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Lambert-Eaton myasthenic syndrome (LEMS)
In Lambert-Eaton syndrome antibodies against voltage-gated calcium channels at the nerve terminal reduce acetylcholine release, causing proximal leg weakness, reduced reflexes and autonomic symptoms — and strength and reflexes improve briefly after short exercise; about half of cases have small-cell lung cancer.
Why it matters in the exam
Questions present a smoker who struggles to rise from a chair and climb stairs, with dry mouth, impotence and constipation, and absent reflexes that return after a muscle contraction. They test the difference from myasthenia (where weakness worsens with effort and starts in the eyes), the typical electrical test (increment after exercise), the antibody (P/Q-type VGCC), and above all the need to search for lung cancer and repeat the search if negative. Common distractors are diagnosing myasthenia and choosing thymectomy, or relying on a single normal scan. The specific symptomatic drug is amifampridine (3,4-diaminopyridine), which prolongs channel opening and increases acetylcholine release. The syndrome sometimes appears months to years before the tumour is visible, so it acts as an early alarm for cancer. Without a tumour think of an autoimmune form, which also occurs in young non-smokers.
Key points
- LEMS is caused by antibodies against presynaptic calcium channels.
- Proximal leg weakness, reduced reflexes and autonomic symptoms, with brief improvement on effort.
- The electrical test shows an increment of 100 percent or more after brief exercise.
- About half have small-cell lung cancer; search by imaging and repeat for 2 years.
- Treatment: the tumour, amifampridine, and if needed IVIG or immunosuppression.
Summary table
| Aspect | LEMS | Myasthenia gravis (for comparison) |
|---|---|---|
| Site of the defect | Presynaptic — voltage-gated calcium channels (VGCC antibodies) | Postsynaptic — acetylcholine receptor |
| Weakness | Proximal, legs more than arms; improves briefly with short effort; eyes only mildly involved | Prominent ocular and bulbar; worsens with effort |
| Reflexes and autonomic system | Reduced or absent reflexes that return after exercise; dry mouth, impotence, constipation | Normal reflexes, no autonomic symptoms |
| Electrical test | Low initial response, and an increment of 100 percent or more after 10 seconds of exercise | Decrement on low-frequency repetitive stimulation |
| Tumour and treatment | Small-cell lung cancer in about half; chest CT, and if negative — PET, with repeat screening for 2 years; treat the tumour, amifampridine, IVIG | Thymoma; pyridostigmine and thymectomy |
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Sources
- Lambert-Eaton Myasthenic Syndrome: Diagnosis and Treatment (Titulaer et al., Lancet Neurology), 2011 — Clinical features, electrodiagnosis and tumour screening
- Updated Diagnostic Criteria for Paraneoplastic Neurologic Syndromes (Graus et al., Neurology: Neuroimmunology and Neuroinflammation), 2021 — High-risk phenotypes
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.