Superior Mediastinum

The superior mediastinum is the upper part of the mediastinal cavity of the thorax, positioned above the pericardial sac. It serves as a major anatomical crossroads where structures from the neck enter the thoracic cavity and distribute to their respective destinations. It is rich in vital vascular, nervous, lymphatic, and respiratory elements, each with complex spatial relationships.

Location and Boundaries

The superior mediastinum is located above the transverse thoracic plane, which is a horizontal line drawn from the sternal angle (angle of Louis) to the intervertebral disc between T4 and T5. It is bordered as follows:

• Anterior: Manubrium of the sternum
• Posterior: Bodies of the first four thoracic vertebrae (T1–T4)
• Superior: Thoracic inlet (bounded by the first thoracic vertebra, first ribs, and the superior border of the manubrium)
• Inferior: Imaginary plane from the sternal angle to T4/T5 intervertebral disc
• Lateral: Mediastinal pleura of the lungs

This compartment connects the root of the neck to the rest of the thoracic cavity and acts as a conduit for major organs and structures traveling between the head and the thorax.

Contents of the Superior Mediastinum

The superior mediastinum contains an intricate collection of structures that can be categorized based on their system. The anatomical relationships among these structures are complex, layered anterior to posterior.

1. Thymus

The thymus is the most anterior structure in the superior mediastinum. In infants and children, it is relatively large and active in the development of T-lymphocytes (adaptive immunity). With age, it undergoes involution and is replaced by fat and fibrous tissue in adults. Its location is behind the manubrium and in front of the major vessels.

2. Veins

Posterior to the thymus lie the large veins that drain blood from the head, neck, and upper limbs into the heart:

• Right and left brachiocephalic veins: These are formed by the union of internal jugular and subclavian veins. They course medially and unite to form the superior vena cava.
• Superior vena cava (SVC): Located at the inferior end of the superior mediastinum, it continues downward into the middle mediastinum and drains into the right atrium.

3. Arteries

Just posterior to the venous structures is the arch of the aorta and its major branches:

• Aortic arch: Begins and ends at the level of T4/T5. It loops posteriorly and to the left over the left main bronchus.
• Brachiocephalic trunk: The first branch of the arch; it bifurcates into the right subclavian and right common carotid arteries.
• Left common carotid artery: The second branch of the arch; it ascends vertically into the neck.
• Left subclavian artery: The third branch; it passes behind the left lung apex and supplies the upper limb.

These arteries are located deeper and slightly to the left of the venous structures, closely related to the trachea and esophagus.

4. Nerves

Several critical nerves pass through the superior mediastinum, coordinating vital autonomic and somatic functions:

• Phrenic nerves (right and left): Arise from C3–C5, descend anterior to the lung roots, and continue through the pericardium to innervate the diaphragm.
• Vagus nerves (right and left): Descend posterior to the lung roots. These give rise to the recurrent laryngeal nerves and contribute to the cardiac, pulmonary, and esophageal plexuses.
• Left recurrent laryngeal nerve: Branches from the left vagus nerve, loops under the aortic arch (posterior to the ligamentum arteriosum), and ascends between the trachea and esophagus to reach the larynx.

5. Trachea

The trachea enters the superior mediastinum from the cervical region and runs anterior to the esophagus. It bifurcates into the left and right main bronchi at the level of the T4/T5 disc (carina). The trachea is flanked laterally by the vagus nerves and the great vessels.

6. Esophagus

The esophagus lies posterior to the trachea and anterior to the vertebral bodies. It enters the thoracic cavity from the neck and continues inferiorly through the posterior mediastinum to reach the stomach via the esophageal hiatus in the diaphragm.

7. Thoracic Duct

The thoracic duct, the largest lymphatic vessel in the body, enters the superior mediastinum from the left side of the thoracic inlet. It ascends behind the esophagus, arches laterally at the T5 level, and drains into the junction of the left subclavian and left internal jugular veins.

8. Lymph Nodes

Several groups of lymph nodes are present in the superior mediastinum. These include:

• Paratracheal nodes
• Brachiocephalic nodes
• Tracheobronchial nodes (superior)

These nodes play a role in filtering lymph from thoracic organs, especially the lungs and trachea.

Anatomical Relationships (Anterior to Posterior)

1. Thymus (or its remnants)
2. Brachiocephalic veins and superior vena cava
3. Aortic arch and its branches
4. Phrenic and vagus nerves
5. Trachea
6. Esophagus
7. Thoracic duct
8. Sympathetic trunks (more lateral)

Functional Importance

The superior mediastinum acts as a highway for key anatomical structures that must travel from the neck to the thorax or vice versa. Each system relies on this passageway for vital connections:

• Cardiovascular: The region ensures vascular continuity between the heart and upper body via the aortic arch and venous return system.
• Respiratory: The trachea courses through this space before bifurcating into the bronchi.
• Nervous: Critical autonomic and somatic pathways, including the vagus, recurrent laryngeal, and phrenic nerves, traverse the superior mediastinum.
• Lymphatic: The thoracic duct facilitates return of lymph into the venous system at the left venous angle.
• Digestive: The esophagus begins its thoracic journey here, continuing into the posterior mediastinum and then into the abdomen.

The complex anatomical layering in the superior mediastinum makes it a vital structure in understanding thoracic cross-sections and in interpreting imaging such as CT or MRI scans.