Diaphragm Disorders

Excerpt

The diaphragm is a vital organ in mammals, serving as the primary muscle for respiration. Diaphragmatic paralysis is the loss of muscular power due to muscle weakness or damage to its nerve supply. Depending on the severity of the paralysis and whether it is unilateral or bilateral, patients can exhibit varied clinical manifestations, ranging from asymptomatic to ventilator-dependent presentations.

Anatomical Structure of the Diaphragm

The diaphragm is a dome-shaped musculofibrous structure between the thoracic and abdominal cavities, constituting the thorax floor 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 visible, the diaphragm functions as separate units (right and left), each with different vascular and nerve supplies.

The diaphragm's peripheral portion is muscular and comprises 3 distinct muscle groups. The sternal group originates from the xiphoid process, the costal group originates from the inner surface of the lower 6 ribs, and the lumbar group originates from 2 crura and arcuate ligaments, which are, in turn, attached to the lumbar vertebra. The diaphragm's central portion comprises strong aponeurotic tendinous ligaments without bony attachments. The diaphragm is C-shaped and has right lateral, middle, and left lateral leaflets. The anterior sternal attachment of the diaphragm is located more cranially compared to the posterior lumbar attachment.

The diaphragm has several openings that allow structures to pass between the thorax and abdomen. At the level of the eighth thoracic vertebra on the right hemidiaphragm, the inferior vena cava enters the thorax from the abdomen through a large opening to join the right atrium. At the level of the tenth thoracic vertebra, a posterior midline opening between the 2 crura of the diaphragm, called the aortic hiatus, allows the descending thoracic aorta to enter the abdomen from the thorax, the thoracic duct to enter the thorax from the abdomen, and the azygos vein to enter the thorax from the abdomen. The esophageal hiatus, located between the fibers of the right crus of the diaphragm, is where the esophagus passes from the thorax to the abdomen (see Image. Anatomy of a Normal Diaphragm).

Nerve and Vascular Supply of the Diaphragm

Diaphragm function is primarily involuntary, with additional voluntary control when needed. The diaphragm is innervated by 2 phrenic nerves originating from cervical nerve roots C3 to C6. The right and left phrenic nerves innervate their respective hemidiaphragms, controlling both sensory and motor functions. The right phrenic nerve courses lateral to the caval hiatus, while the left travels lateral to the pericardium.

Each phrenic nerve divides into 4 trunks—the sternal, anterolateral, posterolateral, and rural. The primary vascular supply for the diaphragm comes from the bilateral phrenic arteries, which are direct branches of the thoracic aorta. Additional blood supply comes from tributaries of the internal mammary and pericardiophrenic arteries. Venous drainage occurs through phrenic veins that empty into the inferior vena cava.

Physiological Function of the Diaphragm

During inhalation, the muscular part of the diaphragm contracts, flattening and expanding the thoracic cavity outward and downward. This diaphragmatic contraction creates negative intrathoracic pressure, facilitating the passive movement of air from the atmosphere into the respiratory system along a pressure gradient. When the diaphragm relaxes, the thoracic cavity constricts, decreasing the subatmospheric pressure and leading to passive air egress from the respiratory system during expiration.

Although external intercostal muscles assist in inspiration, the diaphragm serves as the primary muscle of respiration; thus, diaphragmatic weakness can hinder normal respiratory functions. Paralysis of both hemidiaphragms (ie, bilateral) leads to significant respiratory failure. In contrast, unilateral hemidiaphragm paresis can be asymptomatic due to compensatory function from the opposite side of the diaphragm and recruitment of external intercostal muscles. Voluntary contraction of the diaphragm also increases intraabdominal pressure, supporting essential functions such as vomiting, urination, and defecation while also preventing regurgitation by creating pressure at the lower esophageal sphincter.