Mediastinum - division, borders

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The mediastinum is an area found in the midline of the thoracic cavity. It is surrounded by left and right pleural sacs. The mediastinum is divided into superior and inferior mediastinum by imaginary line which runs from the sternal angle to T4 vertebra.  

The inferior mediastinum is further subdivided into anterior, middle, posterior parts.

Every compartment of the mediastinum contains vital organs, vascular and neural structures that are closely related one to another.  

Borders of the whole mediastinum:  

• Anterior- posterior border of sternum
• Posterior- thoracic vertebra
• Cranial- superior thoracic aperture
• Caudal- diaphragm
• Lateral- pleural cavities  

Superior mediastinum:  

Borders:

• Superior- thoracic inlet, "Superior thoracic aperture”  
• Inferior- continuous with the inferior mediastinum at the level of the sternal angle
• Anterior- manubrium of the sternum  
• Posterior- vertebral bodies of T1-T4  
• Lateral- pleurae of the lungs  

The superior mediastinum contains neural, vascular and respiratory structures passing from adjacent regions of the neck and abdomen:  

• Great systemic blood vessels of the heart - lies within superior mediastinum and their main branches arise there, including the aortic arch and superior vena cava  
• Aortic arch- the 3 major branches of the aortic arch arise within this space:
  • Brachiocephalic artery– right side of the head and neck and right upper limb  
  • Left Common carotid artery– left side of the head and neck  
  • Left Subclavian artery– left upper limb  

• Superior vena cava- has tributaries which are located within this space
• Brachiocephalic veins– draining blood from the upper body  
• Left Superior Intercostal vein– collects blood from left 2nd and 3rd intercostal vein. Drains into the left brachiocephalic vein
• Supreme Intercostal vein– drains the vein from 1st intercostal space directly into brachiocephalic veins
• Azygos vein– receiving blood from right posterior intercostal veins. The left intercostal veins drain first into the hemiazygos and accessory hemiazygos veins, before joining the azygos vein around T7-T9  level  
• Nerves- vagus, phrenic, sympathetic trunk, cardiac nerves  

The Vagus has two distinctive paths, right and left:

• Right vagus nerve– runs parallel to trachea and passes posteriorly to SVC and right primary bronchus
• Left vagus nerve– enters superior mediastinum between left common carotid and left subclavian arteries. It descends anteriorly to the aortic arch, before travelling posterior to left bronchus. The left recurrent laryngeal nerve arises from the left vagus nerve as it passes the aortic arch. It loops under the  arch, to the left of ligamentum arteriosum, before continuing its journey to the larynx in the tracheal-esophageal groove

• Phrenic- from the anterior surface of anterior scalene muscle, phrenic nerves  (C3, C4,C5) enter the superior mediastinum lateral to great vessels. They then descend anteriorly into the middle mediastinum, passing anteriorly to the hilum of the lungs
• Cardiac nerves- originate from superior, middle and inferior cardiac ganglions. Form the superficial and deep cardiac plexi in the superior  mediastinum. The superior plexus sits between the aortic arch and right  pulmonary artery. The deep plexus lies on the surface of the trachea at the point of bifurcation (T4 level)  
• Sympathetic trunk- runs bilaterally to the vertebral bodies along the entire length of the vertebral column  
• Thymus- most anterior structure within the superior mediastinum. It sits against the posterior surface of the sternum and extends into the anterior mediastinum and can often reach into the neck
• Trachea- bifurcates into the primary bronchi posterior to the ascending aorta at the level of T4
• Esophagus- ascends towards the pharynx, which it joins at the level of C6  
• Thoracic duct- passes to the left of esophagus, on its path to the junction of left internal jugular and subclavian veins
• Muscles- sternohyoid and sternothyroid muscles originate from the manubrium. Inferior aspect of longus colli muscle also originates within the superior mediastinum.  

Anterior mediastinum:  

Borders:

• Anterior- body of sternum and transversus thoracis muscles  
• Posterior- pericardium  
• Roof- continuous with superior mediastinum at the level of the sternal angle  
• Floor- diaphragm  
• Lateral- mediastinal pleura (part of the parietal pleural membrane)

Contents:

Contains no major structures. Contains loose connective tissue, “Sternopericardial ligaments”, fat, some lymphatic vessels, lymph nodes, branches of internal thoracic vessels.

In infants and children, thymus extends inferiorly into the anterior mediastinum. However, the thymus recedes during puberty and is mostly replaced by adipose tissue in adults.  

