Dallas, Texas – Lambert-Eaton myasthenic syndrome (LEMS) is a rare autoimmune disorder affecting the neuromuscular junction. The immune system produces autoantibodies against presynaptic P/Q-type voltage-gated calcium channels (VGCCs) on motor nerve terminals. When a nerve impulse arrives, these antibodies reduce calcium entry into the nerve terminal, limiting the release of acetylcholine and weakening muscle contraction. Antibodies may also target N-type calcium channels, which help regulate neurotransmitter release from autonomic nerve terminals.¹
Because myasthenia gravis (MG) is another autoimmune disorder affecting the neuromuscular junction, distinguishing between the two conditions is essential. Although they share certain clinical features, differences in their underlying mechanisms produce distinct patterns of weakness, reflex abnormalities, autonomic symptoms, and responses to physical activity.
LEMS is characterized by a classic triad of proximal muscle weakness, reduced or absent tendon reflexes, and autonomic dysfunction.¹˒² Unlike the fatigability typically associated with MG, patients with LEMS may experience a temporary improvement in muscle strength and reflexes following brief, intense muscular activity. This phenomenon, known as post-exercise facilitation, is an important diagnostic clue.¹
LEMS occurs in two forms. Paraneoplastic LEMS is associated most commonly with small cell lung cancer (SCLC), in which the immune response against tumor-associated VGCCs is thought to trigger the neurological disorder. Nontumoral LEMS occurs without an associated malignancy and behaves as a primary autoimmune condition.¹
Distinguishing LEMS From Myasthenia Gravis
Although LEMS and MG both impair neuromuscular transmission, they affect different parts of the neuromuscular junction. In LEMS, autoantibodies target presynaptic calcium channels, reducing acetylcholine release. In MG, antibodies primarily attack postsynaptic structures, including acetylcholine receptors.¹˒³
These differences result in several clinically important distinctions.
Pattern of muscle weakness. MG frequently begins with ocular symptoms, such as ptosis or diplopia, before involving other muscle groups. LEMS more commonly begins with proximal leg weakness and progresses upward, potentially affecting the oculobulbar muscles as the disease advances.¹˒⁴ In a Dutch comparative study, ocular weakness was the initial symptom in 59% of patients with MG but in none of the patients with LEMS.⁴
Response to physical activity. Muscle weakness in MG typically worsens with sustained activity. By contrast, patients with LEMS may experience transient improvement in strength and reflexes after maximal contraction. Ptosis may also improve following sustained up-gaze, an unusual finding that can help distinguish LEMS from MG.¹˒⁵
Reflexes and autonomic symptoms. Reduced or absent tendon reflexes and autonomic manifestations, particularly dry mouth, are characteristic of LEMS. Other symptoms may include constipation, erectile dysfunction, orthostatic symptoms, altered sweating, and dry eyes.¹˒²
Electrodiagnostic and antibody testing. Patients with LEMS typically have a low resting compound muscle action potential (CMAP), which measures the summed electrical responses of muscle fibers following motor nerve stimulation. The CMAP amplitude often increases substantially after brief exercise or high-frequency repetitive nerve stimulation. Approximately 80% to 90% of patients have antibodies against P/Q-type VGCCs, compared with fewer than 5% of patients with MG.¹
Cancer association and treatment. More than half of LEMS cases are associated with SCLC, making malignancy screening an essential component of the diagnostic evaluation.¹ Unlike MG, which often responds to acetylcholinesterase inhibitors, LEMS generally responds poorly to these drugs when used alone. Amifampridine is the first-line symptomatic treatment.¹˒⁷˒⁸
Epidemiology: Who Is Affected?
