Diaphragmatic Hernias, Acquired 

The diaphragm is the major muscle of respiration. Diaphragmatic excursion and chest wall expansion increase the negative intrathoracic pressure required for inhalation. The sequelae from diaphragmatic rupture and subsequent herniation of intra-abdominal contents are associated with significant morbility and mortality.

Diaphragmatic hernias can be divided into 2 categories: congenital defects and acquired defects. Congenital diaphragmatic hernias (CDH) occur because of embryologic defects in the diaphragm. Most patients with CDH present early rather than late in life; however, a subset of adults may present with a congenital hernia that was undetected during childhood. Acquired diaphragmatic hernias stem from all types of trauma, with blunt forces accounting for the majority.

The first traumatic diaphragmatic hernia was reported by Sennertus in 1541. The first two deaths were described by Ambrose Paré in 1578, one from strangulated bowel.^[1]

Diaphragmatic hernias require a high level of suspicion to detect. Patients can be asymptomatic in up to 53% of hernias from blunt trauma and 44% from penetrating trauma. Routine chest x-ray detects only 33% of hernias when interpreted by the trauma team leader at initial evaluation.^[2] Missed injuries are associated with significant morbidity and mortality.

Frequency

Of patients admitted to the hospital for trauma, 3-5% have a diaphragmatic hernia.^{[3, 4]} The male-to-female ratio is 4:1, with most presenting in the third decade of life. Approximately 0.8-1.6% of patients with blunt trauma sustain a rupture of the diaphragm, accounting for 75% of diaphragmatic hernias. Approximately 69% of hernias are left-sided, 24% are right-sided, and 15% are bilateral.

By far, the most common cause of acquired diaphragmatic disorders is either blunt or penetrating trauma. Motor vehicle accidents are the leading cause of blunt diaphragmatic injury, whereas penetrating injuries result from gunshot or stab wounds. In contrast to earlier work, several recent retrospective studies found 75% of their patients to have tears from penetrating injuries.^{[3, 4]} This may stem from increasing awareness of the part of providers or the ability to detect small tears via minimally invasive methods. Other rare causes of traumatic rupture include labor in women with prior diaphragmatic hernia repair,^[5] and barotrauma during underwater dives in patients with history of Nissen fundoplications.^[6]

The following theories have been postulated to explain the mechanism of rupture for blunt injuries: (1) shearing of a stretched membrane, (2) avulsion of the diaphragm from its points of attachment, and (3) sudden increase in the transdiaphragmatic pleuroperitoneal pressure gradient. The resting pressure differential between the pleural (-5 to -10 cm H₂) and peritoneal (+2 to +10 cm H₂) cavities rises to 100-150 cm H₂ O with a large cough and does not injure the diaphragm. Forces transmitted to the abdomen from blunt trauma can raise the pressure gradient to 1000 cm H₂ O.

Left-sided rupture is more common than right-sided rupture (68.5% vs 24.2%), owing to hepatic protection and increased strength of the right hemidiaphragm. However, the increased prevalence of left-sided hernias may also result from weaknesses in points of diaphragmatic embryologic fusion. Current thought is that the right side has more protection than the left and that it may be slightly stronger than the left. In addition, the liver may provide another protection for the right hemidiaphragm. Children have equal rates of rupture per side, likely due to laxity of liver attachments.^[7]

The pathophysiology of acquired diaphragmatic hernias includes circulatory and respiratory depression secondary to decreased function of the diaphragm, intrathoracic abdominal contents leading to compression of the lungs, shifting of the mediastinum, and cardiac compromise. Smaller diaphragmatic hernias are often not found until months or years later, when patients present with strangulation of intra-abdominal organs, dyspnea, or nonspecific gastrointestinal complaints.

Clinical findings include (1) marked respiratory distress, (2) decreased breath sounds on the affected side, (3) palpation of abdominal contents upon insertion of a chest tube, (4) auscultation of bowel sounds in the chest, (5) paradoxical movement of the abdomen with breathing, and/or (6) diffuse abdominal pain.

Traumatic rupture of the diaphragm requires surgical intervention whether the patient presents immediately or some time after the trauma. The high incidence of concomitant intra-abdominal injuries dictates the need for emergency abdominal exploration in the acute trauma setting after initial resuscitation is accomplished.

Patients who present in the latent phase or long after the trauma require repair because the hernia contents may become strangulated, leading to dead gut, stomach, liver, spleen, or other organs.

The diaphragm is a modified half-dome of musculofibrous tissue that separates the thorax from the abdomen. The thoracic side is covered with parietal pleura, and the abdominal side with peritoneum. Four embryologic components arise during the formation of the diaphragm: the septum transversum, 2 pleuroperitoneal folds, cervical myotomes, and the dorsal mesentery.

Development begins during the third week of gestation and concludes by the eighth week. Failure of the development of the pleuroperitoneal folds, and subsequent muscular migration, results in congenital defects.

The muscular diaphragm originates from the 6 lowest ribs on both sides, from the posterior xiphoid process, and from the external and internal arcuate ligaments. Different structures traverse the diaphragm, including 3 distinct apertures that allow the aorta, esophagus, and vena cava to pass.

The aortic aperture is the lowest and most posterior of the openings, lying at the level of the T12 vertebra. The aortic opening also transmits the thoracic duct and, sometimes, the azygous and hemiazygous veins. Diaphragmatic muscle surrounds the esophageal aperture, which lies at the T10 level. The vena caval aperture is the highest of the 3 openings and lies level to the disk space between T8 and T9.

Because of its combination of fast twitch oxidative-glycolytic and slow twitch oxidative fibers, the diaphragm is a muscle resistant to fatigue. Loss of movement of the diaphragm is indicative of fatigue and can be measured by the contractile force. Many patients with chronic obstructive pulmonary disease (COPD) can live at the brink of fatigue.^[8]

The arterial supply to the diaphragm derives from the right and left phrenic arteries, the intercostal arteries, and the musculophrenic branches of the internal thoracic arteries. Small branches of the pericardiophrenic arteries that course with the phrenic nerve, mainly where the nerves penetrate the diaphragm, provide some arterial blood. Venous drainage is through the inferior vena cava and azygous vein on the right and the adrenal/renal and hemizygous veins on the left.

The diaphragm receives its sole muscular neurologic impulse from the phrenic nerve, which primarily originates from the fourth cervical ramus; however, it also has contributions from the third and fifth rami. (Remember that "C3, C4, and C5 keep the diaphragm alive.") Originating around the level of the scalenus anterior muscle, the phrenic nerve courses inferiorly through the neck and thorax before reaching its end point, the diaphragm. Because the phrenic nerve has such a long course before it reaches its final destination, any process that disrupts the transmission of neurologic impulse through the nerve directly affects the diaphragm.

Relatively no contraindications have been reported for repair of an acquired diaphragmatic hernia. In the trauma setting, the patient must be adequately resuscitated before he or she is transported to the operating room. Many small injuries are discovered during exploratory laparotomy for the repair of other intra-abdominal injuries.

Diaphragmatic hernias should always be repaired. Lack of repair of a diaphragmatic hernia can lead to incarceration and strangulation of intra-abdominal contents or respiratory dysfunction.