Mediastinum — Divisions & Contents
Anatomy · Thorax · lean revision notes
Mediastinum — Divisions & Contents
The mediastinum is the central compartment of the thoracic cavity, sandwiched between the two pleural sacs. For NEET PG it is pure surface-anatomy + contents recall married to high-yield clinical scenarios — the sternal angle (of Louis) landmark, the 4 Ts of anterior mediastinal masses, SVC syndrome, and oesophageal constrictions. Master the planes, then hang every structure on them.
Definition & boundaries
The mediastinum is the space in the median portion of the thorax between the right and left pleural cavities. It extends:
- Superiorly → thoracic inlet (superior thoracic aperture).
- Inferiorly → diaphragm.
- Anteriorly → sternum.
- Posteriorly → bodies of the twelve thoracic vertebrae (T1–T12).
- Laterally → mediastinal pleura on each side.
It contains all thoracic viscera except the lungs: the heart and great vessels, trachea, oesophagus, thymus, nerves (vagus, phrenic, sympathetic chain), the thoracic duct and lymph nodes, and connective tissue.
High-yield: The mediastinum contains everything in the thorax except the lungs. The only midline structure that is NOT in the mediastinum is lung tissue.
Classification (divisions)
The classical (anatomical) subdivision uses a horizontal plane through the sternal angle of Louis (junction of manubrium and body of sternum, at the level of the T4/T5 intervertebral disc). This transverse thoracic plane divides the mediastinum into superior and inferior parts; the inferior is then split by the pericardium into anterior, middle and posterior.
| Division | Boundaries | One-line memory hook |
|---|---|---|
| Superior | Thoracic inlet → sternal angle plane (T4/5) | "Above the angle of Louis" |
| Anterior (inferior) | Sternal body (front) → pericardium (back) | Thymus + fat (small space) |
| Middle (inferior) | The pericardial sac and its contents | Heart + roots of great vessels |
| Posterior (inferior) | Pericardium (front) → vertebrae T5–T12 (back) | Oesophagus + descending aorta |
High-yield: The plane separating the superior from the inferior mediastinum passes through the sternal angle anteriorly and the lower border of T4 / T4–T5 disc posteriorly. This is the single most-tested landmark in the topic.
Mnemonic for the sternal angle plane events (T4/5) — "RAILS & a few extras":
- Rib 2 articulates here (counting starts at the angle).
- Arch of aorta — begins and ends (ascending aorta ends, arch ends → descending begins).
- Azygos vein arches over the right main bronchus to drain into the SVC.
- Bifurcation of trachea (carina, ~T4/5).
- Ligamentum arteriosum / Left recurrent laryngeal nerve hooks.
- Thoracic duct crosses from right to left here.
- Superior–inferior mediastinal boundary.
- Pulmonary trunk bifurcation; start and end of the aortic arch; T2 cardiac plexus level.
High-yield: At the sternal angle: trachea bifurcates (carina), aortic arch begins AND ends, azygos vein arches to join SVC, thoracic duct crosses midline, ligamentum arteriosum lies here, and the second costal cartilage articulates.
Superior mediastinum — contents
A useful anterior-to-posterior layering approach: glands → veins → arteries → airway → food-tube → ducts/nerves at the back.
Order (front → back): Thymus → Veins (brachiocephalic, SVC) → Arteries (arch of aorta + 3 branches) → Trachea → Oesophagus → Thoracic duct → Sympathetic trunks.
| Category | Structures |
|---|---|
| Thymus | Thymus (or its fatty remnant in adults) |
| Veins | Right & left brachiocephalic veins, upper half of SVC, arch of azygos (terminal part) |
| Arteries | Arch of aorta + its 3 branches (brachiocephalic trunk, left common carotid, left subclavian) |
| Airway | Trachea (down to carina) |
| Alimentary | Oesophagus |
| Nerves | Vagus (both), phrenic (both), left recurrent laryngeal, cardiac & sympathetic |
| Lymphatics | Thoracic duct, paratracheal & brachiocephalic nodes |
High-yield: The left brachiocephalic vein is the most anterior major structure in the superior mediastinum (vulnerable in median sternotomy / tracheostomy). The arch of aorta lies behind it.