LEMS is substantially less common than MG. Its annual incidence is estimated to be approximately one-tenth to one-fourteenth that of MG, while its prevalence is about 46 times lower.¹
A nationwide study using Turkish health data identified 159 patients with LEMS. The median age at diagnosis was 60 years, with a range of 16 to 88 years. Women accounted for 55% of cases, and approximately 60% of patients had paraneoplastic LEMS. SCLC accounted for 56% of the associated tumors.⁹
A Japanese nationwide survey estimated LEMS prevalence at 2.7 cases per million people and identified an associated tumor in 47% of patients, most commonly SCLC. Men were more frequently affected by paraneoplastic LEMS, whereas women predominated in the nontumoral autoimmune form.³
Age and sex distributions also vary by subtype. SCLC-associated LEMS typically begins around age 60, and approximately 65% to 75% of affected patients are men. Nontumoral LEMS can develop at any age, with a smaller peak around age 35 and a larger peak near age 60. Women account for most cases of this form.¹
Smoking, unexplained weight loss, and male sex increase the likelihood of an underlying malignancy. Neurological symptoms frequently precede the cancer diagnosis. In an analysis of 227 published cases, SCLC was diagnosed from five years before to six years after the onset of LEMS.¹˒¹⁰ Consequently, eye care professionals may encounter patients with previously undiagnosed LEMS before an associated lung cancer is identified.
Clinical Presentation and Diagnostic Evaluation
Systemic manifestations
Proximal leg weakness is the most common initial complaint. In one large review, approximately 60% of patients presented with leg weakness, whereas diplopia was the initial symptom in only 4%.¹⁰
Weakness generally begins in the proximal muscles, progresses toward the distal muscles, and moves upward from the legs toward the head. This pattern contrasts with the frequent ocular onset of MG. Disease progression may be faster when SCLC is present.¹˒²
Autonomic dysfunction affects approximately 80% to 96% of patients. Dry mouth is the most frequently reported symptom, but patients may also experience erectile dysfunction, constipation, orthostatic symptoms, altered sweating, and dry eyes.¹
Confirming the diagnosis
Diagnosis relies on clinical assessment supported by serological and electrodiagnostic testing.
Approximately 80% to 90% of patients have antibodies against P/Q-type VGCCs. Repetitive nerve stimulation typically reveals a low resting CMAP, a decremental response at low stimulation frequencies, and a substantial increase in CMAP amplitude following brief maximal voluntary contraction or high-frequency stimulation. The post-exercise increment often exceeds 100%.¹
Antibodies against SOX1 can provide additional evidence of an associated SCLC.¹
Once LEMS is confirmed, chest computed tomography (CT) or positron emission tomography imaging is recommended to investigate possible malignancy. Repeat screening every three to six months for at least two years is recommended because the cancer may not be apparent at the initial evaluation.¹
The DELTA-P score can help estimate the risk of underlying SCLC. It assigns one point for each of the following features:
- Age 50 years or older
- Smoking history
- Weight loss of at least 5%
- Bulbar involvement
- Erectile dysfunction
- Karnofsky performance status below 70
Higher scores indicate a greater likelihood of SCLC.¹˒¹¹
Ocular Manifestations of LEMS
How eye involvement differs from MG
Ocular symptoms in LEMS differ from those in MG in both timing and severity. Although ptosis and diplopia can occur, they are less likely to be the initial manifestations of LEMS.
