Diaphragm anatomy

Structure and attachments

• A musculotendinous dome separating the thoracic and abdominal cavities, with a trefoil central tendon fused above to the fibrous pericardium. In expiration the right dome reaches about the 5th rib (pushed up by the liver) and the left dome the 5th intercostal space.
• Origins: sternal (back of the xiphoid), costal (inner surfaces of the lower six costal cartilages and ribs) and lumbar (crura and arcuate ligaments).
• Right crus arises from the bodies of L1–L3, the left crus from L1–L2. Fibres of the right crus loop round the oesophagus.

Openings

• The caval opening lies in tendon, so contraction in inspiration stretches it open, helping venous return.
• The muscular oesophageal hiatus is pinched shut in inspiration and acts as an external sphincter, aiding the lower oesophageal sphincter against reflux.
• The aortic hiatus is not muscular, so diaphragmatic contraction does not compress the aorta. An aneurysm or mass at this hiatus would compress the structures beside the aorta, chiefly the thoracic duct and azygos vein.
• Other routes: greater, lesser and least splanchnic nerves pierce the crura; the sympathetic trunk passes behind the medial arcuate ligament; the superior epigastric vessels pass between the sternal and costal origins (Larrey space); the left phrenic nerve pierces the left dome; lower intercostal nerves enter from the periphery.
• Median arcuate ligament syndrome: a low-lying ligament compresses the coeliac trunk, causing post-meal abdominal pain.

Remember IVC at T8 through tendon, oesophagus with the vagi at T10, aorta with the thoracic duct at T12.

Nerve supply

• Motor: the phrenic nerves (C3, C4, C5) only, each supplying its own half. The diaphragm is a classic example of a muscle with a single motor nerve.
• Sensory: the phrenic nerves also carry sensation from the central diaphragm, its pleura and peritoneum, and the pericardium; the lower intercostal nerves supply the peripheral rim.
• Referred shoulder-tip pain: irritation of the central diaphragm (blood, gas after laparoscopy, subphrenic pus, a ruptured spleen) is carried by the phrenic nerve to C3–C4 and felt in the skin of the shoulder tip, which is supplied by the supraclavicular nerves (C3, C4) from the cervical plexus. Peripheral irritation is referred to the abdominal wall.
• Course of the phrenic nerve: formed at the lateral border of scalenus anterior, it descends on the front of scalenus anterior, enters the thorax behind the subclavian vein, then runs in front of the root of the lung on the lateral surface of the fibrous pericardium with the pericardiacophrenic artery (from the internal thoracic artery). In a median sternotomy or thymectomy, diathermy near this slender artery can injure the nerve.
• Diaphragmatic pleura is supplied centrally by the phrenic nerve and peripherally by the intercostals.

Paralysis

• Unilateral phrenic palsy (stab wound, tumour, cardiac surgery, avulsion at its origin): the hemidiaphragm is raised on X-ray and moves paradoxically upwards on sniffing (sniff test); often little breathlessness at rest.
• Bilateral diaphragmatic paralysis (neuralgic amyotrophy after a viral illness with severe shoulder pain, motor neurone disease, high cervical injury): severe orthopnoea, both hemidiaphragms raised with clear lungs and normal heart, and paradoxical inward movement of the abdomen on inspiration when supine. Vital capacity falls by over 25–30% from sitting to lying because the abdominal contents push the flaccid domes into the chest. Orthopnoea is not always cardiac.
• Cord injury above C3 abolishes diaphragmatic breathing and needs ventilation.

Blood supply

Superior phrenic (thoracic aorta), inferior phrenic (abdominal aorta; also supply the adrenals), musculophrenic and pericardiacophrenic (internal thoracic), and the lower intercostal arteries.

Development

The septum transversum forms in the neck region and its muscle comes from cervical myotomes, which is why the nerve supply is C3–C5 despite the diaphragm's final position.

Hernias

Recesses and subphrenic spaces

• The costodiaphragmatic recess is the slit between the costal and diaphragmatic parietal pleura, below the lower lung border. It reaches the 8th rib in the midclavicular line, the 10th rib in the mid-axillary line and the 12th rib at the back. In splenic or renal surgery through the posterior loin, the pleura can be opened behind the 12th rib.
• Below the diaphragm, the falciform ligament divides the anterior subphrenic (suprahepatic) space into right and left spaces; the coronary and triangular ligaments limit the space behind. Pus after a perforated ulcer or appendicitis collects in these spaces and causes shoulder-tip pain.

Exam trap The IVC passes at T8 through the central tendon; the thoracic duct passes with the aorta at T12.

Exam trap Shoulder-tip pain travels in the phrenic nerve but is felt in the skin of the supraclavicular nerves (C3–C4).

How FCPS asks about this

• Contents of each opening: IVC and right phrenic branches at T8; oesophagus and vagal trunks at T10; aorta, thoracic duct and azygos at T12.
• Vagus passes the diaphragm at: T10 with the oesophagus.
• Shoulder-tip pain after laparoscopic cholecystectomy: phrenic nerve afferents; skin supplied by supraclavicular nerves (C3, C4).
• Artery running with the phrenic nerve on the pericardium: pericardiacophrenic artery.
• Breathless on lying flat, abdomen moving in on inspiration, raised domes, clear lungs: bilateral diaphragmatic paralysis.
• Cause of congenital diaphragmatic hernia: defective pleuroperitoneal membrane, left posterolateral (Bochdalek).
• Structure dividing the right and left anterior subphrenic spaces: falciform ligament.