Diaphragm

Domed fibromuscular sheet that separates the thoracic and abdominal cavities. It's purpose is essentially for inspiration.

Embryology:

• Septum transversum is an involved structure which is formed from mesoderm, and acts as a framework for diaphragmatic development. Forms most of the central tendon.
• Pleuroperitoneal membranes contribute by completely covering the septum transversum and therefore closing connection between thoracic and abdominal parts of the coelom.
• The septum transversum is also invaded by muscle cells derived from the third, fourth and fifth cervical myotomes - meaning they bring the motor supply from the phrenic nerves with them
• Oesophageal mesentery also contributes to the development of the diaphragm by forming the crura

Structure

• Arises from the posterior surface of the xiphoid process, the inner sides of the lowest six ribs, from the fascial arches (medial and lateral arcuate ligaments) and from the upper lumbar vertebrae via the crura. The muscle fibres pass centrally to insert on the central tendon.
• Crura of the diaphragm - musculotendinous structures, embryologically distinct from the remainder of the diaphragm
  • Arise from the anterior surfaces and discs of L1-L3, as well as the anterior longitudinal ligament of the spine
  • The right crus tends to reach slightly lower in its origin from the first three lumbar vertebrae
  • Left crus only from L1 and L2
  • Tendinous fibres are more posteriorly - these parts hold sutures best, so hiatus hernia repair works best posteriorly
  • Quite variable arrangement of fibres at the crura
• Central tendon
  • Interlaced fibres
  • Inseparable from the fibrous pericardium
• Posterior ligaments
  • Median arcuate ligament - tendinous fibres from the medial edge of each crus unite with each other in front of the aorta at T12
  • Medial arcuate ligament
    • Thickening of the psoas fascia
    • Extends from the lower part of the body of the L1/2 vertebrae to a ridge on the anterior surface of the transverse process of L1 (at the lateral border of psoas)
    • Sympathetic trunk passes behind
  • Lateral arcuate ligament
    • Thickening of the anterior layer of lumbar fascia
    • From the transverse process of L1 to the 12th rib (at the lateral border of quadratus lumborum)
    • Subcostal nerve and vessels pass behind

Function

• Contracts during inspiration, which flattens the diaphragm and causes the chest cavity to enlarge
• This creates a vacuum pulling air into the lungs
• During exhalation, the diaphragm relaxes and returns to a dome-like shape, and air is released

Openings:

• Vena caval foramen (T8, just to the right of midline, behind the sixth right costal cartilage, between middle and right leaves of the central tendon)
  • IVC
  • Right phrenic nerve
• Oesophageal opening (T10, usually 2.5cm to left of midline, adjacent to the posterior border of the central tendon, formed in the fibres of the right crus, although can be either crus)
  • The tissue which forms the inferior margin of the oesophageal hiatus also forms the median arcuate ligament across the front of the aorta. This inferior aspect does hold sutures well, but may pick up part of the coeliac plexus.
  • Contents
    • Oesophagus
    • Vagal trunks
    • Oesophageal branches of left gastric artery, veins and lymphatics
  • Technically we should say 'left pillar' and 'right pillar' for the pillars on either side, not 'left and right crus', since it is all 'right crus' in most people
• Aortic opening (T12, in the midline, behind the median arcuate ligament, between the two crura)
  • Aorta
  • Azygos vein to right
  • Thoracic duct leading up from the cisterna chyli in between them
• Other openings:
  • Behind medial and lateral arcuate ligaments
  • Sternocostal foramina
• Other structures piercing the diaphragm
  • Left phrenic nerve through the left dome
  • Hemiazygos vein through the left crus
  • Splanchnic nerves through each crus
  • Sympathetic trunk behind the medial arcuate ligament
  • Subcostal nerve and vessels behind the lateral arcuate ligament
  • Superior epigastric vessels between xiphisternal and costal fibres of the diaphragm

Blood supply

• Mainly inferior phrenic arteries from the aorta just beneath the diaphragm, and entering from abdominal surface
• Superior phrenic arteries are smaller, coming from aorta in thorax
• Pericardiophrenic arteries (internal thoracic) supply the phrenic nerve, pleura and fibrous pericardium
• Anterior blood supply from musculophrenic arteries which are terminal branches of the internal thoracic arteries
• Lower five intercostal arteries and subcostal arteries supply the costal margin

Nerve supply:

• Motor supply is solely from the phrenic nerves
  • Arise from C3, 4 and 5; but mostly C4. Passes inferiorly over the anterior surface of anterior scalene, deep to pre-vertebral fascia. Posterior to subclavian vein and anterior to subclavian artery.
    • Right: descends anteriorly over right lung root; courses along the pericardium of the right atrium; pierces diaphragm at IVC opening
    • Left: descends anteriorly over left lung root; crosses aortic arch and bypasses the vagus nerve; courses along pericardium of LV; pierces the left hemidiaphragm
  • Each half of the diaphragm is supplied by its own phrenic nerve, even the fibres of the right crus that loop to the left around the oesophageal opening are supplied by the left phrenic
  • On reaching the abdominal surface of the diaphragm, both nerves divide into anterior, lateral and posterior branches which run radially, giving off branches that enter the muscle from below
  • Phrenic variants:
    • Pierced scalene
    • Anterior to subclavian artery
    • Additional branches from cervical nerve roots
    • Accessory phrenic nerve from C5 and C6
• Sensory is from intercostal nerves peripherally and phrenic nerves centrally
  • Pain referred to shoulder-tip

Congenital herniae:

• Bochdalek hernia - posteriorly placed, most often seen on left, caused by failure of pleuroperitoneal membrane development
• Morgagni's foramen - junction of costal and xiphoid origins
• See separate topic under 'thoracics'

Incisions in diaphragm

• Radial incisions are less likely to denervate, so are preferred
• Circumferential incisions close to rib cage are unlikely to cause problems
• If dividing right crus, stay posteriorly to stay clear of left hepatic vein
• Left crus can be divided if needed for access