you provide?"

Model Answer:

• Inheritance risk: 50% (autosomal dominant)
• Anticipation: Offspring may have earlier onset and more severe disease
• Congenital DM risk:
  • High due to large maternal CTG expansion (~800)
  • Congenital DM occurs almost exclusively through maternal transmission
  • "Features: Severe hypotonia, respiratory failure, intellectual disability"
  • 25-50% neonatal mortality; survivors have significant disabilities [7]

Obstetric risks:

• Maternal:
  • Worsening weakness during pregnancy
  • Preterm labor
  • Uterine atony and postpartum hemorrhage
  • Anesthetic complications for delivery
• Fetal:
  • Polyhydramnios
  • Reduced fetal movements
  • Risk of congenital DM

Management options:

• Prenatal diagnosis:
  • CVS (10-12 weeks) or amniocentesis (15-18 weeks)
  • Determine if fetus affected and estimate repeat size
• Preimplantation genetic diagnosis (PGD):
  • IVF with genetic testing before embryo transfer
  • Select unaffected embryos
• High-risk obstetric care:
  • Multidisciplinary team (obstetrics, neurology, genetics, anesthesia, neonatology)
  • Serial ultrasounds
  • Delivery planning with neonatal team present

Ethical considerations:

• Reproductive autonomy
• Psychological impact
• Support throughout decision-making

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Scenario 5: Molecular Mechanism

Examiner: "Explain the pathophysiology of myotonia in myotonic dystrophy."

Model Answer:

• DM is an RNA gain-of-function disorder
• Pathogenic mechanism:
  1. CTG expansion in DMPK 3'UTR transcribed into CUG repeat RNA
  2. Mutant RNA forms hairpin structures, accumulates in nuclear foci
  3. Sequestration of MBNL proteins (Muscleblind-like 1, 2, 3)
  4. Loss of MBNL function causes aberrant alternative splicing of multiple genes

Myotonia-specific mechanism:

• CLCN1 (chloride channel-1) mis-splicing: [11]
  • "Normal: Adult isoform with high chloride conductance"
  • "DM: Inclusion of fetal exon 7a → reduced chloride conductance"
  • "Result: Decreased muscle membrane repolarization"
  • Membrane hyperexcitability → sustained muscle contraction → myotonia

Other mis-splicing examples:

• INS-R (insulin receptor): Fetal isoform → insulin resistance
• SCN5A (cardiac sodium channel): Aberrant splicing → conduction defects
• RYR1 (ryanodine receptor): Mis-splicing → weakness

Why this mechanism explains key features:

• Multisystem disease: Many genes mis-spliced across tissues
• Progressive: Somatic instability increases repeat length over time
• Anticipation: Larger repeats cause more severe MBNL sequestration
• No protein loss of function: DMPK protein levels actually normal; disease is RNA-mediated

12. Patient and Family Explanation

What is Myotonic Dystrophy?

Myotonic dystrophy is an inherited condition that affects muscles and several other parts of the body. "Myotonic" means the muscles have difficulty relaxing after they contract - for example, you might notice difficulty letting go when shaking hands, or releasing your grip on a door handle.

What Causes It?

It's caused by a genetic change (mutation) that you inherit from one parent. The mutation involves an expansion of a repeating DNA sequence - like a "genetic stutter." The more repeats you have, the more severe the condition tends to be. There are two types: Type 1 (more common and usually more severe) and Type 2 (often milder).

What Parts of the Body Does It Affect?

While muscle problems are most noticeable, myotonic dystrophy is a multisystem condition:

• Muscles: Weakness (often in hands, feet, and face) and myotonia (stiffness)
• Heart: The heart's electrical system can be affected, which is why regular heart monitoring is very important
• Eyes: Early cataracts are common
• Energy levels: Many people experience excessive sleepiness
• Metabolism: Increased risk of diabetes
• Digestion: Swallowing difficulties or digestive problems in some people

Is There a Cure?

