A 32-year-old woman presents with progressive muscle weakness, ptosis, and diplopia that worsens toward evening. Antibody screening is positive for anti-acetylcholine receptor (AChR) antibodies. Which investigation is most appropriate to confirm the diagnosis of myasthenia gravis and assess severity?

## Investigation of Choice in Myasthenia Gravis ### Clinical Context The patient presents with classic myasthenia gravis (MG) symptoms: fatigable weakness, ptosis, and diplopia. Positive anti-AChR antibodies confirm autoimmune pathology, but functional assessment of the neuromuscular junction is needed. ### Why Repetitive Nerve Stimulation (RNS) is Correct **Key Point:** RNS is the gold standard functional test for neuromuscular junction disorders and demonstrates the pathophysiology of MG. **High-Yield:** In MG, RNS shows a characteristic **decremental response** — progressive decline in compound muscle action potential (CMAP) amplitude with successive stimuli. This reflects: - Depletion of acetylcholine (ACh) vesicles at the presynaptic terminal - Blockade of AChR by antibodies (postsynaptic defect in seropositive MG) - Failure of neuromuscular transmission with repeated activation **Clinical Pearl:** RNS is positive in ~60–70% of generalized MG and ~10% of ocular MG. It is: - Non-invasive - Reproducible - Quantifiable (degree of decrement correlates with severity) - Useful for monitoring disease progression and treatment response ### Diagnostic Criteria Decrement of ≥10% in CMAP amplitude between the first and fourth stimulus is diagnostic of a neuromuscular junction disorder. ### Comparison with Other Investigations | Investigation | Sensitivity in Generalized MG | Specificity | Clinical Use | |---|---|---|---| | **RNS** | 60–70% | High | Functional confirmation; severity assessment | | **SFEMG** | 90–95% | Moderate | Gold standard sensitivity; non-specific (also abnormal in myopathy, neuropathy) | | **Needle EMG** | Low | Low | Excludes myopathy; non-specific | | **MRI of NMJ** | Not applicable | Not applicable | No structural imaging of NMJ; not standard | **Mnemonic:** **RNS = Repetitive Neuromuscular Stimulation** — shows the **decremental defect** that defines NMJ dysfunction. ### Why RNS Over SFEMG? Although SFEMG is more sensitive (90–95%), it is: - Technically demanding and time-consuming - Operator-dependent - Not specific for MG (abnormal in myopathy, neuropathy, motor neuron disease) - Reserved for **diagnostic confirmation when RNS is negative but clinical suspicion is high** RNS is the **first-line functional test** because it is specific, practical, and directly demonstrates the pathophysiology of impaired neuromuscular transmission.