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The mediastinum is the central compartment of the thoracic cavity, lying between the two pleural sacs and containing all thoracic viscera except the lungs.
It extends from the thoracic inlet (above) to the diaphragm (below), and from the sternum (front) to the vertebral column (back).
A transverse plane drawn from the sternal angle to the lower border of T4 vertebra divides it into:
Superior mediastinum (above)
Inferior mediastinum (below)
The inferior mediastinum is further divided into:
Anterior mediastinum – in front of pericardium.
Middle mediastinum – occupied by heart and pericardium.
Posterior mediastinum – behind pericardium and in front of vertebral column.
The superior mediastinum acts as a gateway between neck and thorax, transmitting large vessels, trachea, and esophagus.
From anterior to posterior, its structures are arranged as:
Thymus → Veins → Arteries → Trachea → Esophagus → Thoracic Duct → Vertebral Column.
Contents of Superior Mediastinum:
Thymus
Great veins (brachiocephalic veins, SVC)
Arch of aorta and its branches
Trachea, esophagus, thoracic duct
Vagus, phrenic, and cardiac nerves
Contents of Inferior Mediastinum (Summary):
Anterior: Thymic remnants, fat, lymph nodes, small vessels.
Middle: Heart, pericardium, ascending aorta, pulmonary trunk, SVC, phrenic nerves.
Posterior: Descending aorta, azygos system, thoracic duct, esophagus, vagus nerves, sympathetic trunks, and splanchnic nerves.
Arch of the aorta lies in the superior mediastinum, beginning and ending at the sternal angle (T4).
The trachea bifurcates at the same level — an important landmark both radiologically and surgically.
Left recurrent laryngeal nerve hooks around the arch of the aorta under the ligamentum arteriosum, while the right recurrent laryngeal nerve hooks under the right subclavian artery.
The thoracic duct, the largest lymphatic channel, ascends through the posterior mediastinum, crosses to the left at T5, and ends at the junction of left internal jugular and subclavian veins.
The esophagus passes through the posterior mediastinum, and pierces the diaphragm at T10 level (esophageal hiatus).
(Mnemonic: I 8, 10 Eggs, At 12 → IVC T8, Esophagus T10, Aorta T12).
The descending thoracic aorta gives off:
Posterior intercostal arteries (3rd–11th)
Bronchial arteries
Esophageal arteries
Subcostal arteries
Pericardial and mediastinal branches
The azygos vein system provides a collateral pathway between the superior and inferior vena cava, especially important during venous obstruction.
The phrenic nerves run anterior to the roots of the lungs, while the vagus nerves run posterior to them — a key relation for thoracic surgeries.
The fibrous pericardium is firmly attached to the central tendon of diaphragm and posterior sternum (via sternopericardial ligaments).
The pericardial cavity contains a small amount of fluid that reduces friction between heart movements.
Mediastinal widening on radiographs can indicate:
Aortic aneurysm
Mediastinal tumor
Lymphadenopathy
Hemorrhage after trauma
Mediastinal shift occurs:
Toward lesion in lung collapse.
Away from lesion in pleural effusion or pneumothorax.
The 4 common tumors of anterior mediastinum — “Four Ts”:
Thymoma, Teratoma, Thyroid mass, Terrible lymphoma.
Cardiac tamponade is a life-threatening accumulation of fluid in the pericardial cavity, treated by pericardiocentesis through left 5th intercostal space near sternum.
Referred pain from the pericardium and diaphragmatic pleura is felt at the shoulder tip (C4 dermatome) via the phrenic nerve.
The posterior mediastinum serves as a major communication corridor between the thorax and abdomen — transmitting aorta, esophagus, thoracic duct, and sympathetic pathways.
Knowledge of mediastinal divisions is crucial for interpreting CT, MRI, and chest radiographs, as different diseases localize to characteristic mediastinal compartments.
Case:
A patient with left-sided pneumothorax presents with chest pain and breathlessness.
Anatomical Basis:
The accumulation of air in the left pleural cavity increases intrathoracic pressure, pushing the mediastinum to the opposite side.
This compresses the contralateral lung and great veins, reducing venous return and cardiac output.
In contrast, lung collapse or fibrosis may pull the mediastinum toward the affected side.
Case:
A patient with bronchogenic carcinoma develops swelling of face, neck, and upper limbs with visible dilated chest veins.
