Neurology
Myotonic Dystrophy
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Contents (9)
Myotonic dystrophy (DM) is an autosomal dominant multisystem disorder characterized by progressive muscle weakness, myotonia (delayed muscle relaxation), and systemic manifestations affecting the heart, eyes, endocrine system, and CNS. It is the most common form of muscular dystrophy in adults, with an overall prevalence of 1 in 8,000 individuals and two distinct clinical forms: DM1 (chromosome 19) and DM2 (chromosome 3). The disease exhibits genetic anticipation, where trinucleotide repeat expansions tend to increase in size through successive generations, particularly when transmitted maternally (congenital DM1) or paternally (severe DM2). DM1 typically presents earlier and with greater severity than DM2, making understanding of these distinctions clinically critical.
- CTG trinucleotide repeat expansion in DMPK gene (DM1): Normal individuals have 5-37 CTG repeats; affected individuals have >50 repeats. Expansion occurs in the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene on chromosome 19q13.3. Larger repeat numbers correlate with earlier onset and more severe phenotype. Somatic instability leads to tissue-specific variations in repeat length, explaining variable organ involvement.
- CELF1 and MBNL1 sequestration by toxic RNA: Expanded CTG repeats form stable secondary RNA structures (hairpin loops) that sequester the muscleblind-like (MBNL) proteins, particularly MBNL1 and MBNL2, and increase activity of CELF1 (CUG-BP, Elav-like family member 1). These RNA-binding proteins normally regulate alternative splicing of cardiac troponin T, insulin receptor, chloride channel (ClC-1), and other critical transcripts. Dysregulation of splicing causes the cardinal features of myotonia and systemic manifestations.
- Chloride channel dysfunction and myotonia: Aberrant splicing of the skeletal muscle chloride channel (ClC-1) mRNA leads to production of non-functional channel protein. Reduced chloride conductance impairs muscle membrane stabilization, resulting in myotonia—the characteristic delayed relaxation following voluntary contraction or percussion. Myotonia typically worsens with cold exposure and repeated muscle use (warm-up phenomenon is classic but not universal).
- Systemic RNA toxicity: The pathogenic mechanism extends beyond muscle tissue due to widespread DMPK expression and MBNL sequestration in cardiac myocytes, fibroblasts, and neurons. This explains cardiac arrhythmias (dysregulation of cardiac sodium/potassium channels), cataracts (disrupted lens proteins), testicular atrophy (altered alternative splicing in germline tissues), and cognitive impairment (altered splicing in CNS neurons, particularly affecting tau and APP processing).
- CNBP gene expansion in DM2: DM2 results from CCTG tetranucleotide repeat expansion in intron 1 of the CNBP (Cellular Nucleic acid Binding Protein) gene on chromosome 3q21.3. While the pathogenic mechanism is analogous (MBNL sequestration), DM2 typically has longer repeat numbers but milder phenotype and later onset, a phenomenon not fully explained by repeat size alone, suggesting additional genetic modifiers.
- Autosomal dominant inheritance with genetic anticipation: DM1 and DM2 follow autosomal dominant inheritance patterns. Trinucleotide/tetranucleotide repeat instability during meiosis results in genetic anticipation—successive generations manifest earlier onset and increased severity. Paternal transmission of DM1 expansions typically results in modest size increases; maternal transmission frequently results in dramatic expansions, particularly in generation of congenital DM1.
- Congenital DM1 (neonatal presentation): Occurs in infants born to affected mothers with large CTG expansions (often >1000 repeats). Results from in utero exposure to toxic fetal RNA and maternal DMPK transcript abnormalities. Associated with polyhydramnios, reduced fetal movement, neonatal hypotonia ("floppy baby"), respiratory failure requiring mechanical ventilation, developmental delay, and intellectual disability.
- Age and family history: While DM is genetic, de novo mutations account for approximately 10-15% of DM1 cases, typically presenting with smaller repeat expansions and milder phenotype. Familial cases with known affected relatives carry significantly higher recurrence risk depending on mode of parental transmission.
- Myotonia: Delayed relaxation of skeletal muscles after voluntary contraction or percussion (percussion myotonia demonstrable in thenar eminence or tongue). Patient reports difficulty releasing grip, stiffness improving with repeated use (paradoxical warm-up phenomenon). Worsens in cold environments and with fatigue. More prominent in DM1 than DM2; some DM2 patients have minimal myotonia despite significant weakness.
