Classification of muscular dystrophies

1. Big picture

Muscular dystrophies are inherited, progressive diseases of skeletal muscle caused by defects in structural or functional muscle proteins. They cause progressive muscle weakness, usually with muscle wasting, sometimes pseudohypertrophy, and often markedly elevated creatine kinase (CK).

The key exam sentence:

Muscular dystrophies are hereditary progressive myopathies; they cause weakness without sensory loss, and are classified mainly by the affected protein, inheritance pattern, age of onset and muscle distribution.

They must be distinguished from:

• myositis: inflammatory, often painful, subacute, treatable with immunosuppression;
• myasthenia gravis: fluctuating fatigable weakness, ocular/bulbar, normal CK;
• motor neurone disease: fasciculations, UMN/LMN signs, sensory sparing, neurogenic EMG;
• polyneuropathy: distal weakness + sensory loss + reduced reflexes.

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2. Definition

Muscular dystrophy means a genetically determined progressive degeneration of skeletal muscle due to abnormal muscle proteins.

Pathologically, there is:

• muscle fiber degeneration;
• necrosis;
• regeneration;
• replacement by fat and connective tissue;
• progressive loss of muscle function.

Clinically:

• weakness is usually progressive;
• sensory function is normal;
• CK is often elevated;
• EMG shows a myopathic pattern;
• diagnosis is confirmed by genetic testing and/or muscle biopsy.

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3. General classification of muscular dystrophies