Investigation

• Metabolic myopathies — exclude and treat hypophosphataemia, hypokalaemia, hypocalcaemia and hypomagnesaemia.

• Prolonged effects of muscle relaxants — a prolonged effect of suxamethonium will usually be clinically obvious and should prompt assessment of pseudocholinesterase levels. Suxamethonium effects will also be prolonged in myasthenics. Prolonged effects of non-depolarising muscle relaxants are suggested by a response to an anticholinesterase (neostigmine 0.5 mg by slow IV bolus with an anticholinergic). This should not be attempted if paralysis is complete. Patients with myasthenia gravis will also respond.

• Guillain-Barre syndrome — a lumbar puncture should be performed to confirm raised CSF protein with normal cells. If these features are not found but suspicion is strong, nerve conduction studies may demonstrate segmental demyelination with slow conduction velocities.

• Myasthenia gravis — fatigueable weakness or ptosis suggests myasthenia gravis; response to IV edrophonium

(Tensilon test) and a strongly positive acetylcholine receptor antibody titre confirm this diagnosis. A myasthenic syndrome associated with malignancy (Eaton-Lambert syndrome) involves pelvic and thigh muscles predominantly, tending to spare the ocular muscles.

• Other diagnoses are made largely on the basis of clinical suspicion and specific specialised tests.

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