Diaphragmatic Hernia

Anatomy

The diaphragm separates the abdominal and thoracic cavities and assists in ventilation. The muscular costal, sternal, and lumbar parts of the diaphragm surround a strong central tendon [1]. The costal and sternal muscles insert on the abaxial border of the central tendon, and the lumbar muscles insert on its axial border. The central tendon is approximately Y shaped, with each arm of the Y extending dorsally from the central region up to the 13th rib. The paired lumbar muscles form the diaphragmatic crura. The right crus is larger than the left. Each crus has a bifurcate tendon arising from the bodies of the third and fourth lumbar vertebrae, medial to the psoas minor muscles. The sternal muscle is unpaired and is continuous with the left and right costal muscles. It originates from the xiphoid cartilage, transverse fascia, and the eighth costal cartilages and extends forward to insert dorsally on the body of the central tendon.

The aorta, azygos and hemiazygos veins, and lumbar cistern of the thoracic duct pass through the aortic hiatus.

The thoracic surface of the diaphragm is adjacent to the lungs and is covered by endothelial fascia and pleura, the pleura being continuous with that of the mediastinum. The mediastinal attachment to the diaphragm is median dorsal to the esophagus. Ventral to the esophagus it deflects to the left in a wide semicircle across the left diaphragmatic costal muscles and returns to the midline at the sternum. A pleural reflection (plica vena cava), caudal to the heart around the caudal vena cava, is attached to the diaphragm on the right side. The plica vena cava and the mediastinum form a pouch cranial to the diaphragmatic central tendon that is occupied by the accessory lung lobe. Organs involved in diaphragmatic hernias may occasionally occupy this pouch, rather than the main part of the thoracic cavity. Motor innervation to the diaphragm is by the phrenic nerves [1,2].

Embryology

Embryogenesis of the diaphragm is complex. The transverse septum is the initial incomplete partition between the pericardial and peritoneal cavities. This septum becomes the central tendon of the diaphragm. The dorsal mesentery of the esophagus develops simultaneously with the transverse septum and produces the median portion of the diaphragm. In adults, this mesentery forms the diaphragmatic crura, including the esophageal hiatus and aortic hiatus. The pleuroperitoneal folds develop along the lateral body walls and migrate medially, where they close the pleuroperitoneal canals by fusion with the esophageal mesentery and dorsal portion of the transverse septum, thereby completing the diaphragmatic partition. Finally, as the pleural cavities enlarge, myoblasts from the body wall invade the peripheral border of the diaphragm to form the costal muscles. Failure of development or fusion of any of the aforementioned structures may lead to formation of a pleuroperitoneal or peritoneopericardial hernia [3].

Physiology

The diaphragm contracts during inspiration, pushing caudally on the viscera and displacing the abdominal wall outward [4,5]. At the same time, contraction of the diaphragmatic costal muscles expands the caudal thorax. The diaphragm plays a critical role in the maintenance of negative intrapleural pressure by resisting movement of abdominal viscera into the thorax during inspiration.

Types of Hernias

True diaphragmatic hernias are rare with the exception of hiatal hernias. Herniated viscera in false diaphragmatic hernias (traumatic or congenital) are not contained within a sac but lie free within the pleural cavity or pericardial sac. With peritoneopericardial hernias, the peritoneum is continuous with the visceral layer of the pericardium.

Traumatic Diaphragmatic Hernia

Following trauma, the diaphragmatic costal muscles are more often disrupted than the central tendon, whereas the stronger crural muscles are seldom damaged [6]. Despite early claims to the contrary, left to right distribution is probably uniform in dogs and cats, with sporadic cases having bilateral or multiple tears [6-10]. Tears are oriented circumferentially in about 40% of dogs, radially in 40%, or both in 20%. Cats had a preponderance of circumferential tears (59%) and fewer radial tears (18%) in one report [9]. In another series, tears were left-sided in 44% of cats, right-sided in 38%, ventral in 15%, and multiple in only 1 cat (23%) [8]. In humans, the liver protects the right side of the diaphragm, and left-sided rupture of the tendon is 8 times more frequent than right-sided rupture [11]. In dogs and cats, the liver contacts both sides of the diaphragm and its protective effect is not as marked. The liver is found in the thoracic cavity in over 80% of cases with diaphragmatic hernia [6]. The small intestine, stomach, spleen, omentum, pancreas, colon, cecum, and uterus may be involved, in descending order of frequency. Herniation of a gravid uterus has been reported in a dog [12]. Herniation of a kidney, leading to urothorax has been reported in a cat [13]. Which organs occupy a hernia depend on various factors, including the site of the diaphragmatic tear, proximity of the organs, and range of movement of supporting ligaments or mesentery. The liver, small intestine, and pancreas tend to herniate through right-sided tears, whereas the stomach, spleen, and small intestine more commonly herniate through left-sided tears (Fig. 15-1 and Fig. 15-2)

Figure 15.1. Lateral radiograph from a 9-year-old male neutered cat with a chronic diaphragmatic hernia.