In a Dutch comparative study involving 38 patients with LEMS and 101 patients with MG, 59% of the MG group first noticed ocular symptoms, compared with none of the LEMS group. The investigators concluded that ocular-onset weakness makes LEMS very unlikely.⁴
However, the absence of ocular symptoms at disease onset does not mean the eyes will remain unaffected. Approximately half of patients in a large cohort developed ocular symptoms within 12 months of disease onset, particularly those with SCLC.¹˒²
At the Lahey Clinic in Burlington, Massachusetts, 78% of 23 patients with LEMS had oculobulbar symptoms or signs. Ptosis, diplopia, or both were present in 65%. The investigators cautioned that ocular findings should not be used to exclude LEMS.¹²
A retrospective review from Mayo Clinic examined ophthalmic manifestations more specifically. Reported symptoms included ptosis in 23% of patients, diplopia in 20.5%, decreased vision in 14%, and dry eye in 7%. Examination findings included ptosis in 26%, abnormal ocular motility in 8.5%, strabismus in 8%, pupillary dysfunction in 7%, and dry eye in 2%.¹˒¹³
Overall, ocular weakness in LEMS tends to be milder than in MG, and pronounced ophthalmoplegia is unusual.¹
Ophthalmic findings that may suggest LEMS
Mild, often asymmetric ptosis. In an early neuro-ophthalmic case series, three of five patients had unilateral ptosis measuring no more than 2 to 3 mm.¹⁴
Improvement in ptosis after sustained up-gaze. Ptosis generally worsens with prolonged up-gaze in MG. In LEMS, however, eyelid elevation may transiently improve following sustained up-gaze. This paradoxical response has been described in case reports and may provide a useful diagnostic clue.⁵˒¹⁵
Enhanced ptosis. Manually elevating one eyelid may worsen ptosis in the opposite eyelid in both LEMS and MG. Therefore, this finding does not exclude LEMS.¹⁵
Saccadic facilitation. Eye-movement recordings have demonstrated subclinical increases in saccadic velocity after repeated saccades in patients with LEMS. This finding contrasts with the fatigue expected in MG.¹⁴
Rare isolated ocular presentations. Although uncommon, LEMS can occasionally present predominantly with ocular symptoms.¹⁶ In one reported case, a 59-year-old woman with bilateral ptosis and diplopia was initially diagnosed with seronegative ocular MG. Brief exercise testing subsequently demonstrated a CMAP increment exceeding 100%, and testing revealed markedly elevated P/Q-type VGCC antibodies. Chest CT identified a mediastinal mass, and biopsy confirmed small cell carcinoma.¹⁷
This case illustrates why atypical ocular presentations, particularly those accompanied by unusual examination findings or poor treatment response, warrant further investigation.
What Eye Care Professionals Should Look For
Eye care professionals may be among the first clinicians to encounter patients with LEMS, particularly when ptosis or diplopia develops before the underlying neurological disorder or associated malignancy is recognized.
Several findings should raise suspicion:
- Ocular symptoms accompanied by proximal weakness: Patients may report difficulty climbing stairs, rising from a chair, or performing other activities that require proximal leg strength.
- Autonomic symptoms: Dry mouth, dry eyes, constipation, erectile dysfunction, and orthostatic symptoms may accompany the ocular findings.
- Paradoxical improvement in ptosis: Eyelid elevation that improves rather than deteriorates after sustained up-gaze may suggest LEMS.
- Suspected ocular MG with atypical features: Patients who test negative for relevant MG antibodies or respond poorly to pyridostigmine may require evaluation for alternative diagnoses, including LEMS.
- Risk factors for SCLC: Age 50 years or older, smoking, and unexplained weight loss should increase concern for an underlying malignancy.¹¹
Patients with these features should be referred promptly for neurological assessment, antibody testing, and electrodiagnostic studies.
Recognizing LEMS is also important before surgery. Anesthesia teams should be informed when a patient has the disorder because respiratory failure, although uncommon, may occur in association with paralytic agents used during general anesthesia.¹
Treatment Options
Treatment depends on whether LEMS is associated with malignancy, the severity of symptoms, and the patient’s response to symptomatic therapy.