Middle mediastinum:  

Borders:

• Anterior- anterior margin of the pericardium  
• Posterior- posterior border of the pericardium  
• Lateral- mediastinal pleura of the lungs  
• Superior- imaginary line extending between the sternal angle and T4 vertebrae
• Inferior- superior surface of the diaphragm  

Contents:

The largest subdivision of the inferior mediastinum. It contains several important organs, vessels, nerves and lymphatic structures:  

• Organs- heart and pericardium, tracheal bifurcation, left and right main bronchi  
• Vessels- great vessels that run to and from the heart:
  • Ascending aorta– first part of aorta, arises from aortic orifice. Moves upwards, exiting the fibrous pericardium and entering the superior mediastinum, where it then becomes the aortic arch. The ascending aorta gives rise to two branches, left and right coronary arteries
  • Pulmonary trunk– gives rise to left and right pulmonary arteries. The trunk itself is relatively short and wide, allowing a large volume of blood to pass through it
  • Superior vena cava- returns deoxygenated blood from the upper half of the body. It is formed by the right and left brachiocephalic veins

• Lymphatics- Tracheobronchial lymph nodes are located within the middle mediastinum. They are a group of nodes associated with the trachea and bronchi of the respiratory tract, and are characteristically enlarged in certain lung pathologies.

Posterior mediastinum:  

Borders:

• Lateral- mediastinal pleura  
• Anterior- pericardium  
• Posterior- T5-T12 vertebrae  
• Roof- imaginary line extending between the sternal angle and T4 vertebrae  
• Floor- diaphragm  

Contents:

• Thoracic aorta- descending aorta. Continuation of the aortic arch, begins at T4 vertebra. It descends through the posterior  mediastinum to the left side of the vertebra, becoming more medially located as it descends. At the inferior border of T12, the thoracic aorta becomes the abdominal aorta and passes through the aortic hiatus of diaphragm. The following branches arise from it:
  • Posterior intercostal arteries- supply intercostal spaces
  • Bronchial arteries- paired visceral branches, left arise directly from thoracic aorta, right usually arise from right posterior intercostal
  • Esophageal arteries- unpaired visceral branches, arising from anterior aspect of aorta, supplies esophagus
  • Superior phrenic arteries-arise from anterior aspect of thoracic aorta at the aortic hiatus, supplying the superior aspect of the diaphragm.  

• Esophagus- muscular tube that connects pharynx and stomach. Allowing swallowed food to pass into the digestive system. Passes into the posterior mediastinum from the superior mediastinum, descending posteriorly to the aortic arch. Whilst initially positioned to the right, the esophagus deviates to the left as it moves downwards. It leaves the mediastinum via the esophageal hiatus of the diaphragm
• Esophageal plexus- network of nerves surrounding the esophagus as it descends, consisting of branches from left and right vagus nerves. Immediately above the diaphragm, the fibers of the plexus merge to form the anterior vagal trunk and posterior vagal trunk, which travel along the surface of  the esophagus as it exits the thorax  
• Thoracic duct- largest lymphatic vessel in the body, allowing return of lymph from most of the body (all but the right superior quadrant) into the venous system. Originates from cisterna chyli in the abdomen and enters mediastinum via the aortic hiatus. It ascends to lie directly anterior to the T6-T12 vertebrae, before deviating left as it ascends into the  superior mediastinum. While located in the posterior mediastinum, it receives lymphatic drainage from intercostal spaces and neighboring anatomical structures through a number of branches  
• Azygos system of veins- venous network which drains blood from the body walls and mediastinal viscera. It empties into superior vena cava. It consists of three major veins: azygos vein (formed by the union of right lumbar vein and right subcostal vein),  hemiazygos vein (formed by union of left lumbar vein and left subcostal vein), accessory hemiazygos vein (formed by union of the 4th-8th intercostal veins). At the level of T8 vertebra, the hemiazygos vein combines with the azygos vein. At the level  of T7, the accessory hemiazygos vein drains into the azygos vein.  
• Sympathetic trunks- paired bundles of nerves. Extends from the base of the skull to the coccyx. In the thoracic region, these nerve bundles are known as the thoracic sympathetic trunks. As they descend through the thorax, they lie within  the posterior mediastinum. From these trunks the lower thoracic splanchnic nerves arise, which continues inferiorly to supply the viscera of the abdomen

Sources

Stingl, J., Grim, M., & Druga, R. (2012). ''Regional anatomy''. Galen.

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