The diagnostic label 'diabetes mellitus' refers not to a unique disease but rather to multiple disorders of different causation. Increasing knowledge has allowed us to identify discrete conditions caused by specific genetic abnormalities, while other types of diabetes remain difficult to classify on an etiologic basis. The ADA has published a new etiologic classification of diabetes, an adapted version of which is presented in Figure 6.
Type 1 diabetes (previously insulin-dependent diabetes mellitus (IDDM)) is characterized by p-cell destruction, usually leading to absolute insulin deficiency and associated with a usually juvenile onset, a tendency to ketosis and diabetic ketoacidosis, and an absolute need for insulin treatment. Most patients have type 1A diabetes, which is caused by a cellular-mediated autoimmune destruction of the p-cells of the pancreas, a minority have type 1B diabetes the precise etiology of which is not known.
Type 2 diabetes (previously non-insulin-dependent diabetes mellitus (NIDDM)) is associated with obesity and an onset later in life (although cases in childhood are now being recognized in the USA). Patients, at least initially and often throughout their lives, do not have a need for insulin therapy. The disorder manifests as a result of insulin resistance and relative insulin deficiency. A precise cause (or causes) has not been found. This type of diabetes frequently remains undiagnosed for many years despite affected individuals being at risk of developing serious macrovascular or microvascular complications of the disease. Some patients may masquerade as type 2 diabetic patients, but ultimately are recognized as having a late-onset slowly progressing immune-mediated type 1 diabetes, so called latent autoimmune diabetes in adults (LADA).
Specific monogenetic defects of the p-cell have been identified and usually give rise to maturity-onset diabetes of the young (MODY). MODY is defined as a genetic defect in p-cell function subclassified according to the specific gene involved and is described in detail in Chapter 2 in the section 'Other types of diabetes'.
Diabetes may result from any process that adversely affects the pancreas and such acquired processes include pancreatitis, trauma, pancreatec-tomy and pancreatic cancer. Usually extensive pancreatic damage or removal must occur for diabetes to emerge. Cystic fibrosis, hemochromatosis and fibrocal-culous pancreatopathy may also cause diabetes. Diabetes may also be caused by other endocrine diseases particularly when there is over-secretion of hormones that antagonize the normal effect of insulin (including Cushing's syndrome, acromegaly, pheochromocy-toma). Drugs that have a similar effect (glucocorticoids, diazoxide, thiazides) may also cause diabetes. Diabetes may also occur as a result of certain rare disorders associated with abnormalities of insulin or the insulin receptor, causing extreme insulin resistance and sometimes found in association with acanthosis nigri-cans. These disorders are categorized as insulin resistance syndromes. There is a wide array of other genetic syndromes sometimes associated with diabetes, e.g. Down's, Klinefelter's, Turner's syndromes.
Gestational diabetes mellitus (GDM) is defined as any degree of glucose intolerance with onset or first recognition during pregnancy. Care must be taken to exclude type 2 diabetes that was present before pregnancy and type 1 diabetes diagnosed during pregnancy. The patient's glucose tolerance status needs to be re-classified 6 weeks after giving birth. Deterioration of glucose tolerance occurs during normal pregnancy, especially in the third trimester. The criteria for diagnosing abnormal glucose tolerance in pregnancy have not been agreed worldwide: in the USA the modified O'Sullivan-Mahan criteria have been adopted, but these are at variance with the WHO criteria.
Patients with GDM are at future risk of developing type 2 diabetes.
IFG and IGT refer to a pathophysiologic state between normality and frank diabetes. Patients with IGT may only manifest as hyperglycemic when challenged with an oral glucose load. Between 10 and 15% of IGT patients will develop type 2 diabetes within 5 years, although some may return to normal glucose tolerance. Although patients with IGT do not normally develop the microvascular complications of diabetes, they commonly display other features of the insulin resistance syndrome (also known as syndrome X, the metabolic syndrome or Reaven's syndrome), e.g. hypertension and dyslipidemia, and IGT is associated with a major increase in cardiovascular risk.
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Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...