Lambert-Eaton myasthenic syndrome

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Voltage-gated calcium channels at the presynaptic motor terminal trigger acetylcholine release. Autoantibody-mediated disruption leaves a low baseline quantal output, producing weakness and reduced reflexes. Repeated activation allows calcium to accumulate in the terminal and temporarily improves release, explaining facilitation of strength and reflexes. Autonomic terminals are affected as well, creating dryness, constipation and sexual or pupillary symptoms that are much less characteristic of uncomplicated myasthenia gravis.

Lambert-Eaton syndrome can precede discovery of small-cell lung cancer. One negative CT therefore does not always end surveillance in a high-risk phenotype. Initial contrast chest imaging, sometimes followed by PET-CT and specialist-directed repeat screening, is guided by smoking, age, antibody features and a validated cancer-risk score. Screening duration and interval should follow the current paraneoplastic and lung-cancer multidisciplinary protocol rather than an improvised annual scan schedule.

Treatment targets transmission, tumour and autoimmunity. Amifampridine prolongs presynaptic depolarisation, increases calcium entry and improves acetylcholine release. Because it also increases neuronal excitability, seizure and cardiac precautions matter. Pyridostigmine sometimes adds modest benefit. Cancer therapy can reduce the immune driver; immunoglobulin or immunosuppression is reserved for significant autoimmune disease. Rehabilitation, falls prevention and autonomic care remain important even after power improves.

Key points

Lambert-Eaton myasthenic syndrome is a presynaptic autoimmune junction disorder, usually involving antibodies to P/Q-type voltage-gated calcium channels and reduced acetylcholine release.
Proximal leg weakness is typical: patients struggle to rise from a low chair, climb stairs or walk uphill before upper limbs become significantly affected.
Tendon reflexes are reduced or absent but may briefly return after several seconds of maximal muscle contraction, an examination feature called post-exercise facilitation.
Autonomic symptoms such as dry mouth, constipation, erectile dysfunction, blurred vision and orthostatic light-headedness are common and diagnostically useful.
Strength may transiently improve with exertion before later fatigue, contrasting with the more consistent use-dependent worsening of myasthenia gravis.
Ocular and bulbar symptoms can occur but are often milder or later than in acetylcholine-receptor myasthenia; prominent early ptosis does not by itself exclude Lambert-Eaton disease.
A substantial proportion of cases are paraneoplastic, most importantly with small-cell lung cancer; age, smoking and rapid evolution increase that probability.
Low-frequency repetitive nerve stimulation shows a small compound muscle action potential, while high-frequency stimulation or brief exercise produces a marked increment.
Amifampridine is first-line symptomatic treatment where licensed and commissioned, but dose limits, seizure risk, QT assessment and interacting medicines require specialist prescribing.
Treating an underlying cancer may improve neurological disease; non-tumour or persistent cases may need pyridostigmine, immunoglobulin or immunosuppression through a neuromuscular team.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Paraneoplastic autoimmunity

Small-cell lung cancer commonly expresses antigens that provoke antibodies against presynaptic voltage-gated calcium channels, particularly in older people with smoking exposure.

Non-tumour autoimmune disease

LEMS can occur without detectable cancer through idiopathic loss of immune tolerance, sometimes alongside other autoimmune disorders.

Seronegative junctional disease

A compatible facilitation phenotype and neurophysiology may occur without measurable standard antibodies, requiring careful exclusion of mimics rather than dismissal.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Presynaptic antibody binding
Autoantibodies reduce functional voltage-gated calcium channels at motor and autonomic nerve terminals, thereby impairing transmitter release.
2
Reduced acetylcholine release
Less calcium enters with each impulse, so too little acetylcholine reaches muscle receptors and neuromuscular transmission fails.
3
Activity-dependent facilitation
Repeated activation allows calcium to accumulate in the nerve terminal, temporarily increasing transmitter release, strength and tendon reflexes.
4
Autonomic transmission failure
Similar presynaptic dysfunction in autonomic nerves causes dry mouth, constipation, erectile difficulty, blurred vision and orthostatic symptoms.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Leg-girdle weakness

Difficulty rising, stair climbing and walking dominates early, with relatively preserved sensation and less striking ocular involvement than typical myasthenia.

Reflex facilitation

A previously absent knee jerk becomes elicitable immediately after a forceful brief contraction, reflecting temporary improvement in acetylcholine release.

Autonomic cluster

Persistent dry mouth with constipation, erectile difficulty, impaired sweating or orthostatic symptoms strongly supports a presynaptic junction disorder.

