What is the diaphragm and what does it do?
The thoracic diaphragm is a dome-shaped fibromuscular sheet that separates the thorax from the abdomen. Its convex upper surface forms the floor of the thoracic cavity and its concave under-surface forms the roof of the abdominal cavity. It is the primary muscle of inspiration: on contraction it pulls the central tendon down, increases the vertical diameter of the thorax and lowers intrathoracic pressure so that air is drawn in. It is slightly asymmetric — the left dome sits a little lower than the right.
Beyond breathing, the diaphragm helps the anterior abdominal muscles raise intra-abdominal pressure during micturition, defecation and parturition, and during the Valsalva manoeuvre. It also acts as a thoraco-abdominal pump: descent of the diaphragm compresses the inferior vena cava and abdominal lymphatics and helps push blood and lymph upward toward the heart and the thoracic duct.
- Shape: two domes (right and left hemidiaphragm) joined at a central tendon.
- Insertion: all muscle fibres converge on the central tendon, whose upper surface is partly fused to the fibrous pericardium.
- Main action: muscle of quiet inspiration; assists abdominal straining and venous and lymphatic return.
- Landmark role: the oesophagus, vagus and phrenic nerves, descending aorta and inferior vena cava all cross between chest and abdomen through it.
What are the openings of the diaphragm and their vertebral levels?
StatPearls describes three major and five minor openings. The three major ones, together with their levels and contents, are the most repeated anatomy question on this topic.
| Opening | Level | Position | Structures passing through |
|---|---|---|---|
| Caval opening (vena caval trunk) | T8 | In the central tendon | Inferior vena cava; some branches of the right phrenic nerve |
| Oesophageal hiatus | T10 | Within a sling of fibres from the right crus, to the left of the median plane | Oesophagus; right and left vagal trunks; oesophageal branches of the left gastric vessels; lymphatics |
| Aortic hiatus | T12 | In front of the body of T12, between the crura | Aorta; thoracic duct; azygos vein |
| Opening | Contents |
|---|---|
| Lesser aperture of the right crus | Greater and lesser splanchnic nerves |
| Lesser aperture of the left crus | Hemiazygos vein; greater and lesser splanchnic nerves |
| Behind the medial lumbocostal arch | Sympathetic trunk |
| Foramen of Morgagni — between the sternal and costal parts | Superior epigastric branch of the internal thoracic artery and lymphatics of the abdominal wall |
| Medial and lateral lumbocostal arches | Areolar tissue that, when present, separates the kidney from the pleura |
Where does the diaphragm arise — sternal, costal and lumbar parts?
The peripheral muscle arises from the circumference of the lower thoracic aperture in three groups and converges on the central tendon.
| Part | Origin |
|---|---|
| Sternal | Two fleshy slips from the back of the xiphoid process |
| Costal | Inner surfaces of the cartilages of the lower six ribs on each side, interdigitating with transversus abdominis |
| Lumbar — arches | Medial lumbocostal arch (fascia over psoas major, attached to the side of L1 and the front of its transverse process) and lateral lumbocostal arch (fascia over quadratus lumborum, from the transverse process of L1 to the lower border of the 12th rib) |
| Lumbar — right crus | Anterolateral surfaces of the bodies of the upper three lumbar vertebrae and the intervening discs |
| Lumbar — left crus | Corresponding parts of the upper two lumbar vertebrae |
The medial margins of the two crura form a tendinous arch in front of the aorta called the median arcuate ligament. Muscle fibres of the right crus pass up and to the left to encircle the oesophageal opening like a sling. These fibres act as a physiological sphincter that helps prevent regurgitation of gastric contents into the thoracic oesophagus.
What is the nerve and blood supply of the diaphragm?
| Type | Supply | Notes |
|---|---|---|
| Motor | Right and left phrenic nerves (C3 to C5) | The only motor supply — the whole muscle is paralysed if the phrenic nerve is interrupted |
| Sensory — central part | Phrenic nerve | Supplies the parietal pleura and peritoneum over the central surfaces |
| Sensory — periphery | Lower six intercostal nerves | Supplies the margins of the muscle |
- Arteries: musculophrenic artery (from the internal thoracic), superior phrenic artery (from the aorta), the lower five intercostal arteries with the subcostal artery, and the inferior phrenic artery.
- Veins and lymphatics follow the arteries; the thoraco-abdominal pump compresses abdominal lymph vessels on descent and helps drive lymph up the thoracic duct.
- Phrenic nerve injury: a paralysed hemidiaphragm is pulled up into the chest by negative intrathoracic pressure and shows as an elevated hemidiaphragm on X-ray; unilateral weakness is commoner than bilateral. Compression by a thoracic mass, including lung cancer, is one cause. The right dome normally sits about one intercostal space higher than the left.
