A 42-year-old man from Bangalore presents with progressive weakness of proximal lower limbs and difficulty climbing stairs over 2 months. He has a history of small-cell lung cancer diagnosed 6 months ago. On examination, strength is 4/5 in hip flexors and knee extensors. Repetitive nerve stimulation shows a decremental response (10% amplitude decline). Serum calcium and magnesium are normal. Which of the following best explains the neuromuscular dysfunction in this patient?

Question

A 42-year-old man from Bangalore presents with progressive weakness of proximal lower limbs and difficulty climbing stairs over 2 months. He has a history of small-cell lung cancer diagnosed 6 months ago. On examination, strength is 4/5 in hip flexors and knee extensors. Repetitive nerve stimulation shows a decremental response (>10% amplitude decline). Serum calcium and magnesium are normal. Which of the following best explains the neuromuscular dysfunction in this patient?

Accepted Answer

D. Antibodies against presynaptic voltage-gated calcium channels impair acetylcholine release. ## Diagnosis: Lambert-Eaton Myasthenic Syndrome (LEMS)

This patient presents with the characteristic features of **Lambert-Eaton myasthenic syndrome (LEMS)**:
- **Proximal muscle weakness** (hip flexors, knee extensors) — typical distribution
- **Paraneoplastic association** — small-cell lung cancer (SCLC) in 50–60% of LEMS cases
- **Decremental response on repetitive nerve stimulation** — key electrophysiological finding
- **Normal serum electrolytes** — rules out hypokalemia or hypomagnesemia as causes

## Pathophysiology: Presynaptic Calcium Channel Dysfunction

**Key Point:** LEMS is caused by **IgG autoantibodies against presynaptic voltage-gated calcium channels (P/Q-type and N-type)**, which are essential for acetylcholine release.

### Mechanism of Weakness:

1. **Antibody binding** → calcium channel blockade and complement-mediated destruction
2. **Reduced Ca²⁺ influx** → decreased acetylcholine quantal release
3. **Impaired neuromuscular transmission** → muscle weakness

### Why Decremental Response Occurs:

With each successive stimulation, the already-depleted acetylcholine pool becomes further exhausted, causing progressive decline in compound muscle action potential (CMAP) amplitude — the hallmark of **presynaptic dysfunction**.

**Clinical Pearl:** LEMS differs from myasthenia gravis:
- **Facilitation** (not fatigability) occurs with high-frequency stimulation or brief isometric exercise — calcium channels recover partially with repeated depolarization
- **Autonomic symptoms** are common (dry mouth, impotence, constipation) due to involvement of autonomic presynaptic terminals
- **Proximal weakness** predominates (vs. ocular/bulbar in MG)

**High-Yield:** 3-4-diaminopyridine (3,4-DAP) is the first-line treatment — it blocks presynaptic potassium channels, prolonging depolarization and allowing more time for calcium influx and acetylcholine release.

**Mnemonic: LEMS vs MG**
- **LEMS:** **L**ow ACh release (presynaptic), **E**asily **F**acilitated, **M**alignancy common, **S**mall-cell lung cancer
- **MG:** **M**issing receptors (postsynaptic), **F**atigability, **A**uto-immune, **S**eronegative subset

Answer

A. Postsynaptic acetylcholine receptor blockade by circulating immunoglobulins

Answer

B. Genetic mutation in the acetylcholinesterase gene causing enzyme deficiency

Answer

C. Mitochondrial dysfunction leading to impaired ATP production in muscle fibers

Explanation

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