MANAGEMENT OF HYPERINSULINAEMIA
TABLE 1 Summary of diagnostic tests, reference ranges and interpretation [Equine Endocrinology Group: Recommendations for the Diagnosis and Management of Equine Metabolic Syndrome (EMS) 2022].
Resting (basal) insulin concentration Assay
RIA Immulite 1000
RIA Immulite 1000
RIA Immulite 1000
Immulite 2000 XPi Immulite 2000 XPi Immulite 2000 XPi
Oral sugar test – 60 and/or 90 min insulin concentration RIA
Immulite 2000 XPi
Insulin concentration (μU/mL) <20
20–50 >50 <31
30–75 >75
>45 (0.15 mL/kg) >65 (0.45 mL/kg)
>63 (0.45 mL/kg)
perform an OST, it is currently recommended that the horse under- goes a 3–6 h fast followed by oral administration of either 0.15 or 0.45 mL/kg of corn syrup (Karo® Light, 1 gm/mL carbohydrates; do not use Karo® Lite syrup which has 0.67 gm/mL of carbohydrates) followed by blood collection at 60 and/or 90 min for measurement of serum insulin concentration (EEG, 2022 ). Test performance is im- proved by the administration of the 0.45 mL/kg dose of corn syrup (Jocelyn et al., 2018 ). Ideally, both time points would be measured, but for practical reasons, this may not be possible in a field setting. A recent study performed in non- laminitic ponies demonstrated that measurement of insulin concentration 60 min after adminis- tration of corn syrup was an important predictor of future laminitis risk (Knowles, Elliott, et al., 2023 ; Knowles, Harris, et al., 2023 ). If time constraints pose a challenge in a field setting and if only one blood sample is possible, measurement of insulin at 60 minutes is recommended as it does not adversely affect the accuracy of the test (Clark et al., 2023 ). The interpretation of results to an OST is presented in Table 1 . Season might potentially impact the repeatability of the OST,
however, studies to date have yielded conflicting results (Karikoski et al., 2022 ; Macon, Harris, Bailey, et al., 2022 ; Macon, Harris, Barker, & Adams, 2022 ) One study from Finland found no differences in the insulin responses to the OST across seasons (Karikoski et al., 2022 ), however, a recent study from Kentucky found significant effects of season on insulin responses to an OST in both ID and non- ID horses over a 2 year period (Macon, Harris, Bailey, et al., 2022 ; Macon, Harris, Barker, & Adams, 2022 ). The latter study found that basal and insulin responses to an OST were higher in spring compared to autumn and summer and winter OST responses were higher than the autumn (Macon, Harris, Bailey, et al., 2022 ; Macon, Harris, Barker, & Adams, 2022 ). Clinicians should therefore consider seasonal varia- tion when monitoring horses using the OST. A fed versus fasted state might also potentially impact the insu- lin response to an OST and should be considered when interpreting
Interpretation Nondiagnostic
Suspect ID ID
Non- diagnostic Suspect ID ID
ID ID
results. A study in ponies found that fasted ponies had a significantly higher insulin response to the OST compared to the fed ponies (off- pasture)
(Knowles et al., 2017 ). A recent study performed in ID
horses found no differences in insulin responses to an OST between the fed versus fasted state (Macon et al., 2021 ). These studies indi- cate that clinicians should ensure the same fed versus fasted condi- tions if using the OST as a monitoring tool over time. An oral glucose test (OGT) has been developed and can be uti-
lised in regions where corn syrup is difficult to source (Bertin & de Laat, 2017 ). The diagnosis of hyperinsulinaemia is based on a serum insulin result >80 μU/mL 120 min after the ingestion of 0.75–1 g/kg of glucose powder (de Laat & Sillence, 2017 ). The glucose may be administered via nasogastric tube or offered in a small amount of bran or chaff to be consumed by the horse. It is important to note that some horses may be reluctant to consume the full dose of glu- cose powder. More recently, a novel, palatable carbohydrate pellet has been evaluated (de Laat et al., 2022 ) as a suitable alternative to the traditional OGT but, at the time of writing, the pellets are not commercially available. It is important to note that the diagnosis of hyperinsulinaemia
is not a dichotomous outcome but instead, a continuum of severity (Knowles & Grieve, 2020 ). The results of any test should therefore be interpreted on an individual basis in conjunction with clinical as- sessment. Early diagnosis of hyperinsulinaemia is important as it al- lows for management strategies to be implemented that can reduce the incidence of laminitis (Morgan et al., 2016 ).
Pituitary pars intermedia dysfunction
Pituitary pars intermedia dysfunction (PPID) is a condition that com- monly affects aged horses and results from loss of dopaminergic inhibition of the pars intermedia leading to increases in circulating adrenocorticotropic hormone (ACTH), alpha- melanocyte- stimulating
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