Diagnosing equine pars intermedia dysfunction: are we there yet?

The research article is about diagnosing equine pars intermedia dysfunction (equine Cushing’s disease), the leading equine endocrine disorder. This study focuses on the pathophysiological mechanisms of this disease, its tests, and the effect of seasonal variations on hormone levels in horses.

Understanding Equine Pars Intermedia Dysfunction

• The study covers equine pituitary pars intermedia dysfunction (PID), the most common horse endocrinopathy. Since limited information is available on the pathophysiological mechanisms of this condition, the research focuses on understanding this condition from the hypothalamus-pituitary dopaminergic system’s perspective, and also considering pituitary oxidative stress.
• Various diagnostic tests have been developed over the years for detecting this condition. These include serum cortisol, adrenocorticotropin (ACTH), glucose, insulin concentrations tests, the overnight dexamethasone suppression test (DST), the ACTH and thyrotropin-releasing hormone (TRH) stimulation tests, urinary and salivary corticoids concentrations, and a combination of these tests.
• DST and plasma ACTH are currently the most accepted tests for diagnosing equine PID.

Pitfalls and Complications in Diagnostic Tests

• There have been disagreements on test results for PID, which could be due to variations in animal, test, or environmental conditions, bias on the part of clinicians, and lack of understanding of the equine hypothalamus-pituitary-adrenal (HPA) axis during health and disease states.
• Despite studies that have looked into diurnal variations of plasma cortisol and ACTH concentrations, there is limited info on seasonal fluctuations of equine HPA axis hormones.
• The research shows that seasonal variations in the HPA axis should be seriously considered when the equine pituitary is being evaluated for dysfunction.

Effects of Seasonal Variations

• The study found that healthy horses and ponies have seasonal differences in plasma ACTH concentrations and DST response, which challenges existing reference values for cortisol and ACTH concentrations, timing for diagnosis, results interpretation, and treatment response timing.
• These seasonal differences imply that a single ACTH sample could lead to a false diagnosis due to normal seasonal hormone fluctuations.
• The study recommended that results should be interpreted with caution depending on the season when the sample was collected.

Implications and Future Directions

• More attention should be paid to clinical findings when there is suspicion of PID, even if test results are positive or negative.
• More research should be done with a larger number of horses affected with PID during different seasons.
• More modern research methods should be explored, including molecular level analysis of the hypothalamic and pituitary levels in horses.
• Finally, the research paper proposes a shift in how PID results are interpreted, and suggests coming together as researchers to develop more appropriate diagnostic and therapeutic alternatives.