Etiology

• Trauma: acute abdominal compressive injuries, frequently associated with motor vehicle accidents, less commonly with kicks, falls and fights.
• Failure to perform thoracic radiographs after the acute traumatic episode may result in a delayed diagnosis and chronicity of the condition.

Pathophysiology

• The liver is the most cranially located organ within the abdomen and as the parietal surface of the liver is in contact with most of the diaphragm, the liver is the most commonly herniated organ.
• Other organs that may be herniated, in approximate order of decreasing frequency, are the small intestines, stomach, spleen, omentum, pancreas, colon, cecum and uterus.
• The size and location of the diaphragmatic tear, and the anatomical proximity of adjacent organs, and their mobility associated with the length of their supporting ligament or mesentery will determine which of the abdominal organs become herniated into the pleural cavity.
• Acute abdominal compressive injuries can cause diaphragmatic tears which may be circumferential, radial or a combination of the two. The diaphragmatic costal muscles are most commonly ruptured. The central tendon is less commonly involved, the crural muscles are strong and are rarely ruptured.
• The clinical signs result from the effects of the herniated abdominal organs on the cardiorespiratory system (eg dyspnea, exercise intolerance) and the effects of malpositioning, incarceration, obstruction and strangulation of the herniated abdominal viscera (eg pleural effusion Pleural: effusion , vomiting).
• The reasons for the patient presenting with dyspnea are numerous and inter-related (hypoventilation, ventilation.perfusion mismatch Ventilation-perfusion mismatching , etc).
• Rupture of the diaphragm interferes with the contribution of this structure to the majority of mechanical ventilation at rest, an equilibrium of the pleural and peritoneal pressures, and loss of parietal pleural contact with the lungs.
• Organ herniation, free pleural air (pneumothorax Pneumothorax ) or fluid (hemothorax Hemothorax in the acute case or pleural transudate in the more chronic case) will cause pulmonary compresion and atelectasis.
• Associated injuries such as paracostal abdominal rupture further disrupt effective ventilation and may result in pulmonary atelectasis Lung: atelectasis ; lung contusions caused by the traumatic event of herniated organs will result in further hypoxia.
• Cardiac contusion and myocardial injury can occur at the time of the trauma/organ herniation and may result in dysrhythmias. Myocardial injury may be exacerbated by a dysrhythmia and poor perfusion associated with cardiovascular shock.
• The traumatic event itself may result in shock Shock and multi-organ failure.
• Gastric and intestinal outflow may be partially (or completely) obstructed or their motility affected by their malpositioning, compresion at the diaphragmatic tear and later constriction by the formation of adhesions. This can result in a variety of signs including regurgitation, vomiting, inappetence, weight loss, diarrhea, constipation, etc.
• Gaseous dilation of a herniated stomach will cause severe and rapid compression of the lungs and caudal vena cava resulting in dyspnea and cardiovascular shock that will rapidly prove fatal if not recognized and treated.
• Herniation of the liver may result in hepatic venous stasis due to kinking or compression of the thin walled and low pressure hepatic veins. A small rise in pressure in the hepatic veins and caudal vena cava will result in congestion of the liver lobes with subsequent leakage of hepatic lymphatic fluid from the liver. This fluid accumulates within the pleural cavity (hydrothorax) and/or, less commonly, within the abdomen (ascites).
• Occasionally pleural effusion can occur secondary to hernation of viscera other than the liver.
• Obstruction of the biliary tract can cause jaundice, rupture of the biliary tract can occur uncommonly and will result in bile peritonitis and/or pleuritis.
• Bacteria may proliferate in a herniated, traumatized and/or devitalized liver lobe, a systemic bacteremia and toxemia may occur with release of the vascular obstruction that is achieved with repositioning of the liver lobe in the abdomen although this is unusual.

Timecourse

• Acute or chronic (patients may present within minutes after the traumatic event or many years later).