Diaphragm

Dome-shaped musculotendinous sheet separating the thoracic and abdominal cavities; the primary muscle of respiration.

• Composed of peripheral muscular fibres (sternal, costal, and lumbar parts, named by their origin) converging on a central, non-contractile central tendon.
• The right dome sits higher than the left (typically at the level of the 5th rib/intercostal space anteriorly on the right vs the 5th–6th on the left), due to the underlying bulk of the liver.

• Origin: Two small slips from the posterior surface of the xiphoid process.
• Smallest and least substantial of the three muscular origins.

• Origin: Inner surfaces of the lower six costal cartilages and their adjoining ribs (ribs 7–12), interdigitating with the origin of transversus abdominis.
• Largest muscular component, forming the main peripheral muscular rim.

• Origin: Via two tendinous crura (from the anterolateral surfaces of the upper lumbar vertebral bodies and intervertebral discs) and two arcuate ligaments (thickenings of fascia over psoas major and quadratus lumborum).
• Right crus: Larger and longer, arising from the bodies of L1–L3 vertebrae; its fibres split to encircle the oesophagus (contributing a sphincteric action at the oesophageal hiatus) before most fibres continue to the central tendon.
• Left crus: Smaller, arising from the bodies of L1–L2 vertebrae only.
• Both crura are connected anteriorly by the median arcuate ligament, which arches over the aorta.

• Trefoil (three-leaflet)-shaped, strong aponeurotic sheet into which all the muscular fibres converge and insert.
• Fused with the fibrous pericardium above (the pericardium is attached to the central tendon), a key relationship for understanding the diaphragm-heart interface.
• Non-contractile: contraction of the surrounding muscular fibres pulls the central tendon downward, flattening the dome and increasing thoracic volume.

• Median arcuate ligament: Connects the two crura anteriorly, arching over the aorta as it passes behind the diaphragm (at the aortic hiatus).
• Medial arcuate ligament: Thickened fascia over the upper part of psoas major, extending from the vertebral body to the transverse process of L1 (or L2).
• Lateral arcuate ligament: Thickened fascia over quadratus lumborum, extending from the transverse process of L1 to the 12th rib.
• These ligaments provide origin for diaphragmatic muscle fibres and bridge over the psoas major and quadratus lumborum muscles as they pass from the abdomen into the posterior thoracic/retroperitoneal region.

Mnemonic for levels: "I8, O10, At T12", or simply recall the numerical pattern 8, 10, 12 corresponds to vena cava, oEsophagus, Aorta in that order (alphabetical/ascending order of both letter count and level).

• Caval opening: Lies within the central tendon itself (not muscular), which keeps it patulous (held open) during diaphragmatic contraction, this actually augments venous return to the heart during inspiration, since the opening widens as the central tendon is pulled taut, rather than constricting like a muscular hiatus would.
• Oesophageal hiatus: Surrounded by muscular fibres of the right crus (forming a sling), which contract during inspiration, assisting the lower oesophageal sphincter in preventing reflux, a physiological anti-reflux mechanism that can be disrupted in hiatus hernia.
• Aortic hiatus: The most posterior and fixed of the three openings, lying anterior to the T12 vertebral body, behind the median arcuate ligament rather than through muscular diaphragm tissue proper, the aorta is therefore not compressed by diaphragmatic muscular contraction, appropriately preserving unimpeded systemic blood flow during respiration.

• Greater, lesser, and least splanchnic nerves: pierce the crura directly (typically the greater and lesser through the crus on their respective side).
• Sympathetic trunk: passes posterior to the medial arcuate ligament (deep to it), continuing into the abdomen.
• Superior epigastric vessels: pass through the sternocostal triangle (a small gap between the sternal and costal muscular origins, also known as the space of Larrey), a potential site of diaphragmatic hernia.
• Musculophrenic vessels and small lymphatic channels also cross the diaphragm at various points near its costal margin.

Phrenic nerves:

• Roots: Primarily C4, with contributions from C3 and C5 ("C3, 4, 5 keep the diaphragm alive").
• Arise from the cervical plexus, descend through the neck and thorax (anterior to the root of the lung, between the fibrous pericardium and mediastinal pleura) to reach the diaphragm.
• Sole motor supply to the diaphragm: each hemidiaphragm is supplied only by its own ipsilateral phrenic nerve (no crossover), so a unilateral phrenic nerve lesion paralyses only that hemidiaphragm.

Peripheral sensory supply:

• Central diaphragm (including the pleura and peritoneum immediately overlying it), phrenic nerve (C3–C5); explains referred pain to the shoulder tip (C3–C5 dermatome) from central diaphragmatic irritation (e.g., subphrenic abscess, splenic/hepatic injury with blood tracking under the diaphragm, or referred cardiac pain in some presentations).
• Peripheral diaphragm (costal and some lumbar parts), lower intercostal nerves (roughly T6–T12/subcostal), providing localised sensation corresponding to the overlying chest/abdominal wall segment rather than referred shoulder pain.

• Rich anastomotic network between these sources ensures collateral perfusion of this constantly active muscle.

Inspiration:

• Primary muscle of quiet respiration, responsible for roughly 70–80% of the tidal volume during normal quiet breathing.
• Contraction flattens the dome, pulling the central tendon inferiorly, increasing the vertical (craniocaudal) dimension of the thoracic cavity and lowering intrathoracic pressure, drawing air in.

Abdominal pressure:

• Diaphragmatic contraction, combined with anterior abdominal wall muscle contraction, increases intra-abdominal pressure: essential for the Valsalva manoeuvre, defecation, micturition, childbirth (bearing down), vomiting, and heavy lifting.
• This dual respiratory and abdominal-pressure role means diaphragmatic dysfunction affects far more than breathing alone.

Phrenic nerve injury:

• Can occur from mediastinal tumours/lymphadenopathy compressing the nerve as it runs close to the pericardium, cardiothoracic surgery (particularly cardiac surgery near the pericardium), neck trauma/surgery (its cervical origin), or iatrogenic injury during central line placement.
• Causes unilateral diaphragmatic paralysis, since each side is supplied independently.

Diaphragmatic paralysis:

• Unilateral paralysis is often relatively well tolerated at rest (the contralateral hemidiaphragm compensates significantly), but causes dyspnoea on exertion or when lying flat (orthopnoea), given loss of the affected side's contribution.
• Characteristically shows paradoxical upward movement of the paralysed hemidiaphragm during inspiration (rather than the normal downward movement), visible on fluoroscopy ("sniff test"), passively pushed upward by increased abdominal pressure and the normal side's descent, rather than actively contracting.
• Bilateral paralysis is far more serious, causing significant respiratory compromise and often requiring ventilatory support.

Hiatus hernia:

• Herniation of part of the stomach through the oesophageal hiatus into the thoracic cavity.
• Sliding hiatus hernia (most common, ~95%), the gastro-oesophageal junction itself slides upward through the hiatus, disrupting the normal anti-reflux mechanism (loss of the right crus sling effect and the angle of His) and predisposing to gastro-oesophageal reflux disease.
• Rolling (paraoesophageal) hiatus hernia (less common), the gastro-oesophageal junction remains in its normal position, but the fundus of the stomach rolls up alongside the oesophagus through the hiatus; carries a higher risk of strangulation/volvulus given the abnormal anatomical positioning, even though reflux symptoms may be less prominent than in the sliding type.