Differential diagnosis of muscle weakness

1. Big picture

Muscle weakness is a symptom, not a diagnosis. The first exam task is to decide whether the patient has true loss of power or only fatigue, pain, dyspnea, depression, or deconditioning.

The second task is localization:

Brain/spinal cord
→ anterior horn cell
→ nerve root / plexus / peripheral nerve
→ neuromuscular junction
→ muscle
→ electrolyte/endocrine/systemic cause

The dangerous diagnoses to recognize immediately are:

• Stroke or spinal cord compression
• Guillain-Barré syndrome with respiratory failure
• Myasthenic crisis
• Severe hypo-/hyperkalemia
• Rhabdomyolysis
• Thyrotoxic periodic paralysis
• Sepsis/critical illness myopathy
• Cauda equina syndrome
• Drug/toxin-induced weakness

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