What is hypoglycemia?
Hypoglycemia means low blood sugar (glucose). The blood glucose levels in healthy individuals fluctuate depending greatly on the duration of fasting. The normal range is 70 to 120 mg/dl after an overnight (12 hours) fast. In healthy men, the blood glucose can drop to 55 mg/dl after 24 hours of fasting and to 48 mg/dl after 72 hours of fasting. In healthy women, glucose levels can be as low as 35 mg/dl after only 24 hours of fasting.
Since blood glucose levels can fluctuate widely in healthy subjects, and symptoms of hypoglycemia can be vague and nonspecific, establishing the diagnosis of hypoglycemia as the cause of symptoms is often difficult. When symptoms of hypoglycemia occur together with a documented blood glucose under 45 mg/dl, and the symptoms promptly resolve with the administration of glucose, the diagnosis can be made with more certainty.
What are the symptoms of hypoglycemia?
Hypoglycemia causes the body to release adrenaline. The adrenaline functions to restore and maintain blood glucose levels by mobilizing stored glycogen and fat, converting them into glucose. In turn, the released adrenaline causes symptoms of nervous system stimulation, such as anxiety, sweating, tremor, palpitations, nausea, and pallor. Hypoglycemia also starves the brain of glucose energy, which is essential for proper brain function. Lack of glucose energy to the brain can cause symptoms ranging from headache, mild confusion, and abnormal behavior, to loss of consciousness, seizure, and coma. Severe hypoglycemia can cause death.
Symptoms of hypoglycemia occur at different levels of blood glucose in different patients. In most healthy individuals, symptoms of hypoglycemia may not occur until the glucose level drops below 45 mg/dl. In elderly people, and in patients with chronically high glucose levels (such as patients with poorly controlled diabetes), the symptoms of hypoglycemia can occur at higher glucose levels. In contrast, patients with chronically low blood glucose may experience hypoglycemic symptoms only at very low glucose levels.
What are the causes of hypoglycemia?
Causes of hypoglycemia include drugs, liver disease, surgical absence of the stomach, tumors that release excess amounts of insulin, and pre-diabetes. In some patients, symptoms of hypoglycemia occur during fasting (fasting hypoglycemia). In others, symptoms of hypoglycemia occur after meals (reactive hypoglycemia).
Drug-induced hypoglycemia is seen with the inadvertent or intentional overdose of insulin or oral medications used to lower blood glucose. Blood glucose-lowering pills and insulin are medications used to lower the abnormally high blood glucose levels in patients with diabetes mellitus. Examples of blood glucose-lowering pills are glyburide (Micronase, Diabeta, and Glynase) and glipizide (Glucotrol). Other drugs that can cause hypoglycemia include alcohol, quinine (used in treating malaria), pentamidine (used in treating Pneumocystis carinii pneumonia infection in patients with AIDS), and Vacor (rat poison).
The liver stores energy in the form of glycogen. During fasting, glycogen is broken down and converted into glucose. This glycogen conversion into glucose is important in maintaining the blood glucose level. Therefore, during fasting, maintenance of the blood glucose level depends more on glycogen conversion than on sugars we eat. In severe liver disease, this mechanism can be disrupted, leading to hypoglycemia. Kidney failure also can lead to hypoglycemia, especially when the person is deprived of food.
Insulin is a hormone produced by the pancreas to regulate the blood glucose level. In contrast to adrenaline, insulin decreases the blood glucose level. As the blood glucose level rises after a meal, insulin is released by the pancreas to lower the glucose level. As the blood glucose falls, the insulin released from the pancreas decreases. Therefore, insulin is part of the regulatory system of the body that maintains the blood glucose in the normal range. Insulinoma is a rare tumor of the pancreas that releases a large amount of insulin without regulation. Patients with insulinomas can develop severe hypoglycemia. Special blood and radiological studies (such as CAT scan or magnetic resonance imaging (MRI))are required to diagnose and locate the insulinoma. Treatment consists of removing the tumor with surgery. Other cancers in the body can also produce hormones similar to insulin that cause fasting hypoglycemia.
Reactive hypoglycemia refers to hypoglycemia that occurs after a meal. Reactive hypoglycemia can be seen in patients who have had surgical removal of the stomach (gastrectomy). In the absence of a stomach, glucose in the meal is rapidly absorbed into the blood stream through the intestines, causing sudden hyperglycemia (high blood glucose). In order to correct this sudden hyperglycemia, excessive amounts of insulin are released by the pancreas, which drives the blood glucose down, causing hypoglycemia. The reactive hypoglycemia in gastrectomy patients occurs early, usually within 1 hour after a meal.