Diagnostic testing for pituitary pars intermedia dysfunction in horses.

The research article examines different diagnostic tests for identifying pituitary pars intermedia dysfunction (PPID), a common disease in horses akin to human Cushing disease, assessing their efficacy and proposing a halt in cortisol response to heavy doses of dexamethasone as reliable method for diagnosis.

Introduction to Pituitary Pars Intermedia Dysfunction

• Pituitary Pars Intermedia Dysfunction (PPID) is a slow-developing disorder that can affect horses of all breeds.
• Due to its similarity to human Cushing’s disease, it is sometimes referred to as equine Cushing’s disease.
• The disease affects the pituitary gland, an important part of the endocrine system, responsible for hormone regulation.

Testing for PPID

• In this study, a range of tests were performed on horses suspected to have PPID. The test results of these horses were then compared with those of healthy, control horses.
• These tests focused on the function of the pituitary-adrenocortical function, specifically on the secretion of cortisol, a key hormone that can indicate the presence of PPID.

Inconclusive Tests

• The tests included the measurement of diurnal variations in plasma cortisol concentration, an ACTH stimulation test (using a natural ACTH gel), and a combined dexamethasone suppression test (using a synthetic ACTH).
• All of these tests failed to show a significant difference between the control horses and the horses suspected of having PPID, proving them ineffectual for reliable detection of the disease.

Dexamethasone Suppression Test

• The researchers found a significant suppression of cortisol concentration in response to increasing doses of dexamethasone in control horses. However, this suppression was not observed in horses suspected of having PPID.
• The lack of cortisol suppression in response to dexamethasone thus emerged as a reliable diagnostic marker for PPID in horses.

Conclusion

• The researchers concluded that the dexamethasone suppression test, specifically involving administration of heavy doses of dexamethasone, is a reliable method for differentiating between healthy horses and those with PPID.
• They suggested that this test should be conducted by administering dexamethasone (40 micrograms/kg) either in the evening with sample collection the following morning and noon, or at midnight with sample collection at multiple intervals the next day.