Phrenic vs vagus relation to lung root (classic MCQ): Phrenic nerve passes ANTERIOR to the root of the lung; Vagus passes POSTERIOR. ("Phrenic = Pre/front.")
Anterior mediastinum — contents
A narrow slit between the body of the sternum and the pericardium. Contents are sparse:
- Thymus (or its remnant) extending down from superior mediastinum.
- Loose areolar tissue and fat, sternopericardial ligaments.
- Internal thoracic vessels' branches, lymph nodes (parasternal).
- In children, the thymus may bulge here ("sail sign" on chest X-ray).
Anterior mediastinal mass — the 4 Ts
This is the highest-yield clinical correlate of the whole topic.
| The 4 Ts | Key clue | Notes |
|---|---|---|
| Thymoma / Thymic tumour | Myasthenia gravis, red-cell aplasia, hypogammaglobulinaemia | Commonest primary anterior mediastinal mass in adults |
| Teratoma / germ cell tumour | Fat, calcification, tooth on imaging; raised AFP / β-hCG | Young adults; "germ cell" |
| Thyroid (retrosternal goitre) | Continuity with neck thyroid; tracheal deviation | Most extend from neck |
| Terrible lymphoma | Hodgkin (young) / NHL; B-symptoms | Commonest cause of an anterior mediastinal mass overall in many series |
High-yield: Remember the 4 Ts: Thymoma, Teratoma (germ cell), Thyroid (retrosternal), Terrible lymphoma. Thymoma is the commonest primary anterior mediastinal tumour and is strongly linked with myasthenia gravis (~10–15% of MG patients have thymoma; ~30–50% of thymomas have MG).
Middle mediastinum — contents
The most important compartment volumetrically; it is essentially the pericardium and its contents.
- Heart within the pericardial sac.
- Ascending aorta, lower half of SVC, pulmonary trunk and its bifurcation, pulmonary veins.
- Arch of azygos vein.
- Bifurcation of trachea (carina) and the two main bronchi.
- Phrenic nerves with pericardiacophrenic vessels (descend on the fibrous pericardium — anterior to lung roots).
- Tracheobronchial lymph nodes, bronchial vessels.
High-yield: The phrenic nerve (C3,4,5 — "keeps the diaphragm alive") runs through the middle mediastinum on the fibrous pericardium, anterior to the lung root, accompanied by pericardiacophrenic vessels. The vagus runs posterior to the lung root.
Middle mediastinal masses (exam angle): bronchogenic cysts, pericardial cysts (right cardiophrenic angle "spring-water" cyst), lymph node enlargement (sarcoid, TB, metastasis).
Posterior mediastinum — contents
Lies behind the pericardium, in front of the lower thoracic vertebrae (T5–T12).
Order (front → back): Oesophagus → Descending thoracic aorta → Azygos/hemiazygos veins → Thoracic duct → Sympathetic chains & splanchnic nerves.
| Structure | Key point |
|---|---|
| Oesophagus + oesophageal plexus (vagus) | Anterior vagal trunk = mainly left vagus; posterior trunk = right vagus (due to embryological gut rotation) |
| Descending thoracic aorta | T4 → T12 (becomes abdominal aorta at aortic hiatus) |
| Azygos vein (right), hemiazygos / accessory hemiazygos (left) | Drain posterior thoracic wall |
| Thoracic duct | Largest lymphatic; crosses midline at T4/5 |
| Sympathetic trunks, greater/lesser/least splanchnic nerves | |
| Posterior mediastinal lymph nodes |
High-yield: As they pass through the diaphragm — oesophagus + vagi at T10, (inferior vena cava at T8), aorta + thoracic duct + azygos at T12. Mnemonic: "I 8 (ate) 10 EGGs At 12" → IVC = T8, Esophagus = T10, Aorta = T12.
Thoracic duct course (named, frequently asked)
Begins at the cisterna chyli (L1/L2) → enters thorax through aortic hiatus → ascends on the right side of vertebral column (between aorta and azygos) → crosses to the left at T4/5 → ascends on the left → arches and drains into the junction of left subclavian and left internal jugular veins (left venous angle). Injury below T5 → right chylothorax; injury above T5 → left chylothorax.
High-yield: Thoracic duct injury (e.g., during oesophagectomy) above the T5 crossover causes a LEFT-sided chylothorax; below the crossover causes a RIGHT-sided chylothorax.
