Anatomy, Thorax, Mediastinum

Article Author:
Nathan Stoddard
Article Editor:
David Lowery
Updated:
3/26/2019 4:06:02 PM
PubMed Link:
Anatomy, Thorax, Mediastinum

Introduction

The thoracic mediastinum is the central compartment that runs the length of the thoracic cavity between the pleural sacs of the lungs. It divides into two major parts, the superior and inferior portions, but the inferior portion is commonly referred to by its even further subdivided parts, the anterior, middle, and posterior portions.

Structure and Function

The mediastinum is constructed in a manner that provides compartments to house and support vital structures within the thorax. Below are the borders of each region of the mediastinum.

  • Superior Mediastinum: bordered by the thoracic outlet superiorly, transverse thoracic plane inferiorly, pleural sacs laterally, manubrium of the sternum anteriorly, and first four thoracic vertebral bodies posteriorly
  • Anterior Mediastinum: bordered by the pericardium posteriorly, pleural sacs laterally, and bordered anteriorly by the sternum, left transversus thoracis, and fifth, sixth, and seventh left costal cartilages
  • Middle Mediastinum: formed by the borders of the pericardial sac
  • Posterior Mediastinum: bordered by the pericardium anteriorly, the thoracic surface of the diaphragm inferiorly, the transverse thoracic plane superiorly, the bodies of the fifth to the twelfth thoracic vertebrae posteriorly, and the pleural sacs laterally

Each region of the thoracic mediastinum contains unique structures. Listed below are the different regions of the thoracic mediastinum and a high-level overview of their components.

  • Superior Mediastinum:
    • Organs: thymus, trachea, esophagus
    • Arteries: aortic arch, brachiocephalic trunk, left common carotid artery, left subclavian artery
    • Veins and lymphatics: superior vena cava, brachiocephalic veins, the arch of the azygos, thoracic duct
    • Nerves: left and right vagus nerve, recurrent laryngeal nerves, cardiac nerve, left and right phrenic nerves
  • Anterior Mediastinum:
    • Organs: thymus
    • Arteries: internal thoracic branches
    • Veins and lymphatics: internal thoracic branches, parasternal lymph nodes
    • Nerves: none
  • Middle Mediastinum:
    • Organs: the heart and its great vessel roots
    • Arteries: ascending aorta, pulmonary trunk, pericardiacophrenic arteries
    • Veins and lymphatics: superior vena cava, pulmonary veins, pericardiacophrenic veins
    • Nerve: phrenic nerves, vagus nerves, sympathetic nerves
  • Posterior Mediastinum:
    • Organs: esophagus
    • Arteries: thoracic aorta
    • Veins and lymphatics: azygos vein, hemiazygos vein, thoracic duct
    • Nerve: vagus nerve

Each region of the mediastinum can be considered to serve a primary function.

The superior mediastinum is essentially a conduit space which allows for structures to pass between the head, neck, and thorax. The anterior mediastinum is protective in nature and filled with connective and fatty tissue that provides cushioning and support to the thymus as well as the vital cardiac structures just posterior to it. The middle mediastinum is responsible for housing the heart and great vessel roots. The posterior mediastinum, which can be thought of as a continuation of the superior mediastinum, also serves as a conduit which provides space for passage of structures between the thoracic and abdominal cavities.

Blood Supply and Lymphatics

The thoracic mediastinum houses a large portion of the cardiovascular system including the heart and the great vessels. Due to this anatomical configuration, the thoracic mediastinum has many blood vessels traveling through it. Since the lymphatic system closely integrates into the cardiovascular system, there is also a significant presence of lymphatics in this area.

Traveling within the superior mediastinum is the arch of the aorta and its three associated major branches; the brachiocephalic trunk, the left common carotid and left subclavian arteries. In addition to these large arteries, some smaller branches are present within the superior mediastinum. The thymic branches of the internal thoracic arteries, the proximal portions of the pericardiophrenic arteries, as well as the third and fourth posterior intercostal arteries, all course through the superior mediastinum. For lymphatics in this region, there is the larger terminal portion of the thoracic duct and anterior mediastinal lymph nodes.

Descending inferiorly deep to the lateral borders of the sternum are the two internal thoracic arteries. These arteries exit the anterior thoracic mediastinum inferiorly and branch into the musculophrenic arteries and superior epigastric arteries.[1] Lymphatics in this region include the parasternal, pericardial and superior diaphragmatic lymph nodes. 

The middle thoracic mediastinum, being bounded by the pericardium, contains the coronary arteries and associated branches. The lymphatics within the pericardium are a complex network of vessels that penetrate all layers of the cardiac tissue. These vessels ultimately drain posteriorly towards the pre-tracheal lymph nodes that lie between the aorta and the trachea.[2]  

The posterior thoracic mediastinum is where the descending aorta courses slightly left of midline towards the diaphragm. The most notable arterial branches in this region are the proximal portions of the many intercostal arteries. The lymphatics of this region include the thoracic duct, retrocardiac lymph nodes, diaphragmatic lymph nodes, posterior mediastinal lymph nodes, and prevertebral lymph nodes.

Nerves

In the superior mediastinum, there are two broad categories of nerves passing within the region, nerves that originate superior to the thorax and nerves that originate within the thorax.

In the superior thoracic mediastinum, the primary nerves that originate from somewhere more superior to the thorax are the left and right phrenic nerves (from C3 through C5) and the left and right vagus nerves (from the medulla) and the associated vagal branches such as the esophageal plexus, inferior cervical cardiac branches, thoracic cardiac branches, and recurrent laryngeal nerves. As for the nerves originating from within the thorax, there is the superior portion of the sympathetic trunk (T1 through T4) and its associated sympathetic branches such as the thoracic cardiac and pulmonary branches. 

