Diaphragm and Thoracic Inlet
On this page
Direct answer
"I 8 (ate) 10 eggs at 12" is the mnemonic generations of Indian MBBS students have used for the diaphragm's three great openings: the inferior vena cava at T8 through the central tendon, the oesophagus at T10 through the right crus accompanied by the vagal trunks and the oesophageal branches of the left gastric artery, and the aorta at T12 behind the median arcuate ligament together with the thoracic duct and azygos vein. The phrenic nerve (C3, 4, 5 — "keeps the diaphragm alive") supplies all motor fibres and the central sensation, while the lower six intercostal nerves carry sensation from the periphery, which is why diaphragmatic irritation refers pain to the shoulder tip through C4. Above, the thoracic inlet — bounded by the T1 vertebra, the first pair of ribs and the upper border of the manubrium — transmits the trachea, oesophagus, thoracic duct, both recurrent laryngeal nerves and the great vessels of the neck.
What you must remember
- Opening contents in full: caval opening (T8) — inferior vena cava and branches of the right phrenic nerve; oesophageal hiatus (T10, in the right crus) — oesophagus, anterior and posterior vagal trunks, oesophageal branches of the left gastric vessels (the portosystemic anastomosis of varices); aortic hiatus (T12, behind the median arcuate ligament) — aorta, thoracic duct and azygos vein.
- Phrenic nerve: C3, C4, C5 roots, on the anterior surface of scalenus anterior, motor to the whole diaphragm; irritation of the diaphragmatic peritoneum or pleura refers pain to the shoulder tip (C4 dermatome), the classic sign of a ruptured spleen or subphrenic collection.
- Embryology of the diaphragm: central tendon from the septum transversum, pleuroperitoneal membranes, body wall and oesophageal mesentery; failure of pleuroperitoneal closure gives the left posterolateral Bochdalek hernia (commonest) with pulmonary hypoplasia, while the Morgagni hernia is anterior and right-sided.
- Thoracic inlet boundaries: body of T1 posteriorly, medial border of the first ribs with their cartilages laterally, upper border of the manubrium anteriorly — a kidney-shaped aperture inclined so the lung apices rise above the clavicle.
- Structures crossing the inlet: trachea and oesophagus between trachea and vertebrae with the thoracic duct and left recurrent laryngeal nerve, common carotid and subclavian arteries, internal jugular veins, vertebral arteries, sympathetic trunks and lymphatics.
- Scalenus anterior on the first rib: the subclavian vein and phrenic nerve pass anterior to it; the subclavian artery and brachial plexus pass between scalenus anterior and medius — compression here is the thoracic outlet syndrome of the lower trunk (C8–T1).
- Signs of paralysis: a raised hemidiaphragm on the chest radiograph with paradoxical ascent on sniffing (fluoroscopy); eventration is congenital elevation with thinned but intact muscle.
A walk along the hiatus
Follow the oesophagus from the mediastinum into the abdomen and the anatomy explains three diseases. At T10 the oesophagus passes through the right crus, and the sling fibres around it form the functional lower oesophageal sphincter; a deficient hiatus yields a sliding hiatus hernia, which the screening paper describes as a retrocardiac air-fluid level. The vagal trunks — right posterior, left anterior, reorganised from the oesophageal plexus — accompany it and must be identified during vagotomy and fundoplication. Most consequentially, the oesophageal branches of the left gastric vein pass with the tube: they are part of the portocaval anastomosis, and in portal hypertension they dilate into oesophageal varices that present as haematemesis.
Traps around the openings
Two confusions cost marks. The first is mixing the companions of each hiatus — the duct travels with the aorta at T12, not with the oesophagus, while the left gastric vessels travel with the oesophagus, not the aorta; NBE builds entire option lists from these borrowed companions. A second trap is the phrenic-peripheral split: shoulder-tip referral is phrenic (C4), while rib-margin pain on inspiration is intercostal. Remember also that the right dome stands about half a rib higher than the left because of the liver, so a flat right hemidiaphragm on a radiograph is abnormal, while slight right dominance is not.
Frequently asked questions
What passes through the oesophageal hiatus and at what vertebral level?
The oesophagus, the anterior and posterior vagal trunks and the oesophageal branches of the left gastric artery and vein, at the level of T10 through the right crus.
What is the root value and distribution of the phrenic nerve?
C3, C4 and C5; it is the sole motor supply of the diaphragm and carries sensation from the central diaphragm and the pericardium and mediastinal pleura, with referred pain to the shoulder tip.
Why does diaphragmatic irritation cause shoulder-tip pain?
Because the diaphragmatic peritoneum and central diaphragm share the C4 segment with the skin over the shoulder tip, both served through the phrenic nerve and supraclavicular nerves.
What are the boundaries of the thoracic inlet?
Posteriorly the body of the first thoracic vertebra, laterally the medial borders of the first ribs and their costal cartilages, and anteriorly the upper border of the manubrium sterni.
Which is the commonest congenital diaphragmatic hernia?
The left posterolateral Bochdalek hernia, caused by failure of the pleuroperitoneal membrane to close, presenting with respiratory distress and bowel sounds in the chest.