Muscles - Diaphragm

Question 1

What is the diaphragm?

Answer 1

It is a double-domed musculotendinous sheet located at the inferior-most aspect of the rib cage.

Two main functions:

1) Separates the thoracic from the abdominal cavity
2) Undergoes contraction and relaxation, altering the volume of the thoracic cavity and the lungs, producing inspiration and expiration.

Question 2

Anatomical position and attachments?

Answer 2

The diaphragm is located at the inferior-most aspect of the ribcage, filling the inferior thoracic aperture. It acts as a floor of the thoracic cavity and the roof of the abdominal cavity. The attachments of the diaphragm can be divided into peripheral and central attachments. It has three peripheral attachments:

1) Lumbar vertebrae and arcuate ligaments
2) Costal cartilages of ribs 7-10 (attach directly to ribs 11-12)
3) Xiphoid processs of the sternum

The parts of the diaphragm that arise from the vertebrae are tendinous in structure, and are known as the right and left crura:

1) Right crus - arises from L1-L3 and their intervertebral discs. Some fibres from the right crus surround the oesophageal opening, acting as a physiological sphincter to prevent reflux of gastric contents into the oesophagus.
2) Left crus - arises from L1-L2 and their intervertebral discs.

The muscle fibres of the diaphragm combine to form a central tendon. This tendon ascends to fuse with the inferior surface of the fibrous pericardium. Either side of the pericardium, the diaphragm ascends to form the left and right domes. At rest, the right dome lies slightly highers than the left - this is though to be due to the presence of the liver.

Question 3

Pathways through the diaphragm?

Answer 3

The diaphragm divides the thoracic and abdominal cavities. Thus, any structure that pass between the two cavities will pierce the diaphragm.

There are three openings that act as conduit for these structures:

1) Caval hiatus (T8) - IVC and terminal branches of right phrenic nerve.
2) Oesophageal hiatus (T10) - oesophagus, right and left vagus nerves and oesophageal branches of left gastric artery and vein.
3) Aortic hiatus (T12) - aorta, thoracic duct and azygous vein.

Question 4

Actions?

Answer 4

The diaphragm is the primary muscle of respiration. During inspiration, it contracts and flattens, increasing the vertical diameter of the thoracic cavity. This produces lung expansion, and air is drawn in. During expiration, the diaphragm passively relaxes and returns to its original dome shape. This reduces the volume of the thoracic cavity.

Question 5

Innervation and vasculature?

Answer 5

The halves of the diaphragm receive motor innervation from the phrenic nerve. The left half is innervated by the left phrenic nerve and vice versa. The phrenic nerve is formed by spinal roots C3-C5.

The majority of the arterial supply to the diaphragm is deliver via the inferior phrenic arteries, which arise directly from the abdominal aorta. The remaining supply is from the superior phrenic, pericardiacophrenic, and musculophrenic arteries. The draining veins follow the aforementioned arteries.

Question 7

Clinical relevance - paralysis of the diaphragm?

Answer 7

This is due to interruption to the nervous supply. This can occur in the phrenic nerve, cervical spinal cord or the brainstem. It is most often due to a lesion of the phrenic nerve:

1) Mechanical trauma - ligation or damage to the nerve during surgery.
2) Compression - due to a tumour within the chest cavity.
3) Myopathies - such as myasthenia gravis.
4) Neuropathies - such as diabetic neuropathy.

Paralysis of the diaphragm produces a paradoxical movement. The affected side of the diaphragm moves upwards during inspiration, and downwards during expiration. A unilateral diaphragmatic paralysis is usually asymtomatic and is most often an incidental finding on x-ray. If both sides are paralysed, the patient may experience poor exercise tolderance, orthopnea and fatigue. Lung function tests will show a restrictive deficit.

Management of diaphragmatic paralysis is two-fold. Firstly, the underlying cause must be identified and treated. The second part of the treatment deals with symptomatic relief. This is usually via non-invasive ventilation, such as CPAP (continuous positive airway pressure) machine.