A 52-year-old man with myasthenia gravis presents with progressive proximal muscle weakness and ptosis. His symptoms worsen with repetitive activity and improve with rest. Which investigation is the gold standard for confirming the diagnosis of myasthenia gravis?

Explanation

## Gold Standard Investigation for Myasthenia Gravis **Key Point:** Single-fiber electromyography (SFEMG) is the **gold standard** investigation for confirming myasthenia gravis, with a sensitivity of 90–95% in generalized MG and ~85% in ocular MG — the highest of any single diagnostic test. ### Why SFEMG is the Gold Standard SFEMG measures **jitter** — the variability in the interpotential interval between two muscle fiber action potentials belonging to the same motor unit. In MG: - Impaired neuromuscular transmission → increased jitter - Severe impairment → impulse blocking (missed discharges) - Abnormal jitter is detected even when RNS and serology are negative **High-Yield:** SFEMG characteristics: - Most sensitive electrophysiological test for neuromuscular junction (NMJ) disorders - Detects subclinical NMJ dysfunction - Particularly valuable in **seronegative MG** and **ocular MG** where other tests may be negative - Recognized as gold standard by the American Academy of Neurology (AAN) guidelines ### Comparison of Diagnostic Tests | Investigation | Sensitivity (Generalized MG) | Specificity | Notes | |---|---|---|---| | **SFEMG** | **90–95%** | High | Gold standard; most sensitive; technically demanding | | **RNS** | 60–80% | High | First-line neurophysiological screen; non-invasive; ~50% sensitivity in ocular MG | | **Edrophonium test** | 60–90% | Moderate | Rarely used now; risk of cholinergic crisis | | **Anti-AChR antibodies** | 80–90% (generalized), 50% (ocular) | Very high | Highly specific; negative in seronegative MG | **Clinical Pearl:** While RNS is often performed **first** due to its availability and non-invasive nature, and anti-AChR antibodies are checked early in clinical workup, SFEMG is the **gold standard** because of its superior sensitivity. When the question specifically asks for the "gold standard," SFEMG is the correct answer per Harrison's Principles of Internal Medicine and standard neurology references. ### Diagnostic Hierarchy - **Most specific:** Anti-AChR antibody titre (very high specificity, but misses seronegative cases) - **Most sensitive / Gold standard:** SFEMG (detects NMJ dysfunction in nearly all MG patients) - **First-line practical screen:** RNS (widely available, non-invasive) - **Pharmacological confirmation (historical):** Edrophonium test **Warning:** Do not confuse "first-line investigation" (RNS) with "gold standard" (SFEMG). The stem explicitly asks for the gold standard, which is SFEMG. **Mnemonic: SF = Gold Standard** — **S**ingle-**F**iber EMG is the **Gold Standard** for MG diagnosis. [cite: Harrison's Principles of Internal Medicine, 21e, Ch 454; KD Tripathi 8e Ch 10]