Anatomy of Mediastinum

The anatomical region between the lungs, is a partition separating the two pleural cavities.

It serves as a corridor through which structures from head / neck connect with lower regions of the body.

It contains all of the principle tissues and organs of the chest except lungs.

Important landmarks:

Superior thoracic aperture = opening in the neck
Manubriosternal joint = joint between sternum and manubrium. Sternal angle is here, located at the second rib. Superior mediastinum is above this angle, inferior mediastinum is below this angle (joint).
Inferior thoracic aperture = region that goes around the ribs, is the lower outlet of the thorax, is blocked by diaphragm.

Sternal angle: Angle of Louis

In same plane as between 4^th and 5^th vertebrae.

Boundaries:

Extends from sternum back to vertebral column

Bounded laterally by: pericardium, mediastinal pleura (continguous with those lining thoracic cage).

It is a division of the thoracic cavity: contains heart, thymus, esophagus / trachea portions. Divided as Superior and Inferior (which contains Anterior, middle, and posterior regions).

Heart = in middle mediastinum.

Anterior mediastinum = all in front of heart

Posterior mediastinum = between heart and V column.

Superior mediastinum:

Located above manubriosternal angle
Posterior boundary = First 4 thoracic vertebrae
Above = it is continuous with neck
Below = continuous with anterior / posterior mediastinum
Laterall, limited by parietal (mediastinal) pleura

Trachea = only in one mediastinum, bronchioles in another one. Esophagus in more than one mediastinum.

Contents of superior mediastinum:

Thymus
Aorta (arch)
Superior vena cava
Trachea w. L and R bronchi, but as a structure, carina is at same level of sternal angle (moves with inspiration).
Esophagus
R / L vagus nerve
Phrenic nerves
Thoracic duct

Anterior mediastinum

Bordered by sternum anteriorly and pericardial sac posteriorly
Contains sternopericardial ligament, fat, lymph nodes

Middle mediastinum

Borders: ventrally by anterior mediastinal compartment, dorsally by posterior mediastinum
Structures: pericardium, heart, phrenic nerves, pericardiacophrenic vessels, stems of great vessels – pulmonary artery, aorta, superior / inferior vena cava.

Posterior mediastinum:

Borders: anterior = pericardial sac, posterior = anterior surface of vertebrae
Structures: descending aorta, esophagus, azygous system of veins, vagus nerve, thoracic duct, lymph nodes, thoracic splanchnic nerves.

Wide mediastinum: infection causes it.

Thymus:

Primary lymphoid organ
Located in the lower part of the neck and the anterior part of the superior mediastinum
Lies posterior to manubrium
Extends into the anterior mediastinum
Thymus grows until 5,6 years.
After puberty, has gradual involution and is largely replaced by fat.

Blood supply to thymus: inferior thyroid and internal thoracic artery

Innervation: vagus and sympathetic trunk

Lymphatics: anterior mediastinal nodes

Function: develop and maintenance of the immune system.

Clinical Case: Superior mediastinal mass = Teloma.

Aorta: divided into

Aortic arch = at same line as sternal angle. Where ascending aorta curves back. Above plane of manubriosternal angle, courses posteriorly and laterally.
Ascending aorta = as soon as it leaves the heart. From left ventricle to aortic arch, mostly within pericardial sac.
Descending aorta = curves around back of heart and down. Called thoracic aorta, continuation of aorta on left side of vertebrae, reaches midline at T 12 as it passes through diaphragm to become abdominal aorta.

Major branches of Ascending aorta:

Brachiocephalic: superiorly and to the right of trachea, divides into right subclavian and common carotid artery
Left common carotid: superior and anterior to trachea, anterior to left subclavian artery.
Left subclavian: superior to trachea, lies against left lung and pleura.

Descending aorta:

Esophageal branches (2)
9 posterior intercostals arteries
Pericardial arteries
Bronchial arteries
Subcostal artery on lower border of 12^th rib

Pulmonary trunk and pulmonary arteries:

After pulmonary artery leaves heart, it bifurcates into R and L

Pulmonary Trunk:

Courses upward and to the left
Divides into L and R arteries in concavity of aortic arch
R pulmonary artery is posterior to ascending aorta
L pulmonary artery is anterior to descending aorta. Ligamentum anteriosum = residual from development.

Great veins of the thorax:

Veins run in same pattern as arteries.