- Valsalva link: a held deep breath makes the diaphragm and abdominal muscles raise intra-abdominal pressure, a manoeuvre used to separate right- from left-sided murmurs.
How does the diaphragm develop in the embryo?
The adult diaphragm forms from four embryonic components: the septum transversum, the pleuroperitoneal membranes, the dorsal mesentery of the oesophagus and the mesoderm of the body wall. StatPearls gives the same four parts under diaphragm formation; each contributes a different region.
| Component | Forms (as described in the source) |
|---|---|
| Septum transversum | The anterior central tendon |
| Pleuroperitoneal membranes / folds | The posterolateral parts of the diaphragm, where Bochdalek hernias occur |
| Mesentery of the oesophagus | Contributes to the diaphragm; the pleuroperitoneal folds must fuse with it (and with the septum transversum) by about week 10 |
| Body wall mesoderm | Contributes to the diaphragm (listed as the fourth component) |
Closure of the pleuroperitoneal canals is the critical event. A congenital diaphragmatic hernia (CDH) arises from failed closure or fusion of the pleuroperitoneal membranes and other muscular components between 8 and 12 weeks of gestation. A Bochdalek hernia results when the pleuroperitoneal folds fail to fuse with the septum transversum and the dorsal mesentery of the oesophagus by about the 10th week.
What is a congenital diaphragmatic hernia and what are its types?
CDH is a defect that lets abdominal viscera (stomach, spleen, liver, bowel) enter the chest. The herniated organs compress the developing lung, leading to pulmonary hypoplasia (bilateral but worse on the side of the defect), abnormal remodelling of the pulmonary arterioles and pulmonary hypertension. The incidence is about 1 to 4 per 10,000 live births.
| Type | Site | Frequency / features |
|---|---|---|
| Bochdalek | Posterolateral (pleuroperitoneal fold) | 70–85% of CDH; about 80% are left-sided; intrapleural, so it compresses the lung and causes pulmonary hypoplasia |
| Morgagni | Anterior, parasternal (anteromedial) | About 2–5% of cases; a mediastinal hernia that typically does not cause pulmonary hypoplasia |
| Central tendon defect | Central | Rare; arises from the septum transversum |
| Hiatal hernia and eventration | Oesophageal hiatus / thin muscle sheet | Listed with the six anatomical types of CDH |
- Associations: about 10–15% have a chromosomal anomaly (for example trisomy 13, 18 or 21), and 30–40% have other congenital anomalies, mainly cardiac or central nervous system.
- Right-sided and bilateral defects are less common and carry a worse prognosis.
- Poor prognostic sign: herniation of the liver into the chest.
How is a diaphragmatic hernia diagnosed and managed?
With modern obstetric imaging, roughly 60–70% of CDH cases are diagnosed before birth, usually at the routine 18–24 week anatomy scan. Ultrasound may show abdominal organs in the chest, mediastinal shift, polyhydramnios or an absent stomach bubble in the abdomen. Fetal MRI measures lung volume more accurately.
| Step | Point to remember |
|---|---|
| Antenatal | Serial ultrasound and Doppler; delivery planned at a tertiary centre with neonatal surgery and ECMO; prognosis refined by observed-to-expected lung-to-head ratio and liver herniation |
| Neonatal stabilisation | Gentle ventilation is the cornerstone: preductal saturation 85–95%, peak inspiratory pressure below 25 cm H2O, PEEP 3–5 cm H2O, permissive hypercapnia (PaCO2 45–60 mmHg) |
| Pulmonary hypertension | Suspect with hypoxaemia and a difference between preductal and postductal saturation (right-to-left ductal shunt); echocardiography within 48 hours |
| Surgery | No longer an emergency: repair is usually done 48–72 hours after birth, once the infant is stable; defect closed with sutures or a patch |
What are the high-yield exam points on the diaphragm?
- Levels: IVC T8, oesophagus T10, aorta T12 — the single most repeated fact.
- Vagal trunks go through the oesophageal hiatus; thoracic duct and azygos go through the aortic hiatus; right phrenic branches go with the IVC.
- Phrenic nerve C3–C5 is the sole motor supply; periphery is sensory via lower six intercostal nerves.
- Right crus (L1–L3) forms the oesophageal sling; the median arcuate ligament crosses the aorta at the level of the crura.
- Bochdalek = posterolateral, most common (70–85%), usually left; Morgagni = anterior, rare, no lung hypoplasia.
- Septum transversum forms the central tendon; pleuroperitoneal folds the posterolateral part.
- The foramen of Morgagni transmits the superior epigastric vessels.