Perineal Hernia

What is a perineal hernia?

The pelvic diaphragm is a sheet of muscles that runs from the base of the tail to the floor of the pelvis. These muscles provide an anatomical barrier between the abdominal cavity and the subcutaneous tissues of the perineum, as well as providing support to the rectum.

In dogs, the muscles of the pelvic diaphragm can weaken and separate from the rectum, resulting in a perineal hernia. The cause of this muscle weakening is unknown, but as the condition is seen most commonly in older, entire male dogs, a hormonal cause is suspected. In addition, any condition that causes straining to go to the toilet (for example, prostatic disease) or which increases intra-abdominal pressure (for example, pregnancy) can cause a perineal hernia.

A perineal hernia leads to loss of support to the rectal wall, resulting in difficulty going to the toilet, and risks the herniation of intra-abdominal contents such as the urinary bladder. While perineal hernias can be one-sided, frequently left and right (bilateral) hernias occur.

The most common sign of a perineal hernia is straining and difficulty passing faeces. With a perineal hernia the rectum loses its support and becomes stretched and dilated, which in turn makes it difficult for your pet to pass faeces and which can lead to the accumulation of faecal material and constipation.

It is also common to see a soft swelling under the tail to either side of the anus. This perineal swelling is caused by entrapment of abdominal fat and fluid within the hernia, and can often fluctuate in size.

Occasionally the urinary bladder can flip backwards and become entrapped within the hernia. This will result in your pet acutely straining and often failing to pass urine, and will typically be accompanied by an enlarging perineal swelling. Entrapment of the urinary bladder is an emergency, and immediate veterinary attention is required.

What can be done to treat patients with a perineal hernia?

If you suspect your pet has a perineal hernia you should make an appointment with your vet. Perineal hernias can generally be diagnosed easily in the consulting room by performing a rectal examination. Occasionally, you vet may recommend further imaging such as radiographs or CT, particularly if bladder herniation or other concurrent disease is suspected.

Stool softeners are often prescribed to make it easier for affected patients to pass faeces. If faecal material has accumulated within the dilated rectum, manual evacuation can be performed. Medical management in these cases is usually sub-optimal, and ideally affected patients are referred to a specialist surgeon for hernia repair.

Perineal hernias are repaired by suturing the muscles of the pelvic diaphragm to the muscle surrounding the anorectal wall. Elevation and transposition of the internal obturator muscle from the pelvic floor is routinely performed to achieve a secure hernia repair. As perineal hernias are known to occur more commonly in entire male dogs, castration is usually performed.

Occasionally in severe perineal hernias, the muscles of the pelvic diaphragm are too weak to be effective in the repair, and muscle from elsewhere in the body (typically the back of the thigh) or synthetic mesh has to be utilised.

If there is evidence of serious rectal dilatation, the surgeon may recommend an additional abdominal surgical procedure called a colopexy. This entails pulling the colon and rectum forward within the abdomen, thus restoring the normal cylindrical shape of the rectum, and suturing the colon to the body wall to fix it in its new position. This procedure helps to make it easier for patients to pass faeces.

Bladder herniation will also typically involve an abdominal procedure called a cystopexy at the time of the hernia repair. The bladder is replaced back in its normal position and sutured to the abdominal wall to prevent it from herniating in the future.

What are the risks and benefits of surgery, and what are the outcomes following surgery?

All surgical procedures carry some risk and a number of complications have been reported following this procedure:

Persistent straining. This is one of the most common complications following surgery. Most of the time this is transient and will resolve within a couple of weeks of surgery. If it persists beyond this time, stool softeners can be helpful. In patients with significant rectal dilatation, a colopexy procedure usually proves beneficial.
A perineal hernia developing on the previously unaffected side. It is quite common for perineal hernias to affect both sides. Castration and treatment of any conditions that predispose to straining or increased abdominal pressure may help to prevent the second side failing.
Hernia recurrence. We can see hernia recurrence if the muscles of the pelvic diaphragm are very weak, or if there is persistent straining following surgery. When the internal obturator muscle transposition technique described above is performed by a specialist surgeon, the recurrence rate is less than 10% of cases.
Faecal incontinence. This is a very rare complication of this surgery and is caused by excessive damage to the muscle that surrounds the rectum and anus (the external anal sphincter muscle) or to its nerve supply. In experienced hands this complication is very rare and usually only transient.
Wound complications and infection. The incidence of wound complications such as surgical site infection is higher than that of surgeries performed in other areas of the body, mainly because of the proximity of the anus to the surgical field. However, the incidence of major wound complications remains less than 5%.

In spite of the potential complications of this surgery, the success rates are very high and the majority of patients will see a resolution of their clinical signs. In the hands of an experienced specialist surgeon, the incidence of the above complications is low. If your pet has a perineal hernia, you should talk to your vet about referral for surgery.

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