Hernia

Description

A hernia is defined as the displacement of an organ through a normal aperture (a true hernia) or pathological aperture (false hernia or rupture). The mobile organs of the abdomen are most commonly displaced through hernias involving the abdominal wall, diaphragm, inguinal canal or femoral canal.

In a true abdominal hernia, the displaced organs are contained within a hernial sac which consists of the parietal peritoneum together with any overlying soft tissues. In an external hernia, the skin would form the outermost layer and the hernial sac and its contents would be palpable. Since the parietal peritoneum is torn when ruptures occur, these do not have defined hernial sacs.

Hernias may be congenital or acquired. Congenital hernias usually result from a failure of physiological apertures (such as the umbilical canal) to close fully during embryonic or neonatal development whereas acquired hernias may be related to trauma, wounds or to physiological weakening of tissues with age, pregnancy or chronic straining.

Diagram of a hernial sac (Courtesy of Elspeth Milne)

Pathogenesis

The sequelae of a hernia depend on whether the hernia is reducible and if the herniated organs are incarcerated or strangulated.

Reducible hernia

The contents of the hernial sac can be moved back to their original position. These hernias tend not to cause clinical disease.

Incarceration

If the hernial ring narrows or if the displaced organs expand, the contents of the hernial sac may be trapped or 'incarcerated'. These structures cannot be moved back to their original position and surgical intervention is usually required to increase the size of the hernial ring. Incarcerated hernias also carry a greater risk of strangulation.

Strangulaton

The contents of the hernial sac are trapped and their blood supply is impaired. Initially, only venous drainage is reduced leading to oedema and congestion but infarction may occur if the arterial blood supply is also affected. Strangulated loops of small intestine are unable to maintain an effective barrier against gastro-intestinal bacteria and bacterial endotoxin, leading to peritonitis, sepsis and endotoxic shock. The intestine may also rupture causing acute septic peritonitis.

Types of hernia

• Umbilical hernias are usually congenital and occur in all of the major domestic species.
• Diaphragmatic hernias may be congenital or, much more commonly, diaphragmatic ruptures may occur following blunt abdominal trauma in small animals. Congenital diaphragmaic hernias are an unusual cause of small intestinal incarceration and strangulation in horses, manifesting as severe colic. If the septum transversum (the embryonic structure that forms the diaphragm) fails to develop fully, Peritoneal-Pericardial-Diaphragmatic hernias (PPDH) or Pleuroperitoneal hernias may occur.
• Inguinal/scrotal hernias may be congenital or acquired. Congenital hernias are rare and usually occur in young male dogs with late testicular descent. It is suggested that this process holds the inguinal canal open for longer than normal, allowing abdominal organs to displace into them. Acquired hernias occur in obese animals (in which the canal is held open by the local accumulation of fat) and in older entire bitches (in which the gravid uterus may be herniated). Inguinal hernias also occur in stallions after castration and may cause severe colic.
• Perineal hernias are false hernias where abdominal organs move into space created by the rupture of muscles surrounding the pelvic roof and anus. Older male entire dogs are particularly affected and these often have concurrent enlargement of the prostate gland.
• Ventral abdominal ruptures occur at sites of wound breakdown (after ventral midline coeliotomy) and in older pregnant mares and cows, particularly those suffering from hydrops uteri.
• Traumatic abdominal ruptures may occur at various locations in the abdomen resulting in prepubic ruptures, femoral hernias and paracostal ruptures in cats.
• Omental/mesenteric rents may result in internal hernias causing partial or complete intestinal obstruction.
• Epiploic foramen entrapment is a form of internal hernia affecting horses. Loops of small intestine may become trapped within the epiploic foramen and omental bursa causing complete intestinal obstruction and severe colic. Anecdotally, horses which 'windsuck' are at particular risk of this form of colic and it requires urgent surgical correction.

Volvulus (Courtesy of Bristol BioMed Image Archive)