A hernia is an abnormal protrusion of an organ or part of it through the containing wall of its cavity, beyond its normal
confines. A diaphragmatic hernia is a protrusion of the abdominal viscera through the diaphragm. In the dog and cat, traumatic
diaphragmatic hernias are common, whereas the congenital type is infrequently seen. The diaphragm is not essential for life
as the entire diaphragm can be removed in a newborn cat or dog and the animal will survive.
The diaphragm is a musculotendinous structure that separates the thoracic and the abdominal cavities. The diaphragm projects
into the thoracic cavity like a dome. On the thoracic side it is separated from the pleura by the endothoracic fascia and
on the abdominal side, is separated from the peritoneum by the transversalis fascia. The fascia and serosa are so thin in
the dog that over the tendinous portion they can only be visualized microscopically. Diaphragm attaches to the lumbar vertebrae,
the ribs, and the sternum. Contraction of the diaphragm is a major force contributing to ventilation. The diaphragm is composed
of a U shaped central tendon and 4 muscle groups: the pars sternalis, the pars lumbalis and the paired pars costalis. The
pars lumbalis of the diaphragmatic musculature is formed by the right and left diaphragmatic crura, the right crus being considerably
larger than the left. Seen from the abdominal cavity each crus of the diaphragm is a triangular muscular plate whose borders
produce the tendinous portions. The musculature of the crus medial is the thickest (5-6 mm). The pars costalis on each side
consists of fibers radiating from the costal wall to the tendinous center. The pars sternalis is an unpaired medial part unseparated
from the bilateral costal portions. The diaphragm is composed of only one layer of muscle and two layers of tendon and therefore
is weaker than the multilayered abdominal wall. The central tendon of the diaphragm of the dog is relatively small. In its
tendinous portion, transverse fibers course from one side to the other as a reinforcing apparatus. The motor innervation
of the diaphragm is supplied by the paired phrenic nerves. The phrenico abdominal arteries are the principal blood supply
to the diaphragm. Several structures traverse the diaphragm through one of the three foramens. The caval foramen is located
in the central tendon and allows passage of the caudal vena cava. The esophageal hiatus and aortic hiatus are located in the
pars lumbalis of the diaphragm. The esophageal hiatus allows passage of the esophagus and vagal trunks. The aortic hiatus
is bordered by the paired crural tendons and permits passage of the aorta, azygous vein, and thoracic duct.
The stomach and liver attach by ligaments to the concave peritoneal surface of the diaphragm.
Types of diaphragmatic hernias
• Congenital pleuroperitoneal hernia
• Congenital peritoneopericardial hernia: most common congenital diaphragmatic defect, may remain asymptomatic, associated
with other midline defects: ventricular septal defect, abdominal hernia.
• Traumatic diaphragmatic hernia: the most common form in dogs and cats: 80% of the cases. Nature of the trauma, multisystem
injury, and shock are potential complications in traumatic diaphragmatic hernia
• Hiatal hernia: usually congenital, common in Sharpei, sliding (Figure 1) or paraesophageal.
Diaphragmatic hernia often is missed during the initial assessment after trauma, so a high index of suspicion for this condition
should be maintained in animal that had experienced significant trauma. Clinical findings that suggest diaphragmatic hernia
include dyspnea, tachypnea, cyanosis, paradoxical breathing, and muffled heart and lung sounds. The abdomen may appear empty
on palpation. Auscultation may reveal muffled heart and lung sounds on one side of the chest. Radiographic findings that
support a diagnosis of diaphragmatic hernia include loss of the diaphragmatic silhouette, pulmonary atelectasis, and presence
of fluid dense structures in the chest or the pericardial sac. A gastric or bowel gas pattern within the thoracic cavity
or the pericardial sac confirms the diagnosis. An upper gastrointestinal study might be required to confirm the diagnosis.
Megaesophagus is commonly associated with a hiatal hernia.
Chronic diaphragmatic hernia occurs as a delayed presentation or failure of diagnosis after trauma. Presentation for chronic
diaphragmatic hernia can occur years after the original trauma. Usually presentation is due to entrapment of a liver lobe
that produces significant pleural effusion.