Myotonic Dystrophy: Diagnostic Workup and Multidisciplinary Management
Diagnostic Workup
Confirm the diagnosis with genetic testing for CTG trinucleotide repeat expansion in the DMPK gene on chromosome 19, which is the definitive test for myotonic dystrophy type 1 (DM1). 1, 2
Initial Diagnostic Evaluation
- Genetic testing: Order CTG repeat expansion analysis in the DMPK gene 3' untranslated region—normal individuals have 5-37 repeats, while DM1 patients have >50 repeats (often hundreds to thousands) 2, 3
- Clinical phenotyping: Document myotonia (delayed muscle relaxation), distal then proximal muscle weakness, temporal muscle wasting, facial weakness, and ptosis 3, 4
- Slit-lamp examination: Screen for posterior subcapsular cataracts, which occur in nearly all adult patients 1, 3
Baseline Cardiac Assessment
All patients require comprehensive cardiac evaluation at diagnosis regardless of symptoms, as cardiac conduction abnormalities are the leading cause of sudden death in myotonic dystrophy. 5, 6
- Electrocardiogram (ECG): Obtain baseline to identify PR interval prolongation, QRS widening, or AV block 5, 6
- 24-hour Holter monitoring: Detect intermittent arrhythmias and conduction abnormalities that may be missed on standard ECG 5
- Echocardiography: Assess for left ventricular dysfunction and structural abnormalities 5, 6
Additional Baseline Studies
- Pulmonary function tests: Measure forced vital capacity (FVC) and peak cough flow to establish respiratory baseline 7
- Sleep study: Screen for sleep-disordered breathing, which can occur even with normal pulmonary function 7
- Endocrine evaluation: Check fasting glucose, HbA1c, and testosterone (in males) for insulin resistance and hypogonadism 1, 3
- Neuropsychological assessment: Evaluate for cognitive impairment and executive dysfunction at baseline 3
Cardiac Management
Implement aggressive cardiac surveillance with follow-up intervals no greater than 6 months, as sudden cardiac death from conduction disease can occur even with preserved left ventricular function. 5, 6
Surveillance Protocol
- Annual ECG and echocardiography for all patients, even when asymptomatic with normal baseline studies 6
- Repeat 24-hour Holter monitoring at least annually or more frequently if symptoms develop 5
- Do not assume preserved ejection fraction excludes risk—conduction abnormalities and sudden death occur independently of systolic dysfunction 5
Device Therapy Indications
Implant a permanent pacemaker for third-degree or advanced second-degree AV block at any anatomical level. 6
- Consider pacemaker for any degree of AV block in DM1 due to risk of rapid, unpredictable progression of conduction disease 6
- When pacemaker indication exists with evidence of ventricular arrhythmias or left ventricular dysfunction, implant an ICD instead of pacemaker alone 6
- For patients requiring permanent pacing, consider upgrading to ICD to minimize sudden cardiac arrest risk from ventricular tachycardia, particularly if meaningful survival >1 year is expected (Class IIb recommendation) 5, 6
- Apply standard ICD indications for non-ischemic cardiomyopathy for both primary and secondary prevention 5
Critical Cardiac Considerations
- Maintain high suspicion for bundle-branch reentrant tachycardia in patients presenting with wide QRS complex tachycardia or tachycardia-related symptoms 5, 6
- Avoid medications that prolong QT interval or have proarrhythmic effects, as these patients already have significant cardiac conduction abnormalities 6
- Avoid quetiapine and other QT-prolonging antipsychotics unless absolutely necessary for psychotic symptoms, as they carry significant arrhythmia risk in a population already at 30% risk of sudden cardiac death 6
Respiratory Management
Monitor respiratory function every 6-12 months, as respiratory weakness correlates with disease progression and symptoms are often insidious. 7
Respiratory Assessment
- Medical history: Specifically ask about recurrent chest infections, difficulty clearing secretions, dysphagia, and symptoms of sleep-disordered breathing (morning headaches, daytime somnolence, witnessed apneas) 7
- Forced vital capacity (FVC): Measure in consistent position (sitting or supine) and document position for serial comparison 7
- Peak cough flow (PCF): Assess ability to clear secretions 7
- Sleep study: Perform when symptoms of sleep-disordered breathing are present or when FVC declines 7
Respiratory Interventions
- Initiate non-invasive ventilation (NIV) when sleep-disordered breathing is documented or when FVC falls below 50% predicted 7
- Avoid respiratory suppressants: Use opiates and cough suppressants with extreme caution, especially when FVC <80% 7
- Ensure scheduled vaccinations per national guidelines, including annual influenza and pneumococcal vaccines 7
Multidisciplinary Care Coordination
Establish a multidisciplinary team including neurology, cardiology, pulmonology, ophthalmology, endocrinology, and physical therapy to address the multisystem nature of this disease. 3
Neuromuscular Monitoring
- Assess muscle strength and function every 6-12 months using standardized measures 3
- Monitor for progression of weakness, particularly in distal hand muscles, ankle dorsiflexors, and facial muscles 3, 4
- Screen for dysphagia and aspiration risk, which can impact respiratory function 7
Endocrine Management
- Screen for diabetes mellitus with annual fasting glucose and HbA1c, as insulin resistance is common 1, 3
- Evaluate for hypogonadism in males with testosterone levels and treat if symptomatic 1, 3
Ophthalmologic Care
- Annual slit-lamp examination to monitor cataract progression and plan surgical intervention when vision is impaired 1, 3
Neuropsychiatric Support
- Screen regularly for depression, anxiety, and cognitive decline, which are common in DM1 3
- Provide neuropsychological support and consider cognitive rehabilitation when executive dysfunction impacts daily function 3
Common Pitfalls and Critical Caveats
- Never delay cardiac surveillance—sudden death can be the first manifestation of cardiac involvement, even in young, asymptomatic patients 5, 6
- Do not wait for symptoms to initiate cardiac monitoring—conduction abnormalities progress unpredictably and rapidly 6
- Avoid assuming normal ECG excludes risk—24-hour Holter monitoring is essential as intermittent arrhythmias are common 5
- Be cautious with anesthesia—patients are at high risk for malignant hyperthermia-like reactions and prolonged neuromuscular blockade; consult anesthesiology early for any planned procedures 3
- Screen for aspiration risk before respiratory decline becomes severe—dysphagia can significantly worsen respiratory complications 7
- Recognize that DM1 is distinct from proximal myotonic myopathy (PROMM/DM2)—PROMM lacks CTG expansion and has different inheritance patterns and prognosis 4