- Proximal and distal muscle weakness: Progressive weakness follows characteristic pattern: distal lower extremities and proximal upper extremities in DM1 (notably sparing hip adductors and iliopsoas early). Foot drop and steppage gait common. Ptosis and facial weakness cause characteristic "myopathic facies" with temporal wasting, closed lips, and long narrow face. Proximal leg weakness causes difficulty rising from seated position. DM2 shows more proximal and axial weakness with less distal involvement than DM1.
- Cataracts (anterior subcapsular): Develop in >90% of DM1 patients; often the earliest systemic sign and occasionally presents before myopathic symptoms. Pathognomonic "Christmas tree" iridescent lens opacities appear in cortex. Posterior subcapsular cataracts also occur. DM2 cataracts are posterior and less frequent; anterior cataracts suggest DM1.
- Cardiac involvement: Conduction system disease (PR prolongation, QRS widening) is most common finding; sudden cardiac death risk exists even in asymptomatic early disease. Arrhythmias (atrial fibrillation, ventricular arrhythmias, bradycardia) occur in up to 20% of patients. Dilated cardiomyopathy develops in severe cases. Sudden unexpected nocturnal death in epilepsy (SUDEP)-like events reported. Requires ECG screening in all suspected DM patients.
- Cognitive and behavioral abnormalities: Intellectual disability (more prominent in congenital DM1), cognitive decline, personality changes, apathy, and excessive daytime somnolence occur in up to 50% of DM1 patients. Sleep-disordered breathing and central hypoventilation contribute to fatigue. DM2 exhibits less cognitive involvement but increased risk of depression and anxiety.
- Endocrine dysfunction: Testicular atrophy and infertility (azoospermia or oligospermia) in males; premature ovarian insufficiency in females. Insulin resistance and diabetes mellitus develop in 10-20% of patients. Thyroid dysfunction, hypogonadism, and growth hormone deficiency documented.
- Respiratory involvement: Weakness of respiratory muscles and diaphragm, particularly problematic during sleep or general anesthesia. Central hypoventilation and sleep apnea contribute to hypoxemia. Risk of aspiration due to pharyngeal weakness and swallowing dysfunction.
- Smooth muscle involvement: Gastroesophageal reflux disease, dysphagia, constipation, and diarrhea common due to involvement of GI smooth muscle and autonomic dysfunction.
- Clinical examination demonstrating myotonia: Percussion myotonia (striking thenar eminence produces slow relaxation of adduction); direct observation of myotonia during voluntary muscle contraction (patient makes fist and has difficulty extending fingers). Grip myotonia (cannot quickly release hand grip). Presence of myotonia is essential for diagnosis but may be subtle in DM2.
- Electromyography (EMG): Gold standard diagnostic test. Myotonic discharges appear as characteristic "dive bomber" (descending frequency and amplitude) or "machine-gun" high-frequency positive sharp waves and fibrillations at rest and with needle insertion. Non-myopathic recruitment pattern with small, brief, polyphasic motor unit action potentials indicating myopathic process. Normal nerve conduction studies (helps exclude neurogenic causes).
- Serum creatine kinase (CK) level: Elevated but usually <5× upper limit of normal; helps confirm muscle involvement but non-specific. May be normal in DM2 patients despite significant symptoms.
- Genetic testing (definitive diagnosis): Trinucleotide repeat (CTG) analysis for DM1 (chromosome 19) and tetranucleotide repeat (CCTG) analysis for DM2 (chromosome 3). Positive correlation between repeat number and age of onset; stronger correlation with severity in DM1. Normal: DM1 <37 repeats, DM2 <30 repeats. Intermediate: 38-49 repeats (DM1) requires further testing. Affected: ≥50 repeats (DM1), ≥75 repeats (DM2). Genetic testing is confirmatory and recommended in all suspected cases.
- Cardiac evaluation: 12-lead ECG mandatory in all patients to assess PR interval, QRS duration, and arrhythmias. Echocardiography to assess ejection fraction and detect dilated cardiomyopathy. 24-hour Holter monitor or event monitor if arrhythmias suspected. Cardiac MRI may show structural abnormalities. Risk stratification for ICD placement (indicated for LVEF <35% or high-risk conduction abnormalities).