Figure 15.2. Intraoperative photographs of a chronic ventral diaphragmatic hernia in a cat. A: Herniated viscera is covered by omentum. B: Retraction of the omentum shows herniation of small intestine. C: Reduction of intestine reveals herniation of the quadrate lobe of the liver. The malleable retractor is indicating the gallbladder. Herniation and/or torsion of the liver lobes may lead to temporary or permanent signs of biliary and hepatocellular dysfunction such as obstructive jaundice, ascites, and hepatic encephalopathy.

Congenital Pleuroperitoneal Hernia

Canine congenital pleuroperitoneal hernias are rare, and usually take the form of a defect in the dorsolateral diaphragm, with or without central tendon involvement [14]. The intermediate part of the left lumbar muscle of the crus may be absent, leaving a defect of 1 to 2 cm in diameter, or the defect may be more extensive with both crura and parts of the central tendon absent. An autosomal recessive mode of inheritance of the defect was proposed in a series of related dogs [14]. Congenital pleuroperitoneal hernias have also been diagnosed in cats [15].

Congenital Peritoneopericardial Hernia

Peritoneopericardial hernias commonly contain omentum, liver lobes, gallbladder, and small intestine, as a result of a simple communication between the peritoneal and pericardial cavities (Fig. 15-3). Stomach, colon, falciform ligament, and spleen have also been reported. Cystic lesions in the pericardium have been reported in animals with small peritoneopericardial hernias and pathologic changes of the omentum or liver [16,17].
Figure 15.3. Thoracic radiographs of a 5-month-old male cat with peritoneopericardial hernia. The cat presented with vomiting, decreased exercise tolerance, and hyperpnea. A: Plain radiographs (lateral and ventrodorsal views). Note the enlarged, rounded cardiac silhouette and the marked lung collapse. B: Contrast radiographs of the same cat after oral administration of barium sulfate. Small intestine and colon are present within the pericardial sac. Distention of gut with gas or ingesta, or effusion from herniated structures may lead to pericardial tamponade. C: Plain radiographs 2 months after surgical correction. The lungs were expanded gently over a period of 24 hours to avoid pulmonary reexpansion syndrome.

Causes of Hernias

Up to 85% of diaphragmatic hernias in small animals are traumatic in origin, 5 to 10% are congenital, and the rest are of unknown etiology [6,7,18].

Pleuroperitoneal and peritoneopericardial hernias result from failure of development of the transverse septum and inappropriate patency of the pleuroperitoneal canals, with failure of ingrowth of myoblasts to form the muscular crura [3]. Breed predispositions for peritoneopericardial hernias have been reported for the Weimaraner [19], domestic longhair, and Himalayan cat [20].

Injury to the diaphragm, causing traumatic diaphragmatic hernia, can be either direct or indirect in origin. Blunt force trauma is the most common cause of diaphragmatic hernias in dogs and cats. Direct injuries from thoracoabdominal stab and gunshot wounds are a frequent cause of diaphragmatic injury in humans [21], but are rarely encountered in veterinary patients. Iatrogenic injury of the diaphragm has been reported with thoracocentesis, poorly positioned chest drains, or by extension of a midline celiotomy incision lateral to the xiphoid process.

Blunt force trauma likely to cause diaphragmatic hernia in animals includes motor vehicle trauma and falling from heights. Male dogs 1 to 3 years old are at significantly greater risk for traumatic diaphragmatic hernia, as a consequence of their increased risk of motor vehicle trauma. The mechanism of diaphragmatic rupture in these instances is probably the sudden increase in intra-abdominal pressure at the same time as forced exhalation. Severe injuries of the thoracic cavity and organs have been reported in more than 39% of dogs and cats with musculoskeletal trauma [22,23]. Pulmonary contusions, pleural effusion, hemothorax, pneumothorax, and rib fractures are the most common of these injuries. Between 2% and 5% of animals with fractures have a diaphragmatic hernia [22,24]. Sudden exposure to elevated gravitational forces has been reported to cause diaphragmatic rupture in humans, but is unlikely to be a precipitating factor in veterinary patients. Diaphragmatic hernias have also been reported as a complication of parturition and excessive coughing in humans. Diaphragmatic hernias have been reported occasionally in animals with increased intraabdominal pressure but no obvious precipitating traumatic event.

