Anatomy of the Mediastinum

• Compartmental organization and fascial barriers: The mediastinum is divided into the superior mediastinum (above the sternal angle/T4 vertebral level) and the inferior mediastinum. The inferior mediastinum is further subdivided by the pericardium into anterior mediastinum (ventral to pericardium), middle mediastinum (pericardial sac and contents), and posterior mediastinum (dorsal to pericardium). These compartments are separated by dense fascia that restricts the spread of mediastinal pathology but can lead to compartment syndrome-like effects with rapid fluid accumulation or hemorrhage. The anterior mediastinal compartment (bounded by sternum anteriorly, pericardium posteriorly, and mediastinal pleura laterally) contains adipose tissue, lymph nodes, and is the classic location for anterior mediastinal masses. The middle mediastinum contains the pericardial sac and its contents (heart, roots of great vessels, pericardial nerves and vessels), making it the cardiovascular center. The posterior mediastinum contains the esophagus, descending thoracic aorta, azygos venous system, sympathetic chain, and thoracic duct, and serves as a conduit for posterior structures.

• Vascular and neural relationships: The great vessels (ascending aorta, superior vena cava, inferior vena cava, pulmonary trunk, pulmonary veins) originate or terminate in the mediastinum with complex anatomic relationships that determine hemodynamic flow and surgical accessibility. The superior vena cava courses vertically in the right anteromedial mediastinum, receiving the azygos vein posteriorly before entering the right atrium. The ascending aorta arises from the left ventricle and courses anteriorly and rightward before the aortic arch curves posteriorly and leftward at the level of T4-T5, giving off the brachiocephalic trunk, left common carotid artery, and left subclavian artery. The pulmonary trunk arises from the right ventricle and bifurcates at T5 level into right and left pulmonary arteries. Sympathetic fibers from T1-T4 spinal segments form the cardiac plexus, which innervates the heart; parasympathetic innervation occurs via the vagus nerve (CN X), which courses through the mediastinum bilaterally. The recurrent laryngeal nerve has critical anatomic relationships: the left recurrent laryngeal nerve loops under the aortic arch (making it vulnerable to aortic pathology), while the right loops under the subclavian artery.

• Lymphatic drainage and immune function: The mediastinum contains lymph nodes organized into stations (per American Thoracic Society classification: right paratracheal, left paratracheal, anterior mediastinal, right tracheobronchial, left tracheobronchial, subcarinal, paraaortic, paraesophageal, left pulmonary ligament) that drain the lungs, heart, and thoracic structures. The thoracic duct, which is the largest lymphatic vessel, originates from the cisterna chyli (T12-L1 level) and ascends through the posterior mediastinum between the aorta and azygos vein, coursing to the left at the T5 level and draining into the left venous angle; disruption leads to chylothorax. Mediastinal lymph nodes are critical sites for malignancy staging (lung cancer, lymphoma, metastatic disease) and infection (tuberculous mediastinitis, fungal infections).

• Anterior mediastinal masses (AAAA mnemonic): Airway (thymus, thyroid), Aortic (aortic aneurysm, aortic dissection, aortitis), Anthem (lymphoma), Anteior mediastinal masses (germ cell tumors, teratomas). Additional anterior mediastinal pathology includes thymoma, thymic hyperplasia, hemangioma, lipoma, and cardiac-based masses. In pediatric patients, the anterior mediastinum is the most common site for mediastinal masses (lymphoma, germ cell tumors, neuroblastoma). In adults >40 years, the anterior mediastinum commonly harbors lymphoma, thymoma, and metastatic disease.

• Middle mediastinal masses and cardiac pathology: Pathology arising from or compressing the heart, pericardium, and great vessels, including pericardial effusion, cardiac tamponade, myocarditis, pericarditis, aortic aneurysm, aortic dissection, coronary artery anomalies, and endocarditis. The pericardium can become inflamed (pericarditis), accumulate fluid (pericardial effusion), develop thickening (constrictive pericarditis), or rupture (cardiac tamponade).

• Posterior mediastinal masses: Pathology related to the esophagus (esophageal cancer, esophageal diverticulum), aorta (aortic aneurysm, aortic dissection), neural structures (neuroblastoma, paraganglioma, schwannoma, neurofibromatosis-associated tumors), and lymph nodes (lymphoma, metastatic disease, tuberculosis). Posterior mediastinal masses in children classically include neuroblastoma and other neural crest-derived tumors.

