The diaphragm (derived from the Ancient Greek term ‘diáphragma’ meaning ‘partition’) is a dynamic structure shaped like a parachute, which is composed of skeletal muscle and fibrous tissue. Its two main functions are to separate the two major cavities (acting as the inferior border of the thorax and the superior border of the abdomen) and to aid in respiration. Contraction and relaxation of the diaphragm creates changes in volume and pressure, thus encouraging movement of air in and out of the thoracic cavity.
The diaphragm is anchored in place by a number of tendons and is comprised of two domes known as the left and right hemidiaphragms. It also has a number of diaphragmatic openings transmitting important structures from the thorax to the abdomen.
As previously mentioned, the diaphragm is said to have a left and right hemidiaphragm, the right lying slightly higher than the left due to the large size of the liver. The muscles fibres extend from the xiphoid process and costal margin anteriorly, ribs 6-12 laterally and T12 and upper lumbar vertebrae (and the anterior longitudinal ligament of the vertebral column) posteriorly. These attachments converge at different points to form the following tendinous structures:
· CENTRAL TENDON
o Formed by the convergence of the peripheral attachments, the central tendon is a strong aponeurosis lying slightly anterior within the muscle
o It contains multi-directional muscle fibres which add to its strength
o Continuous with the inferior surface of a small area of fibrous pericardium
· RIGHT + LEFT CRURA
o Are continuous with the anterior longitudinal ligament of the vertebral column
o Arise from the anterior surfaces of the upper lumbar vertebrae
o The right crus is larger and longer than the left crus, and contains some fibres which surround the oesophagus to act as a physiological sphincter
· INTERNAL ARCUATE LIGAMENT (or medial lumbocostal arch)
o Tendinous arch derived from the fascia overlying the superior part of the psoas major muscle
o Attaches to the body and transverse processes of the L1 or L2 vertebra
· EXTERNAL ARCUATE LIGAMENT (or lateral lumbocostal arch)
o Arches across the superior part of the quadratus lumborum muscle
o Attaches to the transverse processes of L1 and to the lower margin of rib 12
The diaphragm has 3 main apertures which allow passage of structures from the thorax to the abdomen. The first is the caval opening which passes through the central tendon. It is found at T8 level and transmits the inferior vena cava and some minor branches of the right phrenic nerve. The second is the oesophageal hiatus found at T10 level. It lies posterior and slightly left of the central tendon and conveys the oesophagus and the anterior and posterior vagal trunks. The third is the aortic hiatus found posteriorly between the left and right crura, which allows the aorta, azygos vein and thoracic duct to pass into the abdomen.
Other smaller apertures include the lesser apertures of the right and left crus, which transmit the right greater and lesser splanchnic nerves and the left greater and lesser splanchnic nerves (along with the hemiazygos vein) respectively. The sympathetic trunk passes into the abdomen posteriorly to the diaphragm under the internal arcuate ligament.
The Diaphragm as a Muscle
Xiphoid process, costal cartilages of ribs 6-12, lumbar vertebrae + arcuate ligaments
Contraction – increases volume of thoracic cavity, decreases intra-thoracic pressure and draws air into lungs
(Passive) Relaxation – decreases volume of thoracic cavity, increases intra-thoracic pressure and air moves out of lungs
Arterial: inferior phrenic (also superior phrenic, musculophrenic + pericardiacophrenic arteries)
Venous: inferior phrenic (also superior phrenic, musculophrenic + pericardiacophrenic veins
(C3, C4, C5 from the cervical plexus)
Also the lower intercostal nerves
Air within the peritoneal cavity, or pneumoperitoneum, can be identified on an erect chest X-ray by the presence of air under the diaphragm. It is a worrying sign as it is an indicator of a perforated hollow viscus, such as a perforated peptic ulcer. However, pneumoperitoneum can also be induced, as in laparoscopic abdominal surgery and is still seen radiologically for up to 48 hours post-operatively.
This often occurs secondary to a lesion of the phrenic nerve, though could also result from interruption of innervation at the cervical spinal cord level or at the brainstem. Causes include compression from tumours (particularly bronchial cancers), trauma (including iatrogenic damage from surgery), neuropathies and myopathies. Paralysis of the diaphragm gives rise to converse diaphragmatic movement, and consequently the diaphragm moves up on inspiration and down on expiration. This can be asymptomatic if a unilateral paralysis is present and is often found incidentally on a chest X-ray. However, if both sides are affected the patient can present with shortness of breath and become easily fatigued. Management of this condition depends on the cause but continuous positive airway pressure (CPAP) can be of benefit.
A hiatus hernia involves abnormal protrusion of the lower oesophagus and/or the stomach through the oesophageal hiatus secondary to weakness or a tear in the diaphragm itself. It is strongly associated with obesity and is a common cause of acid reflux in adults due to an inability to maintain pressure on the lower oesophageal sphincter. If symptomatic, management can range from conservative measures, such as weight loss and avoiding lying flat after meals, to medical management including proton pump inhibitors or H2 receptor antagonists. If the symptoms become very severe surgery may be implicated, namely a procedure called fundoplication, which involves wrapping the fundus of the stomach around the oesophagus to prevent herniation.
A congenital diaphragmatic hernia is a life-threatening condition that occurs when the pleuroperitoneal membrane fails to fuse, leaving a space in which abdominal contents can protrude into the thoracic cavity. The majority of cases occur on the left and can lead to hypoplasia of the developing lung. The most common form of congenital diaphragmatic hernia is known as a Bochdalek hernia, which is due to a posterolateral defect in the diaphragm. Definitive management is surgical.
Innervation to the diaphragm:
C3, 4, 5 keeps the diaphragm alive
DiaPHRagm is innervated by the PHRenic nerve
The diaphragmatic openings, their main constituents and vertebral level can be remembered using this quick rule:
VENA CAVA has 8 letters and the caval opening occurs at T8 level
OESOPHAGUS has 10 letters and the oesophageal hiatus occurs at T10 level. It also transmits the VAGUS NERVE (10 letters) and is the 10th cranial nerve
AORTIC HIATUS has 12 letters and occurs at T12 level
The diaphragm is a thin flat muscle separating the thorax from the abdomen and plays a vital role in respiration. It is derived embryologically by fusion of the septum transversum and the pleuropericardial membrane. It is composed of the two hemidiaphragms (the right is slightly larger due to the liver) and attaches peripherally to the body walls by muscular fibres and tendinous structures, such as the central tendon, right crus and left crus. A number of diaphragmatic apertures transmit important structures, such as the aorta and inferior vena cava, from the thoracic cavity to the abdomen.