The diaphragm

THE DIAPHRAGM (Fig. 4. 5) The diaphragm forms the highly convex floor of the thoracic cage. It arises from vertebral, costal and sternal origins and from the central tendon.

The vertebral part arises from the crura and arcuate ligaments. The right crus is attached to the bodies and discs of L 1 - L 3 vertebrae. The smaller left crus arises from the vertebral body and disc of L1 and L2 vertebrae.

The medial arcuate ligament is a thickening of the fascia over the psoas muscle from the body of L2 to the transverse process of L1 lumbar vertebrae. The lateral arcuate ligament is a thickening of the fascia over quadratus lumborum

Fig. 4. 5 Diaphragm: (a) view from below showing origin and openings; (b) crura and arcuate ligaments; (courtesy of Professor J. B. Coakley); (c) crura of the diaphragm as seen on axial and (d) coronal MRI.

1. Right crus of diaphragm 2. Left crus of diaphragm 3. IVC 4. Aorta 5. Vertebral body 6. Spinal cord 7. Pedicle 8. Lamina 9. Psoas muscle

from the transverse process of L1 vertebrae to the twelfth rib. The median arcuate ligament is the fibrous medial part of both crura behind which the aorta passes; no diaphragmatic muscle arises from this.

The costal part of the diaphragm arises in slips from the lower six costal cartilages.

The sternal part of the diaphragm arises in two small slips of muscle from the posterior surface of the xiphisternum.

The central tendon is, in fact, not central but closer to the sternum. Its midpart is fused w i th the pericardium and its right and left posterior parts extend towards the paravertebral gutters.

Openings in the diaphragm

These are as follows:

• Aortic - at level T12: in fact the aorta passes behind the median arcuate ligament rather than through the diaphragm. The thoracic duct and the azygos vein pass w i th the aorta; • Oesophageal hiatus - at level T10: this is to the left of the midline but is surrounded by fibres of the right crus.

With the oesophagus it transmits the vagal trunks, branches of the left gastric artery, veins and lymphatics; • Caval opening - at level T8: transmits the inferior vena cava, whose adventitial wall is fused w i th the central tendon, and the right phrenic nerve; • Behind the medial arcuate ligament - the sympathetic trunk; • Behind the lateral arcuate ligament - the subcostal nerves and vessels; and • Between sternal and costal origins - the superior epigastric vessels.

Structures that pierce the diaphragm

The structures that pierce the diaphragm are as follows: • Terminal branches of the left phrenic nerve pierce the central tendon; • The greater, lesser and least splanchnic nerves, which pierce each crus; and • The lymph vessels between the abdomen and thorax, which pierce the diaphragm throughout, especially posteriorly.

Blood supply to the diaphragm

The diaphragm is supplied from its abdominal surface by the inferior phrenic arteries from the abdominal aorta. The costal margins are supplied by the intercostal arteries.

Nerve supply to the diaphragm

Right and left phrenic nerves from C3-C5 roots provide the motor supply of the diaphragm. Sensory impulses from the central part of the diaphragm pass w i th the phrenic nerves, and those from the peripheral part w i th the intercostal nerves.

Radiological features of the diaphragm (see Fig. 4. 5)

PA chest radiograph (Fig. 4. 6) The highest point of the right dome is at the sixth intercostal space anteriorly (ranging from the fourth to seventh ribs); it is more accurate to count anterior rather than posterior ribs, as the diaphragmatic dome is nearer to the anterior ribs and the film, and is therefore less subject to distortion by slight angulation of the patient or the beam.

The right dome is higher than the left by 2 cm but the left may be higher than the right in the normal subject, especially w i th swallowed gas in the colon.

The range of movement of the diaphragm w i th respiration is as follows:

• Quiet respiration: 1 cm; and • Deep inspiration /expiration: 4 cm (wide range of normal).

In each case the left hemidiaphragm moves more than the right.

The variation of the diaphragm w i th posture is as follows: • Supine: higher; and • Lateral decubitus: dome on the dependent side is higher.

In dextrocardia, even if the liver is on the right side the left dome of the diaphragm tends to be higher.

Partial reduplication of the diaphragm - known as accessory hemidiaphragm - may occur. This is much commoner on the right side.

Lateral chest radiograph (Fig. 4. 7) The following anatomical details help identify the domes of the diaphragm: • The heart shadow obliterates part of the left dome.

The inferior vena cava may be seen piercing the right dome. • Air within the gastric fundus lies under the left dome.

There is apparent thickness of the diaphragm on radiographs: • With the pleura and peritoneum when there is air in the peritoneum: 2-3 mm thick; and • With the pleura and fundal wall of stomach: 5-8 mm thick.

Curvature of the dome

The perpendicular height of the dome of the diaphragm from a line between costophrenic and cardiophrenic angles is 1. 5 cm.

1. Posterior junctional line 2. Anterior junctional line 3. Azygo-oesophageal line 4. Lateral wall of descending aorta 5. Aortic knuckle 6. Aortopulmonary window 7. Pulmonary trunk/left pulmonary artery 8. Left superior pulmonary vein 9. Left inferior pulmonary artery 10. Left hilar point 11. Right superior pulmonary vein 12. Interlobar artery 13. Right hilar point 14. Trachea 15. Right main bronchus 16. Left main bronchus 17. Azygos vein and position of azygos node 18. Position of left atrial appendage 19. Left ventricle 20. Right atrium 2 1. Inferior aspect of left brachiocephalic vein 22. Medial end of right clavicle 23. Right lateral aspect of manubrium sterni 24. Spinous process of T1 25. Superior surface of clavicle 26. Companion shadow of clavicle 27. Medial aspect of right scapula 28. Coracoid process of right scapula 29. Dome of right hemidiaphragm 30. Dome of left hemidiaphragm 3 1. Stomach bubble 32. Gas in splenic flexure of colon