Last revised by Craig Hacking on 27 Feb 2024

The diaphragm is the dome-shaped skeletal muscle that separates the thoracic cavity from the abdominal cavity, enclosing the inferior thoracic aperture.

On chest imaging, in particular chest radiography, an imaginary anteroposterior midline divides the diaphragm into two halves, forming the left and right hemidiaphragms. The hemidiaphragms are purely descriptive terms and are not discrete anatomical structures.

The muscular fibers of the diaphragm originate around the circumference of the inferior thorax and converge to a common insertion point of the central tendon.

The muscle slips can be grouped according to their origins:

  • sternal: arise from two strips under the xiphoid process

  • costal: arise from the inner surfaces of the lower six costal cartilages and adjoining ribs, interdigitating with the transversus abdominis muscle

  • lumbar: arise from the aponeurotic arches (lumbocostal arches) and from the lumbar vertebrae (forming the crura)

All these muscles insert into the central tendon, a thin but strong aponeurosis. It is situated immediately below and is fused to the pericardium. It is within this central tendon that the vena caval hiatus is located, with the tendon allowing the inferior vena cava (IVC) to remain patent during respiration.

There are two paired posterior tendinous lumbocostal arches:

  • medial lumbocostal arch (medial arcuate ligament): a tendinous arch from the superior anterior thickened psoas major fascia; continuous medially with the ipsilateral crus; attached to the L1/L2 anterolateral vertebral body, and the anterior aspect of the L1 transverse process

  • lateral lumbocostal arch (lateral arcuate ligament): covers quadratus lumborum muscle; attaches medially to the L1 transverse process and attaches laterally to the tip of the 12th rib; this may be discontinuous on CT in up to 11% of people and hence may mimic diaphragmatic rupture 8

The crura are paired tendinous structures of the posterior diaphragm that blend with the anterior longitudinal ligament of the vertebral column:

  • right crus is longer and broader than the left, and arises from the anterior surfaces of the bodies of L1-3

  • left crus arises from the corresponding portions of L1-2

The medial margins of the two crura pass forwards and medially. They meet in the midline to form an arch in front of the aorta called the median arcuate ligament.

Through the diaphragm are a series of three major and some minor apertures that permit the passage of structures between the thoracic and abdominal cavities:

The vertebral levels of the three main diaphragmatic apertures can be remembered by this mnemonic.

  • right inferior phrenic vein into the inferior vena cava (IVC)

  • left inferior phrenic vein into the left suprarenal vein or left renal vein

  • each phrenic nerve (C3-C5) provides the sole motor supply to the ipsilateral hemidiaphragm 9,10

  • sensory innervation is to the central tendon region is supplied by the phrenic nerves 9,10

  • the lower 5-6 intercostal nerves supply proprioceptive fibers to the margins of the diaphragm 4

  • sensation from the peripheral diaphragmatic pleura is supplied by the lower 5-6 intercostal nerves whereas the central pleura is from the phrenic nerve

The phrenic nerve pierces the diaphragm and on its inferior surface divides into several radiating branches to supply it.

  • major role of the diaphragm is inspiratory, but it is also used in abdominal straining

The right dome of diaphragm is usually 2cm higher than the left dome 11.

Diaphragm is seen as echogenic line covering the upper surface of liver and spleen 11.

The diaphragm embryologically develops from four main sources:

  • septum transversum

    • produces most of the central tendon and contributes to the ventral mesentery in the gut

  • cervical myotomes (3rd to 5th):

    • infiltrates the septum transversum with muscle cells

    • carries their own nerve supply from these levels explaining the C3-C5 origin of the phrenic nerve

  • pleuroperitoneal membrane

    • mesodermal folds which connect the septum transversum to the pericardioperitoneal canals

    • separates the peritoneal and pleuropericardial cavities

  • dorsal esophageal mesentery

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