Neonatal hypoglycemia is common, and should be considered in any puppy with nonspecific clinical signs. Hypoglycemia can accompany any neonatal illness that results in anorexia lasting even 24 hours or less. Juvenile hypoglycemia occurs fairly frequently in small-breed dogs younger than 1 year. Hepatic failure and congenital portosystemic shunts can cause hypoglycemia, particularly in young dogs or those with severe hepatocellular dysfunction. Starvation alone is a cause of hypoglycemia by itself only when prolonged (2 weeks or longer), but can contribute to much more rapid development of hypoglycemia in dogs with concurrent illness or preceding cachexia. Sepsis is another relatively common cause of hypoglycemia, and the blood glucose concentration should be measured in any dog that is suspected of having a systemic infection. Exertional hypoglycemia occasionally occurs in dogs after prolonged strenuous exercise; this condition has also been called hunting dog hypoglycemia. Hypoadrenocorticism occasionally causes hypoglycemia to a degree sufficient to cause clinical signs. Hypoglycemia occurs in dogs with hypoadrenocorticism as a result of glucocorticoid deficiency (which may be due to withdrawal of corticosteroids after long-term administration and hypopituitarism), and in these cases, electrolyte abnormalities may be absent. Insulin overdose is a common cause of hypoglycemia in diabetic dogs. Pancreatic islet cell neoplasia (insulinoma) causes severe hypoglycemia due to hypersecretion of insulin. Non–islet cell neoplasia can also cause marked hypoglycemia; hypoglycemia is most likely to be caused by large tumors, such as hepatocellular carcinomas, by splenic neoplasia, or by smooth muscle tumors, particularly of the gastrointestinal tract. Clinicians should avoid diagnosing hypoglycemia based on a single blood glucose measurement in a dog without clinical signs; the diagnosis should always be confirmed by a second fasting blood glucose level.
Physical examination findings typically related directly to hypoglycemia are those related to nervous system dysfunction and include ataxia, weakness, disorientation, collapse, muscle fasciculations, bizarre behavior, seizures, and coma. Hypoglycemia also induces release of catecholamines, which causes nervousness and trembling. In many dogs, hypoglycemia is asymptomatic and clinical signs may be related to the primary disease. In these cases the findings are diverse. Peripheral neuropathy has been reported in a few dogs with insulinomas, so weakness, hyporeflexia, proprioceptive deficits, and muscle atrophy will persist in these dogs even in the absence of hypoglycemia.
When hypoglycemia occurs secondary to an obvious systemic disease such as sepsis or overt hepatic failure, no specific evaluation for hypoglycemia is indicated. However, when the cause of hypoglycemia is not obvious from the history and the results of physical examination, and routine tests such as complete blood count (CBC), serum chemistry panel, and urinalysis, the most likely causes are occult hepatic disease, portosystemic shunt, atypical hypoadrenocorticism, insulinoma, and non–islet cell neoplasia. Abdominal radiographs may identify microhepatica due to liver disease or a portosystemic shunt or a mass indicating non–islet cell neoplasia. Abdominal ultrasound can identify hepatic disease, intraabdominal neoplasia, portosystemic shunt, and occasionally a pancreatic mass consistent with insulinoma. Hepatic function tests such as preprandial and postprandial bile acid measurements are indicated when serum chemistry test results indicate hepatic insufficiency, when a portosystemic shunt is suspected, or if no apparent cause is found. An adrenocorticotropic hormone (ACTH) response test should be considered to evaluate for hypoadrenocorticism in young to middle-aged dogs, particularly if lethargy and gastrointestinal disturbances are part of the history, or if no other apparent cause can be found. Measurement of fasting serum insulin concentration to diagnose insulinoma should be performed only after other diseases have been excluded as possible causes of hypoglycemia. The diagnosis is based on finding a serum insulin level in the middle or above the reference range in a hypoglycemic animal (blood glucose concentration of less than about 50 to 60 mg/dl). It may be necessary to fast the animal 4 hours or longer to induce hypoglycemia. If the insulin concentration is in the middle or above the reference range in the presence of hypoglycemia, a diagnosis of insulinoma is likely since minimal insulin secretion is expected with hypoglycemia. It can be confirmed at exploratory laparotomy.