Lambert Eaton Myasthenic Syndrome

Slide 1 · Lambert–Eaton myasthenic syndrome

Autoimmune presynaptic failure of neuromuscular transmission

Mechanism • clinicopathologic correlation • diagnosis

Lambert–Eaton myasthenic syndrome

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · The disease is a presynaptic autoimmune channelopathy

Autoantibodies interfere with calcium-dependent acetylcholine release at motor terminals.

The principal defect is impaired neuromuscular transmission—not primary skeletal-muscle destruction.

A distinctive physiological finding is improved muscle response after brief activation.

Presynaptic Ca²⁺-channel autoimmunity

Less Ca²⁺-dependent ACh release

Reduced end-plate activation

Painless muscle weakness

Slide 3 · A localization framework for weakness

Muscle contraction depends on a motor axon, neuromuscular junction, and intact muscle fiber.

Failure at the junction can produce weakness despite preserved structural continuity of the nerve and muscle.

In LEMS, the most informative lesion is presynaptic.

Motor axon

Delivers the action potential

Synapse

Transmits via ACh

Muscle fiber

Generates force

Slide 4 · Anatomy: The motor nerve terminal and muscle end plate

Terminal boutons release acetylcholine into the synaptic cleft.

The postsynaptic sarcolemma concentrates acetylcholine receptors in specialized folds.

The distinction between pre- and postsynaptic compartments is diagnostically essential.

Motor nerve terminal

Synaptic cleft

Skeletal muscle (sarcolemma)

ACh released across the cleft

Slide 5 · Normal excitation–secretion coupling

Motor nerve depolarization opens presynaptic voltage-gated calcium channels.

Entering calcium couples the electrical impulse to synaptic-vesicle fusion.

Acetylcholine released from vesicles activates muscle end-plate receptors.

Nerve action potential

Calcium-channel opening

ACh vesicle fusion and release

End-plate depolarization

Slide 6 · Calcium influx is the critical presynaptic trigger

Electrical arrival at the bouton alone is insufficient for normal quantal transmitter release.

Presynaptic calcium entry promotes fusion of transmitter-containing vesicles.

LEMS disrupts this step upstream of the muscle acetylcholine receptor.

Motor nerve terminal

Synaptic cleft

Skeletal muscle (sarcolemma)

ACh released across the cleft

Slide 7 · Acetylcholine must reach a receptive sarcolemma

Release occurs on the neuronal side of the synaptic cleft.

ACh then interacts with receptors clustered on the muscle membrane.

The postsynaptic membrane can be structurally intact but receive too little transmitter in LEMS.

Presynapse

ACh supply and release

Cleft

Neurotransmitter transit

Postsynapse

ACh receptor activation

Slide 8 · The neuromuscular junction is a common disease target

Autoantibodies, inherited defects, and toxins can impair neuromuscular transmission.

The same symptom—painless weakness—may result from lesions at different molecular steps.

Correct presynaptic versus postsynaptic localization guides interpretation.

Neuromuscular transmission defect

Presynaptic

LEMS; Botulinum toxin

Postsynaptic

ACh-receptor or MuSK myasthenia

Slide 9 · LEMS autoantibodies target voltage-gated calcium channels

The relevant antigens are presynaptic calcium channels, principally the P/Q type.

Channel dysfunction reduces calcium entry during motor nerve activity.

Consequently, fewer ACh quanta are available to activate muscle fibers.

Motor nerve terminal

Synaptic cleft

Skeletal muscle (sarcolemma)

Fewer ACh quanta released

Slide 10 · The P/Q-type channel is a functional antigen

P/Q-type voltage-gated calcium channels (CaV2.1) are concentrated in motor nerve-terminal active zones.

Antibodies directed to these channels interfere with efficient transmitter release.

Channel-directed immunity is the central mechanistic distinction from typical ACh-receptor myasthenia.

Motor nerve terminal

Synaptic cleft

Skeletal muscle (sarcolemma)

ACh released across the cleft

Slide 11 · Presynaptic disease: Intact receptors, reduced transmitter

In LEMS, defective ACh output originates at the nerve terminal.

In common myasthenia gravis, impaired postsynaptic reception dominates.

The two disorders can therefore show different behavior during repetitive activation.

