Approach to Muscle Diseases
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Direct answer
Muscle disease presents with proximal, symmetrical, fixed weakness without sensory loss or bladder involvement, and the workup runs creatine kinase, electromyography, selected autoantibodies, imaging, biopsy and genetic panels. Inflammatory myopathies are antibody-defined and treatable: dermatomyositis with its rash and malignancy associations, antisynthetase syndrome with interstitial lung disease, and immune-mediated necrotising myopathy with very high kinase values. Among inherited disease, Duchenne dystrophy, limb-girdle dystrophies, facioscapulohumeral dystrophy, myotonic dystrophy and late-onset Pompe disease carry the examination weight.
What you must remember
- Bedside architecture: proximal weakness with preserved reflexes and sensation; look for Gowers sign, calf pseudohypertrophy, scapular winging, facial weakness and myotonia on grip and percussion.
- Laboratory architecture: creatine kinase markedly elevated in dystrophies, necrotising myopathy and active inflammatory myopathy but normal or mildly raised in steroid, endocrine and congenital myopathies (seizures and falls transiently raise it); electromyography shows short-duration, low-amplitude polyphasic motor units, with myotonic discharges sealing the myotonias.
- Dermatomyositis: heliotrope rash, Gottron papules, shawl and V signs; anti-Mi-2 predicts good steroid response; anti-TIF1-gamma and anti-NXP2 carry adult malignancy risk; anti-MDA5 flags rapidly progressive interstitial lung disease, sometimes with minimal weakness.
- Antisynthetase syndrome: mechanic's hands, interstitial lung disease and arthritis with anti-Jo-1; immune-mediated necrotising myopathy (anti-SRP, anti-HMGCR) causes very high kinase values, often after statins, and needs aggressive immunotherapy.
- Treatment of inflammatory myopathy: corticosteroids with methotrexate or azathioprine as sparing agents, intravenous immunoglobulin especially in dermatomyositis, and malignancy search guided by antibody profile in adults.
- Inherited landmarks: Duchenne muscular dystrophy is an X-linked dystrophinopathy, onset three to five years, with Gowers sign and calf enlargement, treated with corticosteroids (prednisolone or deflazacort) and cardiac and scoliosis surveillance, while gene-based therapies are not yet standard in India. Myotonic dystrophy type 1 — CTG repeat expansion in DMPK with anticipation — adds distal weakness, temporal balding, cataracts, insulin resistance and cardiac conduction disease needing annual electrocardiography; the congenital form affects infants of affected mothers.
- Do not forget late-onset Pompe disease (limb-girdle weakness with disproportionate respiratory failure; treatable with enzyme replacement) and thyrotoxic hypokalaemic periodic paralysis, the commonest adult form.
Common confusion
Steroid myopathy mimics an inflammatory flare: both give proximal weakness, but steroid myopathy has a normal kinase — so in a treated myositis patient with rising weakness and stable kinase, reduce steroids. Dermatomyositis versus antisynthetase disease is a lung question — the latter demands interstitial lung disease screening. Myotonic dystrophy versus myotonia congenita separates systemic features and distal weakness from a pure warm-up myotonia. Distinguish junction from muscle: fatigable ptosis seeks myasthenia; fixed proximal weakness with a high kinase seeks muscle.
Exam-focused takeaway
Three discriminations carry the marks: kinase-high versus kinase-normal myopathy, dermatomyositis antibody to clinical destiny (MDA5 to lung, TIF1-gamma to malignancy), and statin exposure with anti-HMGCR necrotising myopathy. Duchenne facts cluster around Gowers sign, calf pseudohypertrophy and steroid therapy; myotonic dystrophy stems reward anticipation, cataracts and conduction disease. Respiratory weakness out of proportion always triggers a Pompe assay.
Frequently asked questions
Which myositis antibody predicts rapidly progressive lung disease?
Anti-MDA5 in amyopathic or mildly weak dermatomyositis mandates urgent high-resolution CT and interstitial lung disease management.
How does steroid myopathy differ from active myositis?
Steroid myopathy produces painless proximal weakness with a normal creatine kinase; active myositis raises the kinase and inflammatory markers.
What are the systemic features of myotonic dystrophy?
Cataracts, cardiac conduction block, insulin resistance, frontal balding, gonadal atrophy and cognitive change, with anticipation across generations.
What is the standard treatment of Duchenne dystrophy?
Corticosteroids (prednisolone or deflazacort) with cardiac surveillance and multidisciplinary care; gene-based therapies are emerging but not standard in India.
Why must late-onset Pompe disease be recognised?
Because enzyme replacement changes its trajectory, and its disproportionate respiratory failure is easily misfiled as an unexplained myopathy.