• Superior mediastinal masses: Thyroid pathology (goiter, thyroiditis, thyroid cancer), Hodgkin and non-Hodgkin lymphoma, thymoma, lymphadenopathy from infection or malignancy, and superior vena cava syndrome from compression or thrombosis.

• Chest pain: Character, location, and radiation vary by etiology. Aortic dissection typically presents with sudden-onset, severe, tearing chest pain radiating to the back (interscapular region). Pericarditis causes pleuritic (positional), sharp chest pain that worsens with lying supine and improves with leaning forward. Myocarditis may cause chest pain similar to acute coronary syndrome with dyspnea and palpitations. Mediastinitis (usually post-surgical) presents with acute chest pain, fever, and sepsis.

• Dyspnea and respiratory compromise: Mediastinal masses, pericardial effusion with tamponade, aortic aneurysm, or superior vena cava syndrome can compress the trachea, bronchi, or lungs, causing dyspnea, stridor, or orthopnea. Acute aortic dissection may cause dyspnea from aortic regurgitation or pericardial effusion. Posterior mediastinal masses can compress the esophagus (dysphagia) or trachea.

• Superior vena cava syndrome: Facial plethora, upper extremity edema, venous distension (prominent neck and chest wall veins), stridor, dyspnea, and syncope from compression of the SVC by lymphoma, lung cancer, or thymoma.

• Hoarseness and vocal cord paralysis: Left recurrent laryngeal nerve involvement from aortic arch pathology, aortic aneurysm, aortic dissection, or left-sided mediastinal masses (lymphoma, thymic pathology, esophageal cancer) causes hoarseness or vocal cord paralysis (vagus nerve involvement causes complete vocal cord paralysis).

• Palpitations and arrhythmias: Cardiac compression, myocarditis, pericarditis, or direct cardiac involvement by lymphoma or metastatic disease.

• Syncope and hypotension: Cardiac tamponade from pericardial effusion, aortic dissection with aortic regurgitation causing acute decompensation, or massive hemorrhage from aortic rupture.

• Fever and sepsis: Mediastinitis (post-surgical, esophageal perforation, descending necrotizing mediastinitis from oral infection) presents with fever, chest pain, dyspnea, and septic shock.

• Horner syndrome: Disruption of the sympathetic chain in the posterior mediastinum by Pancoast tumor (lung apex), neuroblastoma, or other posterior masses causes miosis, ptosis, anhidrosis, and enophthalmos.

• Physical examination findings: Blood pressure discrepancy between upper extremities (aortic dissection), diminished or absent pulses (aortic dissection with branch vessel involvement), new aortic regurgitation murmur (aortic dissection), pericardial friction rub (pericarditis), muffled heart sounds (pericardial effusion/tamponade), elevated JVP with Kussmaul sign (tamponade or constrictive pericarditis), leg edema and chest wall venous distension (SVC syndrome).

• Chest X-ray (CXR): Initial imaging modality that may reveal mediastinal widening, mass, cardiac silhouette enlargement, pleural effusion, or signs of pulmonary edema. Mediastinal widening (>8 cm at T4 level on frontal view) suggests aortic pathology, hemorrhage, or masses. CXR is insensitive for small masses and cannot characterize mediastinal fat from pathologic tissue reliably.

• Computed tomography (CT) of the chest with IV contrast: Gold standard for mediastinal imaging, providing excellent delineation of mediastinal compartments, masses, and great vessels. IV contrast is essential for evaluating aortic pathology, distinguishing mediastinal fat from pathology, and assessing vascular involvement. CT can characterize mass density (fat-density vs. soft tissue), enhancement patterns, and relationships to adjacent structures. CT angiography (CTA) is the primary imaging for aortic dissection, superior vena cava syndrome, and pulmonary embolism, with sensitivity >95% and specificity >98% for aortic dissection.

• Magnetic resonance imaging (MRI) of the chest: Excellent for soft tissue characterization, particularly useful for neurogenic tumors (schwannoma, neurofibromatosis), cardiac imaging, and evaluating spinal cord involvement in posterior mediastinal masses. MRI avoids ionizing radiation and provides superior tissue contrast but is contraindicated with metallic implants and takes longer than CT.

