Diagnostic Options for Equine Cushing’s Syndrome, or Pituitary Pars Intermedia Dysfunction (PPID)
The clinical entity of PPID has been recognized for several decades and several aspects of endocrine dysfunction have been identified in affected horses. Pituitary pars intermedia dysfunction (PPID) occurs when oxidative stress reduces tonic dopaminergic inhibition of pars intermedia melanotropes, giving rise to hyperplasia, microadenomas, and macroadenomas.
Detecting excessive endogenous plasma adrenocorticotropic hormone (ACTH) derived from the abnormal pars intermedia is the most common diagnostic test for PPID. Although comparable in performance to endogenous ACTH, the overnight dexamethasone suppression test (ODST) is more laborious and is therefore falling out of favor. Sensitivities of both endogenous ACTH and the ODST are acceptably high in equids with advanced PPID, but not in mild cases. A more sensitive option is the thyrotropin-releasing hormone (TRH) stimulation test (measuring ACTH response).
Endogenous ACTH has supplanted endogenous cortisol measurement.
All PPID tests may yield false positives in animals with severe systemic illness, stress, or pain.
Rather than avoiding autumn (mid-July to mid-November) testing, exploit heightened seasonal responsiveness of the hypothalamic-pituitary-adrenal axis to increase diagnostic test sensitivity. Currently, seasonal reference ranges are only established for endogenous ACTH.
Endogenous ACTH concentration
- Collect EDTA plasma at any time of day
- From mid-November to mid-July, a basal ACTH concentration > 30 pg/mL may support a diagnosis of PPID
- From mid-July to mid-November, a basal ACTH concentration > 50 pg/mL may support a diagnosis of PPID
Overnight dexamethasone suppression test
- Collect baseline serum cortisol sample in the late afternoon
- Administer dexamethasone at 40 µg/kg IM (20 mg to a 500 kg horse)
- Collect serum cortisol sample(s) between 15 – 19 hours later
- Failure of cortisol suppression < 30 nmol/L supports PPID
Thyrotropin-releasing hormone (TRH) stimulation test
- Do not perform within 12 hours after an oral sugar test (OST) or a grain meal due to blunting of pituitary ACTH responses to TRH
- Collect a baseline EDTA plasma sample for ACTH measurement
- Administer 0.5 mg (horses < 250 kg) or 1.0 mg (horses > 250 kg) of TRH IV
- Collect an EDTA plasma sample for ACTH measurement exactly 10 minutes after TRH administration
- A post-TRH ACTH concentration > 110 pg/mL may support a diagnosis of PPID
PPID and Insulin Dysregulation
Insulin dysregulation is common and is likely associated with an increased risk of laminitis and poorer long-term prognosis; testing for insulin dysregulation is recommended in all PPID cases. Many horses with PPID will acquire altered glucose metabolism due to insulin resistance. Affected horses may have increased circulating concentrations of insulin as an indicator of the increased insulin secretion required to maintain normal blood glucose. Some horses will develop pronounced elevations of insulin accompanying high-normal or elevated serum concentrations of glucose.
For practical clinical application, insight as to the presence of insulin resistance can be made by measurement of insulin in a baseline serum sample, with the specimen collected and harvested in a manner to have a good quality glucose result. The sample should not be collected within 4 hours after a meal composed of grain or other rapidly absorbed carbohydrate. Aside from the timing of sampling in reference to a grain meal, there are no restrictions to feeding of hay. Of the tests described in this section, the insulin assay is of the least sensitivity and specificity for diagnosis of PPID. Insulin resistance is also a component of the condition in horses currently referred to as Equine Metabolic Syndrome.
PPID versus Equine Metabolic Syndrome (EMS)
At the time of diagnosis, horses with PPID are >15 years of age, and usually older than 20 years. The typical clinical signs include hirsutism, loss of muscle, and accumulation of fat in the neck. Insulin resistance is common and some horses may develop diabetes mellitus. There is susceptibility for laminitis.
Horses with EMS tend to be of middle-age (<15 years). The physical appearance is one of generalized or regional (cresty neck) obesity. There is increased risk for development of laminitis. Contributing factors include diets of a high glycemic index (lush new pasture or grain diet), and limited exercise. There are likely to be genetic factors with predisposition for glucose intolerance. When evaluated for endocrine dysfunction, these horses have insulin resistance but normal results for pituitary-adrenocortical function. It is possible that some of these horses are in early stages of PPID.
Diagnostic Options for Equine Metabolic Syndrome (EMS)
Equine Metabolic Syndrome (EMS) is defined as the triad of generalized and/or regional adiposity, insulin dysregulation, and either clinical or subclinical laminitis. Genetic predispositions towards abnormal glucose and insulin metabolism, combined with environmental and physiologic factors including obesity, carbohydrate-rich diets, and concurrent illnesses, culminate in insulin resistance, deranged glucose metabolism, and dyslipidemia. Resting hyperinsulinemia develops as pancreatic β-cells compensate, however mildly affected horses may have a normal insulin concentration unless challenged with oral or intravenous glucose. Hyperinsulinemia is a major risk factor for laminitis and subclinical laminar damage may be ongoing for years prior to recognition. Recognition and management of EMS before the onset of laminitis is the goal.
Resting insulin and glucose concentration
- Collect serum sample either in the fed state or after 6-12 hours of fasting
- Fasted: insulin concentration > 145 pmol/L (20 μU/mL) supports insulin dysregulation
- Fed: insulin concentration > 360 pmol/L (50 μU/mL) supports insulin dysregulation and between 145 – 360 pmol/L (20 – 50 μU/L) suggests possible insulin dysregulation
- Persistent hyperglycemia (> 6.9 mmol/L; 125 mg/dL) indicates partially decompensated insulin resistance (if hyperinsulinemic) or diabetes mellitus (if normoinsulinemic)
Oral sugar test (Karo® Light Syrup)
- More sensitive than measuring resting glucose and insulin concentration
- Leave only 1 flake of hay in stall or dirt pen after 10pm the night prior to testing
- The following morning, administer 0.15 mL/kg Karo® Light Syrup orally using 60-mL catheter-tip syringes (e.g. 75 mL per 500 kg)
- Collect serum sample between 60 – 90 minutes after syrup administration
- Normal response if insulin concentration < 320 pmol/L (45 μU/mL); insulin dysregulation supported if insulin concentration > 430 pmol/L (60 μU/mL); mild insulin dysregulation supported if insulin concentration is between 320 – 430 pmol/L (45 – 60 μU/mL)
- Excessive glucose response if > 6.9 mmol/L (125 mg/dL)
- Keep sample chilled and separate serum (1.0 mL), ideally within 4 hours of collection; store serum either refrigerated (no longer than overnight) or frozen, and ship on ice.
On April 27, 2016 the MSU VDL hosted a webinar on Equine Endocrine Testing. Questions and Answers from that session are available and cover a range of topics.