Pathophysiology

Regardless of the etiology of the diaphragmatic defect, clinical signs related to diaphragmatic hernia are largely associated with the following: failure of the thoracic bellows to produce adequate negative inspiratory pressure, a mass effect from the abdominal organs present within the thorax or pericardium, and incarceration or strangulation of the herniated structures. Respiratory manifestations of diaphragmatic hernia may be further exacerbated by the presence of a pleuroperitoneal effusion or dilation of the gastrointestinal tract. Because of the nature of automobile trauma, multisystem injury and shock are potential complications in traumatic diaphragmatic hernia.

Respiratory Consequences of Diaphragmatic Hernia

Dyspnea and tachypnea are the most common clinical signs in diaphragmatic hernias. In addition to diaphragmatic dysfunction, rupture of the diaphragm causes loss of parietal pleural contact with the lungs and impairs the animal's ability to generate negative intrathoracic pressure. Pressures in the thoracic and abdominal cavities equalize and abdominal and thoracic wall muscles take over ventilation. Pain, rib fractures, and flail chest may limit thoracic expansion in patients with polytrauma. Pulmonary atelectasis following herniation of abdominal organs results in hypoventilation, ventilation/perfusion mismatch, and hypoxia. Dyspnea can also result from pleural effusion [25]. Effusions are commonly reported in association with diaphragmatic hernias, and result from obstruction to lymphatic drainage, inflammation of herniated or displaced organs, and leakage of bile, urine, and feces. Although in most cases the effusion is present in both the peritoneal and pleural cavity, in some instances abdominal organs seal the diaphragmatic defect and the effusion may be restricted to the pleural cavity [13,26]. Gastric tympany should be considered as a cause of respiratory compromise in cases where breathing deteriorates rapidly, particularly in animals known to have a diaphragmatic hernia.

Cardiovascular Consequences of Diaphragmatic Hernia

Cardiovascular abnormalities most commonly result from peritoneopericardial hernias. Pressure of herniated viscera within the pericardial sac may cause cardiac tamponade and right-sided congestive heart failure as a result of compression of the thin-walled right atrium and ventricle. Hypotension, tachycardia, venous distension, ascites, and pleural effusion (including chylothorax) may be seen. Concomitant cardiac defects in animals with congenital disease may produce a variety of hemodynamic effects.

Gastrointestinal Consequences of Diaphragmatic Hernia

The main effects of diaphragmatic hernias on the abdominal viscera are incarceration, obstruction, and strangulation. These effects result from pressure applied by the edge of the diaphragmatic tear as the organs pass over it, from formation of fibrous adhesions and strictures, or from malpositioning or torsion.

Incarceration of the stomach and intestine in a diaphragmatic hernia may result in partial or complete obstruction. Gastric tympany may cause rapid interference with cardiorespiratory function by compression of the caudal vena cava and lungs, and can be rapidly fatal. Recurrent vomiting, leading to dehydration, acid-base and electrolyte derangements, altered cardiac electrical conduction, and muscle weakness, was reported as a consequence of gastric outflow and duodenal obstruction in one dog [27]. Severe compromise of blood supply can also induce ischemic necrosis, intestinal perforation, and abscessation. In some instances, viscera may rupture, leading to bile peritonitis and pleuritis, fecal contamination of the body cavities, and uropleuritis.