- Ophthalmologic examination: Slit-lamp examination essential for detecting cataracts, including preclinical anterior subcapsular opacities characteristic of DM1. Fundoscopy to exclude other retinal abnormalities.
- Sleep study (polysomnography): Consider in symptomatic patients with excessive daytime somnolence or apnea-like episodes to assess for obstructive or central sleep apnea and hypoventilation.
- Diagnostic criteria: Clinical diagnosis requires myotonia (myotonic discharge on EMG) plus proximal/distal muscle weakness plus systemic features (cataracts, cardiac abnormalities, cognitive changes) in context of positive genetic testing. DM1 vs DM2 distinction made by genetic testing (chromosome location and repeat type differ).
- Myotonia management: Mexiletine (anti-arrhythmic with membrane-stabilizing properties) is first-line agent; initial dose 150 mg daily divided into 2-3 doses, titrate to 300-600 mg daily based on tolerance and efficacy. Mechanism involves stabilization of muscle fiber membrane potential by blocking sodium channels. Monitor ECG for QT prolongation and arrhythmias. Procainamide alternative if mexiletine ineffective or contraindicated. Phenytoin and quinine largely abandoned due to adverse effects. Cold avoidance and avoiding fatigue provide symptomatic relief.
- Muscle weakness and fatigue: No disease-modifying therapy exists for underlying dystrophy. Strength training and aerobic exercise improve functional capacity and fatigue without exacerbating myotonia (unlike some older beliefs). Occupational and physical therapy essential for maintaining function and addressing adaptive needs. Modafinil (100-400 mg daily) or methylphenidate may improve cognitive fatigue and daytime somnolence; evidence mixed but often tried empirically.
- Cardiac management: Beta-blockers and ACE inhibitors for patients with dilated cardiomyopathy and reduced ejection fraction (similar to non-ischemic cardiomyopathy management). Permanent pacemaker indicated for symptomatic bradycardia or advanced AV block (PR >250 ms, QRS >120 ms associated with conduction disease progression risk). Implantable cardioverter-defibrillator (ICD) indicated for LVEF ≤35% or sustained VT. Antiarrhythmic agents (avoid Class IC agents; beta-blockers preferred) for symptomatic arrhythmias. Requires close cardiology collaboration and serial ECG monitoring.
- Respiratory support: Sleep apnea management via CPAP or BiPAP if indicated by sleep study. Consider non-invasive ventilation (BiPAP) for nocturnal hypoventilation. Avoid general anesthesia and central nervous system depressants (high aspiration and respiratory failure risk); if anesthesia necessary, require anesthesia consultation and appropriate monitoring. Respiratory muscle training may provide modest benefit.
- Cataract surgery: Phacoemulsification and intraocular lens implantation when cataracts interfere with vision. Surgical risk generally low but requires anesthesia awareness of myotonic dystrophy risk.
- Endocrine management: Screen for diabetes mellitus and manage with standard therapy; insulin resistance may necessitate metformin or thiazolidinediones. Testosterone replacement for hypogonadal males if appropriate. Screen for thyroid dysfunction and treat if present.
- Genetic counseling and family screening: Refer all patients and at-risk family members for genetic counseling before and after testing. Pregnant women with DM1 warrant high-risk obstetric monitoring (polyhydramnios, fetal movement assessment). Preimplantation genetic diagnosis (PGD) available for family planning.
- Monitoring and surveillance: Annual cardiology evaluation recommended (ECG minimum; consider echo every 2 years). Annual eye exams. Screen for diabetes and thyroid disease. Assess respiratory function periodically (spirometry). Monitor cognitive status and depression screening given psychiatric comorbidity risk.
- Sudden cardiac death: Most serious life-threatening complication. Results from conduction system disease progression leading to high-degree AV block or ventricular arrhythmias. Risk elevated even in asymptomatic patients with ECG abnormalities. Requires ICD consideration in higher-risk patients; responsible for significant mortality in DM population. All patients require ECG screening; progression may occur insidiously without symptoms.
- Respiratory failure: Can occur acutely (perioperative period, pneumonia, sleep apnea exacerbation) or chronically (progressive diaphragmatic weakness). Central hypoventilation and obstructive sleep apnea increase risk. Aspiration pneumonia common due to pharyngeal weakness and dysphagia. Sleep-disordered breathing can cause sudden nocturnal desaturation events.
