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M01.05.001 Superior Mediastinum
The mediastinum is the central compartment of the thoracic cavity, serving as a conduit for critical structures traversing the thorax. Understanding the superior mediastinum is crucial for students preparing for the USMLE Step 1, as it houses vital anatomical and functional components.
Definition and Division of the Mediastinum
Definition
- The mediastinum is located between the two pleural sacs of the thoracic cavity.
- It contains most of the thoracic organs, including the heart, great vessels, trachea, esophagus, and thymus.
Division
The mediastinum is divided into:
- Superior Mediastinum
- Extends from the superior thoracic aperture to the sternal angle (T4 vertebrae).
- Inferior Mediastinum
- Subdivided into the anterior, middle, and posterior regions, extending to the diaphragm.
Borders of the Superior Mediastinum
| Border | Structure |
|---|---|
| Superior | Thoracic inlet |
| Inferior | Sternal angle to T4 vertebra |
| Anterior | Manubrium of the sternum |
| Posterior | Vertebral bodies of T1-T4 |
| Lateral | Pleurae of the lungs |
Contents of the Superior Mediastinum
1. Great Vessels
| Vessel | Details |
|---|---|
| Arch of Aorta | Gives rise to: |
| – Brachiocephalic artery: Supplies the right side of head, neck, and upper limb. | |
| – Left common carotid artery: Supplies the left side of the head and neck. | |
| – Left subclavian artery: Supplies the left upper limb. | |
| Superior Vena Cava | Formed by the union of the brachiocephalic veins. |
| Drains blood from the upper body via tributaries such as: | |
| – Left superior intercostal vein | |
| – Supreme intercostal vein | |
| – Azygos vein |
2. Nerves
| Nerve | Details |
|---|---|
| Vagus Nerve | Right and left branches: |
| – Right vagus: Passes posterior to the right bronchus. | |
| – Left vagus: Loops under the aortic arch (left recurrent laryngeal nerve). | |
| Phrenic Nerve | Derived from C3-C5, descends lateral to great vessels to the diaphragm. |
| Cardiac Nerves | Form the superficial and deep cardiac plexuses. |
| Sympathetic Trunk | Runs bilaterally along the vertebral bodies. |
3. Other Structures
| Structure | Details |
|---|---|
| Thymus | Most anterior structure; extends into the anterior mediastinum. |
| Trachea | Bifurcates at the sternal angle into primary bronchi. |
| Esophagus | Ascends towards the pharynx, posterior to the trachea. |
| Thoracic Duct | Drains lymph into the venous system near the left subclavian vein. |
| Muscles | – Sternohyoid and sternothyroid muscles originate from the manubrium. |
| – Inferior longus colli muscle also resides here. |
Key Points to Remember
- Great vessels in the superior mediastinum are crucial for systemic and pulmonary circulation.
- Vagus and phrenic nerves have unique anatomical courses critical for innervation.
- The thymus is a central lymphoid organ important in early immune function.
- The thoracic duct is the primary lymphatic drainage pathway.
- Clinical relevance: Aortic aneurysms, superior vena cava syndrome, and mediastinal masses often involve these structures.
Figures and Diagrams
- Borders of the Mediastinum: Depicts anatomical boundaries.
- Great Vessels: Highlights major arteries and veins.
- Course of the Left Vagus Nerve: Shows relationship with the aortic arch.
- Thymus and Adjacent Structures: Demonstrates position in the superior mediastinum.
USMLE Tip
Focus on the clinical correlations of mediastinal anatomy:
- Compression of the left recurrent laryngeal nerve can cause hoarseness (Ortner’s syndrome).
- Obstruction of the superior vena cava can lead to facial swelling and distended neck veins.
References
- TeachMeAnatomy: Superior Mediastinum Anatomy
- Gray’s Anatomy for Students
- Moore’s Clinical Anatomy