Hepatic Consequences of Diaphragmatic Hernia

The liver may be injured at the time of diaphragmatic injury, or one or more lobes of the liver may herniate into the thoracic cavity. The latter may occur in both traumatic and congenital diaphragmatic hernia. Disease resulting from liver lobe herniation occurs as a result of hepatic congestion, hepatic necrosis, liver lobe torsion, or obstruction to biliary flow. A pressure gradient of up to 12 mm Hg normally exists across the hepatic sinusoids, between the portal vein and the hepatic vein. The caudal vena cava and hepatic veins are thin-walled, low-pressure vessels and are easily compressed. When the liver herniates or undergoes torsion, compression of these vessels results in occlusion to hepatic venous outflow. A rise in pressure in the hepatic veins or caudal vena cava relative to the intrahepatic sinusoidal pressure results in hepatic congestion, dilation of hepatic lymphatic vessels, and extravasation of copious quantities of high-protein hepatic lymph. Rapid accumulation of hydrothorax, ascites, or a combination may result. Hydrothorax and ascites develop in about 30% of animals with liver herniation [6,7,10]. The fluid typically is a modified serosanguineous transudate. In addition to fluid extravasation, the liver may undergo severe and sometimes irreversible changes (Fig. 15-4). Hepatonodular myelolipomatosis has been reported in a cat with herniation of liver lobes into a peritoneopericardial hernia [28]. Diaphragmatic rupture with concomitant biliary tract injury, bile peritonitis, and bile pleuritis has been reported in dogs. Two cats with chronic diaphragmatic hernia were presented mainly for icterus resulting from extrahepatic biliary obstruction [29]. Although not yet reported in the setting of diaphragmatic hernias, bacterial proliferation (particularly of naerobic organisms) is known to occur in devitalized liver, leading to sepsis and death.


Figure 15.4. Darkened and fibrotic liver lobes (arrow) that have been reduced from a chronic diaphragmatic hernia in a cat. No clinical signs of hepatic dysfunction were present, but a preoperative biochemistry panel revealed markedly elevated activities of serum alanine aminotransferase and alkaline phosphatase. The non herniated portion of the liver has undergone hyperplasia (open arrow). The discolored liver lobes were assessed as being viable despite the chronic changes and they were not resected. The cat made a complete recovery.

Pleural Effusion

Pleural effusion occasionally results from damage to viscera other than the liver. Hemothorax, chylothorax, urothorax, pneumothorax, and pyothorax have been reported. Pleural effusion may also occur as a result of right-sided congestive heart failure in animals with cardiac tamponade.

Mortality associated with Diaphragmatic Hernia

Prior studies have indicated that approximately 15% of animals die before presentation for anesthesia and surgical correction of traumatic diaphragmatic hernia. Preoperative deaths are due to hypoventilation, shock, multiorgan failure, and cardiac dysrhythmias. Restraint for examination, radiography, and pleural or peritoneal drainage may cause decompensation of critically compromised patients. Mortality may also result in animals with chronic hernias from sudden dilation of the stomach or intestine with ingesta or gas. The effect of acute gaseous distension of the stomach is equivalent to a tension pneumothorax. Emergency decompression of the stomach with a hypodermic needle or stomach tube may be required.

Anesthetic induction is a critical phase of management, and any undue delay in intubation and controlling ventilation can cause death. Induction of anesthesia and surgical intervention in patients that have not been adequately stabilized is likely to be the main reason for the poor outcomes observed previously in animals sent to surgery within 24 hours of injury [9,36]. More recent studies indicate that survival rate is between 80% and 94% and prognosis should be equally good for patients operated on within the first 24 hours, as long as appropriate adjunctive treatment is given [8,37,38].

Postoperative complications that result in death fall into two groups and may result from the hernia, the surgical procedure, or the underlying traumatic event. In the first 24 hours after surgery, death is caused by hemothorax, pneumothorax, pulmonary edema, shock, pleural effusion, and sudden cardiac arrest. Deaths occurring later after surgery tend to be a result of rupture, obstruction, or strangulation of the gastrointestinal tract, secondary changes in herniated organs, or diseases unrelated to hernia.

Mortality may occur in up to 14% of dogs and cats undergoing surgery for peritoneopericardial hernia [19,20,30]. Mortality has resulted from cardiac arrest and persistence of preoperative signs such as chylothorax and vomiting. One animal developed ascites and signs of hepatic encephalopathy as a result of hepatic cirrhosis, presumably as a result of hepatic ischemia during herniation [19]. The author has also encountered this complication in an Australian cattle dog with a chronic diaphragmatic hernia [15].

Complications of Diaphragmatic Hernia Repair

Complications reported after repair of diaphragmatic hernias result from changes to diaphragmatic function, physical impingement on structures passing through or near the diaphragm, changes in pleural and peritoneal pressure, and pulmonary reinflation.

