IN-DEPTH: GERIATRIC MEDICINE/METABOLICS


been associated with PPID in aged horses.14 Fast- ing conditions are achieved by leaving only one flake of hay with the horse after 10 PM and drawing blood the next morning, or feeding as normal in the morn- ing and then collecting the blood sample after 4 PM. An oral sugar test (OST) has been introduced to


better assess postprandial hyperinsulinemia in equids,15 and this test detects insulin dysregulation in horses with normal fasting insulin concentra- tions. The test is performed by fasting the horse as described above and then administering corn syrupa at a dosage of 0.15 mL/kg PO (75 mL for a 500-kg horse). Syrup is given by mouth with the use of 60-mL catheter-tip syringes, and blood samples are collected for glucose and insulin measurements 60 and 90 minutes later. The owner can administer the corn syrup before the veterinarian arrives.


Interpretation


● Results are normal if the insulin concentration is 45 U/mL (radioimmunoassay) at 60 and 90 minutes.


● Results indicate hyperinsulinemia if the insu- lin concentration is 60 U/mL at 60 or 90 minutes.


● Results are considered equivocal if the insulin concentration is 45 to 60 U/mL at 60 or 90 minutes. Testing is repeated at a later time, or the combined glucose-insulin test should be considered.


● Results indicate an excessive glucose response if the glucose concentration is 125 mg/dL at 60 or 90 minutes.


Owners sometimes express concern about admin- istering corn syrup to horses with suspected hyper- insulinemia or laminitis. Problems have not been encountered in the author’s experience, but a two- step approach can be adopted if necessary. The first step is to measure the fasting insulin concen- tration. If the concentration is 20 U/mL, then this result provides evidence of insulin dysregula- tion and further diagnostic testing is optional. It is still helpful to perform an OST in a horse with modest hyperinsulinemia to assess insulin re- sponses to oral sugars and to make diet recommen- dations, but this is not essential. If the fasting insulin concentration is 20 U/mL, then the OST is recommended to complete the evaluation, and the owner can be reassured that the test is safe to per- form in their horse. Resting triglyceride and leptin concentrations can provide additional information in the management of endocrine disorders in aged horses. Hypertriglyc- eridemia has been identified as a predictor of lami- nitis risk in ponies, with cutoff values of 57 and 94 mg/dL established from two studies of the same population.2,3 A lower cutoff value of 27 mg/dL has been proposed for horses by McCue et al16 after reviewing data from an EMS genetics study, and


breed-specific reference ranges may be forthcoming. Leptin concentrations can be measured for diagnos- tic purposes if blood samples are sent to the Cornell Animal Health Diagnostic Laboratory (Ithaca, New York). When horses of approximately the same body condition score were compared, horses with high leptin concentrations (12 ng/mL) had signifi- cantly lower insulin sensitivity than those with nor- mal leptin concentrations (2 ng/mL).14 Higher- than-normal leptin concentrations (4 ng/mL) indicate that adipose tissues are abnormal and se- creting excessive amounts of leptin. Other markers of metabolic dysregulation may be incorporated into diagnostic panels in the future. Wooldridge et al17 recently validated an ELISA for high-molecular-weight adiponectin and reported concentrations of 3.6  3.9 g/mL (mean  SD) and 8.0  4.6 g/mL for obese and lean horses, respec- tively. Serum amyloid A 18 and ferritin19 concen- trations are also associated with obesity and insulin resistance and may provide additional evidence of metabolic dysregulation.


4. Medical Management of PPID


Pergolide mesylateb is administered to horses with PPID to restore dopaminergic inhibition of mela- notrophs. This ergot alkaloid drug binds to D2 re- ceptors and inhibits proopiomelanocortin hormone synthesis.20 Pergolide is prescribed at a starting dose of 0.002 mg/kg (1 mg for a 500-kg horse), and the dosage range extends to 0.01 mg/kg (5 mg for a 500-kg horse). Approximately 30% of horses ex- hibit anorexia when treatment is initiated, and this problem can be avoided by starting with half of the dose for the first 2 days. Cyproheptadine can be administered in combina-


tion with pergolide at a dosage of 0.25 mg/kg PO q 12 hours.8 When these drugs were compared as single treatments, pergolide was more effective. Seven- teen of 20 horses (85%) with PPID improved after pergolide treatment, compared with only two of seven horses treated with cyproheptadine.20 Cypro- heptadine antagonizes serotonin, which is thought to be a stimulatory neurotransmitter for pars inter- media melanotrophs, and mild sedation is an occa- sional side effect of treatment. The author considers cyproheptadine therapy as an additional treatment when the horse reaches a pergolide dos- age of 0.006 mg/kg (3 mg/d for a 500-kg horse). The goals of medically managing PPID in an aged


horse depend on disease severity. In early PPID, the pergolide dosage should be adjusted until plasma ACTH concentrations return to reference range, with adjustments after 30 days. However, this goal cannot be attained in some advanced cases. Clinical signs are important to monitor in both sit- uations, and the primary goal when treating ad- vanced PPID is to achieve a clinical response. This response can sometimes be seen when pergolide is administered at a lower dosage over a long period of time, so even owners with financial limitations


AAEP PROCEEDINGS  Vol. 59  2013 307


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