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Diagnostic testing for pituitary pars intermedia dysfunction in horses

N O Dybdal1, K M Hargreaves, J E Madigan

  • 1Department of Pathology, School of Veterinary Medicine, University of California, Davis 95616.

Journal of the American Veterinary Medical Association
|February 15, 1994
PubMed
Summary

The dexamethasone suppression test reliably distinguishes pituitary pars intermedia dysfunction in horses. This equine Cushing disease test is crucial for accurate diagnosis and management of affected horses.

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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.

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  • 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.