Diagnosing EGS is challenging as no single clinical sign is specific for all three forms of the disease (acute, subacute and chronic) and clinical signs exhibited are often diverse, varying depending on the severity or form of the disease. Therefore the diagnosis is often presumptive, based on epidemiological information, case history and clinical signs. Repeat veterinary examinations may be necessary before a diagnosis can be made.
For cases of acute and subacute EGS, reaching a diagnosis is important to distinguish them from other causes of colic that may be treatable, as many of the clinical signs observed may also occur in a substantial proportion of colic cases.
Confirming the diagnosis of EGS is also important for informing the management of other horses on the same premises.
CONFIRMING THE DIAGNOSIS OF EGS
Unfortunately, the most reliable way to diagnose EGS is by conducting a thorough post mortem examination. Microscopic examination of tissues can confirm the presence of the nerve degeneration characteristic of EGS.
In the absence of a non-invasive method for definitive diagnosis of EGS in the live horse, microscopic examination of gastrointestinal tract biopsy samples is the most widely accepted method of confirming the diagnosis. Most commonly, intestinal biopsies are obtained during exploratory abdominal surgery, conducted with the horse under a general anaesthetic. Abdominal surgery also allows the exclusion of some surgical colic diagnoses, which can have similar presenting clinical signs. However, surgery is expensive and often referral to an equine hospital with surgical facilities is required. Recently, a procedure for obtaining intestinal biopsies in the standing horse (under sedation, without the need for a general anaesthetic) using key-hole surgery has been described. In this study, samples of sufficient quality to provide diagnostic information were obtained in 77% of cases.
OTHER TESTS USED IN THE DIAGNOSIS OF EGS
There are no other tests which are specific for EGS; however there are a number of tests that may be of use in providing evidence to support a diagnosis of EGS or in helping to eliminate other diagnoses.
- EGS cannot be diagnosed using routine blood tests; however they can be useful to determine level of dehydration in EGS cases
- routine blood tests can also help in some cases to rule in or out other causes of colic
- recently, increased concentrations of the inflammatory markers serum amyloid A (SAA) and plasma fibrinogen have been reported in EGS cases
- one study found that urine samples from EGS cases had significantly higher protein and creatinine concentrations, a higher specific gravity, and a significantly lower pH (more acidic urine)
- results of urinalysis may be helpful in supporting a diagnosis of EGS, but as these changes can occur in other diseases, they are not specific indicators of EGS
Abdominal fluid samples
- analysis of fluid from the abdominal cavity (peritoneal fluid) can be undertaken on samples obtained by performing a “belly tap” (abdominocentesis) – this is a common procedure in the examination of colic cases
- in EGS cases it may not be possible to obtain peritoneal fluid via abdominocentesis
- changes in peritoneal fluid may be useful in ruling in or out other causes of colic but are not specific for EGS
Phenylephrine eye drops
- one of the most commonly used non-invasive tests is the phenylephrine eye drops test
- a dilute solution of phenylephrine is applied to one eye and within a short period (around 30 minutes), the angles of both upper eyelashes are compared
- this test can reverse the droopy upper eyelid (ptosis) in EGS cases
- a difference in eyelash angle of 22° or more between the phenylephrine-treated eye and the untreated control eye provides evidence to support a diagnosis of EGS
- to ensure correct interpretation of this test it is critical to ensure that the horse has not been sedated, even several hours previously, because sedation will lead to increased responses
- horses without EGS can also exhibit some response to this test; therefore consideration of case history and other clinical signs is vital in interpreting results of the test
- ultrasound examination of the abdomen may be useful for ruling in or out other causes of colic
- acute EGS cases may have distended small intestine which can be recognised on abdominal ultrasound
- rectal examination is one of the most common diagnostic tests used in the investigation of any colic case
- in acute EGS, distended small intestine may be evident on rectal examination
- in subacute cases, a secondary impaction may be detected on rectal examination o impactions are unusual in young horses kept at pasture, so together with case history and other clinical signs, this may provide some evidence to support a diagnosis of EGS
- rectal biopsy is a minimally invasive procedure that can readily be performed in the standing horse, without the requirement of specialist equipment or surgical facilities
- in EGS cases, nerves in the rectum are relatively mildly affected and only a small tissue sample can be obtained via rectal biopsy
- because it is difficult to obtain tissue samples containing enough nerve cells, a negative result from standard rectal biopsy does not rule out a diagnosis of EGS
- a positive result from a rectal biopsy provides strong evidence to support a diagnosis of EGS