Diaphragm Disorders

Excerpt

The diaphragm is a vital organ for mammals as it is the primary muscle for respiration. Diaphragmatic paralysis is the loss of its muscular power and can arise from either weakness of the muscle itself or damage to its nerve supply. Depending on the severity of the paralysis and whether it is unilateral or bilateral, patients can have varied clinical manifestations. A patient may be asymptomatic, while another may be ventilator dependent.

The diaphragm is a dome-shaped musculo-fibrous structure located between the thoracic and abdominal cavities. It constitutes the floor of the thorax and the roof of the abdomen. The word “diaphragm” is derived from the Greek words “dia,” meaning in between, and “phragma,” meaning fence. Although a clear anatomical distinction is not evident, the diaphragm functions as two, separate units (right and left), each with different vascular and nerve supplies. The peripheral portion of the diaphragm is muscular and is composed of three distinct muscle groups. The sternal group originates from the xiphoid process, the costal group originates from the inner surface of the lower six ribs, and the lumbar group originates from two crura and arcuate ligaments which are, in turn, attached to the lumbar vertebra. The central part is made of strong aponeurotic tendinous ligaments without any bony attachment. It is C-shaped and has right lateral, middle, and left lateral leaflets. The anterior sternal attachment of diaphragm is located more cranially compared to posterior lumbar attachment. During inhalation, the muscular part of the diaphragm contracts making it flatter and expands the thoracic cavity outward and downward. This creates negative intrathoracic pressure leading to passive movement of air from atmosphere to the respiratory system along the pressure gradient. When the diaphragm relaxes, the thoracic cavity constricts decreasing the subatmospheric pressure and leading to passive egress of air from the respiratory system during expiration.

Although external intercostal muscles aid in inspiration, the diaphragm is the primary muscle of respiration, and its weakness can impede the respiratory functions. Paralysis on both sides of the hemidiaphragm will cause significant respiratory failure, but paresis of one hemidiaphragm can be asymptomatic due to compensatory function from the other half of diaphragm and recruitment from external intercostal muscles. Voluntary contraction of the diaphragm will also increase the intraabdominal pressure and aid in other vital functions like vomiting, urination, and defecation. It also prevents regurgitation by creating pressure at the lower esophageal sphincter.

The diaphragm has several openings allowing the structures to pass from the thorax to the abdomen. At the level of the eight thoracic vertebrae on the right hemidiaphragm, there is a large opening through which the inferior vena cava enters thorax from the abdomen to join the right atrium. At the level of the ten thoracic vertebrae, there is a posterior midline opening between the two crus of the diaphragm called aortic hiatus through which the descending thoracic aorta enters the abdomen from the thorax, the thoracic duct enters the thorax from the abdomen, and the azygous vein enters the thorax from the abdomen. Between the fibers of the right crus of the diaphragm, there is the esophageal hiatus through which the esophagus locates from the thorax to the abdomen.

The diaphragm functions primarily involuntarily with additional voluntary control when needed. It is innervated by two phrenic nerves originating from cervical nerve roots C3 to C6. Right and left phrenic nerves to innervate the respective hemidiaphragm and control both sensory and motor functions. The right phrenic nerve is located lateral to the caval hiatus, and the left phrenic nerve is located lateral to the pericardium. Each phrenic nerve divides into four trunks, the sternal, anterolateral, posterolateral and crural trunks. The main vascular supply for the diaphragm comes from the bilateral phrenic arteries, which are direct branches of the thoracic aorta.The diaphragm is also supplied by tributaries from the internal mammary arteries, and from pericardiophrenic arteries. Venous drainage occurs through phrenic veins, which drain into the inferior vena cava.