Mediastenum

Reading File

Finding Sources

Reading File

Loading Image

I now have everything needed for a thorough, well-illustrated answer. Here it is:

The Mediastinum

Source: Gray's Anatomy for Students; Grainger & Allison's Diagnostic Radiology

Definition and Boundaries

The mediastinum is a thick midline partition within the thoracic cavity. Its boundaries are:

Anteriorly: sternum
Posteriorly: bodies of the thoracic vertebrae
Superiorly: superior thoracic aperture (opening into the root of the neck)
Inferiorly: diaphragm
Laterally (on each side): mediastinal part of the parietal pleura

The pleural cavities lie on either side of it, and each lung remains attached to it via a root formed by the airway, pulmonary vessels, lymphatics, and nerves.

Subdivisions

A horizontal plane through the sternal angle and the TIV/TV intervertebral disc divides the mediastinum into two major parts:

Fig. 3.5 - Lateral view showing divisions of the mediastinum (Gray's Anatomy for Students)

Fig. 3.59 - Subdivisions of the mediastinum (Gray's Anatomy for Students)

1. Superior Mediastinum

Boundaries:

Anterior: manubrium of sternum
Posterior: bodies of T1-T4 vertebrae
Superior: oblique plane from jugular notch to T1 (continuous with root of neck)
Inferior: sternal angle / TIV-TV disc plane

Contents:

Structure	Notes
Thymus	Most anterior structure; large in children, involutes with age
Right and left brachiocephalic veins	Join to form SVC
Left superior intercostal vein	Drains into left brachiocephalic vein
Superior vena cava (SVC)
Arch of aorta + 3 branches	Brachiocephalic trunk, left common carotid, left subclavian
Trachea	Midline, bifurcates at carina (T4-5 level)
Esophagus	Posterior to trachea
Phrenic nerves	Pass anterior to lung roots
Vagus nerves	Pass through here on way to abdomen
Left recurrent laryngeal nerve	Hooks around arch of aorta
Thoracic duct

Fig. 3.89 - Structures in the superior mediastinum (Gray's Anatomy for Students)

2. Inferior Mediastinum

Divided into 3 parts by the pericardium:

2a. Anterior Mediastinum (smallest)

Boundaries:

Anterior: body of sternum
Posterior: pericardial sac
Superior: sternal angle / TIV-TV plane
Inferior: diaphragm
Lateral: mediastinal pleura on each side

Contents: Inferior extension of the thymus gland, fat, connective tissue, lymph nodes, mediastinal branches of internal thoracic vessels, and sternopericardial ligaments.

2b. Middle Mediastinum

Contents (the pericardium and heart):

Pericardium - a fibroserous sac consisting of:
- Fibrous pericardium - tough outer connective tissue layer
- Serous pericardium - thin inner layer with parietal layer (lining fibrous pericardium) and visceral layer/epicardium (on the heart surface); between them is the pericardial cavity containing a small amount of fluid
Heart and roots of the great vessels
Phrenic nerves and pericardiacophrenic vessels
Various smaller nerves and vessels

2c. Posterior Mediastinum

Boundaries:

Anterior: pericardial sac and diaphragm
Posterior: bodies of mid and lower thoracic vertebrae
Superior: TIV-TV disc plane (continuous with superior mediastinum above)
Inferior: diaphragm
Lateral: mediastinal pleura on each side

Contents:

Structure	Key Relations
Esophagus	Begins at C6; passes through esophageal hiatus at T10; moves left and anterior as it descends
Thoracic aorta	Left of esophagus; gives off posterior intercostal arteries
Azygos system of veins	Drains posterior thoracic wall; azygos on right, hemiazygos on left
Thoracic duct	On right side inferiorly, crosses to left more superiorly; posterior to esophagus
Sympathetic trunks	Run alongside vertebral bodies
Thoracic splanchnic nerves	Arise from sympathetic ganglia; pierce diaphragm to reach abdomen

Important esophageal relations in the posterior mediastinum:

Right side: covered by mediastinal parietal pleura
Left side: thoracic aorta
Posterior: thoracic duct (right side inferiorly), hemiazygos veins, right posterior intercostal vessels
Anterior (below carina): right pulmonary artery, left main bronchus, then immediately posterior to the left atrium (separated only by pericardium) - clinically important in left atrial enlargement causing dysphagia

Summary Table

Compartment	Key Boundary	Key Contents
Superior	Above sternal angle (TIV/V disc)	Aortic arch + 3 branches, SVC, brachiocephalic veins, trachea, esophagus, thymus, phrenic/vagus/recurrent laryngeal nerves, thoracic duct
Anterior	Behind sternum, in front of pericardium	Thymus (inferior part), fat, lymph nodes, sternopericardial ligaments
Middle	Pericardium itself	Heart, pericardium, origins of great vessels, phrenic nerves
Posterior	Behind pericardium, in front of vertebrae	Esophagus, descending aorta, azygos veins, thoracic duct, sympathetic trunks, splanchnic nerves

Clinical Relevance

Mediastinal widening on CXR: suggests aortic aneurysm/dissection, lymphoma, or haematoma
Superior mediastinal syndrome (SVC compression): facial/arm swelling, venous distension - caused by tumour, lymphoma, or thrombosis
Left atrial enlargement: displaces the esophagus posteriorly - detected on barium swallow
Left recurrent laryngeal nerve palsy: caused by any mass at the aortic arch level (e.g. Pancoast tumour, lymph node enlargement) producing hoarseness
Mediastinitis: infection spreading from esophageal perforation or neck is serious because the mediastinum has no fascial barriers to contain spread
Mediastinal masses (classic "4 T's" of anterior mediastinum): Thymoma, Teratoma, Terrible lymphoma, Thyroid mass
Gray's Anatomy for Students, pp. 165, 220, 252
Grainger & Allison's Diagnostic Radiology, p. 44

This is a shared conversation. Sign in to Orris to start your own chat.