Equine Metabolic Syndrome (EMS)

Equine Metabolic Syndrome Syndrome (EMS)

EMS was often previously referred to as Atypical Equine Cushing’s disease or Peripheral Cushing’s disease. EMS is a syndrome of obesity, insulin resistance (IR) and laminitis. Obesity is not a significant factor on its own and appears to be strongly linked to genetic predisposition especially in some of the susceptible British breeds of ponies. Obesity includes abnormal weight distribution including “cresty” neck or bulging supraorbital fat pads. Insulin resistance is the crucial factor in EMS and results in significant hyperinsulinaemia. Laminitis is the most common disease expression associated with EMS and is often seasonal, occurring during the summer months when animals have access to lush summer pastures. The spectrum of EMS varies from overly obese horses that become insulin resistant during the summer months resulting in seasonal hyperinsulinaemia and increased risk of summer pasture associated laminitis to the other end of the spectrum where insulin resistance and hyperinsulinaemia are present all year round and laminitis can occur at any time. Hirsutism or abnormal coat shedding are not usually found in EMS and this helps in the differentiation with Equine Cushing’s Syndrome (ECS). Horses are also usually younger than those with ECS. Basal cortisol levels may be significantly increased above the reference range. Endogenous ACTH is expected to be within the normal range or borderline high. Animals test negative on an overnight Dexamethasone. test, so this is a useful test to differentiate EMS and ECS except in the autumn when false positives may occur due to seasonal variation.

Diagnosis of EMS is obtained by confirming insulin resistance and showing hyperinsulinaemia in a basal serum Insulin sample ideally taken in the morning before feeding (fasted sample). Insulin levels are usually raised on basal samples, but if there is doubt, a dynamic test of insulin resistance can be used, e.g. the combined glucose-insulin test. Ensure horse is fasted or not fed any carbohydrate prior to collection of samples and the horse has not exercised that day. Elevated insulin values without having fed a carbohydrate meal is likely to be caused by insulin resistance.

Combined Glucose/Insulin Test

Fast the animal overnight and begin the test about 9am.

  1. Take basal blood sample and measure glucose with handheld glucometer.
  2. Inject bolus of glucose (150mg/kg); use 40% or 50% glucose.
  3. Immediately after glucose administration, give 0.1IU of soluble Insulin/kg.
  4. Collect blood samples and analyse with glucometer at the following times after completion of Insulin administration: 1,5,25,35,45,60,75,90,105,120,135, 150 minutes.

Interpretation

Blood glucose concentration should return to baseline within 45 minutes in healthy animals. If blood glucose remains elevated for longer than 45 minutes then some degree of insulin resistance is present. The test may be shortened to just 60 minutes if the glucose concentration has returned to normal.