Medicine – Lambert–Eaton Myasthenic Syndrome

Lambert–Eaton myasthenic syndrome (LEMS) is an autoimmune disorder of the presynaptic neuromuscular junction. It causes impaired release of acetylcholine from motor nerve terminals and therefore produces characteristic muscle weakness.

It is especially important because it may occur as a paraneoplastic syndrome, classically in association with small-cell lung carcinoma.

1. Autoimmune Mechanism

In LEMS, autoantibodies are directed mainly against presynaptic P/Q-type voltage-gated calcium channels on the motor nerve terminal.

Normally, when a nerve impulse reaches the presynaptic terminal, voltage-gated calcium channels open and allow calcium to enter.

The calcium influx then triggers release of acetylcholine into the neuromuscular junction.

2. Effect of Calcium-Channel Antibodies

When these calcium channels are impaired by autoantibodies, less calcium enters the nerve terminal.

This causes reduced acetylcholine release.

The sequence is:

Antibodies against presynaptic Ca²⁺ channels → reduced Ca²⁺ entry → reduced acetylcholine release → impaired neuromuscular transmission → muscle weakness.

3. Paraneoplastic Association

LEMS has a particularly strong association with small-cell lung carcinoma (SCLC).

Small-cell carcinoma can express proteins resembling neuronal voltage-gated calcium channels.

The immune response directed against the tumour can therefore cross-react with calcium channels at the neuromuscular junction.

This produces a classic paraneoplastic neurological syndrome.

4. Non-Paraneoplastic LEMS

Not every patient with LEMS has cancer.

Some cases are purely autoimmune and may occur without an underlying malignancy.

However, because of the strong association with SCLC, patients with newly diagnosed LEMS generally require appropriate assessment for an underlying malignancy, particularly lung cancer.

5. Proximal Muscle Weakness

The characteristic weakness is predominantly proximal rather than distal.

The lower limbs are often affected first and most severely.

Patients may therefore complain of difficulty:

Rising from a chair.

Climbing stairs.

Getting out of bed.

Walking for prolonged periods.

Upper-limb proximal weakness may develop later.

6. Lower Limb Predominance

LEMS commonly affects the pelvic-girdle and thigh muscles before the upper limbs.

Patients may initially describe their legs as heavy or weak.

This lower-limb proximal pattern is an important clue when distinguishing LEMS from some other neuromuscular-junction disorders.

7. Facilitation with Exercise

A very characteristic feature of LEMS is that muscle strength may temporarily improve after brief exercise.

This is called facilitation or post-exercise improvement.

Repeated nerve activity allows calcium to accumulate within the presynaptic nerve terminal.

Even though calcium-channel function is reduced, the accumulated calcium allows more acetylcholine to be released.

Therefore:

Brief exercise → increased presynaptic Ca²⁺ accumulation → increased acetylcholine release → temporary improvement in strength.

8. Weakness with Sustained Activity

The original note states that weakness improves with exercise and then weakens again with sustained exercise.

This is a useful general concept.

The most characteristic finding is initial facilitation after brief activity. With prolonged or repeated exertion, the improvement may not persist, and fatigue can again become apparent.

Therefore, remember:

LEMS → brief exercise improves strength.

This contrasts with the classic fatigability of myasthenia gravis.

9. Reduced Reflexes

An important feature not included in the original notes is reduced or absent tendon reflexes.

Patients often have hyporeflexia or areflexia.

However, reflexes may temporarily improve after the patient contracts the relevant muscle for several seconds.

This phenomenon is another example of post-activation facilitation.

10. Autonomic Symptoms

LEMS frequently affects the autonomic nervous system because voltage-gated calcium channels also participate in autonomic neurotransmission.

Common autonomic symptoms include:

Dry mouth.

Constipation.

Erectile dysfunction.

Reduced sweating.

Orthostatic symptoms.

Dry mouth is particularly common.

11. Ocular and Bulbar Symptoms

Ocular symptoms such as ptosis and diplopia can occur, but they are usually less prominent than in myasthenia gravis.

Bulbar symptoms such as dysarthria or dysphagia may also occur but are generally less dominant.

This helps distinguish the typical clinical patterns of the two diseases.