Currently, there is no cure or treatment that stops the condition from progressing. However, there is much we can do to help:

• Heart monitoring: Regular ECGs to detect heart rhythm problems early. Some people need a pacemaker to keep their heart rhythm safe.
• Breathing support: Some people benefit from a breathing machine at night to help with sleep quality and energy.
• Medication: For excessive sleepiness, modafinil can help. For severe muscle stiffness, mexiletine may be prescribed.
• Eye surgery: Cataract surgery when needed.
• Physiotherapy: Exercise programs to maintain strength and mobility.
• Annual check-ups: To monitor for complications and intervene early.

Will I Pass It to My Children?

Because myotonic dystrophy is inherited in an "autosomal dominant" way, each of your children has a 50% (1 in 2) chance of inheriting the condition. Importantly, the condition often gets more severe in each generation - this is called "anticipation."

Genetic counseling before pregnancy is very important. Options include:

• Prenatal testing during pregnancy
• Preimplantation genetic diagnosis (IVF with testing before embryo transfer)
• Choosing not to have biological children
• Acceptance of the 50% risk

What About Surgery and Anesthesia?

This is very important: If you ever need surgery or anesthesia, always inform the medical team that you have myotonic dystrophy. People with this condition are at higher risk from anesthesia, particularly breathing problems afterward. The team will need to take special precautions, and you may need closer monitoring after the procedure.

What's the Outlook?

The progression varies greatly between individuals. Some people have very mild disease with near-normal life expectancy, while others have more significant disability. The most important thing is regular monitoring (especially of your heart) and addressing complications early. Working with a specialist neuromuscular team gives you the best outcomes.

Where Can I Get Support?

• Muscular Dystrophy UK: https://www.musculardystrophyuk.org/
• Myotonic Dystrophy Foundation: https://www.myotonic.org/
• Genetic counseling services through your neurology or genetics clinic
• Patient support groups (local and online)

13. Key Learning Points

1. Myotonic dystrophy is the most common adult muscular dystrophy, with two genetic types (DM1 and DM2) sharing RNA toxicity mechanisms but differing in distribution of weakness.

2. DM1 is caused by CTG repeat expansion in DMPK gene; DM2 by CCTG expansion in CNBP gene. Both follow autosomal dominant inheritance with anticipation.

3. RNA gain-of-function through MBNL protein sequestration and aberrant alternative splicing underlies multisystem pathology. Myotonia results from CLCN1 mis-splicing reducing chloride conductance.

4. Cardiac conduction disease is the leading cause of sudden death. Annual ECG is mandatory; pacemaker indicated for PR ≥240ms or significant conduction blocks.

5. Anesthetic complications pose major risk: Succinylcholine is absolutely contraindicated (risk of myotonic crisis). Extreme sensitivity to sedatives and anesthetics requires specialized perioperative management.

6. Congenital myotonic dystrophy occurs almost exclusively through maternal transmission with large CTG expansions (> 1,500 repeats), causing severe neonatal hypotonia, respiratory failure, and intellectual disability.

7. Clinical diagnosis clues: "Hatchet face," frontal balding, grip myotonia, percussion myotonia (thenar eminence), characteristic "Christmas tree" cataracts, and distal-predominant weakness (DM1) vs proximal weakness with pain (DM2).

8. Genetic testing is diagnostic: PCR and Southern blot for CTG repeats (DM1) or repeat-primed PCR for CCTG (DM2). Somatic mosaicism means blood testing may underestimate severity.

9. Multidisciplinary management is essential: Neurology, cardiology, respiratory, ophthalmology, endocrinology, genetics, anesthesia, and rehabilitation services all play roles.

10. No disease-modifying treatment exists, but symptomatic management (modafinil for sleepiness, NIV for respiratory insufficiency, pacemakers for heart block, mexiletine for severe myotonia) improves quality of life and reduces mortality.

