12th August 2024

Lambert-Eaton myasthenic syndrome (LEMS)

LEMS is caused by antibodies to voltage-gated calcium channels (VGCC) on the presynaptic membrane, leading to decreased release of acetylcholine into the synapse. It can occur as a primary autoimmune disorder, but the majority of cases are paraneoplastic, with more than half related to small cell lung cancer (SCLC). Most (60-75%) patients are male, with mean age of onset in the 6ᵗʰ decade.

It is characterised by predominantly proximal muscle weakness which improves with exercise (post-exercise facilitation). Oculobulbar symptoms are common – diplopia, ptosis, dysarthria and dysphagia are all seen. Autonomic symptoms are also prominent, with constipation, dry mouth, altered perspiration and erectile dysfunction in men reported. Reflexes are typically absent.

In addition to antibody testing, EMG may be carried out demonstrating increasing compound muscle action potentials (CMAPs).

The mainstay of treatment should be identification and treatment of the underlying malignancy. Symptomatic relief may be afforded by 3,4-diaminopyridine (amifampridine) and immunosuppression.