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Diaphragm anatomy

Diaphragm embryology

  • Develops from 4 sources
    1. Septum Transversum – gives rise to Central Tendon
    2. Pleuroperitoneal Membranes (Mesodermal folds) – close the connection between the thoracic and abdominal parts of the Coelom
    1. Oesophageal Mesentery – also contributes to the development of the Diaphragm
    2. Cervical Myotomes
      • Muscle cells derived from the 3rd, 4th and 5th Cervical Myotomes
      • Muscle cells carry their own nerve supply with them – hence the motor supply from the Phrenic Nerves
  • Failure of Pleuroperitoneal Membrane development – most common cause of Congenital Diaphragmatic hernia
    • The defect (Bochdalek’s Foramen) is posteriorly placed
    • Clinically manifests more often on left side probably due to the presence of the liver on the right
  • Morgagni’s Foramen – possible small hernia site at the junction of the costal and xiphoid origins

Overview

  • Derivative of the Inner (Transversus) layer of the muscles of the body wall
    • Fibres arise in continuity with Transversus Abdominis from within costal margin
  • Completed behind the costal margin by fibres arising from the Arcuate Ligaments and the Crura
  • From circumference of oval origin, fibres arch upwards into pair of domes and then descend to a Central Tendon which lies at the level of the xiphisternal joint
    • Right dome – ascends in full expiration as high as nipple line (4th space)
    • Left dome – ascends as high as 5th rib
  • Viewed from side: diaphragm resembles inverted J – long limb extending up from Crura (upper lumbar vertebrae) and the short limb attached to xiphisternum (T8 level)
  • About 55% of muscle fibres (and 65% of intercostal muscle fibres) are slow twitch fatigue-resistant type

Attachments

  • Sternal part
    • Arises from the posterior surface of the xiphoid process
  • Costal part
    • Arises from the inner surfaces of ribs 7–12 and their costal cartilages
  • Lumbar part
    • Right crus: from L1–L3 (or L4) vertebral bodies and intervertebral discs
    • Left crus: from L1–L2 (or L3) vertebral bodies and discs
    • Median arcuate ligament: spans over the aortic hiatus
    • Medial arcuate ligaments: arch over the psoas major
    • Lateral arcuate ligaments: arch over the quadratus lumborum
  • Insertion
    • All fibres converge into the central tendon of the diaphragm

Crura

  • Crura are the strong tendons attached to the anterolateral surfaces of the bodies of the upper lumbar vertebrae

  • Muscle fibres radiate from each crus, overlap, and pass vertically upwards before curving forwards into the central tendon

  • Median Arcuate Ligament

    • Formed from the union of tendinous fibres from the medial edge of each crus in front of the Aorta at level of T12

  • Medial Arcuate Ligament

    • Thickening of the Psoas Fascia
    • Extends from the side of the L1 or L2 vertebral body to a ridge on the anterior surface of L1 vertebra transverse process at the lateral margin of Psoas

  • Lateral Arcuate Ligament

    • Thickening of the anterior layer of the Lumbar Fascia on the front of Quadratus Lumborum
    • Extends across to the middle of the lower margin of the 12th rib

  • Central Tendon

    • Anteriorly situated middle leaf fused on each side to lateral leaves that extend back towards the paravertebral gutters
    • Inseparable from the fibrous pericardium – same embryological origin
    • Attached to the Falciform Ligament
    • NOT attached to the Phreno-Oesophageal Ligament

  • Right Crus

    • Fixed to upper 3 Lumbar vertebrae and intervening discs
    • Some fibres pass up on the abdominal surface of fibres from Left Crus and surround oesophageal orifice in a sling-like loop

  • Left Crus

    • Fixed to upper 2 Lumbar vertebrae

Openings in the Diaphragm

Aortic Opening

  • Opposite T12 vertebra
  • Midline
  • Behind Median Arcuate Ligament
  • Transmits:

Oesophageal Opening

  • Opposite T10 vertebra
  • 2.5cm to the left of midline
  • Behind the 7th Left Costal Cartilage
  • Lies in the fibres of the Left Crus
  • Sling of fibres from the Right Crus loop around it
  • Transversalis fascia on the under-surface of the diaphragm extends up through the opening and blends with the endothoracic fascia above the diaphragm
    • Attached to the diaphragm 2-3cm above the GOJ – i.e. the Phreno- oesophageal Ligament
    • Phreno-oesophageal Ligament– stretched in the sliding hiatus hernia
  • Transmits:
    • Oesophagus
    • Vagal Trunks
      • Anterior vagal trunk
        • Comprised mainly of left vagal fibres from the oesophageal plexus in the posterior mediastinum
          • Lies in contact with the anterior oesophageal wall, usually nearer to its right margin
      • Posterior vagal trunk
        • Comprised mainly of right vagal fibres
          • Lies in loose tissue a little behind and to the right, NOT in contact with the posterior surface of the oesophagus
    • Left Gastric Artery (Oesophageal branches)
    • Veins
      • Venous drainage – passes caudally to the Portal venous system and cranially to the Azygos venous system – major site of portal-systemic anastomosis
    • Lymphatics

