What is the most likely diagnosis in a 51-year-old man with progressive proximal muscle weakness, creatine kinase around 1500 U/L, mild troponin T elevation, positive antinuclear antibody, macrocytosis, elevated creatinine, normal cardiac ischemia evaluation, and elbow flexion contractures?

Myotonic Dystrophy Type 1

The most likely diagnosis is myotonic dystrophy type 1 (DM1), not an idiopathic inflammatory myopathy. The constellation of early-onset atrial arrhythmias (age 43), elbow flexion contractures, distal muscle atrophy (forearm and thigh muscles), macrocytosis, elevated creatinine, and the pattern of weakness strongly suggests a hereditary muscular dystrophy rather than an inflammatory process.

Key Distinguishing Features Against Inflammatory Myopathy

While this patient has several features that could superficially suggest inflammatory myopathy—elevated CK (1500 U/L), positive ANA (1:160), and proximal weakness—the clinical picture is fundamentally inconsistent with idiopathic inflammatory myopathies 1:

Why This Is NOT Polymyositis or Dermatomyositis:

• Cardiac involvement pattern: The fixed split second heart sound and history of atrial fibrillation/flutter at age 43 indicate structural cardiac disease with conduction abnormalities, not the myocarditis or diastolic dysfunction typical of inflammatory myopathies 2

• Joint contractures: The 15-degree elbow flexion contractures are characteristic of myotonic dystrophy, not inflammatory myopathies where contractures develop late from chronic weakness

• Muscle atrophy pattern: Atrophy of upper arms AND thighs with distal involvement is atypical for inflammatory myopathies, which primarily cause proximal weakness without early atrophy 2

• Macrocytosis: The MCV of 99 fL is unexplained by inflammatory myopathy but is a recognized feature of myotonic dystrophy

• Low-normal AST with elevated ALT: AST of 10 IU/L (below normal range) with ALT of 98 IU/L is unusual for muscle disease; in inflammatory myopathies, both transaminases are typically elevated proportionally 2

Why This Is NOT Immune-Mediated Necrotizing Myopathy:

Although recent literature describes anti-HMGCR IMNM with statin exposure 3, 4, 5, 6, this patient:

• Has no statin exposure history
• Has CK elevation of only 1500 U/L (IMNM typically shows CK >10,000 U/L) 2, 7
• Has a chronic, insidious course with contractures and cardiac disease predating the weakness
• Lacks the acute/subacute progressive weakness characteristic of IMNM 7

Clinical Algorithm for Myotonic Dystrophy Type 1

The diagnosis should be confirmed with:

1. Genetic testing for CTG repeat expansion in the DMPK gene (chromosome 19q13.3)—this is the definitive diagnostic test

2. EMG showing myotonic discharges—the electrical myotonia is pathognomonic even when clinical myotonia is absent

3. Clinical myotonia assessment—test for grip myotonia (inability to quickly release hand grip) and percussion myotonia (thenar eminence percussion causing sustained thumb contraction)

4. Cardiac evaluation:

   • 24-hour Holter monitoring for conduction defects
   • Consider pacemaker/ICD evaluation given the atrial arrhythmias and risk of sudden cardiac death
   • Echocardiogram already performed showed no ischemia, which is appropriate

5. Ophthalmologic examination for posterior subcapsular cataracts (present in >90% of DM1 patients)

Critical Management Implications

This distinction has profound implications for morbidity and mortality:

• Cardiac monitoring is paramount: DM1 patients have high risk of sudden cardiac death from conduction abnormalities; the patient needs ongoing cardiology follow-up, not immunosuppression

• Avoid immunosuppression: Starting corticosteroids and immunosuppressive agents (as would be done for inflammatory myopathy 2) would expose this patient to unnecessary toxicity without benefit

• Anesthesia precautions: DM1 patients have increased sensitivity to anesthetics and risk of malignant hyperthermia—critical for any future procedures

• Family screening: This is an autosomal dominant condition with anticipation; genetic counseling is essential

• Multisystem monitoring: Screen for diabetes, respiratory insufficiency, and gastrointestinal dysmotility

Common Pitfalls to Avoid

The positive ANA (1:160) and elevated troponin T are red herrings. Low-titer ANA can be seen in up to 15% of healthy individuals and in various chronic conditions. The troponin elevation likely reflects chronic myocardial involvement from DM1, not acute coronary syndrome—the cardiac evaluation appropriately showed no ischemia. Recent case reports confirm that troponin can be elevated in inflammatory myopathies 3, but this further emphasizes the importance of not reflexively attributing all muscle disease with positive autoantibodies to inflammatory myopathy.

The elevated CK should not automatically trigger a myositis workup. The level of 1500 U/L is modest and consistent with chronic muscular dystrophy. The EULAR/ACR criteria for inflammatory myopathies 1 require elevated CK as only one component, and the absence of other supportive features (skin manifestations, myositis-specific antibodies, appropriate weakness pattern) makes inflammatory myopathy unlikely.