• Echocardiography: Transthoracic echocardiography (TTE) evaluates the heart, pericardium, and great vessels, detecting pericardial effusion, tamponade physiology, wall motion abnormalities (myocarditis), and aortic regurgitation. Transesophageal echocardiography (TEE) provides superior visualization of the aorta and posterior structures and is useful for aortic dissection diagnosis and guidance during procedures. M-mode and Doppler interrogation assess ventricular function and hemodynamic significance.

• Electrocardiography (ECG): Pericarditis shows diffuse ST elevation (concave), PR depression, and absence of reciprocal ST depressions. Myocarditis may show ST-segment changes, T-wave inversions, or arrhythmias. Aortic dissection may show ischemic changes if dissection involves coronary ostia.

• Laboratory studies: Elevated troponin and elevated BNP/NT-proBNP suggest myocardial involvement. Elevated D-dimer raises suspicion for aortic dissection or pulmonary embolism (though nonspecific). Inflammatory markers (ESR, CRP) are elevated in pericarditis, myocarditis, and aortitis. Lactate is elevated in septic mediastinitis. Blood cultures are obtained for suspected mediastinitis or endocarditis.

• Pericardiocentesis: Performed for pericardial effusion with hemodynamic compromise (tamponade) or diagnostic purposes. Fluid analysis includes cell count (exudate vs. transudate by Light criteria), glucose, protein, LDH, cytology (malignancy), Gram stain and culture (infection), and specific testing (TB culture, viral PCR, fungal stains).

• Biopsy and histopathology: Core needle biopsy or excisional biopsy of anterior mediastinal masses (via mediastinoscopy, VATS, or thoracotomy) is required for definitive diagnosis of lymphoma, thymoma, germ cell tumors, and other malignancies. Mediastinoscopy (Chamberlain procedure for anterior/middle mediastinal nodes; left anterior mediastinotomy for left paratracheal/subaortic nodes) provides tissue for staging and diagnosis. Video-assisted thoracic surgery (VATS) allows biopsy with less morbidity than thoracotomy.

• Specific diagnostic criteria: Aortic dissection is confirmed by CTA, MRA, or TEE showing an intimal flap with true and false lumens (sensitivity of CTA >99%). Pericarditis diagnosis requires two of: pleuritic chest pain, pericardial friction rub, ECG changes (diffuse ST elevation, PR depression), or pericardial effusion. Cardiac tamponade physiology is confirmed by echocardiography showing RA/RV collapse and Doppler demonstrating pulsus paradoxus (>10 mmHg drop in SBP with inspiration). Mediastinitis diagnosis is clinical (fever, chest pain post-operatively) and confirmed by imaging (CT showing pneumomediastinum, fluid collection) and positive blood/fluid cultures.

• Aortic dissection: Acute aortic syndrome (dissection, intramural hematoma, penetrating aortic ulcer) requires urgent evaluation. Type A dissection (involving ascending aorta) is a surgical emergency requiring immediate cardiothoracic surgery with goal of heart rate control (beta-blocker target HR <60 bpm, e.g., esmolol, metoprolol) and blood pressure control (target SBP 100-120 mmHg with nitroprusside, nicardipine, or hydralazine); ascending aortic repair/replacement is definitive treatment with graft interposition or valve-sparing root replacement. Type B dissection (descending aorta distal to left subclavian artery) is typically managed medically with aggressive blood pressure and heart rate control unless complicated (rupture, malperfusion, severe aortic regurgitation), in which case thoracic endovascular aortic repair (TEVAR) is performed.

• Aortic aneurysm: Thoracic aortic aneurysm (TAA) management depends on size, growth rate, and etiology. Elective repair is indicated for ascending aortic aneurysm >5.5 cm (or >5 cm with bicuspid aortic valve, Marfan syndrome, or Ehlers-Danlos syndrome) and descending TAA >5.5-6 cm. Medical management with beta-blockers (target HR <60 bpm) or ARBs/ACE inhibitors slows aneurysm growth. Surgical repair (graft replacement, root replacement) or **endovascular repair (TEVAR

Airway and vascular compression (emergencies)