Where is transmission disrupted?

LEMS

Presynaptic Ca²⁺ entry / ACh release

Myasthenia gravis

Postsynaptic ACh receptor function

Slide 12 · Antibody-mediated failure follows a causal sequence

Presynaptic antigen recognition is upstream of reduced calcium-dependent release.

Reduced ACh output diminishes end-plate activation.

Failure to reliably activate muscle fibers manifests as fluctuating strength.

Anti–P/Q-type VGCC response

Less presynaptic Ca²⁺ entry

Reduced ACh quantal release

Weakness and fatigability

Slide 13 · Why the muscle remains weak despite neural impulses

A motor impulse may arrive but fail to release enough ACh.

The postsynaptic response may not consistently reach the threshold for muscle-fiber excitation.

The clinical deficit reflects transmission failure rather than a primary muscle-fiber lesion.

Motor nerve terminal

Synaptic cleft

Skeletal muscle (sarcolemma)

Fewer ACh quanta released

Slide 14 · Reduced quantal content links antibody to weakness

ACh is released in discrete vesicular packets (quanta).

Presynaptic calcium-channel inhibition decreases the number released per impulse.

Electrophysiologic responses provide functional evidence of this reduced transmitter output.

Lower calcium availability

Fewer ACh vesicles released

Smaller end-plate response

Fewer muscle fibers activated

Slide 15 · Brief contraction can transiently improve transmission

Muscle strength in LEMS often improves after several seconds of activity.

Repeated impulses transiently enhance residual calcium availability in the terminal.

More transmitter release produces post-activation facilitation.

QUALITATIVE SCHEMATIC — NOT A PATIENT TRACE

Successive responses

Amplitude

Post-activation increment

Slide 16 · Post-activation facilitation identifies a presynaptic defect

Baseline calcium-dependent release is impaired in LEMS.

Short bursts of impulses temporarily improve the probability of ACh release.

An increased evoked muscle response immediately after activation supports presynaptic localization.

Brief intense activation

Residual terminal Ca²⁺ rises

ACh release increases

Muscle response increases

Slide 17 · Facilitation is not the same as sustained endurance

The improvement after brief contraction is transient.

Prolonged exertion may still cause fatigue and functional limitation.

Record the timing of examination: Strength and reflexes can vary immediately after activity.

Rest

Brief contraction

Transient improvement

Slide 18 · Typical electrophysiology contains both decrement and increment

A low resting compound muscle action potential (CMAP) suggests low baseline transmitter release.

Low-frequency repetitive nerve stimulation often produces decrement.

Brief exercise or high-frequency stimulation elicits an increment—a diagnostic clue.

Low CMAP

Resting response

Decrement

Low-frequency trains

Increment

Post-activation / rapid trains

Slide 19 · Low-frequency stimulation can produce decrement

Repeated slow stimuli may progressively exhaust an already limited release reserve.

The evoked muscle response decreases across the stimulus train.

Decrement alone does not reliably distinguish LEMS from myasthenia gravis.

SCHEMATIC — not quantitative data

Successive responses

Amplitude

Slide 20 · A post-exercise increment is the discriminating response

After brief maximal contraction, the evoked response can markedly increase.

This facilitation reflects enhanced presynaptic ACh release.

Interpret the finding with the clinical phenotype and antibody results.

QUALITATIVE SCHEMATIC — NOT A PATIENT TRACE

Successive responses

Amplitude

Post-activation increment

Slide 21 · Electrophysiology must be interpreted as a pattern

No isolated waveform feature should replace careful clinical examination.

Resting CMAP reduction, low-rate decrement, and post-activation increment are complementary.

Technical quality, muscle selection, and activation protocol influence interpretation.

Clinical suspicion

Resting motor response

Repetitive stimulation / exercise

Serology and integrated diagnosis

Slide 22 · Autoimmune LEMS exists with or without malignancy

One form accompanies an underlying neoplasm (paraneoplastic LEMS).

Another occurs without a detected tumor and may coexist with autoimmune disease.

The neuromuscular physiological defect is shared by both forms.

LEMS: Autoimmune presynaptic failure

Paraneoplastic

Associated with malignancy, commonly SCLC

Non-tumor-associated

May accompany other autoimmune diseases

Slide 23 · Small-cell lung carcinoma is the dominant tumor association

Underlying cancer in about half of patients.

