Diagnostic testing for pituitary pars intermedia dysfunction in horses

Area of Science:

• Veterinary Endocrinology
• Equine Internal Medicine

Background:

• Pituitary pars intermedia dysfunction (PPID) is a common, progressive endocrine disorder in horses.
• PPID shares clinical and biochemical similarities with human Cushing disease, often termed equine Cushing disease.
• Accurate diagnostic methods are essential for timely intervention and management of PPID.

Purpose of the Study:

• To evaluate the efficacy of various pituitary-adrenocortical function tests in distinguishing horses with PPID from healthy controls.
• To identify a reliable diagnostic test for equine Cushing disease.

Main Methods:

• Comparison of diurnal plasma cortisol variations between control and PPID-affected horses.
• Assessment of ACTH stimulation tests and combined dexamethasone suppression-ACTH stimulation tests.
• Evaluation of a dexamethasone suppression test using increasing dexamethasone doses (5-40 µg/kg) and timed plasma cortisol measurements.

Main Results:

• Diurnal cortisol variations and standard ACTH stimulation tests did not differentiate between control and PPID horses.
• A significant dose-dependent cortisol suppression by dexamethasone was observed in control horses (P < 0.001), but not in PPID horses.
• The dexamethasone suppression test at 40 µg/kg, with specific sampling times, reliably distinguished between PPID and control horses.

Conclusions:

• Standard diagnostic tests for PPID are insufficient for accurate differentiation.
• The dexamethasone suppression test, particularly at a 40 µg/kg dose with appropriate timing, is a reliable diagnostic tool for equine Cushing disease.
• This validated test aids in the accurate diagnosis and management of pituitary pars intermedia dysfunction in horses.