Q1
LONG ANSWER
10M
★ High-yield
Define the mediastinum and describe its divisions with their contents. Add a note on the structures at the level of the sternal angle.
Definition
The mediastinum is the space in the thoracic cavity between the two pleural sacs, containing all the thoracic viscera except the lungs.
| Boundary | Structure |
|---|---|
| Anterior | Sternum and costal cartilages |
| Posterior | Twelve thoracic vertebrae |
| Lateral | Mediastinal pleura on each side |
| Superior | Thoracic inlet |
| Inferior | Diaphragm |
Divisions
Superior Mediastinum — Contents
| Category | Structures |
|---|---|
| Arteries | Arch of the aorta and its three branches — brachiocephalic, left common carotid, left subclavian |
| Veins | Right and left brachiocephalic veins, upper half of the superior vena cava, arch of the azygos |
| Nerves | Vagus, phrenic, left recurrent laryngeal, cardiac nerves, sympathetic chain |
| Viscera | Thymus, trachea, oesophagus |
| Other | Thoracic duct, lymph nodes, areolar tissue |
- Arranged in four layers from front to back — thymus, veins, arteries, then trachea and oesophagus
Inferior Mediastinum — Contents
| Division | Contents |
|---|---|
| Anterior | Remnant of the thymus, lymph nodes, fat, sternopericardial ligaments, branches of the internal thoracic vessels |
| Middle | Heart within the pericardium; roots of the great vessels; phrenic nerves with the pericardiacophrenic vessels; bifurcation of the trachea and the two main bronchi; pulmonary vessels; tracheobronchial lymph nodes |
| Posterior | Descending thoracic aorta, oesophagus, thoracic duct, azygos and hemiazygos veins, vagus nerves, splanchnic nerves, sympathetic chain, lymph nodes |
Structures at the Level of the Sternal Angle (T4/T5)
- Beginning and end of the arch of the aorta
- Bifurcation of the trachea at the carina
- Ligamentum arteriosum, and the left recurrent laryngeal nerve hooking round it
- Arch of the azygos vein entering the superior vena cava
- Boundary between superior and inferior mediastinum
- Second costal cartilage articulates here — the starting point for counting ribs
- Thoracic duct crosses from right to left
- Superficial and deep cardiac plexuses
- Mnemonic — "ratplant", or simply remember it is the busiest plane in the thorax
Relations of the Oesophagus and its Constrictions
| Constriction | Distance from the incisor teeth | Cause |
|---|---|---|
| Cricopharyngeal (upper) | 15 cm | Cricopharyngeus — the narrowest point |
| Aortic | 22.5 cm | Arch of the aorta |
| Bronchial | 27.5 cm | Left main bronchus |
| Diaphragmatic | 40 cm | Oesophageal hiatus |
- Foreign bodies lodge and strictures form at these constrictions, and they are the sites of difficulty in passing an instrument
The Trachea and its Relations
- Extends from the lower border of the cricoid cartilage (C6) to the carina (T4/T5); about 10–12 cm long
- Kept patent by 15–20 C-shaped cartilages, deficient posteriorly where the trachealis muscle lies against the oesophagus
- Anterior relations in the neck — isthmus of the thyroid over the 2nd to 4th rings, inferior thyroid veins, jugular arch
- Lateral — lobes of the thyroid, common carotid arteries, recurrent laryngeal nerves in the tracheo-oesophageal groove
- In the thorax — the arch of the aorta and the brachiocephalic artery in front, the oesophagus behind
- Tracheostomy is performed through the 2nd to 4th rings, after retracting or dividing the thyroid isthmus
Applied Aspects
- Mediastinal shift — toward the lesion in collapse and fibrosis; away from it in pleural effusion and tension pneumothorax. Assessed by the position of the trachea and the apex beat
- Superior vena caval obstruction — usually from bronchial carcinoma or lymphoma; causes engorged non-pulsatile neck veins, facial oedema and dilated chest wall veins
- Mediastinitis is dangerous — infection tracks down from the neck along the fascial planes, or follows oesophageal perforation
- Anterior mediastinal masses — the four Ts: Thymoma, Teratoma, Thyroid (retrosternal), Terrible lymphoma
- Mediastinoscopy through a suprasternal incision samples the paratracheal and tracheobronchial nodes in staging lung cancer
- Pneumomediastinum — air in the mediastinum from oesophageal or airway rupture; gives surgical emphysema in the neck and Hamman crunch on auscultation
- Retrosternal goitre may compress the trachea and cause stridor; the thyroid descends into the superior mediastinum along the pretracheal fascia
All questions in Thorax
- Q1Define the mediastinum and describe its divisions with their contents. Add a note on the structures at the level of the sternal angle. ★
- Q2Describe the chambers of the heart, its conducting system and its blood supply. Add a note on the surface marking of the valves. ★
- Q3Describe the lungs with their lobes, fissures and root. Explain bronchopulmonary segments and add a note on the pleura. ★
- Q4Describe the diaphragm under origin, openings, nerve supply and actions. Add a note on its applied anatomy. ★
- Q5Describe the intercostal space with its muscles, vessels and nerves. Add a note on the thoracic cage and applied aspects. ★
- SN1Describe the coronary circulation with its branches and peculiarities. ★
- SN2Describe the pericardium with its layers and sinuses. Add a note on cardiac tamponade. ★
- SN3Describe the course and distribution of the phrenic nerve. Explain the referred pain to the shoulder. ★
- SN4Describe the azygos system of veins and its clinical importance. ★
- SN5Describe the course and drainage of the thoracic duct. Add a note on chylothorax. ★
- SN6Describe the formation, course and relations of the superior vena cava. Add a note on SVC obstruction. ★
- SN7Describe the breast with its lymphatic drainage. Add a note on the surface anatomy of the thorax.