The associated malignancy is most often small-cell carcinoma of the lung.

Recognizing LEMS may therefore reveal an occult malignancy.

LEMS

Immune channelopathy

About half

Underlying malignancy

SCLC

Most frequent tumor type

Slide 24 · Paraneoplastic immune recognition is a proposed mechanism

Small-cell lung cancer may express calcium-channel antigens also found at motor terminals.

A tumor-induced immune response is thought to cross-react with neuronal channels.

This explanation is biologically plausible but is presented by Robbins as a hypothesis.

Tumor-expressed channel antigen

Antitumor immune response

Cross-reactive channel antibody

Presynaptic ACh-release defect

Slide 25 · Neurologic symptoms can precede cancer detection

Lambert–Eaton weakness can arise before a tumor has been diagnosed.

The interval may extend for years in some patients,.

An initially negative malignancy evaluation does not eliminate future cancer risk.

LEMS symptoms

Initial screening

Possible later SCLC diagnosis

Slide 26 · Cancer surveillance is part of syndrome evaluation

Confirming the neuromuscular syndrome should trigger assessment for an occult malignancy.

Chest imaging is central because of the SCLC association.

Follow-up screening strategy should be individualized using specialist protocols and risk factors.

Clinical + physiological LEMS

Evaluate for underlying SCLC

Address a detected malignancy

Continue follow-up when appropriate

Slide 27 · Non-tumor-associated LEMS may cluster with autoimmunity

A substantial fraction of patients have no identified neoplasm.

Vitiligo and thyroid disease as examples of associated autoimmunity.

These associations reinforce an immune mechanism but are not themselves diagnostic of LEMS.

LEMS

Autoimmune presynaptic dysfunction

Thyroid disease

Possible autoimmune association

Vitiligo

Another described association

Slide 28 · Clinical triad: Proximal weakness, hyporeflexia, dysautonomia

The characteristic pattern combines proximal limb weakness with depressed tendon reflexes.

Autonomic dysfunction—often dry mouth—provides an important localizing clue.

The full triad need not be simultaneously prominent in every patient.

Limb weakness

Usually proximal; Legs early

Tendon reflexes

Reduced or absent

Autonomic features

Dry mouth and other symptoms

Slide 29 · Weakness commonly begins in the proximal legs

Hip-girdle and thigh muscles are often most affected early.

Patients may struggle to climb stairs or rise from a chair.

The distribution can resemble other causes of proximal weakness unless reflexes and autonomic signs are assessed.

Hip flexion

Proximal motor task

Chair rise

Functional limitation

Stairs

Common patient complaint

Slide 30 · Upper-limb and cranial weakness may emerge later

Proximal weakness can extend beyond the lower limbs.

Oculobulbar symptoms may develop; They are generally less dominant initially than in classic myasthenia gravis.

Severe cases can involve respiratory muscles, requiring careful clinical assessment.

Proximal leg weakness

Additional limb involvement

Possible ocular / bulbar symptoms

Severe: Respiratory involvement

Slide 31 · Autonomic dysfunction reflects channel involvement beyond motor terminals

Presynaptic calcium channels also participate in autonomic neurotransmission.

Reduced transmitter release can produce non-motor manifestations.

These findings help differentiate LEMS from many primary myopathies.

Autonomic neuronal channels affected

Reduced neurotransmitter output

Abnormal autonomic function

Dry mouth and related symptoms

Slide 32 · Dry mouth is a valuable diagnostic clue

Xerostomia is a commonly reported autonomic symptom.

Constipation, altered sweating, and erectile dysfunction may also occur.

A targeted autonomic review can change the localization of otherwise nonspecific proximal weakness.

Dry mouth

Commonly reported

Gastrointestinal

Constipation

Other

Sweating / sexual function

Slide 33 · Tendon reflexes may increase after activation

Hyporeflexia or areflexia is characteristic at rest.

A brief voluntary contraction may transiently make a reflex more readily elicited.

Compare reflex assessment before and immediately after activation when clinically appropriate.

At rest: Reduced

Brief contraction

Transient facilitation

Slide 34 · Bedside examination should test for reversibility

Assess baseline power in affected proximal muscle groups.