- Malignant hyperthermia-like reaction: Increased anesthetic sensitivity; myotonic dystrophy patients show exaggerated response to volatile anesthetics and succinylcholine (avoid). Rhabdomyolysis can occur perioperatively. Requires regional anesthesia when feasible; if general anesthesia necessary, use non-triggering agents and avoid succinylcholine.
- Cognitive decline and psychiatric illness: Progressive cognitive impairment, dementia in severe cases. High prevalence of depression, anxiety, and personality disorders. Excessive daytime somnolence impairs quality of life and safety. Requires neuropsychological assessment and psychiatric management.
- Aspiration and dysphagia complications: Pharyngeal and esophageal muscle involvement with smooth muscle dysfunction increases aspiration risk, particularly for solids and liquids. Aspiration pneumonia can be life-threatening. Modified diet, swallowing evaluation, and speech therapy often necessary.
- Falls and fractures: Progressive weakness and myotonia increase fall risk, compounded by cognitive changes and myotonia-induced balance impairment. Fractures associated with disability and further functional decline.
- Infertility and reproductive dysfunction: Testicular atrophy results in azoospermia or severe oligospermia in males; assisted reproductive technology often necessary. Premature ovarian insufficiency in females. Congenital DM1 in offspring of affected mothers carries significant morbidity/mortality risk.
- Diabetic complications: Development of insulin-dependent diabetes in subset of patients; standard diabetic complications (nephropathy, neuropathy, retinopathy) can occur. Glucose management often challenging due to insulin resistance.
DM1 typically follows slowly progressive course over decades with variable severity depending on CTG repeat number and genetic background. Larger CTG expansions (>1000) predict congenital DM1 with infantile-onset presentation and poorer long-term outlook; many affected infants require respiratory support in neonatal period. Patients with repeat numbers 50-100 usually manifest adult-onset DM1 with slower progression. Average disease duration from onset to wheelchair dependence approximately 20 years in classic DM1. Some patients remain ambulatory into later decades; others experience rapid disability progression.
**Congenital D
- The handshake stem: a patient who cannot let go after shaking the examiner's hand, with frontal balding, ptosis, temporal wasting, and a long thin face, is DM1 until proven otherwise. Grip and percussion myotonia plus a hatchet face are the highest-yield descriptors.
- The one association examiners test: cardiac conduction disease. A 12-lead ECG is the single best next step in any newly suspected or newly diagnosed patient — PR prolongation and QRS widening precede symptoms, and sudden death may be the first event. The ACC/AHA/HRS bradycardia and conduction delay guideline specifically addresses pacing in neuromuscular disease, where progression is unpredictable and the threshold for permanent pacing is lower than in the general population.
- Anticipation with a parent-of-origin rule: CTG repeats expand across generations. The great majority (>90%) of congenital DM1 is maternally transmitted, though rare paternally transmitted cases are documented — a floppy, ventilator-dependent neonate with talipes equinovarus and a mother with unrecognized grip myotonia and cataracts is the classic vignette. Ask the mother to make a fist.
- Buzzword cluster to recognize instantly: dive-bomber discharges on EMG, Christmas tree iridescent posterior subcapsular/cortical cataracts on slit lamp, and the warm-up phenomenon (myotonia improves with repeated use). The iridescent posterior lens opacity occurs in both DM1 and DM2 and is therefore a disease marker, not a subtype discriminator.
- Anesthesia is a testable trap: succinylcholine and volatile agents are avoided; exaggerated sensitivity to opioids, benzodiazepines, and neuromuscular blockers causes prolonged apnea and postoperative respiratory failure disproportionate to the surgery performed.
- Common distractor — myotonia congenita: CLCN1 chloride channel mutation (Thomsen/Becker) gives myotonia with muscle hypertrophy and a herculean build, but no fixed or progressive weakness (the recessive Becker form may show transient weakness after initiating contraction that resolves with warm-up), no cataracts, no arrhythmia, no endocrine disease. Paramyotonia congenita (SCN4A) shows paradoxical myotonia that worsens with exercise and cold. Multisystem involvement is what makes it dystrophy.
- DM1 versus DM2: DM2 (PROMM, CCTG repeat in CNBP) is proximal-predominant, milder, lacks a congenital form, and does not show reliable repeat-size/severity correlation. Distal-predominant weakness points to DM1.