A 38-year-old woman with a family history of periodic paralysis presents with acute flaccid paralysis and serum K⁺ = 6.8 mEq/L (normal 3.5–5.0). You suspect hyperkalaemic periodic paralysis with abnormal sodium channel function. Which investigation is most appropriate to directly assess the effect on action potential generation and conduction?

Explanation

## Rationale for Investigation Choice **Key Point:** Intramuscular microelectrode recording during potassium challenge directly measures action potential parameters and reveals the electrophysiological defect in sodium channel–mediated periodic paralysis. ### Pathophysiology of Hyperkalaemic Periodic Paralysis Hyperkalaemic periodic paralysis is caused by mutations in the **SCN4A gene** (skeletal muscle sodium channel Nav1.4). Elevated extracellular K⁺ causes: 1. **Depolarization of RMP** — reduces the voltage difference between RMP and threshold 2. **Sodium channel inactivation** — sustained depolarization locks Na⁺ channels in the inactivated state 3. **Loss of excitability** — muscle becomes inexcitable despite depolarization (paradoxical paralysis) ### Why Intramuscular Microelectrode Recording? | Investigation | Measures | Detects Sodium Channel Defect | Direct Electrophysiology | |---|---|---|---| | **Intramuscular microelectrode** | Action potential amplitude, threshold, rate of rise, inactivation kinetics | **Yes** — shows reduced AP amplitude, prolonged inactivation | **Yes** — direct voltage recording | | Serum CK/myoglobin | Muscle damage markers | No — indirect | No — biochemical | | Genetic sequencing | SCN4A mutation presence | Yes — confirms diagnosis | No — DNA level, not functional | | Repetitive nerve stimulation | Neuromuscular transmission | No — tests acetylcholine, not Na⁺ channels | No — NMJ, not muscle membrane | **High-Yield:** Intramuscular microelectrode recording during K⁺ challenge can demonstrate: - **Reduced action potential amplitude** (smaller depolarization) - **Prolonged inactivation phase** (Na⁺ channels stay inactivated longer) - **Reduced excitability** despite RMP depolarization (paradoxical inactivation) - **Spontaneous depolarizations** and afterdepolarizations ### Clinical Pearl While **genetic sequencing confirms the diagnosis** (SCN4A mutation), it does **not** explain the functional electrophysiological defect or why the patient is paralysed despite hyperkalaemia. Intramuscular microelectrode recording **functionally demonstrates** the sodium channel inactivation that causes paralysis — the mechanistic investigation. **Warning:** Do not confuse **genetic diagnosis** (identifies mutation) with **functional investigation** (explains the electrophysiological mechanism). The question asks for investigation of the **effect on action potential generation** — only direct recording achieves this. ### Mnemonic: **SCAR** — Sodium Channel Action Recording - **S** — Sodium channel defect (SCN4A) - **C** — Confirm with microelectrode - **A** — Action potential parameters - **R** — Recording during K⁺ challenge