Anatomical Basis:
The SVC lies in the superior mediastinum, and its obstruction (by tumor or lymph nodes) blocks venous drainage from the upper body.
Collateral flow occurs through the azygos system, internal thoracic, and vertebral veins, but venous congestion causes cyanosis and edema of face and arms.
Case:
A middle-aged man complains of chest pain radiating to the back and difficulty swallowing.
Anatomical Basis:
An aneurysm of the arch of the aorta in the superior mediastinum may compress:
Trachea → cough, dyspnea.
Esophagus → dysphagia.
Left recurrent laryngeal nerve → hoarseness of voice.
Sympathetic chain → Horner’s syndrome (ptosis, miosis, anhidrosis).
Case:
A trauma patient’s chest X-ray shows a widened mediastinum.
Anatomical Basis:
Widening may indicate aortic rupture, lymphadenopathy, or hematoma within the loose areolar tissue of the mediastinum.
Immediate imaging and surgical exploration are essential to rule out aortic dissection.
Case:
A young adult presents with cough, chest heaviness, and venous congestion.
Anatomical Basis:
Tumors in specific mediastinal compartments cause characteristic syndromes:
Anterior mediastinum: thymoma, teratoma, thyroid mass, lymphoma (the “4 Ts”).
Middle mediastinum: pericardial cysts or lymphadenopathy compressing heart/great veins.
Posterior mediastinum: neurogenic tumors compressing sympathetic chain or esophagus.
Case:
A patient after blunt chest trauma develops neck swelling with a crackling sensation on palpation.
Anatomical Basis:
Rupture of alveoli or tracheobronchial tree allows air to escape into the mediastinum, which then spreads to subcutaneous tissue of neck and face, producing surgical emphysema.
Air may track along fascial planes into the neck and retroperitoneum.
Case:
Post-esophageal surgery, a patient develops fever, chest pain, and difficulty breathing.
Anatomical Basis:
Infection spreads through loose connective tissue of mediastinum — an area continuous with neck fascial planes.
This leads to acute suppurative mediastinitis, which may cause sepsis or pericardial infection.
Rapid diagnosis and drainage are crucial.
Case:
A patient with tuberculosis develops hypotension, distended neck veins, and muffled heart sounds.
Anatomical Basis:
Fluid accumulation in the pericardial cavity (middle mediastinum) compresses the heart, impairing ventricular filling — a condition known as cardiac tamponade.
It is relieved by pericardiocentesis, performed through the left 5th intercostal space near the sternum.
Case:
A patient with difficulty swallowing is found to have a posterior mediastinal mass.
Anatomical Basis:
The esophagus lies in the posterior mediastinum, closely related to the aorta, left bronchus, and heart.
Compression or invasion by an aneurysm, tumor, or enlarged lymph nodes leads to dysphagia.
Case:
After esophageal surgery, a patient develops milky pleural effusion.
Anatomical Basis:
Injury to the thoracic duct (posterior mediastinum) causes leakage of chyle (lymph rich in fat) into the pleural cavity.
This results in chylothorax, which may require surgical ligation of the duct.
Case:
A patient with right heart failure shows a paratracheal shadow on chest X-ray.
Anatomical Basis:
The azygos vein in the posterior mediastinum becomes distended due to back pressure.
It provides an important collateral pathway between superior and inferior vena cava.
Case:
A middle-aged woman with myasthenia gravis presents with chest fullness and venous congestion.
Anatomical Basis:
The thymus, located in the anterior and superior mediastinum, may enlarge or develop a tumor (thymoma).
It compresses the SVC or trachea, and is strongly associated with autoimmune neuromuscular disorders.
Case:
A patient reports regurgitation and retrosternal pain after meals.
Anatomical Basis:
Herniation of the stomach through the esophageal opening of the diaphragm (T10) brings part of the stomach into the posterior mediastinum, causing reflux and pain.
Case:
A patient has unilateral ptosis, miosis, and facial anhidrosis.
Anatomical Basis:
Compression of the sympathetic trunk in the posterior mediastinum by a tumor (neurogenic or bronchogenic) disrupts sympathetic pathways to the head and neck.
Case:
A patient with pericarditis complains of pain radiating to the left shoulder and neck.
Anatomical Basis:
The fibrous pericardium and diaphragmatic pleura are supplied by the phrenic nerve (C3–C5).
Inflammation in the middle mediastinum causes referred pain to the shoulder tip area corresponding to the C4 dermatome.
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