Richard A. Read, BVSc(Hons), PhD, FACVSc
Professor, Small Animal Surgery, Murdoch University, Murdoch, Western Australia, Australia
A hernia can be defined as the protrusion of an organ or part through a defect in the wall of the anatomical cavity in which it lies. Changes in function of both the body cavity and the herniated contents can be important in herniation. These changes may be insignificant but in some cases may result in serious pathophysiological consequences that can lead to acute severe illness and in some cases to the death of the animal. This usually occurs when an organ that is herniating through the defect in the abdominal wall becomes trapped (incarcerated hernia) and/or its blood supply becomes compromised (strangulated hernia).
Incarceration of hernial contents involves the development of adhesions between the contents and the hernial sac or ring. These adhesions may not cause any functional disturbance per se but if they result in occlusion of the lumen of the bowel, a rapid build-up of fluid and gas can occur with important effects on fluid and electrolyte balance. Bladder obstruction in perineal hernias can also result in post-renal uraemia and fluid retention.
Strangulation involves interruption to or obstruction of the vascular supply to the contents of the hernia. Vascular obstruction of hernial contents can occur in two ways. Firstly if incarceration leads to serious luminal obstruction, the distension of the obstructed organ with fluid and gas may eventually result in sufficient intramural pressure to obstruct the vascular supply. Necrosis of the wall (e.g., of the bowel or bladder) will then follow. Secondly, incarceration may directly compromise the vascular supply to the herniated organ resulting in necrosis and rupture. This may then lead on to blood or body fluid loss, toxaemia or septicaemia depending on the organ involved.
Principles of Herniorrhaphy
Four main aims of hernia repair can be identified:
1. Return of viable contents to their normal anatomical location (within the abdomen)
2. Secure closure of the neck of the hernia, thus preventing recurrence
3. Obliteration of any redundant tissue in the sac
4. Use of the patient's own tissues wherever possible
Because the hernial contents are often friable, adequate exposure is essential in all herniorrhaphies. Both the contents and the ring must be exposed sufficiently to allow accurate visual inspection so that the extent of adhesions and their relationship to the vascular supply of the hernia can be properly evaluated. This also allows careful assessment of the viability of the herniated tissue and facilitates its resection if required. The hernial ring may need to be enlarged to allow adequate exposure--this is often required in inguinal hernia repair.
Once all adhesions have been broken down and the viable hernial contents replaced in their normal position, closure by direct apposition of local tissues is the preferred technique. Some local dissection may be required to decrease the tension on the closure. Good examples of this technique are the internal obturator and superficial gluteal muscle flaps used in perineal hernia repair.
Hernias that Present as Emergencies
1. Inguinal Hernia
Inguinal hernias may be direct (direct outpouching of peritoneum and abdominal contents adjacent to the inguinal canal) or indirect (outpouching of peritoneum (vaginal process) and abdominal contents through the inguinal canal.
Indirect inguinal hernia in dogs occurs predominantly in young males and mature females. This type of hernia is also referred to as a scrotal hernia. Emergency situations usually occur in the young male group because the inguinal canal is longer and more narrow in males than it is in females. As a result of this anatomical difference, herniated contents are more likely to become strangulated in males than in females. In addition, the circulation to the testis may become compromised by compression of the spermatic artery and vein.
Signs of scrotal herniation are closely related to the pathophysiology of incarceration and strangulation, so these include local pain, swelling and inflammation. Specific presenting signs depend on the specific abdominal structure that has herniated and become compromised. If small intestine is involved, gastrointestinal signs may predominate. If omental fat alone is involved, pain, swelling and discolouration of the overlying skin may be the primary signs. Testicular swelling and pain are usually present.
Diagnosis is centered on identifying the abnormal local signs and then using palpation and ultrasound to identify the hernial contents. If needle aspiration is considered important then it should be performed under ultrasound guidance.
Supportive therapy will vary with individual cases--if small intestine has become strangulated and has ruptured, signs of local sepsis, peritonitis and in some cases Systemic inflammatory response syndrome (SIRS/septic shock) may be present and aggressive fluid therapy with crystalloids, colloids and antibiotics will be indicated, with surgery performed as soon as the patient is sufficiently stable. Surgical treatment may involve dealing with local and peritoneal contamination with bowel contents. In other cases, surgery will be more routine.
Owners of dogs with scrotal herniation should be counseled to have the dog neutered at the time of hernia correction. The incidence of recurrence of the hernia or persistent swelling of he testis is significant as it is difficult to judge how much to narrow down the inguinal canal when the local tissues are inflamed and swollen.
2. Traumatic Abdominal Hernia
Most traumatic hernias are caused by blunt trauma and involve the ventrolateral caudal abdominal (inguinal or prepubic areas) and paracostal regions. In the latter case, rupture of the diaphragm may also occur. The direction of the traumatic force and resultant changes in intra-abdominal pressure influence the location of the hernia. If the abdominal muscles are contracted at the time of trauma and the glottis remains open, the increase in intra-abdominal pressure will be minimal and avulsion injuries to minimally-elastic structures (e.g., pre-pubic tendon). In contrast, a sudden increase in intra-abdominal pressure may result in abdominal wall rupture. Direct local trauma at the site of the injury may tear the abdominal musculature. Patients with pelvic/pubic fractures may also have herniation through the prepubic or inguinal regions.
Crushing, rupture or avulsion of intra-abdominal organs may occur with blunt trauma, but sharp trauma (such as bite wounds, or gunshot wounds) carries a higher risk of perforation and laceration of intra-abdominal structures.
The masking of the signs of traumatic herniation by the swelling and contusions caused by the trauma itself can pose a very real problem in diagnosing traumatic herniation. Extent of organ herniation is very variable--gravity may help to maintain the abdominal viscera in their normal location, so that external swelling is minimal. Alternatively, the contents may migrate significant distances in the subcutaneous space, so that the site of the swelling may be very misleading as to the location of the hernia.
Other clinical signs reflect the extent of damage to abdominal organs--hypovolaemia due to blood loss (liver, spleen damage) or fluid sequestration into obstructed or strangulated bowel. Concurrent pelvic fractures are common and the clinical signs associated with these injuries may be the most obvious.
Radiography is an important diagnostic tool, due to the limitations in thorough abdominal palpation in the painful, traumatised patient. Subcutaneous emphysema may be present. Definitive signs of herniation are absence of abdominal organ from its normal position, an obvious discontinuity in the abdominal wall, and displacement of abdominal organs into the subcutaneous space. Ultrasonography may be helpful to differentiate fluid-filled hernias from other soft tissue masses. Diagnostic peritoneal lavage should be used in any traumatic hernia case where serious injury to internal organs is suspected.
The choice of surgical approach and the precise location and extent of the surgical incision are very important in facilitating the definitive repair. For the emergency patient, a midline approach should be used so that access is afforded to all possible sites of herniation and all abdominal viscera are accessible for examination and repair of injured structures as required.
3. Perineal Hernia
Perineal hernia is usually a chronic condition but a small subset of patients will present as emergency cases due to prolapse of the bladder and/or prostate into the hernia, resulting in urinary obstruction. The dogs will usually present with a history of straining to urinate and as the pressure of the bladder increases the perineal swelling becomes very hard.
Diagnosis is assisted by the above history and confirmed by either needle drainage of urine via the perineum or relieving the urine via urethral catheter if possible. Once the pressure is relieved, rectal examination should confirm the presence of the hernia.
Treatment options include standard herniorrhaphy techniques but in some cases an abdominal approach to perform cystopexy and colonopexy can be sufficient to eliminate the clinical signs of straining to defaecate and stranguria.
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