Repeat selected maneuvers after several seconds of maximal activation.

A transient rise in strength or reflex amplitude supports—but does not independently confirm—LEMS.

Establish baseline

Brief maximal activation

Reassess strength / reflexes

Integrate with electrophysiology

Slide 35 · Painless fatigue localizes poorly without further clues

Disorders at the neuromuscular junction commonly produce painless weakness.

Proximal distribution alone also occurs in myopathies and some neuropathies.

LEMS-specific clues are post-activation facilitation, autonomic features, and depressed reflexes.

Weakness

Common to multiple disorders

Facilitation

Presynaptic clue

Autonomic + reflex

Strengthen LEMS suspicion

Slide 36 · A focused history can raise suspicion for LEMS

Ask about progressive leg weakness and difficulty with stairs or chair rise.

Elicit dry mouth, bowel symptoms, and other autonomic complaints.

Review prior malignancy, smoking exposure, and the time course of neurological symptoms.

Motor

Function and progression

Autonomic

Dry mouth / constipation

Cancer risk

History and risk factors

Slide 37 · Serum channel antibodies support an autoimmune diagnosis

Most patients have detectable antibodies to P/Q-type VGCC.

A positive result supports the diagnosis in a compatible clinical and neurophysiological setting.

Antibody testing should be interpreted alongside the full syndrome—not in isolation.

Clinical phenotype

Proximal weakness ± autonomic signs

Electrophysiology

Presynaptic response pattern

Anti–P/Q VGCC

Supportive serology

Slide 38 · A negative channel-antibody result does not exclude LEMS

Reviews report antibodies in approximately 90% of affected patients—not all patients.

Seronegativity therefore cannot replace careful electrodiagnostic evaluation.

The probability of disease depends on the entire presentation and objective physiological findings.

Strong clinical suspicion

Negative / indeterminate serology

Perform expert electrodiagnosis

Integrated final interpretation

Slide 39 · Resting CMAP amplitude provides a baseline

Compound muscle action potentials reflect the summed electrical response of activated muscle fibers.

LEMS often has low initial CMAP amplitudes before exercise.

The response may become larger after activation when transmitter output transiently increases.

QUALITATIVE SCHEMATIC — NOT A PATIENT TRACE

Successive responses

Amplitude

Post-activation increment

Slide 40 · Low-rate repetitive stimulation is not pathognomonic

A decremental response can occur in LEMS during low-frequency stimulation.

Myasthenia gravis also produces decremental responses.

The decisive distinction often requires testing the response to brief exercise or rapid stimulation.

SCHEMATIC — not quantitative data

Successive responses

Amplitude

Slide 41 · Post-activation increment reflects physiologic reserve

Transient accumulation of calcium can partially bypass reduced resting entry.

The resulting increased ACh release recruits additional muscle fibers.

An increment is expected immediately after activation; Timing matters.

QUALITATIVE SCHEMATIC — NOT A PATIENT TRACE

Successive responses

Amplitude

Post-activation increment

Slide 42 · Electrodiagnosis requires correct timing and technique

Obtain baseline recordings before facilitating the muscle.

Compare low-frequency responses with those after brief activation.

Poor activation, temperature effects, and technical artifacts can mislead interpretation.

Baseline CMAP

Low-rate repetitive stimulation

Brief maximal exercise

Immediate repeat CMAP

Slide 43 · An integrated diagnostic sequence limits false conclusions

Clinical localization leads to targeted neurophysiology and serology.

A coherent triad is more persuasive than one isolated abnormality.

Tumor evaluation follows LEMS recognition rather than substituting for neurologic confirmation.

