Learning Objectives
Identify the anatomical boundaries and contents of the Mediastinum. Distinguish between Anterior, Middle, and Posterior mediastinal masses, and understand the pathophysiology of Mediastinitis and Pneumomediastinum for the USMLE Step 1.
1. Mediastinal Masses by Compartment
The mediastinum is the central compartment of the thoracic cavity. While some pathologies (lymphoma, abscess) can appear anywhere, most masses follow a predictable anatomical distribution.
| Compartment | Common Pathologies | Clinical Note |
|---|---|---|
| Anterior | The 4 T’s:
1. Thymus (Thymoma) 2. Teratoma 3. Terrible Lymphoma 4. Thyroid (Substernal) |
Thymoma is associated with Myasthenia Gravis and Pure Red Cell Aplasia. |
| Middle | Metastases, Hiatal hernia, Bronchogenic cysts. | Lymphadenopathy from lung cancer is a very common middle mediastinal finding. |
| Posterior | Neurogenic tumors (Neurofibroma, Schwannoma), Esophageal cancer, Multiple Myeloma. | An esophageal mass can extend into the middle mediastinum. |
2. Mediastinitis
Mediastinitis is a serious inflammatory condition typically resulting from surgery, infection spread, or trauma.
| Type | Common Causes | Clinical Features |
|---|---|---|
| Acute | Post-op cardiothoracic procedures, Boerhaave syndrome (esophageal rupture), Odontogenic infection. | Fever, tachycardia, leukocytosis, chest pain, and sternal drainage. |
| Chronic (Fibrosing) | Proliferation of connective tissue, often due to Histoplasma capsulatum. | Can lead to Superior Vena Cava (SVC) Syndrome due to compression. |
Activity
3. Pneumomediastinum
This occurs when air tracks into the mediastinal space. It is often diagnosed clinically by the presence of air under the skin (subcutaneous emphysema).
| Mechanism | Clinical Presentation | Classic Sign |
|---|---|---|
| Ruptured pulmonary bleb, trauma, iatrogenic injury, or esophageal rupture. | Chest pain, dyspnea, voice change, and subcutaneous emphysema. | Hamman Sign |
Activity:
High-Yield Clinical Pearls:
- Hamman Sign: This is a “crunching” sound heard over the heart during systole when air is present in the mediastinum.
- Boerhaave Syndrome: Transmural esophageal rupture due to violent retching. It is a medical emergency that leads to a rapid onset of acute mediastinitis.
- Thymoma Association: Always screen a patient with a thymoma for symptoms of Myasthenia Gravis (ptosis, diplopia, muscle weakness).