Paraneoplastic context

Older age, tobacco exposure, weight loss, cough or rapid neurological progression increases the likelihood of associated small-cell lung cancer.

Bulbar or respiratory declineRed flag

Weak swallow, aspiration, falling cough strength or ventilatory symptoms need urgent admission even though severe crisis is relatively unusual.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Repetitive nerve stimulationFirst step
Why
Demonstrate a presynaptic neuromuscular-junction transmission pattern.
Interpretation and limitations
Low baseline amplitude with a substantial increment after brief exercise or high-frequency stimulation supports Lambert-Eaton syndrome; technique and temperature affect sensitivity.
02
P/Q-type voltage-gated calcium-channel antibodies
Why
Support the autoimmune diagnosis and paraneoplastic assessment.
Interpretation and limitations
A positive result is helpful in a concordant syndrome but is not perfectly specific for cancer status; seronegative clinically proven disease can occur.
03
CT chest with contrast
Why
Search for small-cell lung cancer at diagnosis.
Interpretation and limitations
Review with the lung-cancer team; if initial imaging is negative but risk remains, PET-CT or interval surveillance may be required under specialist guidance.
04
Pulmonary function and bulbar assessment
Why
Identify uncommon but serious respiratory or swallowing compromise.
Interpretation and limitations
Serial vital capacity, cough and swallow safety matter more than a single normal saturation when generalised weakness progresses.
05
ECG and electrolyte profile
Why
Provide safety data before amifampridine and investigate arrhythmic or seizure risk.
Interpretation and limitations
Correct potassium, magnesium and calcium, review QT interval and interacting drugs, and involve cardiology if conduction or structural disease complicates treatment.
06
Broader neuropathy and myopathy screen
Why
Exclude competing causes when sensory signs, pain or enzyme elevation is present.
Interpretation and limitations
Blood count, glucose, thyroid, B12, CK and paraprotein studies are selected from phenotype; abnormal sensory conduction argues against an isolated junction disorder.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Myasthenia gravis

Prominent fluctuating ocular-bulbar weakness with preserved reflexes favours myasthenia; leg-predominant weakness, autonomic symptoms and post-exercise reflex return favour LEMS.

Myopathy

Symmetrical proximal weakness can resemble LEMS, but muscle enzyme, electromyographic and biopsy patterns differ and autonomic symptoms are less characteristic.

Peripheral neuropathy

Sensory loss and length-dependent distal weakness support neuropathy; LEMS generally preserves sensation despite clinically depressed reflexes.

Botulism

Acute descending weakness, fixed dilated pupils, gastrointestinal exposure or wound context favours botulinum toxin rather than chronic autoimmune junctional disease.

Additional chapter-specific clues

Alternative localisation

Sensory loss, length-dependent pain, upper motor-neurone signs or a persistently normal neurophysiology study should prompt neuropathy, myopathy or central causes.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01LocalisationIdentify presynaptic junction failureFirst stepAn adult develops progressive proximal weakness with reduced reflexes.

1Ask about transient exercise improvement, dry mouth, constipation, erectile function, ptosis, dysphagia, smoking and cancer symptoms, then map power and sensation.
2Test reflexes before and immediately after brief maximal contraction, while distinguishing true facilitation from inconsistent technique or patient effort.
3Refer to a neuromuscular service for repetitive stimulation and calcium-channel antibodies, avoiding a diagnostic trial of high-risk medicine without confirmation.

02Cancer searchDetect the paraneoplastic driverLambert-Eaton syndrome is confirmed or strongly suspected.

1Arrange contrast chest imaging promptly and assess smoking, age, weight loss and rapidity of onset with respiratory and oncology colleagues.
2If initial CT is unrevealing, use the specialist risk score to select PET-CT and repeat surveillance interval rather than declaring non-paraneoplastic disease immediately.
3Treat detected small-cell cancer through the lung multidisciplinary team while coordinating neurological medicines and anticipating chemotherapy-related nerve or marrow toxicity.

03Symptomatic treatmentImprove acetylcholine release safelyWeakness limits mobility or daily function after diagnostic assessment.

1Check seizure history, ECG, electrolytes and interacting medicines before starting licensed amifampridine at a low divided dose through specialist prescribing.
2EscalationIncrease gradually within the product maximum, documenting walking, chair rise and adverse effects rather than escalating solely from subjective fatigue.
3Consider pyridostigmine as adjunct and provide physiotherapy, falls and autonomic support, recognising that symptomatic drugs do not replace cancer surveillance.

04Immune escalationTreat persistent autoimmune disabilityEscalationFunction remains poor despite tumour therapy and optimised symptomatic treatment.