Phenotype + examination

Neurophysiological confirmation

VGCC serology

Malignancy assessment

Slide 44 · LEMS and myasthenia gravis affect opposite sides of the synapse

Feature

Lambert–Eaton syndrome

Myasthenia gravis

Dominant site

Presynaptic calcium channels

Postsynaptic AChR or MuSK

Transmission defect

Reduced ACh release

Reduced ACh receptor function

Brief activity

Strength may improve

Weakness commonly worsens

Typical context

SCLC association

Thymic abnormalities often associated

Slide 45 · Clinical discrimination: LEMS versus myasthenia gravis

Clinical clue

LEMS

Typical AChR myasthenia

Early emphasis

Proximal legs

Ocular weakness often prominent

Reflexes

Often reduced / absent

Usually maintained

Autonomic features

Common

Not a defining feature

Activation

Brief facilitation

Exertional fatigability

Cancer link

Small-cell lung cancer

Thymoma in a subset

Slide 46 · Electrophysiologic discrimination requires increment testing

Test feature

LEMS

Myasthenia gravis

Resting CMAP

Often reduced

Often preserved

Low-rate repetitive train

Decrement may occur

Decrement may occur

After brief contraction

Marked facilitation possible

No characteristic marked increment

Diagnostic principle

Presynaptic release failure

Postsynaptic receptor dysfunction

Slide 47 · Botulism is another presynaptic disorder—but by a different mechanism

Botulinum neurotoxin blocks vesicular ACh release from presynaptic terminals.

LEMS instead involves antibodies to presynaptic calcium channels.

Exposure history, clinical tempo, autonomic findings, and electrodiagnosis distinguish the mechanisms.

Presynaptic reduction of ACh release

LEMS

Antibody to Ca²⁺ channel

Botulism

Toxin blocks vesicle-release machinery

Slide 48 · Congenital myasthenic syndromes require a different framework

Congenital syndromes result from inherited defects in synaptic proteins.

Clinical onset can be perinatal or later depending on the affected gene.

Genetic disease differs from the acquired autoimmune pathogenesis of LEMS.

Neuromuscular junction dysfunction

LEMS

Acquired autoimmune disease

Congenital syndromes

Inherited synaptic-protein defects

Slide 49 · Primary myopathy can resemble proximal LEMS weakness

Proximal weakness and impaired chair rise are not unique to the neuromuscular junction.

Autonomic symptoms and exercise-related reflex facilitation favor LEMS localization.

Serological and electrodiagnostic studies can distinguish junctional dysfunction from primary muscle disease.

Phenotype

Similar proximal weakness

Distinguish

Reflexes and autonomic signs

Confirm

Neurophysiology + serology

Slide 50 · A positive VGCC antibody without the syndrome is insufficient

A laboratory result must be interpreted against pre-test probability.

Compatible clinical and electrophysiological findings increase diagnostic confidence.

Do not diagnose or exclude LEMS using serology as the sole criterion.

Assess phenotype

Check objective NMJ physiology

Interpret serology in context

Reach a clinical diagnosis

Slide 51 · Treatment targets transmitter release, immunity, and cancer

Symptomatic treatment improves effective presynaptic ACh release.

Immune-directed treatment addresses antibody-mediated dysfunction when indicated.

If SCLC is present, tumor-directed treatment is a core component of care.

Symptomatic

Enhance ACh release

Immunologic

Reduce immune-mediated effects

Oncologic

Treat associated tumor

Slide 52 · Amifampridine enhances presynaptic transmitter release

Amifampridine (3,4-diaminopyridine) blocks presynaptic potassium channels.

Prolonged depolarization allows more calcium entry through remaining functional channels.

The resulting increase in ACh release can improve strength and function.

Presynaptic K⁺-channel block

Prolonged nerve-terminal action potential

Enhanced Ca²⁺ entry

Increased ACh release

Slide 53 · Symptomatic therapy has important safety considerations

Amifampridine is a treatment for LEMS, not a cure for autoimmune channel dysfunction.

The FDA label includes seizure-related precautions and contraindications.

Prescription choice, dosing, and monitoring require patient-specific specialist evaluation.

Benefit

More effective ACh release

Risk

Seizure precautions

Oversight

Specialist prescribing / monitoring

Slide 54 · Immunotherapy addresses the autoimmune component

Immunosuppressive approaches analogous to those used in myasthenia gravis.

Selected patients may receive immunomodulation when symptomatic measures are insufficient.

Choice depends on severity, clinical context, and concurrent tumor management.

Persistent autoimmune dysfunction

Consider immune-directed treatment

Reduce pathogenic antibody effect

Reassess function and adverse effects

Slide 55 · The associated malignancy changes management priorities

LEMS can be a presenting clue to small-cell lung carcinoma.

Cancer assessment should not be delayed by partial neurologic improvement.

