Diagnosing equine pituitary pars intermedia dysfunction

Equine pituitary pars intermedia dysfunction (PPID), also called equine Cushing’s disease, is caused by an adenoma of the pituitary pars intermedia (PIA). This produces excess secretions of a variety of closely related peptides, including adrenocorticotropic hormone (ACTH), which causes adrenocortical hypertrophy.

PPID is typically a disease of the older equine (over 15 years), although horses as young as seven years have reportedly been diagnosed. There is no breed or sex predisposition. The most common clinical signs of PPID include hirsutism (long curly hair coat), weight loss (usually muscle, particularly along the topline), lethargy, laminitis, polyuria and polydipsia. Additional signs may include hyperhidrosis, exercise intolerance, pot belly, increased susceptibility to chronic infections and fat deposits on the top of the neck, tailhead and also above the eyes.

Approximately 30 percent of horses with PPID also show signs of insulin dysregulation. In these horses, blood insulin levels are high because tissues have a decreased insulin response. Insulin dysregulation is the hallmark of equine metabolic syndrome (EMS), and PPID and EMS can coexist at the same time in a single animal. Knowledge of insulin status is important for assessing laminitis risk and informing dietary management.

How is PPID diagnosed?

Diagnosis of PPID involves the clinical signs present alongside diagnostic testing. Early diagnosis of this condition is crucial to avoid the negative consequences of the disease process; however, this can be challenging as clinical signs can be vague.

Early diagnosis of this condition is crucial to avoid the negative consequences of the disease process; however, this can be challenging

No ideal diagnostic tests exist for PPID. Instead, a variety of tests, or a combination of tests, can be used for diagnosis. A combination of tests helps to avoid the high rates of false positive and false negative results noted with individual tests. This article will break down and evaluate the different diagnostic tests for PPID.

Basal cortisol

Basal cortisol is not very useful as a diagnostic tool as levels are often within or below the normal range unless the animal is acutely stressed due to pain, possibly caused by an attack of laminitis.

Insulin

Basal levels are frequently raised, but insulin alone should not be used to diagnose PPID. This is because insulin levels can also be raised with stress, pain (eg laminitis), obesity and EMS. Furthermore, ponies are inherently insulin resistant, and so levels are naturally raised.

Endogenous ACTH

A baseline ACTH is currently considered the most reliable test for PPID. Horses do not need to be fasted before taking a baseline ACTH, but they should not have consumed grain or a carbohydrate meal within 12 hours prior to the test.

Endogenous ACTH (eACTH) is quite stable in EDTA plasma, and the sample may be sent to the laboratory chilled on ice packs. During testing, eACTH is usually measured by chemiluminescent methodology, such as immulite, and reference ranges are adjusted for the seasons as described by Copas and Durham (2012). This is because eACTH levels are often increased in the autumn months of August, September and October (Copas and Durham, 2012). eACTH concentrations can vary with stress, illness, exercise and, sometimes, diet, so testing the horse at their home environment when they are healthy and not stressed is ideal. Acute laminitis may also result in high eACTH, and testing should be delayed until laminitis or concurrent disease has resolved.

eACTH concentrations can vary with stress, illness, exercise and, sometimes, diet, so testing the horse at their home environment when they are healthy and not stressed is ideal

A baseline eACTH test is good at detecting moderate to more advanced cases of PPID but may not pick up horses early in the disease process. In cases of PPID, the eACTH concentration may be increased to over 30pg/ml. It is important to note that ACTH production by equine pituitary tumours can be variable, and some pituitary adenomas do not result in elevated ACTH.

In cases of suspected PPID where the eACTH is not elevated, the thyrotropin-releasing hormone (TRH) response test can be used. This has been shown to have a higher diagnostic accuracy than basal eACTH alone (Beech et al., 2007). Pituitary adenoma cells seem to lose receptor specificity for hypothalamic-releasing hormones. In most cases, corticotrophs (ACTH-producing cells) are abnormally stimulated by TRH, causing increased ACTH production by the PIA.

TRH eACTH stimulation test

As above, a baseline eACTH sample is collected before TRH is administered intravenously (0.5mg for horses under 250kg and 1.0mg for horses over 250kg). Another eACTH sample is then collected 10 and/or 30 minutes later. In cases of PPID, the eACTH is usually over 120pg/ml at 10 minutes post TRH or over 65pg/ml at 30 minutes post TRH.

It is important to consider that TRH is currently unlicensed in the UK; however, other tests that were previously used for diagnosing PPID have mostly fallen out of favour. This is due to a combination of variable results and the seasonal variation having an effect in some tests. These tests are also included below.

Overnight dexamethasone suppression test

The overnight dexamethasone suppression test was considered the best test to use to diagnose PPID, but it has a significant disadvantage. This is the fact that it should not be used in the autumn months (August, September and October) because an increased number of healthy horses will have increased cortisol post-dexamethasone and thus increased false positives during this period. However, it can be used at other times and is considered an economic and efficient test.

Protocol

1. Take a basal blood sample at about 5pm
2. Inject 0.04mg/kg dexamethasone intramuscularly
3. Take a further blood sample 20 hours later
4. Label samples clearly and request cortisol from the laboratory

Interpretation

Healthy horses should suppress cortisol levels to below 30nmol/l. Please note that normal horses may give false positive results if the test is done in the autumn months (August, September and October).

TRH stimulation test

The TRH stimulation test may be helpful in confirming a diagnosis of PPID in horses; however, a study suggests that the TRH stimulation test alone was not able to distinguish between normal and PPID cases (Eiler et al., 1997).

Protocol

1. Take 5ml of clotted or heparinised blood
2. Intravenously inject 1mg of TRH slowly over one minute
3. Take two further samples of 5ml clotted or heparinised blood at 15 minutes and 60 minutes post TRH injection
4. Label samples clearly and request cortisol from the laboratory

Interpretation

Normal horses show up to a 20 percent increase in cortisol concentration at 15 minutes post TRH stimulation (mean 17 percent increase), but levels usually return to baseline values by 60 minutes (mean 5 percent decrease) (Beech and Garcia, 1985). Horses with PPID show more than a 50 percent rise in cortisol concentrations 15 minutes post TRH (mean 90 percent increase) and levels often remain elevated at 60 minutes (mean 58 percent increase).

Urine cortisol:creatinine ratio

Examining the horse’s urine cortisol:creatinine ratio (UCCR) is a very sensitive screening test to exclude PPID, but it must not be used to make a diagnosis. This is because it is affected by the same factors as cortisol levels. Dilute urine in cases with polyuria/polydipsia may not give reliable results. A morning urine sample is collected for analysis. Normal horses have a UCCR of less than 20 x 10⁶.

Final thoughts

A diagnosis of PPID requires a combination of detailed history, clinical signs and a combination of diagnostic tests

The combined dexamethasone suppression/TRH stimulation test with cortisol measured is no longer used. Since PPID and EMS can coexist, it is important to test for insulin dysregulation. In conclusion, a diagnosis of PPID requires a combination of detailed history, clinical signs and a combination of diagnostic tests. The effect of season and the current health status of the individual should also be taken into account.