Which single feature best distinguishes myasthenia gravis (MG) from Lambert-Eaton myasthenic syndrome (LEMS)?

Explanation

## Distinguishing LEMS from MG **Key Point:** The hallmark electrophysiological finding that separates LEMS from MG is the **incremental response** (post-tetanic potentiation) on high-frequency repetitive nerve stimulation (RNS). ### Pathophysiology Basis LEMS is caused by **antibodies against presynaptic voltage-gated calcium channels (VGCC)**, leading to reduced acetylcholine (ACh) release. During high-frequency stimulation, calcium accumulates in the presynaptic terminal, partially overcoming the calcium channel blockade and allowing more ACh quanta to be released — hence the **incremental response**. In contrast, MG involves **postsynaptic acetylcholine receptor (AChR) antibodies**, causing receptor blockade and complement-mediated destruction. High-frequency stimulation in MG produces a **decremental response** (worsening transmission) because the already-reduced safety margin deteriorates further with repeated stimulation. ### Comparative Table | Feature | MG | LEMS | | --- | --- | --- | | **Antibody target** | AChR (postsynaptic) | VGCC (presynaptic) | | **RNS at low frequency (2–3 Hz)** | Decremental response | Decremental response | | **RNS at high frequency (20–50 Hz)** | Further decrement | **Incremental response** ✓ | | **Post-tetanic potentiation** | Absent | **Present** ✓ | | **Ocular involvement** | Common (ptosis, diplopia) | Rare | | **Autonomic symptoms** | Absent | Common (dry mouth, constipation, impotence) | | **Proximal vs. distal weakness** | Proximal + ocular | Proximal (legs > arms) | | **Associated malignancy** | Rare (thymoma) | Common (SCLC, other cancers) | **High-Yield:** The **incremental response on high-frequency RNS** is the **gold-standard electrophysiological discriminator** between LEMS and MG. This finding is present in ~80% of LEMS patients and is virtually pathognomonic. **Clinical Pearl:** LEMS patients often present with leg weakness and autonomic dysfunction; MG patients present with ocular symptoms and bulbar weakness. However, clinical overlap exists — the EMG finding is the definitive separator. **Mnemonic:** **LEMS = Incremental** (think "Calcium accumulation → more ACh released → incremental improvement"). **MG = Decremental** (think "Fewer receptors available → worsening with use").