1Reconfirm adherence, diagnosis, cancer activity and another neuromuscular contributor before attributing all weakness to immune persistence.
2Discuss intravenous immunoglobulin for substantial short-term need and steroid-sparing immunotherapy for selected non-paraneoplastic or refractory disease.
3Monitor infection, blood and organ toxicity with a named neurology team and repeat objective strength measures to stop ineffective long-term immunosuppression.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

First-line licensed symptomatic therapy where locally commissioned, increasing presynaptic acetylcholine release and improving strength.

Amifampridine

The licensed adult start is 15 mg daily in 3–4 divided doses, increased by 5 mg every 4–5 days to a maximum 60 mg daily; no single dose should exceed 20 mg. In mild renal or hepatic impairment start 5 mg twice daily, and in moderate-to-severe impairment start 5 mg once daily, increasing by 5 mg no more often than every 7 days under the SmPC.

Contraindicated with epilepsy, uncontrolled asthma and specified QT-prolonging or narrow-therapeutic-window combinations; review ECG, electrolytes, paraesthesia and abdominal effects. NHS access and funding vary, and NHS England does not routinely commission amifampridine phosphate outside its stated policy route.

Reduces acetylcholine breakdown and may add symptomatic benefit in some patients.

Pyridostigmine

Use a low oral dose titrated under the BNF and neuromuscular team, usually as adjunct rather than relying on it as the only effective treatment.

Diarrhoea, cramp, salivation and bradycardia limit dosing; renal impairment and concurrent conduction disease require particular care.

Can provide temporary improvement in severe autoimmune Lambert-Eaton syndrome.

Intravenous immunoglobulin

For acute Lambert-Eaton syndrome, current NHS England criteria advise 1 g/kg initially, with a further 1 g/kg only for deterioration or no response, using the specialist local weight and infusion policy.

Infusion reactions, thrombosis, haemolysis, renal injury and aseptic meningitis require screening and monitoring; it does not remove the need to search for cancer.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Occult cancer progression

Neurological symptoms may precede recognition of small-cell lung cancer, so inadequate surveillance can delay potentially consequential oncological treatment.

Bulbar and respiratory weakness

Severe disease can impair swallowing, cough and ventilation, causing aspiration or respiratory failure despite leg symptoms being more typical.

Falls and autonomic injury

Proximal weakness and orthostatic hypotension cause collapse, fractures and restricted mobility, while constipation and urinary symptoms add morbidity.

Treatment-related cardiac or seizure risk

Symptomatic and immune medicines can affect cardiac conduction, seizure threshold and infection risk, requiring specialist selection and monitoring.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Record chair-rise time, walking distance, proximal power, reflex facilitation, bulbar function and falls at each meaningful treatment review.
Continue small-cell lung cancer surveillance at the specialist-selected interval after negative first imaging, documenting when and why surveillance ends.
During amifampridine titration review seizures, palpitations, syncope, ECG context, electrolytes, paraesthesia and gastrointestinal effects.
Assess dry mouth, constipation, erectile symptoms and orthostatic blood pressure, using practical autonomic measures that do not worsen cardiac risk.
Monitor infection, blood count, liver and other agent-specific toxicity during immunosuppression or cancer treatment with shared specialty ownership.
Reassess respiratory and swallowing safety whenever weakness accelerates, because relative rarity of crisis does not make aspiration impossible.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Reflexes can return

Post-exercise facilitation is unusually visible: calcium accumulates presynaptically and transiently restores a tendon response that was absent at rest.

Dry mouth discriminates

Autonomic dysfunction is central to Lambert-Eaton disease and can help distinguish it from uncomplicated myasthenia even before neurophysiology.

One negative scan is insufficient

Neurological disease can precede radiologically detectable small-cell cancer, so risk-guided interval surveillance is part of treatment.

Exercise has two phases

Strength may briefly facilitate immediately after effort before ordinary fatigue emerges, which explains apparently conflicting patient descriptions.

Treatment can reveal cancer response

Neurological improvement after effective tumour therapy supports a paraneoplastic driver, although symptomatic and immune care may still be required.

11Common pitfallsFrequent interpretation and management errors.

01
Calling absent reflexes peripheral neuropathy without testing facilitation.
02
Assuming prominent dry mouth is merely a medicine adverse effect.
03
Stopping cancer investigation after one negative chest CT.
04
Using amifampridine without seizure, ECG and interaction review.
05
Treating proximal weakness while ignoring falls and autonomic symptoms.
06
Assuming Lambert-Eaton syndrome can never impair breathing or swallowing.
07
Continuing immunosuppression without objective evidence of benefit.