Vena Caval Foramen

  • Opposite T8 vertebra
  • Right of the midline
  • Behind the 6th Right Costal Cartilage
  • Lies between the middle and right leaves of the Central Tendon; fibres of which fuse firmly with the adventitial wall of the IVC
  • Transmits:
    • Inferior Vena Cava
  • Right Phrenic Nerve passes through Central Tendon alongside IVC at its opening

Other Structures and Openings

  • Hemiazygos Vein – passes through Left Crus
  • Greater, Lesser and Least Splanchnic Nerves – pierce each Crus
  • Sympathetic Chain– behind Medial Arcuate Ligament
  • Subcostal Nerve and Vessels – behind Lateral Arcuate Ligament
  • Left Phrenic Nerve – pierces muscle of Left Dome
  • Neurovascular bundles of 7th to 11th Intercostal Spaces – pass between digitations of Diaphragm and Transversus Abdominis into the neurovascular plane of the abdominal wall
  • Superior Epigastric Vessels – pass between xiphisternal and costal (7th) fibres of the Diaphragm
  • Extraperitoneal Lymph vessels on the abdominal surface pass through Diaphragm to lymph nodes lying on its thoracic surface – mainly in posterior mediastinum
  • At posterior part of Diaphragm – may be a gap between lowest costal fibres and those arising from Lateral Arcuate Ligament
    • Posterior surface of kidney and its peri-renal fascia is separated from the pleural only by areolar tissue of the Endothoracic Fascia

Blood Supply

  • Lower 5 Intercostal and Subcostal arteries: Supplies costal margin
  • Right and Left Inferior Phrenic arteries (from the Abdominal Aorta): Supplies main mass of fibres from the Crura
  • Pericardiacophrenic and Musculophrenic branches of the Internal Thoracic Artery and Superior Phrenic branches of the Thoracic Aorta: Small contributions

Nerve Supply

Phrenic Nerve

  • Arrises from the Cervical Plexus
  • Left and Right Phrenic Nerves (C3, 4, 5 but predominantly C4) – sole motor nerve supply
    • Course of Phrenic Nerves:
      • Arise principally from C4 in the neck
      • Passes down over the Anterior Scalene muscle
        • Deep to the Prevertebral Fascia
        • Passes from lateral to medial over the Anterior Scalene surface
      • Crosses in front of the Subclavian Artery and its Internal Thoracic Artery branch
      • Across dome of the pleura behind the Subclavian Vein
        • Can sometimes pass anterior or even through the Subclavian Vein
        • May contain C5 fibres that pass anterior to the Subclavian Vein
        • Can sometimes pass posterior to the Internal Thoracic Artery
      • Lies lateral to the Ascending Cervical Artery (branch of the Inferior Thyroid Artery or Thyrocervical Trunk)
        • Ascending Cervical Artery runs up on the Prevertebral Fascia
      • Crosses anterior to the Vagus
      • Runs through the mediastinum in front of the lung root
        • Each nerve is in contact laterally with the mediastinal pleura throughout the entire course
    • Right Phrenic Nerve
      • Related medially with venous structures throughout its thoracic course
        • Right Brachiocephalic Vein, Superior Vena Cava, Pericardium over the Right Atrium, and the Inferior Vena Cava
        • i.e. The Right Phrenic Nerve is lateral to the above structures
      • Reaches under-surface of Diaphragm by piercing the Central Tendon adjacent to the Caval Opening

  • Left Phrenic Nerve
    • Related medially to arterial structures throughout its thoracic course
      • Left Common Carotid Artery, Left Subclavian Artery
    • Crosses the arch lateral to the Superior Intercostal Vein; Anterior to the Vagus Nerve
    • Runs laterally down the pericardium over the Left Ventricle towards the Apex of the heart
    • Reaches under-surface of Diaphragm by piercing the muscular part just to the left of the pericardium

  • Both
    • On reaching abdominal surface of Diaphragm, divides into Anterior, Lateral and Posterior branches which run radially giving off branches that enter the muscle from below
  • Fibres of the Right Crus that loop to the left around the Oesophageal Opening: supplied by the Left Phrenic Nerve
  • Lower intercostal nerves give proprioceptive fibres to Diaphragm periphery
  • Phrenic Nerves – proprioception to central part
  • About 2/3rd of the Phrenic Nerve fibres are motor to the Diaphragm
    • Rest are sensory to the:
      • Diaphragm (except the most peripheral parts, which receive intercostal afferent fibres)
      • Mediastinal pleura
      • Fibrous pericardium
      • Parietal layer of the serous pericardium
      • Central parts of the diaphragmatic pleura and peritoneum
        • Pain from diaphragmatic part of peritoneum classically felt in shoulder tip (C4)
        • Pain from thoracic surfaces supplied by the Phrenic Nerve (pleura and pericardium) is usually only vaguely located there
  • Diaphragm moves in respiration under central control of cell groups in the region of the Nucleus of the Tractus Solitarius

 

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