Oesophageal anatomy & relations (high-yield)
The oesophagus runs from C6 (cricoid) to T11 (cardia). Its three constrictions are favourite MCQ material — they are sites of foreign-body impaction, stricture and carcinoma:
| Constriction | Level | Distance from incisors | Cause |
|---|---|---|---|
| 1. Cricopharyngeal (upper sphincter) | C6 | ~15 cm | Cricopharyngeus muscle — narrowest point |
| 2. Broncho-aortic | T4 | ~22.5 cm | Crossed by arch of aorta + left main bronchus |
| 3. Diaphragmatic | T10 | ~40 cm | Oesophageal hiatus / LES |
High-yield: The narrowest part of the oesophagus is the cricopharyngeal sphincter (~15 cm from incisors) — the commonest site of foreign-body lodgement. Distances "15, 22.5, 40" cm are classic exam numbers.
SVC syndrome — applied anatomy
The superior vena cava is formed by the union of the right and left brachiocephalic veins behind the right first costal cartilage and drains into the right atrium; the azygos vein joins its posterior aspect at T4. The SVC is a thin-walled, low-pressure vessel surrounded by rigid structures (sternum, trachea, right main bronchus, lymph nodes), making it easily compressible.
Causes → bronchogenic carcinoma (most common, esp. right-sided small-cell) > lymphoma > metastatic mediastinal nodes > thymoma/teratoma > fibrosing mediastinitis > thrombosis (catheters/pacemakers).
Features → facial & upper-limb swelling/plethora, dilated neck veins (non-pulsatile, raised JVP), distended chest-wall collaterals, headache, Pemberton's sign (facial congestion on raising arms).
High-yield: Commonest cause of SVC obstruction = bronchogenic carcinoma (small-cell type). If obstruction is above the azygos entry, blood reroutes via azygos; if below the azygos, collaterals are more extensive and symptoms worse.
Mediastinal shift — causes
Displacement of mediastinal structures from the midline, detected clinically by tracheal/apex-beat shift and confirmed on chest X-ray.
Stepwise reasoning → Is the lesion PUSHING or PULLING?
- Shift AWAY from the lesion (push) → tension pneumothorax, massive pleural effusion, large diaphragmatic hernia, large tumour.
- Shift TOWARDS the lesion (pull) → collapse/atelectasis, pneumonectomy, pulmonary fibrosis/agenesis, old extensive TB.
| Mediastinum shifted | Mechanism | Classic example |
|---|---|---|
| Towards the abnormal side | Loss of volume (pull) | Lung collapse, fibrosis, pneumonectomy |
| Away from the abnormal side | Added volume/pressure (push) | Tension pneumothorax, massive effusion |
High-yield: Tension pneumothorax → trachea/mediastinum shift AWAY from affected side (emergency, needle decompression 2nd ICS MCL or 5th ICS for current ATLS). Collapse/fibrosis → shift TOWARDS the lesion.
Diagnosis & investigation of choice
- Chest X-ray (PA + lateral): first-line; localises mass to anterior/middle/posterior compartment. Lateral film is key for compartment localisation.
- Contrast-enhanced CT thorax: investigation of choice for mediastinal masses — defines compartment, vascularity, fat/calcification (teratoma), continuity with thyroid.
- MRI: best for neurogenic (posterior) tumours and vascular/spinal cord involvement.
- Tumour markers: AFP & β-hCG (germ cell), LDH (lymphoma).
- Biopsy: mediastinoscopy (paratracheal/subcarinal nodes), EBUS-TNA, anterior mediastinotomy (Chamberlain procedure), CT-guided core biopsy.
High-yield: CECT thorax is the imaging investigation of choice for a mediastinal mass. MRI is preferred for posterior/neurogenic tumours (assess intraspinal extension — "dumb-bell" tumour).
Compartment-wise mass differentials (must-know table)
| Compartment | Common masses |
|---|---|
| Anterior | 4 Ts — Thymoma, Teratoma/germ cell, Thyroid (retrosternal goitre), Terrible lymphoma; parathyroid |
| Middle | Lymphadenopathy (lymphoma, sarcoid, TB, mets), bronchogenic cyst, pericardial cyst, vascular (aneurysm) |
| Posterior | Neurogenic tumours (commonest) — schwannoma, neurofibroma, ganglioneuroma, neuroblastoma; oesophageal lesions; descending aortic aneurysm; meningocele |
High-yield: Posterior mediastinal masses are predominantly NEUROGENIC (arise from sympathetic chain / intercostal nerves). In children, the commonest posterior mediastinal mass is a neuroblastoma/ganglioneuroma.
