News|Articles|October 30, 2026

Lambert-Eaton Myasthenic Syndrome: how eye care professionals differentiate it from myasthenia gravis

Key Takeaways

Lambert-Eaton Myasthenic Syndrome (LEMS) is a presynaptic disorder, which explains how it differs clinically from MG.
Eye signs are usually mild and late, but they are common enough to matter. Ocular-onset weakness makes LEMS very unlikely.
Several clues should prompt neurologic referral from the eye clinic: ptosis or diplopia with proximal weakness, autonomic symptoms, ptosis that improves with sustained up-gaze, “ocular MG” that is antibody-negative or responds poorly to pyridostigmine, and small cell lung cancer risk factors (age 50 or older, smoking, weight loss).
Diagnosis rests on P/Q-type VGCC antibodies (present in 80%–90% of patients) and a low compound muscle action potential that rises by more than 100% after brief exercise.
Recognizing LEMS can lead to earlier detection of lung cancer and changes treatment.

Lambert-Eaton myasthenic syndrome is a rare autoimmune disorder of the neuromuscular junction that should be differentiated from myasthenia gravis, another neuromuscular junction disorder.

Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disorder of the neuromuscular junction. Patients produce autoantibodies against presynaptic P/Q-type voltage-gated calcium channels (VGCCs) on motor nerve terminals. When a nerve impulse arrives, these antibodies limit calcium entry into the terminal, so fewer vesicles release acetylcholine and muscle contraction weakens.¹ Antibodies also may bind N-type channels, which govern transmitter release from autonomic nerve terminals. Because myasthenia gravis (MG) is also an autoimmune disease of the neuromuscular junction, the correct diagnosis has to be ferreted out.

Is it LEMS or is it MG?

The autoantibody activity reflects the autonomic symptoms that set LEMS apart from myasthenia gravis (MG).¹

LEMS presents with the characteristic triad of proximal limb weakness, reduced/absent tendon reflexes, and autonomic dysfunction.^1,2 Because the LEMS defect is presynaptic, patient strength and reflexes may briefly improve after a short burst of maximal effort. This post-exercise facilitation contrasts with the fatigability typical of MG in some patients.¹

LEMS has two forms. In paraneoplastic LEMS, the immune response is thought to begin against VGCCs expressed by a tumor, most often small cell lung cancer (SCLC). In nontumoral LEMS, there is no cancer, and the disease behaves as a primary autoimmune condition.¹

How LEMS differs from MG

The defects differ between the two diseases. In contrast to the targeted molecules on the presynaptic membrane in LEMS, in MG, the autoantibodies go after molecules on the postsynaptic membrane.^1,3 This difference results in several clinical differences:

Pattern of weakness. MG usually begins in the extraocular muscles and spreads downward. LEMS begins in the legs and ascends, reaching the oculobulbar muscles in disease.^1,4 In a Dutch comparison, ocular weakness was the first symptom in 59% of patients with MG and in none with LEMS.⁴
Response to effort. Weakness in MG worsens with sustained activity. In LEMS, strength and reflexes may briefly improve after maximal contraction. Ptosis may also lessen, rather than worsen, after sustained up-gaze.^1,5
Reflexes and autonomic function. Reduced or absent tendon reflexes and autonomic symptoms such as dry mouth are hallmarks of LEMS.^1,2
Testing. In LEMS, the resting compound muscle action potential, one of the earliest and most commonly used waveforms in clinical nerve studies, is defined asthe summed action potentials of the muscle fibers, recorded by a surface electrode over the muscle after the motor nerve supplying that muscle is stimulated.⁶ Its amplitude roughly reflects how many muscle fibers respond. The compound muscle action potential (CMAP), an electrodiagnostic waveform, is low and often more than doubles after brief exercise; 80% to 90% of patients with LEMS have P/Q-type VGCC autoantibodies in contrast to fewer than 5% of patients with MG.¹
Cancer and treatment. More than half of LEMS cases are associated with SCLC, so the diagnosis should prompt a tumor search.¹ Acetylcholinesterase inhibitors help many patients with MG but usually produce little improvement in LEMS. Amifampridine is the first-line symptomatic therapy.^1,7,8

