Hypoglycemic seizures in a Shetland pony

A 12-year-old, 195 kg Shetland pony broodmare had eight seizures between May 29 and August 7, 1979. Plasma glucose levels during three of these seizures were markedly depressed (16, 18 and 19 mg/100 ml). Serum insulin levels were elevated during two of the seizures (86.0 and 97.7 microU/ml). Although a fasting hypoglycemia was not demonstrated, plasma glucose values during a normal day were abnormal; a plasma glucose level of 42 mg/100 ml was noted eight hours post-feeding. Serum insulin values obtained during an oral glucose tolerance test and intravenous glucagon tolerance test were consistent with organic hyperinsulinism of a pancreatic origin. An intravenous glucose tolerance test and a twenty-four hour fast failed to differentiate between the subject and control ponies with respect to plasma glucose and serum insulin levels. Gross and microscopic pathological evaluation of the pancreas revealed a single 2.0 mm adenoma of pancreatic islet cell origin and hyperplasia of islet cells, predominantly beta cells. This is the first report of hypoglycemic seizures in a horse as a result of a pancreatic neoplasm and associated hyperinsulinism.