Diagnosis of Lambert-Eaton myasthenic syndrome: Muscle fatigue with normal manual muscle testing, areflexia, and electrodiagnostic findings.

OBJECTIVE

This study aimed to identify clinically relevant features for the diagnosis of Lambert-Eaton myasthenic syndrome (LEMS) and to elucidate factors associated with diagnostic delay in LEMS.

METHODS

Medical records of 12 patients with LEMS and 20 with acetylcholine receptor antibody-positive generalized myasthenia gravis were retrospectively reviewed. Intervals from symptom onset to first primary care visit and to definitive diagnosis were analyzed. In patients with LEMS, correlations between diagnostic intervals and clinical parameters, including manual muscle testing (MMT) sum scores and ulnar compound muscle action potential (CMAP) amplitudes, were assessed.

RESULTS

All patients with LEMS reported muscle fatigue or general malaise, yet 58% demonstrated normal strength on MMT. Tendon reflexes were absent or reduced in 91.7% of patients. Diplopia and dry mouth were each observed in 41.7% of cases. Reduced ulnar CMAP amplitudes were identified in 91.7% of patients. The median interval from symptom onset to diagnosis was significantly longer in LEMS than in generalized acetylcholine receptor antibody-positive generalized myasthenia gravis (7.5 months vs. 1.5 months; p < 0.0001). A significant positive correlation was found between MMT sum scores and the interval from symptom onset and diagnosis (rs = 0.6403, p = 0.0338, n = 11).

CONCLUSION

Diagnostic delay in LEMS was associated with the absence of objective muscle weakness on MMT. Key diagnostic features of LEMS included subjective muscle fatigue, reduced or absent tendon reflexes, and decreased CMAP amplitudes, despite normal MMT findings.