Treating the underlying tumor
In paraneoplastic LEMS, management of the underlying cancer is a priority. Effective cancer treatment may also improve neuromuscular symptoms.¹
Amifampridine
Amifampridine, also known as 3,4-diaminopyridine, is the first-line symptomatic treatment for LEMS. It blocks presynaptic potassium channels, prolonging nerve-terminal depolarization and allowing voltage-gated calcium channels to remain open longer. This increases acetylcholine release and improves neuromuscular transmission.⁷˒¹⁸
In a phase 3 randomized withdrawal trial involving 38 patients, amifampridine phosphate was significantly more effective than placebo on the Quantitative Myasthenia Gravis score and the Subject Global Impression. The most common adverse events included oral and digital paresthesias, nausea, and headache.¹⁹
A separate trial of the base formulation also demonstrated sustained treatment efficacy.¹⁸ A Cochrane review found moderate- to high-quality evidence that amifampridine improves muscle strength and CMAP amplitude.²⁰
Typical total daily doses range from 30 to 80 mg in divided doses, although dosing must be individualized. Seizures have been reported at daily doses exceeding 100 mg.¹ Amifampridine is approved by the US Food and Drug Administration for adults and pediatric patients aged six years and older.²¹
Pyridostigmine
Pyridostigmine, an acetylcholinesterase inhibitor, generally provides less benefit in LEMS than in MG. A randomized crossover trial found no additional benefit when pyridostigmine was added to 3,4-diaminopyridine. Pyridostigmine alone also did not significantly outperform placebo.⁸
Intravenous immunoglobulin and plasma exchange
For rapidly progressive disease, intravenous immunoglobulin (IVIG) may provide short-term improvement. In a randomized crossover trial, a regimen of 1 g/kg daily for two days improved limb strength. The effect peaked two to four weeks after treatment and gradually declined over the following eight weeks, alongside reductions in VGCC antibody levels.²²
Plasma exchange may also benefit patients with severe weakness.¹
Long-term immunosuppression
For patients whose symptoms persist despite symptomatic treatment, long-term immunosuppression may be appropriate. Prednisolone combined with azathioprine is commonly used, particularly in nontumoral LEMS.
In a single-center cohort of 47 patients with nontumoral LEMS, approximately 90% required combination therapy, and 43% achieved sustained clinical remission within three years. However, only 14% achieved both clinical and pharmacological remission.²³
Because immunosuppression could theoretically impair the immune response against an associated tumor, intensive immunotherapy is generally used more often in patients without cancer.¹
Prognosis
The prognosis of paraneoplastic LEMS depends largely on the underlying malignancy. Interestingly, patients with SCLC and LEMS have been reported to survive longer than patients with SCLC alone. This survival advantage remained evident after adjustment for known prognostic factors in a prospective comparison.²⁴
Conclusion: Recognizing LEMS Through Its Ocular Signs
Lambert-Eaton myasthenic syndrome is rare, and its ocular manifestations are often mild and develop later than its systemic symptoms. As a result, ptosis, diplopia, and other eye findings may be mistaken for MG or attributed to aging.
Nevertheless, ocular symptoms occur in a meaningful proportion of patients, and rare cases may initially present with predominantly ophthalmic complaints.¹³˒¹⁶˒¹⁷ Recognizing the combination of ocular abnormalities, proximal muscle weakness, autonomic dysfunction, and paradoxical improvement in ptosis after sustained up-gaze can help eye care professionals identify patients who require further neurological evaluation.
Timely recognition is particularly important because LEMS may precede the diagnosis of SCLC by months or years. Appropriate referral can therefore lead not only to an accurate neuromuscular diagnosis and effective treatment but also to earlier detection of an associated malignancy.
A Patient’s Perspective: The Importance of Awareness
Price Woolridge, founder and board president of the Lambert-Eaton LEMS Family Association and a patient with LEMS, emphasizes the challenges associated with recognizing this uncommon condition:
“Given how small our patient population is, clinical experience is often lacking due to the scarcity of patients. This leads to frequent misdiagnosis, because MG, LEMS, and other autoimmune disorders have overlapping symptoms. I have experienced symptoms also present in MG. And, though proximal weakness and the inability to walk were my first symptoms, I’ve also experienced vision problems, blurry and double vision. What confounds these symptoms, as well as other LEMS symptoms, is that the severity can come and go, often with little rhyme or reason. An increased understanding of this disease by physicians will give those who live with this disorder a better chance at a correct diagnosis and a treatment plan that’s most effective.”
Information about the LEMS Family Foundation can be accessed at the website: lemsfamily.org
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References
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