Complications & clinical correlates
- SVC syndrome (above).
- Mediastinitis — acute (oesophageal perforation, Boerhaave syndrome, post-cardiac surgery) → broad mediastinum, surgical emergency; chronic/fibrosing (histoplasmosis, TB).
- Pneumomediastinum — air tracking along fascial planes (asthma, ventilation, oesophageal rupture) → Hamman's crunch on auscultation.
- Recurrent laryngeal nerve palsy — left RLN hooks under the arch of aorta near the ligamentum arteriosum; involved by aortic aneurysm, bronchial carcinoma, mediastinal nodes (Ortner's syndrome = cardiovocal syndrome → hoarseness from left atrial enlargement / aortic pathology).
- Tracheo-oesophageal compression → stridor, dysphagia.
High-yield: A hoarse voice in a chest patient = think left recurrent laryngeal nerve (long course, hooks under the arch of aorta). The right RLN hooks under the right subclavian artery in the neck, so it is rarely affected by thoracic disease.
Recently asked / exam angle
- "Plane separating superior from inferior mediastinum passes through?" → Sternal angle / lower border of T4 (T4–T5 disc).
- "Structure NOT in the superior mediastinum?" → distractors like heart (middle), descending aorta (posterior).
- "Commonest anterior mediastinal tumour associated with myasthenia gravis?" → Thymoma.
- "Commonest posterior mediastinal tumour?" → Neurogenic (schwannoma/neurofibroma).
- "Narrowest part of oesophagus / distance of cricopharyngeal sphincter from incisors?" → ~15 cm, cricopharyngeus.
- "Commonest cause of SVC obstruction?" → Bronchogenic (small-cell) carcinoma.
- "Thoracic duct crosses midline at?" → T4/T5. Injury above → left chylothorax.
- "Level of tracheal bifurcation (carina)?" → T4/T5 (deeper, ~T6 in expiration on imaging is a known nuance).
- "Phrenic vs vagus at lung root?" → Phrenic anterior, vagus posterior.
- "Diaphragmatic openings levels?" → IVC T8, oesophagus T10, aorta T12.
- "Mediastinum shifts away from lesion in?" → Tension pneumothorax / massive effusion.
- Image-based: lateral CXR with retrosternal opacity = anterior mediastinal mass → think 4 Ts.
Rapid revision
- Mediastinum = everything in thorax except the lungs; divided by the sternal angle (T4/5) plane into superior + inferior (anterior/middle/posterior).
- Superior mediastinum order (front→back): thymus → veins → arch of aorta + 3 branches → trachea → oesophagus → thoracic duct → sympathetic chain.
- Anterior = 4 Ts (Thymoma, Teratoma, Thyroid, Terrible lymphoma); thymoma ↔ myasthenia gravis.
- Middle mediastinum = pericardium + heart + great-vessel roots + carina + phrenic nerves.
- Posterior = oesophagus, descending aorta, azygos/hemiazygos, thoracic duct, sympathetic chain; masses are mostly neurogenic.
- Phrenic nerve anterior, vagus posterior to lung root; phrenic = C3,4,5.
- Thoracic duct crosses midline at T4/5; injury above → left chylothorax, below → right chylothorax; drains into left venous angle.
- Oesophageal constrictions: 15 cm (cricopharyngeal, narrowest), 22.5 cm (broncho-aortic), 40 cm (diaphragmatic).
- Diaphragm openings: IVC T8, oesophagus + vagi T10, aorta + thoracic duct + azygos T12 ("I ate ten eggs at twelve").
- SVC obstruction — commonest cause = bronchogenic (small-cell) carcinoma; look for Pemberton's sign.
- Mediastinal shift: collapse/fibrosis pull towards; tension pneumothorax / massive effusion push away.
- CECT thorax = imaging of choice; MRI for posterior/neurogenic (dumb-bell) tumours; left RLN hooks under arch of aorta → hoarseness (Ortner's).