Who is affected

LEMS is uncommon. Its annual incidence is roughly one-tenth to one-fourteenth that of MG, and its prevalence is about 46 times lower.¹ A nationwide Turkish database study confirmed 159 cases. The median age at diagnosis was 60 years (range, 16-88), 55% of patients were women, and about 60% had the paraneoplastic form, with SCLC accounting for 56% of those tumors.⁹ A Japanese nationwide survey estimated the prevalence at 2.7 per million and found a tumor in 47% of patients, most often SCLC. Men predominated in the paraneoplastic group and women in the autoimmune group.³

Age and sex differ by subtype. In SCLC-associated LEMS, the median age at onset is about 60 years, and 65% to 75% of patients are men. Nontumoral LEMS can begin at any age, with a smaller peak around age 35 and a larger one near 60 years, and most of these patients are women.¹

Smoking, weight loss, and male sex raise the likelihood of an underlying cancer.¹ Neurologic symptoms usually appear before the tumor is found. In an analysis of 227 published cases, cancer was diagnosed from 5 years before to 6 years after LEMS onset.^1,10 An eye care professional evaluating new ptosis or diplopia may therefore encounter such a patient before a cancer is diagnosed.

Systemic features and diagnosis

Leg weakness is the most common first complaint, reported by 60% of patients in one large review; diplopia was the presenting symptom in only 4%.¹⁰ Weakness typically spreads from the proximal to distal muscles and from the legs upward toward the head, the reverse of the craniocaudal pattern seen in MG. It progresses faster when SCLC is present.^1,2

Autonomic dysfunction affects 80% to 96% of patients. Dry mouth is the most frequent complaint; others include erectile dysfunction, constipation, orthostatic symptoms, altered sweating, and dry eyes.¹

Diagnosis rests on clinical features confirmed by serology and electrodiagnostic testing; as mentioned, 80% to 90% of patients have P/Q-type VGCC antibodies in LEMS. On repetitive nerve stimulation, the resting CMAP is low, shows a decrement at slow stimulation rates and rises, often by more than 100%, after 10 seconds of maximal voluntary contraction or high-rate stimulation. Antibodies to SOX1 favor an associated SCLC.¹

Once LEMS is confirmed, chest computed tomography (CT) or positron emission tomography imaging is recommended, with repeat screening every 3 to 6 months for at least 2 years.¹ The DELTA-P score assigns 1 point each for age 50 years or older, smoking, weight loss of 5% or more, bulbar involvement, erectile dysfunction, and a Karnofsky performance score below 70. Higher scores indicate greater SCLC risk.^1,11

How LEMS affects the eye

Ocular involvement in LEMS differs from that in MG in timing and severity. In a Dutch comparison of 38 patients with LEMS and 101 with MG, 59% of patients with MG first noticed eye symptoms, whereas no patient with LEMS did. The authors concluded that ocular-onset weakness makes LEMS very unlikely.⁴ However, eye findings become common as the disease evolves. In a large cohort, about half of patients developed ocular symptoms within 12 months of onset, particularly those with SCLC.^1,2 At the Lahey Clinic in Burlington, MA, 78% of 23 patients had oculobulbar symptoms or signs, and 65% had ptosis, diplopia, or both; the investigators cautioned that such findings should not be used to rule out LEMS.¹²

A Mayo Clinic retrospective review focused specifically on ophthalmic findings.¹³ Reported symptoms included ptosis (23%), diplopia (20.5%), decreased vision (14%), and dry eye (7%). Examination signs included ptosis (26%), abnormal ocular motility (8.5%), strabismus (8%), pupillary dysfunction (7%), and dry eye (2%).^1,13 Overall, ocular weakness in LEMS tends to be milder than in MG, and frank ophthalmoplegia is very unusual.¹

Several examination findings can point toward LEMS:

Mild, often asymmetric ptosis. In an early neuro-ophthalmic series, three of five patients had unilateral ptosis of no more than 2 to 3 mm.¹⁴
Lid elevation after sustained up-gaze. In MG, ptosis typically worsens with prolonged up-gaze. In LEMS, ptosis may transiently improve, a paradoxical response described in case reports.^5,15
Enhanced ptosis. Manually lifting one lid can worsen ptosis of the fellow lid in LEMS and MG, so this sign does not exclude LEMS.¹⁵
Saccadic facilitation. Eye-movement recordings have shown subclinical increases in saccadic velocity after repeated saccades, the opposite of the fatigue expected in MG.¹⁴