12. LEMS versus Myasthenia Gravis

The two disorders both affect neuromuscular transmission but at different sites.

LEMS is presynaptic.

The autoantibodies attack voltage-gated calcium channels, reducing acetylcholine release.

Myasthenia gravis is mainly postsynaptic.

The immune system commonly attacks acetylcholine receptors or related postsynaptic proteins, impairing the muscle’s response to released acetylcholine.

13. Pattern of Weakness

In LEMS:

Proximal lower-limb weakness predominates.

Strength may initially improve with brief exercise.

Reflexes are often reduced.

Autonomic symptoms are common.

In myasthenia gravis:

Ocular and bulbar weakness are common.

Weakness generally worsens with repeated activity and improves with rest.

Reflexes are usually preserved.

Autonomic symptoms are not characteristic.

14. Electrophysiology

Nerve-conduction studies and repetitive nerve stimulation can support the diagnosis.

At rest, the compound muscle action potential may be reduced.

After brief exercise or high-frequency stimulation, there can be a marked increment in response amplitude.

This electrophysiological facilitation reflects increased acetylcholine release after repeated presynaptic stimulation.

15. Antibody Testing

Blood testing may detect antibodies against P/Q-type voltage-gated calcium channels.

A positive result supports the diagnosis when the clinical pattern is compatible.

However, diagnosis still depends on integrating symptoms, examination, electrophysiology, antibody results, and assessment for an underlying malignancy.

16. Investigation for Small-Cell Lung Cancer

Because LEMS may precede recognition of SCLC, patients should be evaluated for an underlying lung malignancy when clinically appropriate.

This may involve thoracic imaging and sometimes repeated surveillance if initial assessment is negative but clinical suspicion remains significant.

17. Treatment of the Underlying Tumour

When LEMS is associated with small-cell lung carcinoma, treating the malignancy is a crucial part of management.

Improvement in tumour control may also improve the neurological syndrome.

18. Amifampridine

A major symptomatic treatment for LEMS is amifampridine (3,4-diaminopyridine).

It blocks presynaptic potassium channels and prolongs depolarisation of the motor nerve terminal.

This allows more calcium to enter and increases acetylcholine release.

The sequence is:

K⁺ channel blockade → prolonged depolarisation → ↑ Ca²⁺ entry → ↑ acetylcholine release → improved muscle strength.

19. Other Treatment

Selected patients may require additional treatment such as pyridostigmine, although its benefit is usually less striking than in myasthenia gravis.

Immunomodulatory treatments may also be used in autoimmune disease, including corticosteroid-sparing therapy, intravenous immunoglobulin, or other specialist-directed therapy depending on severity.

20. Lambert–Eaton Syndrome – Note Form

Definition: autoimmune presynaptic neuromuscular-junction disorder.

Antibody target: P/Q-type voltage-gated calcium channels on the presynaptic motor nerve terminal.

Mechanism: reduced calcium entry → reduced acetylcholine release.

Classic malignancy: small-cell lung carcinoma.

Weakness: predominantly proximal.

Distribution: lower limbs usually affected more than upper limbs.

Exercise: strength characteristically improves temporarily after brief exercise.

Reflexes: reduced or absent but may improve after muscle activation.

Autonomic symptoms: dry mouth, constipation, erectile dysfunction, impaired sweating and orthostatic symptoms.

Ocular symptoms: can occur but are usually less prominent than in myasthenia gravis.

Electrophysiology: incremental response after exercise or high-frequency stimulation.

Treatment: treat underlying malignancy when present; amifampridine is an important symptomatic therapy.

Key Clinical Pattern

Remember LEMS as:

Proximal lower-limb weakness + reduced reflexes + autonomic symptoms + strength improves with brief exercise.

The mechanism is:

Presynaptic voltage-gated Ca²⁺ channel antibodies → ↓ acetylcholine release.

The classic association is:

Lambert–Eaton syndrome → small-cell lung carcinoma.

And the easiest comparison is:

LEMS → gets stronger initially with exercise + reflexes reduced + autonomic symptoms common.

Myasthenia gravis → gets weaker with repeated activity + reflexes preserved + ocular symptoms prominent.