Decrement on Low-Frequency Repetitive Nerve Stimulation Is Not Synonymous With Myasthenia Gravis

Lambert-Eaton myasthenic syndrome (LEMS) is a rare presynaptic neuromuscular junction disorder that may be mistaken for peripheral neuropathy, radiculopathy, inflammatory myopathy, or myasthenia gravis. We report a female in her early 40s with two months of progressive, predominantly proximal lower-limb weakness, areflexia, and dry mouth, without ocular or bulbar symptoms. Routine laboratory testing and creatine kinase were unremarkable, while spinal imaging showed degenerative abnormalities that did not explain the clinical pattern. Nerve conduction studies demonstrated low compound muscle action potential amplitudes with preserved sensory responses and no demyelinating features. Low-frequency repetitive nerve stimulation produced a decrement, whereas brief exercise caused a marked increase in compound muscle action potential amplitude. Serum P/Q-type voltage-gated calcium channel antibodies were positive, confirming LEMS. Pyridostigmine produced partial symptomatic improvement. Amifampridine was subsequently initiated, followed by intravenous immunoglobulin, after which the patient demonstrated marked improvement in lower-limb strength and walking ability. Initial malignancy screening was negative, and longitudinal surveillance was planned. This case emphasizes that a decrement on low-frequency stimulation is not specific for myasthenia gravis and that low resting motor amplitudes with marked post-exercise facilitation strongly indicate a presynaptic neuromuscular junction disorder.