Lambert-Eaton myasthenic syndrome
Description
- Antibody-mediated presynaptic neuromuscular junction disorder
- Autoantibodies against P/Q-type voltage-gated calcium channels (VGCC) on the presynaptic terminal -> dec Ca2+ influx -> dec quantal release of acetylcholine
- ~50-60% paraneoplastic, classically small cell lung cancer
The clinical triad
1. Proximal limb weakness, legs >> arms - difficulty rising from a chair, stairs, waddling gait
2. Autonomic dysfunction - dry mouth (most common, ~75%), constipation, erectile dysfunction, postural hypotension, impaired sweating
3. Areflexia or hyporeflexia
The signature: post-exercise facilitation
- Power improves and reflexes return after 10-15 s of sustained maximal contraction
- Mechanism: repetitive stimulation -> Ca2+ accumulates in the terminal -> transmitter release recovers
- *This is the opposite of myasthenia gravis, where sustained effort causes fatigue*
LEMS vs myasthenia gravis
| LEMS | Myasthenia gravis | |
|---|---|---|
| Site | Presynaptic (VGCC) | Postsynaptic (AChR/MuSK) |
| Onset pattern | Legs first, proximal | Ocular/bulbar first |
| Ocular involvement | Mild, late, transient | Early and prominent (ptosis, diplopia) |
| Bulbar/respiratory | Less prominent | Common; crisis typical |
| Reflexes | Absent/reduced | Normal |
| Autonomic features | Prominent (dry mouth) | Absent |
| Effect of exercise | Improves (facilitation) | Worsens (fatigue) |
| RNS low frequency (3 Hz) | Decrement | Decrement |
| RNS high frequency / post-exercise | Increment >60-100% | No increment |
| Baseline CMAP amplitude | Low | Normal |
| Tumour | SCLC ~50-60% | Thymoma ~10-15% |
| Response to pyridostigmine | Poor | Good |
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