Diaphragmatic hernia in horses is uncommon but not rare. Most referral practices are presented with one or more cases annually. The causes are associated principally with trauma to either the thorax or abdomen, but the injury also can occur after exertion such as that experienced by stallions during breeding. Congenital diaphragmatic hernia can occur in neonates when closure of the crura does not occur. Blunt trauma such as falls, kicks, or collisions can result in tearing or rupture of the diaphragm.
Diaphragmatic hernia also can occur without an obvious cause and may precipitate signs of colic, mild to moderate elevations in respiratory rate, or toxic shock secondary to bowel incarceration or strangulation. The size of the tear within the diaphragm is an important determinant of the clinical signs manifested, in that chronic tears may go unnoticed by clients until such time as the abdominal viscera dislocate into the thorax and become compromised. Acute tears in the diaphragm may be associated with hemorrhage into both the abdominal and thoracic cavities. Fractures of the ribs in the caudal thorax have resulted in the “sawing” or incising of sharp bone edges at the fracture site through adjacent diaphragmatic musculature, creating a discontinuity in the sheetlike muscular surface through which abdominal structures eventrate.
Clinical Signs of Diaphragmatic Hernia
Clinical signs of a diaphragmatic hernia may include a moderate to severe increase in respiratory rate, concurrent hemorrhagic shock, colic, and endotoxic shock. If the horse has colic, a rectal examination may suggest a relative absence of normal abdominal viscera. Auscultation may reveal absence of breath sounds in the ventral thorax, and thoracic percussion can define similar areas of decreased aeration. Some horses with diaphragmatic hernias can show relatively few clinical signs, because little pulmonary or bowel compromise occurs. Occasionally, diaphragmatic hernias are discovered incidentally at post mortem, supporting the notion that they can be benign in horses not subjected to heavy exertion.
Diagnosis of Diaphragmatic Hernia
The diagnosis of a diaphragmatic hernia is based largely on a clinical suspicion often confirmed during an exploratory celiotomy of a horse that presents with signs of colic. Before surgery, suspicions of a diaphragmatic hernia can be confirmed by thoracic radiography or ultrasonographic examination. Thoracic radiographs can reveal obliteration of the ventral views of the heart and posterior vena cava and the presence of lines that represent gas-filled loops of bowel within the thorax. The use of ultrasound can demonstrate abdominal viscera in direct contact with the lungs or heart, without obscuring the latter structures from view. During ultrasound examination of the thorax the clinician may observe aberrant structures filled with fluid and/or gas, or may observe peristaltic movements of bowel within the thorax. If the patient presents with signs of colic, the peritoneal fluid obtained from abdominal paracentesis does not consistently reflect bowel compromise because abnormal fluid may be confined to the thoracic cavity.
Treatment of Diaphragmatic Hernia
Treatment of diaphragmatic hernia depends on the degree of clinical compromise. Some horses with a chronic hernia show few clinical signs. In these cases, treatment is optional and, if elected, involves surgical repair. Surgical correction can be complicated by the need for concurrent repair of fractured ribs or treatment of intestinal compromise. The herniated bowel must be returned to the abdomen and strangulating or obstructing lesions of either small or large intestine must be corrected. Surgical repair of the diaphragm often requires the use of surgical “mesh” to obliterate the hernia because primary closure of the muscular diaphragm is usually not a feasible option. Postoperative care includes medical management of any associated clinical disorders and stall confinement until the hernia repairs. In horses that show an immediate response to treatment, the prognosis is relatively favorable.