Hypoglycemia is blood glucose (ideally plasma glucose) level below its normal lower limit, which is generally accepted as approximately 70 mg/100 ml (3.9 mmol/Liter) of blood during fasting. So, in other word hypoglycemia is blood glucose level below 70 mg/100 ml. Hypoglycemia can be defined as “appearance of certain symptoms (headache, fatigue, confusion etc.) that disappear or are relieved promptly when blood glucose level is raised”.
Common causes of hypoglycemia:
Hypoglycemia is most commonly caused by antidiabetic medications (medicines used for treatment of diabetes), consumption of large quantity of alcohol and some other drugs. The antidiabetic medications that commonly cause hypoglycemia are insulin (especially if high dose is used), sulfonylureas (like glimepiride, glipizide, gliclazide, glibenclamide etc.) and other oral hypoglycemic drugs. Drugs that can sometimes cause hypoglycemia are quinine, pentamidine, sulfonamides etc.
Other than drugs certain other diseases like insulinoma, non-β-cell tumors, and organ failure, gastric surgery (may be immediately after surgery or even many years later hypoglycemia may develop), certain hormone deficiencies, certain inherited metabolic disorders, severe sepsis etc. can also cause hypoglycemia.
Clinical manifestations of hypoglycemia:
Hypoglycemia cause two types of symptoms neuroglycopenic symptoms and neurogenic (can also be called autonomic) symptoms of hypoglycemia. Neuroglycopenic symptoms of hypoglycemia are generally due to direct lack of glucose in CNS (central nervous system) and include symptoms like confusion, fatigue, behavioral changes etc. and in severe cases seizure, loss of consciousness and in extreme cases death can occur, especially if hypoglycemia is very severe and prolonged and without any proper treatment. Neurogenic symptoms of hypoglycemia are due to physiologic changes but mediated by CNS and include palpitations, tremor, anxiety etc. Sweating, hunger, and paresthesia are cholinergic symptoms of hypoglycemia, but they are also included in neurogenic symptoms of hypoglycemia. All these are non specific symptoms and occur in other diseases also.
The clinical signs (sign is what physician elicit after examining a patient) of hypoglycemia are pallor (paleness of skin), raised heart rate, increased blood pressure and diaphoresis.

