IN-DEPTH: ENDOCRINOLOGY IN GERIATRICS


with concurrent ID and PPID are highly susceptible to endocrinopathic laminitis, often with the addi- tional complication of subsolar abscesses. Age is also an important indicator of risk of endocrine disease. The author does not make a diagnosis of PPID in horses younger than 14 years of age in the absence of strong supporting clinical and diagnostic evidence. There are several factors that can in- crease adrenocorticotrophic hormone (ACTH) con- centration other than PPID, making test results alone unreliable (see below). Thrifty horses will have a more profound seasonal activation of pars intermedia response than a nonthrifty horse.5–7 Both baseline ACTH and ACTH after thyrotropin- releasing hormone (TRH) administration may be significantly above the reference interval in easy keepers during the fall, without additional clinical or pathological signs of PPID. In other words, false positive tests for PPID are common in thrifty horses in the fall. Although these horses warrant close monitoring over time, the author would not choose to treat these horses as having PPID if their only clinical signs can be attributed to ID, such as lami- nitis, obesity, or regional adiposity. Although a diagnosis of PPID in a horse younger


than 14 years should not be made without strong additional evidence, PPID does occur in younger animals. In addition to the age-related typical PPID, there is a very uncommon, early onset PPID that likely has a strong genetic component. These early onset cases are most often rapidly progressing and, in the author’s experience, have clear clinical and diagnostic indications of PPID. The horse in Figure 1 was initially observed to shed abnormally at 11 years of age. Antemortem testing strongly supported a diagnosis of both ID and PPID, with an abnormal dexamethasone suppression test, plasma ACTH test, and serum insulin concentration. Di- agnosis of PPID was confirmed at 13 years of age at postmortem examination by the presence of a Grade 5 pars intermedia adenoma.


Clinical Signs: What Are the Clinical Signs That Make You Suspect an Endocrinopathy?


Clinical signs: If obesity, regional adiposity, easy keeper, laminitis of unknown origin, or seasonal laminitis are the clinical signs, one should be con- cerned with ID. A horse with any of these clinical signs needs to


have objective and subjective measures of body weight, body condition, and regional adiposity as part of the examination. Body condition scores and neck circumference measures should be recorded in the medical record, along with a body weight either by scale or weight tape. Multiple measurement strategies are preferred, as the body condition scor- ing system is not as accurate in obese animals.8 Although obesity, either generalized or regional, is a primary clinical sign of EMS, not all horses with EMS are obese and not all obese horses have EMS. Therefore, one can think of obesity as a biomarker of


180 2018  Vol. 64  AAEP PROCEEDINGS


ID and its presence should trigger one to perform a diagnostic test to assess insulin regulation to deter- mine laminitis risk. Examination of hoof growth characteristics, hoof heat and pulse, hoof pain as assessed by hoof testers, and gait analysis should be part of the evaluation of horses with these clinical signs. Digital photographs are ideal for recording changes in hoof growth patterns. Hoof radiographs are highly recommended. Diagnostic testing to evaluate insulin regulation should include an oral sugar test and/or an assessment of tissue-level insu- lin resistance (e.g., ITT). If an obese horse that is an easy keeper has a


normal oral sugar test and/or a normal ITT, it may be a metabolically healthy obese horse or the test results may be a false negative. Either way, you are going to manage this horse similarly. Because it isn’t possible to determine if an obese horse with normal insulin regulation testing is going to prog- ress to ID (is in a pre-ID, pre-EMS state), the most prudent approach is to consider this horse at higher than normal risk of laminitis. Work with the client to adjust diet and environment to facilitate weight reduction in the horse. A therapeutic plan will in- clude dietary management, hoof care, and possibly pharmaceutical interventions, as will be discussed in a subsequent session.9 Monitor the horse with scheduled assessments of the insulin axis (OST or ITT) and full examinations of its feet, because both of these will help in establishing laminitis risk more than body weight or condition alone. If this horse is young (14 years) and has no


clinical signs that are not a direct outcome of obesity or laminitis, then diagnostic tests for PPID may not be necessary, other than to provide a baseline. Testing for PPID should be included in the work up of all aged ID suspects or those with additional clinical signs that suggest PPID. This is critical because horses with concurrent ID and PPID are going to have a different treatment plan and a dif- ferent prognosis with a greater risk of laminitis than those with either disease alone.


Clinical Signs: If an Aged Horse Is Shedding Ab- normally, Then One Should Rule Out PPID If a horse that is in its middle teens or older is shedding late, incompletely, or not at all, chances are that it has PPID and a laboratory test will sup- port the diagnosis. These are not difficult cases to diagnose, although it may be necessary to perform a dynamic endocrine test (e.g., TRH stimulation test) earlier in disease progression.


Clinical signs: If the horse has less specific clinical signs including muscle atrophy, bacterial infections, changes in behavior, abnormal sweating, polyuria, infertility, exercise intolerance, ligament, or tendon injuries you might suspect early PPID. A logical approach to diagnosis is critical to avoid mislabeling the horse with PPID, resulting in lifelong treatment or failing to treat a horse with true PPID.


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