Definition and Classification of Hypoglycemia
It is difficult to know the exact prevalence of hypoglycemia (usually defined as blood glucose levels less than 70 mg/dL or 3.9 mmol/L) because many different classifications have been used in studies over the years. In addition, developments in new technologic methods over the past decades have made changes in how hypoglycemia is determined. Traditionally, hypoglycemia was defined as the presence of the Whipple triad, which included:
Low blood glucose
Symptoms and signs associated with low blood glucose levels
Resolution of these symptoms and signs by carbohydrate ingestion.
Some studies categorize hypoglycemia as major or minor episodes. Major hypoglycemia typically suggests life threatening levels requiring third-party assistance. Minor hypoglycemia, presenting with symptoms such as tachycardia, sweating and dizziness can be treated with simple sugars. Minor hypoglycemic episodes frequently go unrecognized in older adults because they may present with nonspecific symptoms such as brief episodes of lightheadedness or vertigo. However, in frail older patients, they can lead to dangerous falls with fractures, decline in quality of life and increased mortality. Symptomatic vs. silent are other ways of classifying hypoglycemia. Hypoglycemic unawareness is a common cause of silent hypoglycemia and is highly prevalent in older adults. Thus, it is important to look carefully for the possibility of unrecognized hypoglycemia in older patients.
Physiologic Responses to Hypoglycemia
In healthy adults, when blood glucose levels fall (usually below 70 mg/dL or 3.9 mmol/L), multiple responses are triggered, and euglycemia is quickly restored. In response to hypoglycemia, insulin secretion from the pancreas will first decrease as the initial response. Next, the pancreas will increase glucagon production as counterregulation. The liver then detects the decrease in insulin and the increase in glucagon and responds by increasing both glycogenolysis and gluconeogenesis. The adrenal glands next produce epinephrine, which acts on muscle, fat and kidney to decrease glucose clearance. Should glucagon be deficient, this epinephrine response becomes particularly significant. The adrenals, along with the peripheral nervous system, which detects hypoglycemia, then mediate an autonomic response via neurotransmitters. Acetylcholine triggers hunger and diaphoresis while norepinephrine triggers arousal with tremor and palpitations. These are the primary life-saving responses to hypoglycemia. As secondary responses, cortisol and growth hormone are also released, but these are less critical in terms of adaptive responses to hypoglycemia.