Lambert–Eaton myasthenic syndrome

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Figure. A 72-year-old female smoker presented with morning fatigue, discrete left ptosis and absent reflexes in lower limbs. Top panel: Electromyogram showing evoked motor response of abductor of the fifth finger after stimulation of the left ulnar nerve with a 50-mA current at 50 Hz. Bottom panel: CT of thorax of the same patient 5 years later.

Repetitive electric stimulation at 50-Hz applied to the ulnar nerve at the wrist normally elicits from the abductor of the fifth finger an evoked response of ≥ 5 mV, which may increase no more than 50%. In the case we describe, the electromyogram (EMG) showed that the initial low-amplitude potential was at 0.54 mV and increased progressively to 1.82 mV, a change of 237%. This EMG pattern is a diagnostic feature of Lambert–Eaton myasthenic syndrome (LEMS).

When the patient first presented, she had an unremarkable plain chest radiograph. An initial CT of the thorax showed noncalcified mediastinal lymphadenopathy. Lymph node biopsy revealed a malignant neuroendocrine tumour, but the patient refused treatment. Five years later, a repeat CT of the thorax (Fig. 1) showed calcified subcarinal lymphadenopathy (small arrow) and a new 3.1-cm nodule in the left lower lobe (large arrow). These findings are consistent with a primary bronchogenic carcinoma, which had not been apparent initially. The level of antibodies to the P/Q-type voltage-gated calcium channels was high on presentation, at 1595 (normal < 20) pmol/L; it was still elevated 5 years later, at 881 pmol/L.

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Fig. 1: CT of thorax 5 years after presentation, showing calcified subcarinal lymphadenopathy (black arrow) and new 3.1-cm nodule in left lower lobe (white arrow).

LEMS is most often a paraneoplastic syndrome associated with small-cell carcinoma, which it may precede. Like myasthenia gravis, it is an autoimmune disorder. Unlike myasthenia gravis, the weakness improves with exertion, and oculobulbar involvement is rare except for ptosis.

As in our case, EMG is diagnostic of LEMS when it shows incremental increases in muscle action potential in response to repetitive nerve stimulation. EMG helps to differentiate LEMS from other causes of cancer-associated weakness, such as myopathy and polyneuropathy. Detection of antibodies to the P/Q-type voltage-gated calcium channels expressed by the tumour and present on the presynaptic neuron of the neuromuscular junction is also diagnostic of LEMS.

Monique D'Amour Department of Neurology Gilles Gariepy Department of Pathology Joseph Braidy Department of Respiratory Diseases Hôpital Saint-Luc Centre hospitalier universitaire de Montréal Montréal, Que.