Eaton-Lambert syndrome - Neuromuscular junction disorders

Eaton-Lambert syndrome

Definition

A rare paraneoplastic syndrome usually associated with small-call carcinoma (SCC) of the bronchus, causing muscle weakness.

Aetiology/pathophysiology

Antibodies directed against the presynaptic voltagegated calcium channels have been detected. Around 70% have an underlying tumour, almost always SCC of the lung.

Clinical features

Proximal muscle weakness, particularly of the hips, later affecting the shoulders. Ptosis may occur. The ocular and bulbar muscles are typically spared. Unlike myasthenia gravis, weakness tends to be worst in the morning and improve with exercise. Reflexes are reduced, but normalise with exercise.

Investigations

Nerve conduction studies show an incremental response when a motor nerve is repetitively stimulated, in direct contrast to the findings in myasthenia gravis (where there is a decremental response).

Chest X-ray and CT screening for an underlying tumour.

Specific serum antibodies can be detected.

Management

Treatment of the underlying tumour can lead to improvement. Plasmapheresis and intravenous immunoglobulin may be used, and drugs which increase acetylcholine release from presynaptic terminals appear to have symptomatic benefit.