Colic Diagnosis in the Horse

Diagnosis

Many different diagnostic tests can be used to diagnose the cause of equine colic, which may have greater or lesser value in certain situations. The most important distinction to make is whether the condition should be managed medically or surgically. If surgery is indicated, then it must be performed with utmost haste, as delay is a dire prognostic indicator.

History

A thorough history is always taken, including age, sex, recent activity, diet, any recent dietary changes, and routine anthelmintic treatment. However, the most important factor is time elapsed since onset of clinical signs, as this has a profound impact on prognosis, and the type of treatment that will be undertaken.

Cardiovascular Parameters

Heart rate rises with progression of colic, in part due to pain, but mainly due to decreased circulating volume, decreased preload, and endotoxemia. The rate should be measured over time, and its response to analgesic therapy ascertained. A pulse that continues to rise in the face of adequate analgesia is considered a surgical indication. Mucous membrane colour can be assessed to appreciate the severity of haemodynamic compromise. Reddening of membranes reflects worse prognosis, and cyanotic membranes indicate a very poor chance of a positive outcome.

Laboratory tests can be performed to assess the cardiovascular status of the patient. Packed Cell Volume (PCV) is a measure of hydration status, with a value 45% being considered significant. Increasing values over repeated examination are also considered significant. The total protein (TP) of blood may also be measured, as an aid in estimating the amount of protein loss into the intestine. Its value must be interpreted along with the PCV, to take into account the hydration status. Blood lactate levels are useful in determining severity of disease, and as a prognostic indicator; levels between 1-2mmol/L are considered normal, while levels above 5.7mmol/L are considered significant. "Colic scores" that combine several parameters can be relatively accurate prognostic indicators, although most laboratory tests have limited use in terms of specific diagnosis.

Rectal Examination

Repeated rectal examinations are a cornerstone of colic diagnosis, as many large intestinal conditions can be definitively diagnosed by this method alone. Other non-specific findings, such as dilated small intestinal loops, may also be detected, and can play a major part in determining if surgery is necessary.

Naso-gastric Intubation

Passing a Naso-Gastric Tube (NGT) is useful both diagnostically and therapeutically. Fluid is refluxed from the stomach, and any more than 2 litres of fluid is considered to be significant. Increased fluid is generally a result of backing up of fluid through the intestinal tract, due to a downstream obstruction. This finding is important as it represents a relatively advanced stage of colic, and is often a surgical indication. Therapeutically, gastric decompression is important, since horses are unable to vomit. If fluid build up occurs, gastric rupture may occur, which is inevitably fatal.

Abdominocentesis

The extraction of fluid from the peritoneum can be useful in assessing the state of the intestines. A sanguinous fluid can be caused by an infarction, which indicates surgery is necessary. However, sanguinous fluid can also be caused by external trauma (e.g. rib fractures), middle uterine artery rupture in post-foaling mares, or by inadvertent bleeding caused by the procedure itself. A cloudy fluid is suggestive of an increased number of white blood cells, which indicates the disease is relatively advanced. The protein level of abdominal fluid can be analysed, and may also give information as to the integrity of intestinal blood vessels. Elevated lactate levels in the sample can also give an indication of the degree of compromise to bowel, particularly as a peritoneal:peripheral lactate ratio. Peritoneal fluid that contains food material can indicate rupture of the gastro-intestinal tract, although care should be taken that intestine has not been punctured inadvertently. A normal peritoneal fluid sample does not rule out a strangulating lesion. For example, in the case of a diaphragmatic hernia, the strangulating gut is contained within the thoracic cavity, so will not affect fluid within the abdominal cavity. A similar situation is true of intussuception, where the strangulating gut is contained with another piece of non-strangulating gut.

Abdominal Distension

Any degree of abdominal distension is usually indicative of a condition affecting the large intestines, as distension of structures upstream of here would not be large enough to be visible externally.

Auscultation

Auscultation of the abdomen, usually performed in a four quadrant approach, can be a useful tool. Auscultation of the ventral abdomen can also be useful in regions where sand impaction is common. Increased gut sounds are not usually found with major changes, and may be indicative of spasmodic colic, or impending diarrhea. A decreased amount of sound, or no sound, may be suggestive of serious changes. Trapped gas, particularly in the caecum, can often be heard by "pinging", where a flick of the finger against the skin over the affected area causes a sharp sound audible through the stethoscope. This sound is sometimes compared to the ringing sound made by a rubber ball hitting a solid surface.

Faecal Examination

The amount of faeces produced, and its character can be helpful, although as changes often occur relatively distant to the anus, changes may not be seen for some time. In areas where sand colic is known to be common, or if the history suggests it may be a possibility, faeces can be examined for the presence of sand, often by immersion in water, or simply by its texture. The presence of diarrhoea is common in sand colic, and can be seen in horses with enteroliths. Otherwise, diarrhoea is usually indicative of a non-surgical condition, although it can be associated with life threatening conditions such as salmonellosis.

Abdominal Ultrasound

Ultrasonographic evaluation of the abdomen is extremely useful in characterizing certain components of the disease process. The amount and character of free abdominal fluid can be determined, as well as the determination of a specific place for safe, high-yield abdominocentesis. The appearance of small intestine, including distension, wall thickness and motility (or lack thereof, often seen as sedimentatioon of digesta) can be extremely important in the decision for surgical or medical therapy. The large colon and cecum can be evaluated for wall thickness (particularly useful in cases of right dorsal colitis), fluidy contents (colitis/diarrhea), and sometimes displacement. The presence of mesenteric vessels associated with the large colon is generally associated with displacement. The normal anti-mesenteric vessels of the cecum can be used to trace its course. Ventral displacement of the spleen with obscuring of the left kidney is associated with nephro-splenic displacement. Visualization of sacculated large bowel immediately ventral to the liver or spleen, or non-sacculated large bowel in the ventral abdomen suggests displacement. The stomach can be evaluated for distension and abnormalities of the wall. Abdominal ultrasound is useful in detecting diaphragmatic or inguinal herniation. Abnormalities of the liver or kidneys, both potential causes of false colic, are often detectable with ultrasound.