A 32-year-old woman presents with progressive muscle weakness, ptosis, and diplopia that worsens towards evening. Antibody testing is ordered. Which investigation is most specific for confirming the diagnosis of myasthenia gravis?

Explanation

## Investigation of Choice for Myasthenia Gravis ### Pathophysiology of MG Myasthenia gravis is an autoimmune disorder of the neuromuscular junction. The majority of patients (~80%) develop antibodies against the **nicotinic acetylcholine receptor (nAChR)** at the motor end plate. These antibodies bind to nAChR, causing complement-mediated destruction and reduced acetylcholine binding capacity. ### Why Anti-nAChR Antibodies? **Key Point:** Anti-nAChR antibodies are present in 80–90% of generalized MG and 50% of ocular MG, making them the **gold standard confirmatory test**. **High-Yield:** The presence of anti-nAChR antibodies: - Confirms autoimmune MG - Correlates with disease severity - Guides immunosuppressive therapy - Has high specificity (>95%) for MG ### Diagnostic Algorithm for MG ```mermaid flowchart TD A[Clinical suspicion of MG]:::outcome --> B[Edrophonium/Neostigmine test or<br/>Repetitive nerve stimulation]:::action B --> C{Positive?}:::decision C -->|Yes| D[Serum anti-nAChR antibodies]:::action C -->|No| E[Consider MuSK antibodies<br/>or other diagnoses]:::action D --> F{Antibodies present?}:::decision F -->|Yes| G[Autoimmune MG confirmed]:::outcome F -->|No| H[Seronegative MG<br/>or alternative diagnosis]:::outcome ``` ### Comparison of Antibody Tests in MG | Antibody Type | Prevalence | Sensitivity | Specificity | Clinical Significance | |---|---|---|---|---| | **Anti-nAChR** | 80–90% generalized; 50% ocular | High in generalized | >95% | Gold standard; indicates receptor blockade | | **Anti-MuSK** | 40% seronegative MG | Moderate | >95% | Seronegative MG; often more severe | | **Anti-AChE** | <5% | Very low | Variable | Not routinely tested; historical interest only | **Clinical Pearl:** Seronegative MG (10–20% of cases) may have anti-MuSK antibodies or remain double-seronegative; these patients still respond to immunosuppression. ### Confirmatory Investigations (Supporting Role) - **Repetitive nerve stimulation (RNS):** Shows decremental response in 60–70% of generalized MG - **Single-fiber electromyography (SFEMG):** Most sensitive (95%) but non-specific; confirms neuromuscular junction disorder - **Edrophonium test:** Rapid reversal of symptoms (now rarely used due to cardiac risk) - **CT chest:** Rules out thymoma (present in ~10% of MG) **Key Point:** Anti-nAChR antibodies are the **most specific confirmatory test** because they directly identify the pathogenic mechanism.