• Mediastinal mass syndrome on anesthetic induction: loss of negative intrathoracic pressure, bronchial smooth-muscle tone, and diaphragmatic descent allows a bulky anterior mass to collapse the trachea/main bronchi or compress the right heart and pulmonary artery — signaled by inability to ventilate or cardiovascular collapse after paralysis. Orthopnea, stridor, or supine syncope predicts it; maintain spontaneous ventilation, semi-upright position, and consider awake/rigid bronchoscopy or extracorporeal support standby.
• Superior vena cava syndrome: extrinsic compression or thrombosis raises upper-body venous pressure; facial plethora, Pemberton sign, collateral chest wall veins. Laryngeal edema, stridor, or depressed consciousness (cerebral edema) makes it a true emergency requiring urgent radiation or endovascular stenting.
• Cardiac tamponade: malignant or hemorrhagic pericardial fluid raises intrapericardial pressure above right-heart filling pressure; pulsus paradoxus, electrical alternans, right-chamber diastolic collapse on echo. Immediate pericardiocentesis.

Infectious and traumatic

• Acute mediastinitis: esophageal perforation (Boerhaave), odontogenic/retropharyngeal spread along fascial planes, or post-sternotomy contamination; fever, crepitus, Hamman crunch, pneumomediastinum on CT. Surgical emergency — drainage plus broad-spectrum antibiotics; deep sternal wound infection is a recognized Society of Thoracic Surgeons reportable complication.
• Chylothorax: thoracic duct injury during posterior mediastinal or esophageal surgery; milky, triglyceride-rich pleural fluid, with progressive lymphopenia and malnutrition.

Neurologic and treatment-related

• Recurrent laryngeal or phrenic nerve injury: invasion or surgical transection — hoarseness/weak cough, or elevated hemidiaphragm with orthopnea.
• Horner syndrome and spinal cord involvement: sympathetic chain or dumbbell extension of a neurogenic tumor through the intervertebral foramen; MRI before resection.
• Aortic repair complications: per the ACC/AHA 2022 aortic disease guideline, malperfusion, aortic regurgitation, tamponade, and stroke complicate acute type A dissection; spinal cord ischemia with paraplegia follows extensive descending repair or TEVAR from intercostal/*Adamkiewicz* sacrifice.
• Therapy sequelae: tumor lysis syndrome with lymphoma induction, and years later radiation-associated constrictive pericarditis, coronary disease, and secondary breast/lung malignancy (NCCN survivorship surveillance).

• Compartment predicts pathology: anterior — the four Ts (thymoma, teratoma/germ cell, thyroid goiter, "terrible" lymphoma); middle — pericardial and great-vessel disease, bronchogenic cysts, nodal disease; posterior — neurogenic tumors, esophageal and descending aortic disease. The single most useful next step after an abnormal chest radiograph is contrast-enhanced CT of the chest.
• Thymoma is the association examiners love: myasthenia gravis with anti-acetylcholine receptor antibodies, plus paraneoplastic pure red cell aplasia and hypogammaglobulinemia (Good syndrome). Any adult with new MG deserves cross-sectional chest imaging.
• Young man, anterior mass: check AFP and β-hCG before biopsy — markedly elevated AFP means nonseminomatous germ cell tumor, treated with chemotherapy rather than upfront resection. Fat plus calcification or teeth on CT is a teratoma.
• Do not give empiric glucocorticoids to a bulky anterior mediastinal mass before tissue diagnosis: steroids lyse lymphoma and can render histology uninterpretable. Common distractor answer.
• Left recurrent laryngeal nerve loops under the aortic arch at the ligamentum arteriosum; the right loops under the subclavian artery. Hoarseness therefore implicates left-sided arch, aortopulmonary window, or left hilar pathology — never a right-sided lesion via the arch.
• Sudden tearing chest pain with a widened mediastinum: CT angiography per the ACC/AHA 2022 aortic disease guideline, with rate control before vasodilation (beta blocker such as esmolol first, then nicardipine or nitroprusside) to avoid reflex tachycardia and increased aortic wall shear. Type A goes straight to cardiac surgery. Anticoagulation/thrombolysis is the classic wrong answer.
• Pediatric posterior mediastinal mass = neuroblastoma or other neural crest tumor; look for elevated urine HVA/VMA, opsoclonus-myoclonus, and paraspinal calcification, and obtain MRI to exclude intraspinal extension.
• Thoracic duct crosses to the left at about T5; injury yields milky, triglyceride-rich chylothorax. Hamman crunch with pneumomediastinum after forceful emesis means Boerhaave — get water-soluble contrast esophagography or CT, not endoscopy first.