A 52-year-old man with known myasthenia gravis presents with acute worsening of generalized weakness, difficulty swallowing, and respiratory distress. He has been on pyridostigmine monotherapy for 2 years. Before initiating immunosuppression, the clinician needs to exclude a thymoma, which is present in 30–50% of generalized MG cases. Which is the most appropriate investigation of choice for detecting thymoma in this patient?

Explanation

## Imaging for Thymoma Detection in Myasthenia Gravis **Key Point:** High-resolution computed tomography (HRCT) of the chest is the gold standard investigation for detecting and staging thymoma in patients with myasthenia gravis, with sensitivity >95% for masses >1 cm. ### Why HRCT is the Answer HRCT provides superior spatial resolution and soft-tissue contrast compared to chest X-ray, allowing: - Detection of small thymomas (even <1 cm) - Assessment of local invasion (pleura, pericardium, great vessels) - Evaluation of mediastinal lymph nodes - Staging of thymoma (Masaoka-Koga classification) - Guidance for surgical planning **High-Yield:** All patients with generalized MG should undergo chest imaging to screen for thymoma, as: - Thymoma is present in 30–50% of generalized MG - Thymoma-associated MG has worse prognosis and requires thymectomy - Early detection improves surgical outcomes - Imaging findings influence immunosuppressive strategy ### Comparison of Chest Imaging Modalities for Thymoma | Imaging Modality | Sensitivity | Specificity | Spatial Resolution | Cost | Radiation | Clinical Role | |---|---|---|---|---|---|---| | **Chest X-ray (CXR)** | 40–60% | Moderate | Poor (mediastinal detail) | Low | Low | Screening; detects large masses only | | **HRCT chest** | >95% | High | Excellent | Moderate | Moderate | Gold standard; staging and surgical planning | | **MRI chest** | 90–95% | High | Excellent | High | None | Alternative if HRCT contraindicated; less available | | **PET-CT** | 80–90% | Moderate | Good | High | High | Not first-line; useful for metastatic disease | | **Transthoracic echo** | <50% | Low | Poor | Low | None | Cannot assess anterior mediastinum adequately | **Clinical Pearl:** - CXR may appear normal in up to 40–60% of thymomas because the thymus is a mediastinal structure with poor contrast on plain radiographs - HRCT is mandatory in all generalized MG; ocular MG has lower thymoma incidence (~5–10%) but imaging is still recommended - MRI is an alternative if HRCT is contraindicated (e.g., renal impairment precluding contrast) but is less widely available ### Diagnostic Algorithm for Thymoma Screening in MG ```mermaid flowchart TD A[Diagnosis of MG confirmed]:::outcome --> B{Generalized or ocular?}:::decision B -->|Generalized| C[HRCT chest with contrast]:::action B -->|Ocular| D[HRCT chest recommended]:::action C --> E{Thymoma detected?}:::decision D --> E E -->|Yes| F[Staging: Masaoka-Koga]:::action E -->|No| G[Thymic hyperplasia?]:::decision F --> H[Surgical consultation]:::action G -->|Yes| I[Consider thymectomy]:::action G -->|No| J[Medical management alone]:::action H --> K[Thymectomy + adjuvant therapy]:::action ``` **Mnemonic:** **HRCT = High-Resolution CT is the standard** — it detects thymomas that CXR misses in 40–60% of cases. [cite:Harrison 21e Ch 378]