Pulmonary dysfunction and edema are perhaps the most serious complications directly related to surgical repair of diaphragmatic hernias. Pulmonary reexpansion syndrome is a well described entity in humans and was first reported in a cat following diaphragmatic hernia repair in 1978 [39]. Pulmonary reexpansion injury is commonly reported in humans following evacuation of pneumothorax, sometimes after just a few hours of lung collapse. It has since been identified as a major risk in many studies of small animal patients with diaphragmatic hernia. Pulmonary injury may result from barotrauma owing to overly zealous reinflation. Reexpansion pulmonary edema results from increased permeability of the pulmonary capillary bed. Experimental studies suggest that an inflammatory response follows mechanical injury to the alveolar-capillary membrane and is complicated by reperfusion injury. Reexpansion injury may be avoided by ensuring slow reexpansion by evacuation of air from the thorax using a negative pressure less than minus 10 mm Hg. Lungs should be reinflated slowly over a period of 12 to 24 hours.40 Reexpansion injury is not just a complication of diaphragmatic hernia repair but may potentially occur in any animal undergoing pulmonary reexpansion after chronic lung collapse, for instance following drainage of pleural effusion or resection of a large space-occupying lesion of the thorax.

Hiatal hernia is a recognized complication of diaphragmatic hernia repair. Proposed mechanisms by which this occurs include physical damage to the esophageal hiatus, abnormal tension on the central tendon of the diaphragm and hence the hiatus, increases in intra-abdominal pressure as a result of loss of abdominal domain following chronic diaphragmatic hernia, and alterations in vagal activity as a result of surgical trauma and organ repositioning.

Loss of abdominal domain is not, strictly speaking, a complication of diaphragmatic hernia repair, but it is a complicating factor that may impact the success of surgery. Loss of abdominal domain refers to reduction in the capacity of the abdominal cavity as a result of abdominal myofascial retraction in the presence of chronically reduced volume of abdominal contents. Muscles of the abdominal wall contract and lose the capacity to stretch immediately back to their original length. The fascial sheaths undergo remodeling. Muscles may also undergo fibrosis and retraction if they were damaged at the time of the original trauma. Therefore, following repositioning of herniated organs, direct closure of the celiotomy incision may be impossible, or may result in dangerously elevated intra-abdominal pressures. Development of hiatal hernias subsequent to repair of chronic diaphragmatic hernias is likely to result, at least in part, from elevated intra-abdominal pressure secondary to loss of domain. Strategies for overcoming the restrictions imposed by loss of domain include myofascial releasing incisions and mesh expansion [41]. The author has used relaxing incisions of the external rectus sheath parallel to the celiotomy incision to assist abdominal closure following repair of chronic diaphragmatic hernia in small animal patients.

Development of ascites immediately after surgery may signal an elevation in hepatic venous drainage pressure caused by repositioning the liver during herniorrhaphy or by chronic liver disease.

Gastric ulceration was a postoperative complication in two dogs that had chronic traumatic diaphragmatic hernia with intrathoracic adhesions to incarcerated liver [42].

Hyperthermia was reported as a complication of peritoneopericardial hernia repair in 54% of cats in one study [20]. The authors speculated that this may be a result of hepatic manipulation and reperfusion. Hyperthermia has also been reported as a consequence of other major hepatic manipulations such as intrahepatic shunt attenuation in dogs and repair of hepatic injuries in humans.

Prognosis

Traumatic Diaphragmatic Hernia

The overall survival rate for animals diagnosed as having diaphragmatic hernias in the published literature is 52 to 94%. Two significant peaks in mortality were noted in a previous review. Survival rates for 40 dogs operated on within 24 hours was 67% and for 8 dogs operated on after 1 year was 37% [18,36,43]. Death in the early group was due entirely to shock and trauma and in the late group was largely unrelated to the hernia. By delaying surgery until 1 to 3 weeks after trauma, survival rates were greater than 90%. More recent reports, however, suggest that prognosis is equally good for animals operated on early, as long as the patient is stabilized appropriately prior to surgery, and following repair of chronic hernias [8,37].

Recurrence rates of traumatic hernia, initially repaired with interrupted sutures of polyglactin 910 through an intercostal exposure, have been reported as 4% for dogs and 5% for cats [44]. Recurrence, in man and animals, is usually a result of a combination of tissue weakness and loss of abdominal domain, leading to elevated intra-abdominal pressures.

Congenital Diaphragmatic Hernia

A surgical success rate of greater than 80% has been reported for repair of peritoneopericardial hernia in dogs and cats [19,20,30].