Correct Answer: B. Thymoma
Thymoma is the most likely diagnosis in a patient presenting with ptosis, muscle weakness improving with rest (fatigability), and dysphagia — the classic triad of myasthenia gravis (MG), which is associated with thymoma in 30–50% of cases. The engorged thoracic veins and mediastinal mass on CT point to an anterior mediastinal mass, the characteristic location of thymoma. Thymomas arise from the epithelial cells of the thymus gland and are the most common primary anterior mediastinal tumor in adults. The association between thymoma and MG is pathophysiologically linked: thymomas produce autoantibodies against the acetylcholine receptor (AChR), causing neuromuscular blockade. In Indian clinical practice, thymoma-associated MG is a common presentation in tertiary care centers. The CT finding of a well-defined anterior mediastinal mass in a patient with MG symptoms is virtually pathognomonic for thymoma. Thymomas are classified by the WHO classification (A, AB, B1, B2, B3) and by Masaoka staging; however, the clinical presentation here is diagnostic. Treatment involves thymectomy, which can lead to remission of MG in many patients, especially if performed early.
Why the other options are wrong
A. Small cell lung cancer — SCLC typically presents with a central hilar or mediastinal mass and is associated with Lambert-Eaton myasthenic syndrome (LEMS), not classic MG. LEMS presents with proximal muscle weakness and autonomic symptoms (dry mouth, constipation), not the ocular/bulbar symptoms (ptosis, dysphagia) seen here. SCLC is a peripheral lung malignancy, not an anterior mediastinal mass. The clinical triad of ptosis + fatigability + dysphagia is pathognomonic for MG-associated thymoma, not LEMS. C. Adenocarcinoma — Lung adenocarcinoma typically arises in the lung periphery, not the anterior mediastinum. It is not associated with MG or the classic neuromuscular symptoms (ptosis, fatigability, dysphagia) described. Adenocarcinoma may cause superior vena cava syndrome with engorged veins, but the combination of MG symptoms + anterior mediastinal mass is not typical. The anterior mediastinal location and MG association strongly favor thymoma over lung adenocarcinoma. D. Pancoast tumor — Pancoast tumors are apical lung tumors that cause Horner syndrome (miosis, ptosis, anhidrosis) and brachial plexus invasion (arm pain, weakness), not the generalized fatigable weakness and dysphagia of MG. Pancoast tumors are located at the lung apex, not the anterior mediastinum. The clinical presentation of MG with an anterior mediastinal mass is incompatible with Pancoast tumor pathology. The engorged thoracic veins and mediastinal location rule out an apical lung lesion.
High-Yield Facts
- Thymoma is the most common primary anterior mediastinal tumor in adults and is associated with myasthenia gravis in 30–50% of cases.
- MG-thymoma triad: ptosis, fatigability (weakness improving with rest), and dysphagia; caused by autoantibodies against acetylcholine receptors.
- Anterior mediastinal mass on CT is the radiological hallmark of thymoma; located in the mediastinum between the lungs, anterior to the pericardium.
- Thymectomy is the definitive treatment for thymoma and can induce remission of MG in up to 80% of patients if performed early.
- WHO classification of thymoma (A, AB, B1, B2, B3) and Masaoka staging (I–IV) determine prognosis; most thymomas are low-grade (A, AB, B1).
- Superior vena cava syndrome (engorged thoracic veins) can occur with large thymomas due to mass effect on mediastinal vessels.
Mnemonics
MG-Thymoma Triad: PFD Ptosis, Fatigability (improves with rest), Dysphagia. When you see this triad + anterior mediastinal mass on imaging, think thymoma. Anterior Mediastinal Masses: 4 Ts Thymoma, Teratoma, Terrible lymphoma, Thyroid. Thymoma is the most common in adults; remember it's the anterior mediastinal location that clinches the diagnosis.
NBE Trap
NBE pairs ptosis with Pancoast tumor (Horner syndrome) to trap students who confuse ocular ptosis with apical lung pathology. The key discriminator is fatigability (weakness improving with rest) and dysphagia, which are MG-specific, not Horner-specific. Additionally, the anterior mediastinal location on CT rules out Pancoast (apical lung) and SCLC (hilar/central).
Clinical Pearl
In Indian tertiary care centers, thymoma-associated MG is a common presentation in patients aged 40–60 years. Early recognition and referral for thymectomy can dramatically improve outcomes; many patients achieve drug-free remission post-thymectomy. Always image the mediastinum in any patient with MG symptoms.
_Reference: Robbins Ch. 11 (Neoplasia, Mediastinal Tumors); Harrison Ch. 375 (Myasthenia Gravis); Bailey & Love Ch. 52 (Thoracic Surgery)_