A 52-year-old man with small-cell lung cancer presents with progressive leg weakness, areflexia, and dry mouth. EMG shows a decremental response at 2 Hz RNS, but an incremental response at 50 Hz RNS. Which feature best distinguishes his condition from autoimmune myasthenia gravis?

Explanation

## Clinical Presentation: LEMS with Malignancy **Key Point:** The patient's clinical and electrophysiological findings are diagnostic of **Lambert-Eaton myasthenic syndrome (LEMS)**. The **incremental response at high-frequency RNS** is pathognomonic. The **best discriminating feature from MG is the presence of autonomic dysfunction**. ### Why Autonomic Symptoms Distinguish LEMS from MG LEMS is caused by **presynaptic voltage-gated calcium channel (VGCC) antibodies** that block calcium influx. This affects not only the neuromuscular junction but also **autonomic nerve terminals**, leading to: - Dry mouth (reduced salivation) - Constipation (reduced GI motility) - Impotence (autonomic dysfunction in erectile tissue) - Reduced sweating - Orthostatic hypotension In contrast, **MG involves only postsynaptic AChR antibodies**, which do not affect autonomic terminals. Therefore, **autonomic symptoms are absent in MG**. ### Comparative Clinical Features | Feature | MG | LEMS | | --- | --- | --- | | **Autonomic dysfunction** | **Absent** ✗ | **Present** ✓ | | **Ocular symptoms** | **Common** (ptosis, diplopia) | Rare | | **Bulbar weakness** | **Common** | Rare | | **Proximal leg weakness** | Present | **Prominent** | | **Areflexia** | Absent (reflexes normal) | **Present** ✓ | | **Associated cancer** | Rare (thymoma) | **Common** (50–60%) | | **Incremental RNS response** | Absent | **Present** ✓ | **High-Yield:** **Autonomic dysfunction is virtually pathognomonic for LEMS** and is absent in MG. The triad of **muscle weakness + areflexia + autonomic symptoms** is the clinical hallmark of LEMS. **Clinical Pearl:** The presence of **dry mouth and constipation** in a patient with proximal weakness and areflexia should immediately raise suspicion for LEMS, especially if associated with malignancy. Conversely, a patient with MG presenting with dry mouth should prompt investigation for a secondary cause. **Mnemonic:** **LEMS = Autonomic involvement** (think "VGCC blockade affects both motor AND autonomic terminals"). **MG = Motor-only** (think "AChR antibodies affect only the motor endplate"). ### Why This Patient Has LEMS 1. **Incremental RNS response at 50 Hz** — diagnostic of LEMS 2. **Decremental response at 2 Hz** — seen in both LEMS and MG, but combined with incremental response at high frequency, confirms LEMS 3. **Areflexia** — suggests presynaptic dysfunction (LEMS), not postsynaptic (MG) 4. **Autonomic symptoms** — dry mouth, constipation — pathognomonic for LEMS 5. **Small-cell lung cancer** — LEMS is associated with malignancy in 50–60% of cases; MG is not