Isolated ocular presentations are rare but documented.¹⁶ In one report, a 59-year-old woman with bilateral ptosis and diplopia was initially diagnosed with seronegative ocular MG. Brief exercise testing showed a CMAP increment of more than 100%, P/Q-type VGCC antibodies were markedly elevated, and a chest CT showed a mediastinal mass, and biopsy confirmed small cell carcinoma.¹⁷

What eye care professionals should look for

Ptosis or diplopia in a patient who also reports trouble with stairs, rising from a chair, or proximal fatigue
Autonomic clues, such as dry mouth, dry eye, constipation, or erectile dysfunction
Ptosis that improves, rather than worsens, after sustained up-gaze
Suspected ocular MG that is antibody-negative or responds poorly to pyridostigmine, since acetylcholinesterase inhibitors usually provide little benefit in LEMS¹
Risk factors for SCLC, including age 50 or older, smoking, and unexplained weight loss¹¹

Patients with these features warrant prompt neurologic referral for antibody and electrodiagnostic testing. Before oculoplastic or strabismus surgery under general anesthesia, the anesthesia team should know the diagnosis, because respiratory failure in LEMS, although uncommon, is often related to the use of paralytic agents.¹

Treatment options

Treating the tumor. In paraneoplastic LEMS, treating the underlying cancer is the priority and often improves neuromuscular symptoms.¹

Amifampridine (3,4-diaminopyridine) (Firdapse, Catalyst Pharmaceuticals). This agent blocks presynaptic potassium channels, prolonging depolarization of the nerve terminal so that VGCCs stay open longer and more acetylcholine is released.^7,18 In a phase 3 randomized withdrawal trial of 38 patients, amifampridine phosphate was significantly better than placebo on the Quantitative Myasthenia Gravis score and the Subject Global Impression. The most common adverse events were oral and digital paresthesias, nausea, and headache.¹⁹

A trial of the base formulation showed that the drug’s strength is maintained.¹⁸ A Cochrane review found moderate- to high-quality evidence that the drug improves muscle strength and CMAP amplitude.²⁰ Typical daily doses range from 30 to 80 mg in divided doses; seizures have been reported at daily doses over 100 mg.¹ In Australia, amifampridine is recommended as first-line symptomatic therapy and is approved for adults and for children aged 6 years and older.⁷The FDA²¹ approved amifampridine for adults and pediatric patients 6 years and older.

Pyridostigmine. Acetylcholinesterase inhibition helps LEMS less than MG. A randomized crossover trial in which pyridostigmine was added to 3,4-diaminopyridine provided no further benefit, and pyridostigmine alone did not differ from placebo.⁸

Intravenous immunoglobulin (IVIG) and plasma exchange. For rapidly progressive disease, 1 g/kg daily of IVIG for 2 days improved limb strength in a randomized crossover trial. The effect peaked at 2 to 4 weeks and declined up to 8 weeks along with decreased VGCC antibody levels.²² Plasma exchange can help patients with severe weakness.¹

Long-term immunosuppression. When symptoms persist despite symptomatic therapy, prednisolone combined with azathioprine is commonly used, mainly in nontumor LEMS. Among 47 such patients followed at one center, about 90% needed combination therapy, and 43% achieved sustained clinical remission within 3 years. Only 14%, however, reached both clinical and pharmacologic remission.²³ Because immunosuppression could theoretically weaken the immune response against a tumor, aggressive immunotherapy is used more often in patients without cancer.¹

Prognosis. In paraneoplastic LEMS, outcome depends largely on the cancer. Notably, patients with SCLC and LEMS survive longer than patients with SCLC alone, an advantage that held after adjustment for known prognostic factors in a prospective comparison.²⁴

Conclusion

LEMS is rare, and its eye signs are usually mild and appear late, which makes them easy to attribute to MG or to aging. Still, ptosis, diplopia, dry eye, and pupillary changes occur in a meaningful share of patients, and in a few, the eye is where the disease first manifests.^13,16,17 An eye care professional who pairs an ocular finding with a history of proximal weakness or autonomic symptoms, and who recognizes ptosis that improves with sustained up-gaze, can prompt testing that leads both to a neuromuscular diagnosis and to earlier detection of an associated lung cancer.