Constipation is a term used to describe absent, infrequent or difficult passage of faeces (stools). Constipation results in the retention of faeces within the colon or rectum (large bowel). Animals with constipation may exhibit signs of pain when trying to pass faeces (defecate), and this may be accompanied by excessive straining. If there is prolonged constipation the faeces become impacted in the colon and rectum. This is known as obstipation. With repeated bouts of constipation or obstipation the colon can become distended and loses its ability to propel faeces towards the rectum in a condition called megacolon, literally meaning large colon.

Causes of constipation

Cats can be reluctant to defecate because of behavioural reasons or medical reasons, such as pain. Behavioural reasons may include reluctance to use a dirty litter tray, competition for litter trays, dislike of the litter tray substrate used and association with an unpleasant experience whilst defecating. Pain in the lower back, tail head, hips or knees may make adoption of the squatting posture and straining uncomfortable. Foreign bodies within the colon or anal area will also result in painful defecation, as will anal sac infections, or abscesses (eg, cat bites) around the anus. Inflammatory conditions of the bladder may also make defecation painful.

Dehydration

The colon is the main area of the gastrointestinal tract from which water is reabsorbed. Cats that are dehydrated will reabsorb more fluid from the colon, resulting in very dry, hard faeces and constipation. One of the most common conditions with which this is associated is chronic renal failure. Cats with chronic renal failure lose excessive amounts of water in their urine, and although their water consumption increases, they are often still dehydrated and can therefore become constipated.

Neurological

The nerves that control the contraction of the bowel may be damaged as a result of trauma, eg, road traffic accidents or tail-pull injuries. In some cases, faecal incontinence (leaking out) may arise, but in others, faecal retention is seen, and may also be accompanied by urinary retention (inability to pass urine).

Obstruction to the colon

Pelvic canal deformities can give rise to narrowing of the pelvic canal and compression of the colon where it passes through the pelvic canal. Although birth defects or developmental problems such as nutritional deficiencies can result in narrowing, the most common cause is trauma resulting in a fractured pelvis. Faeces build up behind the area of constriction and megacolon can ultimately develop.

As well as narrowing from external pressure, an area of the colon itself may become narrowed. This is termed a stricture, and may be associated with scar tissue in the bowel wall (eg, from a previous injury or surgery), or some types of tumour that can infiltrate the bowel wall. Tumours may also cause an obstruction within the interior of the colon, as can foreign bodies, preventing passage of faeces past them.

Idiopathic megacolon

Although the above factors may be identified as causes of chronic constipation which progress to megacolon, in the vast majority of cases, no underlying cause is identified, and the condition is known as idiopathic megacolon. In idiopathic megacolon, the smooth muscle that normally contracts to propel the faeces towards the rectum loses its ability to do so. This is thought to be caused by a failure in neuromuscular transmission - a failure to transfer the command electrical impulses from the nerves to the smooth muscle.

A more extensive list of conditions that may cause constipation is given in Table 1.

Signs of constipation

A decreased frequency of defecation, or the production of small, hard dry stools are the signs most commonly observed. Increased trips to the litter tray, accompanied by non-productive straining are also common findings, but it may be difficult to decide whether the cat is trying to urinate or defecate. In cases of obstipation, the cat may pass faecal fluid, rather than formed faeces. This arises as a result of the retained faeces irritating the lining of the large intestine and stimulating fluid secretion. This can be mistaken for diarrhoea. Cats may lose their appetite and start vomiting as the condition progresses. In severe cases cats may collapse and appear very cold (hypothermic).

Diagnostic tests

In addition to obtaining the history and performing a physical examination, veterinary surgeons may perform further diagnostic tests. Blood tests are useful to check for underlying systemic problems such as kidney disease, or calcium or potassium abnormalities that can lead to muscle weakness. Urine tests enable kidney or bladder diseases to be detected. X-ray examination can help with the identification of back, hip and pelvic problems, and allow the degree of the distension of the colon to be ascertained (see Figure 1).

Further investigations that may be performed include:

•  Barium enema - introduction of fluid into the colon that can be seen clearly on an X-ray image.    Physical abnormalities in the colon can then be identified.

•  Colonoscopy - visual examination of the colon using an endoscope

•  Colonic biopsy - collection of small amount of tissue from the colon for examination under a microscope.

Treatment

Fluid therapy

An intravenous drip may be required for constipated cats to overcome dehydration. This may also enable important electrolytes (substances vital for cells to function normally) such as potassium to be given. Longer term home-care to improve the cat's hydration may consist of changing from dried food to wet food, adding water to the food, or subcutaneous fluid administration.

Laxatives

Laxatives are medicines used to loosen bowel contents to allow defecation. The are classified as bulk-forming, emollient, lubricant, hyperosmotic or stimulant, according to their mode of action (see Table 2). All cats should be well hydrated before receiving them, as they retain water within the colon.

Enemas

Mild constipation may be alleviated with a lubricant laxative such as Micralax. More severely affected cats are likely to require general anaesthesia for a warm water or saline enema (introduction of fluid into the rectum), with manual evacuation.

Prokinetic agents

Prokinetic drugs act to improve the contraction of the bowel, thereby propelling digesta from the stomach towards the rectum. They act by improving neuromuscular transmission and are useful in mild to moderate cases of megacolon. The most effective of these drugs, Cisapride, has now been withdrawn. Unfortunately, the replacement human drugs that are available target different neurotransmission sites and have not been as effective for cats. It is hoped that new human drugs currently available in other countries may soon be available in the United Kingdom, although their use in cats would be 'off licence'.

Surgery

Surgery is indicated if medical management is unable to resolve the constipation. Surgery consists of removal of the dilated portion of colon known as a subtotal colectomy, and may be performed with or without preservation of the ileo-colic valve. This value acts a one way gate between the last portion of the small intestine, the ileum, and beginning of the large intestine (colon) (see Figure 2). Advantages and disadvantages associated with these procedures are given in Table 3.

Prognosis

Chronic constipation is a relatively frequent problem encountered in feline practice. Although milder cases can often be successfully managed medically, many cats do eventually require surgery. It is hoped that future work will shed light on the causes of idiopathic megacolon, and that investigations with new prokinetic drugs will benefit medical management.