Treatment of the detected tumor may be relevant to the neurologic syndrome as well as oncologic prognosis.

Recognize LEMS

Detect associated malignancy

Tumor-directed care

Longitudinal neurologic review

Slide 56 · Follow-up should track both neurologic and oncologic outcomes

Document limb function, bulbar/respiratory symptoms, and autonomic complaints.

Assess treatment tolerability and the durability of improvement.

For patients without detected cancer, surveillance decisions follow specialist cancer-risk assessment.

Function

Strength and daily activities

Therapy

Response / adverse effects

Cancer

Assessment and surveillance

Slide 57 · Case: An older adult with progressive proximal leg weakness

Several months of difficulty climbing stairs and rising from a chair.

Examination shows reduced tendon reflexes and dry mouth.

Strength improves immediately after several seconds of maximal contraction.

Motor

Proximal leg weakness

Reflex

Reduced at rest

Autonomic

Xerostomia

Slide 58 · Case interpretation: Localize before naming the tumor

The combined pattern strongly suggests presynaptic neuromuscular-junction dysfunction.

Obtain electrodiagnostic testing and anti–P/Q-type VGCC serology.

If LEMS is confirmed, assess for occult small-cell lung cancer.

Clinical LEMS phenotype

Neurophysiology + channel serology

Cancer evaluation

Individualized treatment

Slide 59 · Case: An autoimmune background without known malignancy

A patient with thyroid autoimmune disease develops proximal weakness.

Resting reflexes are diminished; An increment appears after brief activation.

Absence of a known tumor does not rule out the LEMS diagnosis.

Suspected LEMS

Assess NMJ

Confirm physiology + channel antibodies

Assess context

Check for malignancy and autoimmune disease

Slide 60 · Case interpretation: Negative initial imaging is not the endpoint

A compatible clinical/electrophysiologic syndrome remains diagnostically meaningful.

Negative initial chest imaging should be interpreted in light of evolving cancer risk.

Clinical follow-up is needed regardless of whether an associated cancer is identified.

LEMS recognized

Initial evaluation

Planned follow-up

Slide 61 · Misconception: All myasthenic weakness worsens with brief effort

Typical myasthenia gravis can worsen with repeated use.

In LEMS, a short burst of activity can improve strength and electrical response.

This contrast follows directly from the location of the transmission defect.

Response immediately after brief activity

LEMS

Facilitation possible

Typical MG

Fatigable weakness / decrement

Slide 62 · Misconception: LEMS can be dismissed without a visible tumor

Non-neoplastic LEMS and autoimmune comorbidities.

Neurologic symptoms may predate discovery of SCLC.

Clinical assessment must treat the neurologic diagnosis and the malignancy search as related but distinct tasks.

Confirm presynaptic disorder

Evaluate tumor association

Do not equate negative scan with no LEMS

Reassess risk over time

Slide 63 · Synthesis: Three mechanisms explain the disease

Anti–P/Q-type VGCC antibodies reduce presynaptic calcium-dependent ACh release.

Short bursts of activation transiently enhance release and muscle response.

Paraneoplastic recognition of channel antigens is a proposed route to immune initiation.

Autoimmunity

Presynaptic channel target

Facilitation

Activity raises ACh release

Paraneoplasia

Frequently linked to SCLC

Slide 64 · Synthesis: A clinically recognizable phenotype

Proximal weakness, diminished reflexes, and autonomic complaints suggest LEMS.

Electrophysiologic facilitation and channel antibodies provide confirmatory evidence.

The possibility of small-cell lung carcinoma changes clinical evaluation.

Recognize

Proximal weakness + autonomic signs

Confirm

RNS / exercise + VGCC serology

Investigate

Associated SCLC

Slide 65 · Selected scientific references

Source

Contribution

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. 2026. Chapter 27.

Primary pathology and parent neuromuscular-junction framework

Lipka AF, Verschuuren JJG. Handb Clin Neurol. 2024;200:307–325.

Clinical phenotype, antibodies, screening, electrodiagnosis

Ivanovski T, Miralles F. Degener Neurol Neuromuscul Dis. 2019;9:27–37.

Diagnostic physiology and clinical differentiation

FDA. FIRDAPSE (amifampridine) prescribing information. 2025.

Drug mechanism, indications and safety