14. References

1. Bird TD. Myotonic Dystrophy Type 1. 1999 Sep 17 [updated 2024 Nov 14]. In: Adam MP, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2025. PMID: 20301344

2. Gutiérrez Gutiérrez G, et al. Clinical guide for the diagnosis and follow-up of myotonic dystrophy type 1, MD1 or Steinert's disease. Neurologia (Engl Ed). 2020 Apr;35(3):185-206. doi:10.1016/j.nrl.2019.01.001. PMID: 31003788

3. Santoro M, et al. Myotonic dystrophy type 1: role of CCG, CTC and CGG interruptions within DMPK alleles in the pathogenesis and molecular diagnosis. Clin Genet. 2017 Oct;92(4):355-364. doi:10.1111/cge.12954. PMID: 27991661

4. Kleefeld F, et al. Myotonic Dystrophy Type 2. 2006 Sep 21 [updated 2025 Sep 25]. In: Adam MP, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2025. PMID: 20301639

5. Itoh H, et al. CTG repeat length underlying cardiac events and sudden death in myotonic dystrophy type 1. Eur Heart J Open. 2024 Sep 18;4(5):oeae078. doi:10.1093/ehjopen/oeae078. PMID: 39391712

6. Mathieu J, et al. Anesthetic and surgical complications in 219 cases of myotonic dystrophy. Neurology. 1997 Dec;49(6):1646-50. doi:10.1212/wnl.49.6.1646. PMID: 9409361

7. Lanni S, Pearson CE. Molecular genetics of congenital myotonic dystrophy. Neurobiol Dis. 2019 Dec;132:104533. doi:10.1016/j.nbd.2019.104533. PMID: 31326502

8. Kusumoto FM, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. Circulation. 2019 Aug 20;140(8):e382-e482. doi:10.1161/CIR.0000000000000628. PMID: 30586772

9. Osborne RJ, Thornton CA. RNA-dominant diseases. Hum Mol Genet. 2006 Oct 15;15 Spec No 2:R162-9. doi:10.1093/hmg/ddl181. PMID: 16987879

10. Cerro-Herreros E, et al. AntimiR treatment corrects myotonic dystrophy primary cell defects across several CTG repeat expansions with a dual mechanism of action. Sci Adv. 2024 Oct 11;10(41):eadn6525. doi:10.1126/sciadv.adn6525. PMID: 39383229

11. Louis JM, et al. Expression levels of core spliceosomal proteins modulate the MBNL-mediated spliceopathy in DM1. Hum Mol Genet. 2024 Nov 5;33(21):1873-1886. doi:10.1093/hmg/ddae125. PMID: 39180495

12. Ginjupalli VKM, et al. Cardiac Involvement in Myotonic Dystrophy Type 1: Mechanisms, Clinical Perspectives, and Emerging Therapeutic Strategies. Int J Mol Sci. 2025 Nov 13;26(22):10992. doi:10.3390/ijms262210992. PMID: 41303475

13. Peric S, et al. Cerebral involvement and related aspects in myotonic dystrophy type 2. Neuromuscul Disord. 2021 Aug;31(8):681-694. doi:10.1016/j.nmd.2021.06.002. PMID: 34244019

14. Jenquin JR, et al. Furamidine Rescues Myotonic Dystrophy Type I Associated Mis-Splicing through Multiple Mechanisms. ACS Chem Biol. 2018 Sep 21;13(9):2708-2718. doi:10.1021/acschembio.8b00646. PMID: 30118588

15. González-Martínez I, et al. Peptide-conjugated antimiRs improve myotonic dystrophy type 1 phenotypes by promoting endogenous MBNL1 expression. Mol Ther Nucleic Acids. 2023 Sep 5;34:102024. doi:10.1016/j.omtn.2023.09.001. PMID: 37744174

16. Itoh H, et al. Cardiac Conduction Disorders as Markers of Cardiac Events in Myotonic Dystrophy Type 1. J Am Heart Assoc. 2020 Sep;9(17):e015709. doi:10.1161/JAHA.119.015709. PMID: 32812471

17. Tanner MK, et al. Targeted splice sequencing reveals RNA toxicity and therapeutic response in myotonic dystrophy. Nucleic Acids Res. 2021 Feb 26;49(4):2240-2254. doi:10.1093/nar/gkab022. PMID: 33503262

18. Arandel L, et al. Reversal of RNA toxicity in myotonic dystrophy via a decoy RNA-binding protein with high affinity for expanded CUG repeats. Nat Biomed Eng. 2022 Feb;6(2):207-220. doi:10.1038/s41551-021-00838-2. PMID: 35145256

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