Paraneoplastic manifestation of small-cell bronchial carcinoma due to defective acetylcholine release at the neuromuscular junction
Aetiology
Strong association with underlying small cell carcinoma
A smaller proportion of cases are autoimmune without underlying malignancy
Pathophysiology
Antibodies against voltage activated Ca2+ channels in the motor neuron terminal result in reduced Ca2+ entry in response to depolarisation → reduced ACh release
Clinical presentation
Proximal limb muscle weakness, sometimes with ocular/bulbar muscles, some absent tendon reflexes
Weakness tends to improve after a few minutes of muscular contraction (exertion), and absent reflexes return
Investigations
Repetitive nerve stimulation
Detection of underlying malignancy
Management
Pharmacological
Potassium channel blockers (e.g. 3,4-diaminopyridine aka amifampridine) increase the release of ACh by prolonging the action potential in the motoneurone terminal
Immunosuppression recommended for severe cases - prednisolone and a steroid-sparing agent e.g. azathioprine
Surgical
Depends on the nature of any malignancy discovered
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