Left braciocephalic vein: formed by confluence of internal jugular and subclavian. Courses to join the right brachiocephalic vein to form the Superior vena cava behind the intercostals space of Rib 1.
Right brachiocephalic vein: similar to left. Cources vertically behind manubrium.
Inferior vena cava. Pierces the diaphragm at T8 and immediately enters the pericardium / heart.

Compression of superior vena cava = superior vena cava syndrome caused by tumor putting pressure on smaller vessels.

Thoracic duct:

Drains all of body below diaphragm and left side of thorax
Right lymphatic duct has 3 branches:
Right jugular = drains the right side of the head and neck.
Subclavian = drains right upper limb.
Bronchomediastinal = drains the right side of the thorax.

Thoracic duct:

Origin is in abdomen.

Ascends through aortic hiatus of diaphragm and ascends between azygous vein and aorta.

Crosses to the left at T4-5 to ascent behind the esophagus into the neck where it joins internal jugular and subclavian veins. Note that duct goes Left at same plane T4-5, sternal angle.

Chylothorax: duct is ruptured and fluid drains into thorax. Thus, duct is close to vertebrae, so fracture of spinous process may injure thoracic duct.

Trachea:

Behind the aorta.

Located in superior mediastinum.

D shaped tubes supported by cartilage
Courses from larynx to bifurcation at T4-5 at Carina, level with sternal angle.
Anteriorly = great vessels and thymus
Posteriorly = esophagus
Aortic arch is to the left of bifurcation
Main bronchi = left bronchos anterior to aorta, Right bronchus inferior to azygous arch
Blood supply from bronchial arteries and inferior thyroid arteries
Innervated by pulmonary plexus, vagus, and sympathetic trunk.

Esophagus:

courses through superior and posterior mediastinum
Continuous with pharynx in neck
Pierces diaphragm at T10 to join stomach
UUpper portion in thorax, slightly left of midline
At lower end, passes through diaphragm to left of midline
Pushed toward midline by aortic arch
Is a muscular tube that is striated in the neck, mixed smooth / striated in middle, and smooth muscle in lower third.

Associations: Anterior = trachea, L bronchus, pericardium. Posteriorly = Vertebral bodies, thoracic duct, intercostals arteries

Constrictions: 1^st = at junction of esophagus with pharynx, 2^nd = where aorta and left bronchus compress it, 3^rd = near gastric end. Tumors will grow here more often.

Blood supply: In neck = inferior thyroid arteries / veins

In thorax = Bronchial vessels, aorta, pericardial vein. Has anastomoses with inferior thyroid and left gastric arteries.

Innervation: R and L vagus nerve innervates striated muscle (recurrent branch of vagus)

Smooth muscle by parasympathetic vagal.

Esophageal plexus: surrounds lower thoracic esophagus = mostly vagal fibers, some sympathetic. See main branch superior / anterior and are joined by plexus.

Left atrium is right up against esophagus. Can swallow barium to detect whether left atrium is enlarged (will show up in X ray)

Nerves in mediastinum:

Phrenic nerves:

Motor fibers to all of diaphragm (C3-5)
Sensory fibers to parietal pleura, pericardium, central diaphragm
Associated with artery in pericardium
Right phrenic: descends along side great veins, is anterior to the root of the lungs
Left phrenic: along left side of subclavian artery, crosses left side of aortic arch anterior to root of lung, along left side of pericardial sac. Is on top of pericardium.

Vagus:

L vagus descends close with left common corotid artery, loops around aortic arch and gives off branch called recurrent branch. Other branch continues to esophagus = anterior vagus that supplies anterior esophagus.
R vagus: no aortic arch, so loop around subclavian artery and ascends back to neck to become posterior vagus.
Vagus is deep to phrenic

Azygous system of veins:

A group veins in posterior mediastinum formed by azygous, hemiazygous, accessory hemiazygous, and left superior intercostal veins
Is a way for blood to be shunted out of vena cavas. If have obstruction in inferior vena cava, blood can to around to superior vena cava to reach atrium. Important when have tumors / obstructions. Similar system in abdomen.
Formed by Right superior intercostal vein and Right ascending lumbar vein. Hemiazygous, azygous, and accessory azygous are between these.
Accessory azygous branches off superiorly at T8.
Azygous ends at T4-5 at superior vena cava (see sternal angle importance again). Hemiazygous joins azygous at T9. Accessory (formed by intercostals) joins azygous at T8. Azygous on R side, hemiazygous and accessory on L side.
Left superior intercostals vein: instead of draining into azygous, drains into brachiocephalic vein.