The anterior mediastinum does not contain any major named nerves. 

The middle mediastinum is densely innervated from the autonomic nervous system as well as somatically innervated by the phrenic nerves. Sympathetic innervation comes from branches of the sympathetic trunk around the T2 through T4 levels while parasympathetic innervation derives from branches of the left and right vagus nerves. The somatic innervation to this middle mediastinal region is via branches of the left and right phrenic nerves to just the fibrous and parietal pericardial layers.[3] Because the borders of the middle mediastinum are the borders of the pericardium itself, only a small portion of the nerves mentioned truly reside within the middle mediastinum.

The posterior mediastinum contains many autonomic and somatic nerves. Autonomic nerves include the sympathetic from the levels T5 to T12, and its associated sympathetic branches as well as parasympathetic innervation from the left and right vagus nerves in the form of the esophageal plexus. The somatic nerves in the posterior mediastinum are the intercostal nerves.

Muscles

Although often overlooked, there are many critical muscular structures within the thoracic mediastinum. In the superior thoracic mediastinum is the skeletal and smooth muscle of the esophagus and the left and right inferior oblique part of longus cervicis which assist in mobilizing the cervical spine. In the anterior thoracic mediastinum on the posterolateral sternum originates the transversus thoracis muscles which help in depressing the ribs. In the middle thoracic mediastinum lies arguably one of the most important muscles, the heart. And just posterior to the heart, in the posterior thoracic mediastinum, lies the distal smooth muscle portion of the esophagus. Within blood vessels, especially the large and named vessels such as the aorta, there are many layers of smooth muscle that allow for cardiovascular homeostasis and, although not named as individual muscles, make up a respectable portion of muscular tissue within the mediastinum. 

Physiologic Variants

Regularly, there is a minimal level of variation in the compartmentalization and organization of the thoracic mediastinum between individuals. The structures held within the thoracic mediastinal compartments are highly subject to variability, though. Below are some physiologic variants that can affect and/or be seen in structures within the thoracic mediastinal compartments.

Superior Mediastinum[4]:

  • Right brachiocephalic trunk replaced with right common carotid and right subclavian artery coming directly off the aorta   
  • Patent ductus arteriosus
  • Tracheoesophageal fistula

Anterior Mediastinum[5]:

  • Pectus excavatum

Middle Mediastinum:

  • Heart side dominance (based on which coronary artery gives rise to the posterior interventricular branch)
  • Patent foramen ovale
  • Tetralogy of Fallot

Posterior Mediastinum:

  • Esophageal stenosis
  • Esophageal atresia

Clinical Significance

The Thoracic Mediastinum is a clinically significant region of the human body. Vital cardiopulmonary structures reside within this region. Below is an abbreviated list of significant clinical correlations.

The superior thoracic mediastinum is clinically relevant due to its large vessels and nerves. Penetrating wounds to this area have a high likelihood of affecting important vessels or nerves resulting in critical damage on impact, as well as during extraction.[6] The arch of the aorta, the site of a potential aneurysm, runs through the center of the superior thoracic mediastinum and is where the entirety of the of cardiac output, travels before being dispersed throughout the body. An aneurysm in the arch of the aorta is dangerous and if untreated for long enough can dissect and result in near instant death. The superior thoracic mediastinum also houses parts of the esophagus and trachea, which are both conduits that commonly are obstructed and injured by ingestion and inhalation of foreign substances.

The anterior mediastinum may appear clinically benign, but it is responsible for many clinical considerations. The thymus is in the superior portion of the anterior mediastinum, and this organ is notorious for its root in diseases such as myasthenia gravis, pure red cell aplasia, and thymus cancer.[3] Additionally, the anterior mediastinum is located directly posterior to the sternum and is therefore vulnerable to trauma to the anterior thorax which can result in an intrathoracic or thymic hematoma.

The middle mediastinum is extremely clinically significant as it contains the pericardium, heart, and great vessel roots. Myocardial infarction, pericardial effusion, cardiac tamponade, Tetralogy of Fallot, and cardiomegaly are just a few cases of pathology which isolate to the middle thoracic mediastinum.

The posterior mediastinum piggybacks a large amount of its clinical importance from the structures descending from the superior thoracic mediastinum. The descending aorta, autonomic nervous networks, the extensive lymphatics, and the esophagus are all capable of causing vast systemic dysfunction in the presence of pathology.  Descending thoracic aortic aneurysm, thoracic duct obstruction, and distal esophageal-related dysphagia are problems that can quickly manifest into life-threatening situations. 

These examples are just a few of the many clinical correlations of the thoracic mediastinum. These compartments and their structures are often involved in disease and trauma, and therefore understanding the distinct regions of the thoracic mediastinum, the contained structures, landmarks, and physiologic variants are all beneficial to an effective clinician at all levels.


References

[1] Shahoud JS,Burns B, Anatomy, Thorax, Internal Mammary (Internal Thoracic) Arteries . 2019 Jan     [PubMed PMID: 30726022]
[2] Brakenhielm E,Alitalo K, Cardiac lymphatics in health and disease. Nature reviews. Cardiology. 2019 Jan     [PubMed PMID: 30333526]
[3] Yasuda M,Osaki T,Fukuich Y,Kobayashi K,Iwata T,So T, Anterior mediastinal tumor as a solitary lymph node metastasis of occult thyroid carcinoma. Journal of surgical case reports. 2019 Feb     [PubMed PMID: 30792843]
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[5]     [PubMed PMID: 30592107]
[6]     [PubMed PMID: 28393789]