Lambert-Eaton Myasthenic Syndrome (LEMS)

Diagnosis

Diagnosing LEMS is often difficult since leg weakness, arm weakness, and fatigue are common complaints. LEMS is more likely to be suspected in people with cancer, though it is often mistaken for myasthenia gravis (MG) due to overlapping symptoms. The presence of a dry mouth is sometimes a helpful clue.

Neurologists will ask questions and conduct a physical exam to evaluate muscle strength and tendon (muscle stretch) reflexes, both of which are usually reduced in LEMS. In some patients, reflexes briefly improve after 10 seconds of muscle activation. These findings are clues for the diagnosis.

The diagnosis is often made in the electromyography (EMG) laboratory, where patients may undergo nerve conduction studies for various causes of weakness. The nerve responses are usually low, but after 10 seconds of exercise, they may increase significantly. In LEMS, the response usually increases by more than 60% (often >100%), a finding considered diagnostic of LEMS.

Doctors may also order a blood test designed to detect antibodies to voltage-gated calcium channels (VGCC) on the nerve side of the nerve-muscle junction (presynaptic), before or after EMG. Positive test results support a diagnosis of LEMS, but some individuals lack these antibodies.

Once a diagnosis of LEMS is confirmed, cancer screening is important. A CT scan of the chest is recommended to look for small cell lung cancer. If negative, a PET-CT and ongoing CT follow-up are advised to look for an underlying cancer. Current practice recommends patients with LEMS should be screened for lung cancer with chest CT (and FDG-PET if available) at diagnosis and again at 3–6 months if initial screening is negative, as nearly all associated cancers are detected within the first year. Further routine screening beyond 1–2 years is generally not recommended unless new symptoms or risk factors arise.

Additional reading

• Titulaer MJ, Lang B, Verschuuren JJ. Lambert-Eaton myasthenic syndrome: from clinical characteristics to therapeutic strategies. Lancet Neurol. 2011 Dec;10(12):1098-107. doi: 10.1016/S1474-4422(11)70245-9. PMID: 22094130.
• Titulaer MJ, Soffietti R, Dalmau J, Gilhus NE, Giometto B, Graus F, Grisold W, Honnorat J, Sillevis Smitt PA, Tanasescu R, Vedeler CA, Voltz R, Verschuuren JJ; European Federation of Neurological Societies. Screening for tumours in paraneoplastic syndromes: report of an EFNS task force. Eur J Neurol. 2011 Jan;18(1):19-e3. doi: 10.1111/j.1468-1331.2010.03220.x. Epub 2010 Sep 29. PMID: 20880069; PMCID: PMC3086523.
• Varon MC, Dimachkie MM. Diagnosis and treatment of Lambert-Eaton myasthenic syndrome. Practical Neurology (US). 2024;23(3):26-28,47.

Last revised December 2025.