A raised level of glucose in the blood; a sign that diabetes is out of control. Many things can cause hyperglycaemia. It occurs when the body does not have enough insulin or cannot use the insulin it does have to turn glucose into energy. Signs of hyperglycaemia are a great thirst, a dry mouth and a need to urinate often. For people with type 1 diabetes, hyperglycaemia may lead to diabetic ketoacidosis (diabetic coma).
Diabetes mellitus and lesser forms of glucose intolerance, particularly impaired glucose tolerance (IGT), can now be found in almost every population in the world and epidemiological evidence suggests that, without effective prevention and control programmes, diabetes will likely continue to increase globally1World Health Organization. Prevention of diabetes mellitus. Technical Report Series no. 844. WHO, Geneva, 1994..
Diabetes is recognized as a group of heterogeneous disorders with the common elements of hyperglycaemia and glucose intolerance due to insulin deficiency, impaired effectiveness of insulin action, or both2Harris M, Zimmet P. Classification of diabetes mellitus and other categories of glucose intolerance. In International Textbook of Diabetes Mellitus second edition, ed. Alberti K, Zimmet P, DeFronzo R, Keen H (Honorary). Chichester, England: John Wiley and Sons Ltd. 1997:9-23..
Diabetes mellitus is classified on the basis of aetiology and clinical presentation of the disorder into four types:
Type 1 diabetes results from cellular-mediated autoimmune destruction of pancreatic islet beta-cells causing the loss of insulin production3Atkinson M, Maclaren N. The pathogenesis of insulin-dependent diabetes mellitus. N Engl J Med 1994; 331:1428-1436.. It ranks as the most common chronic childhood disease in developed nations4LaPorte R, Matsushima M, Chang Y. Prevalence and incidence of insulin-dependent diabetes. In Diabetes in America, second edition, ed. NDDG NIH 1995:37-46., but occurs at all ages5Molbak A, Christau B, Marner B, Borch-Johnsen K, Nerup J. Incidence of insulin-dependent diabetes mellitus in age groups over 30 years in Denmark. Diabet Med 1994; 11:650-655. and the clinical presentation can vary with age6Groop L, Bottazzo G, Doniach D. Islet cell antibodies identify latent type I diabetes in patients aged 35-75 years at diagnosis. Diabetes 1986; 35:237-241. 7Gleichmann H, Zörcher B, Greulich B, Gries F, Henrichs H, Bertrams J, Kolb H. Correlation of islet cell antibodies and HLA-DR phenotypes with diabetes mellitus in adults. Diabetologia 1984; 27:90-92..
Type 1 diabetes in an adult may masquerade as type 2 diabetes at presentation with a slow deterioration in metabolic control, and subsequent progression to insulin dependency. This form is called latent autoimmune diabetes mellitus in adults (LADA)8Tuomi T, Groop LC, Zimmet PZ et al. Antibodies to glutamic acid decarboxylase reveal latent autoimmune diabetes mellitus in adults with a non-insulin-dependent onset of disease. Diabetes 1993; 42(2):359-362.. LADA falls within type 1 autoimmune diabetes, but in a slowly progressive form, in the new World Health Organization (WHO) classification.
The predominant cause of hyperglycaemia in type 1 diabetes is the autoimmune destruction of the beta cells, which leads to absolute dependence on insulin treatment and a high rate of complications typically occurring at relatively young ages. Type 1 diabetes, therefore, places a particularly heavy burden on the individual, the family and the health services. Find out more
Type 2 diabetes is characterized by insulin resistance and relative insulin deficiency, either of which may be present at the time that diabetes becomes clinically manifest9Reaven GM, Bernstein R, Davis B, Olefsky J. Non-ketotic diabetes mellitus: Insulin deficiency or insulin resistance? Am J Med 1976; 60:80-88. 10Tuomilehto J, Lindstrom J, Eriksson J, Valle T, Hamalainen H, Ilanne-Parikka P, Keinanen-Kiukaanniemi S, Laakso M, Louheranta A, Rastas M, Salminen V, Uusitupa M, Finnish Diabetes Prevention Study Group. Prevention of type 2 diabetes mellitus by changes in lifestyle among subjects with impaired glucose tolerance. N Engl J Med 2001; 344:1343-1350.. The specific reasons for the development of these abnormalities are not yet known.
The diagnosis of type 2 diabetes usually occurs after the age of 40 years although the age of onset is often a decade earlier in populations with a high diabetes prevalence11Zimmet P, Dowse G, Finch C, Serjeantson S, King H. The epidemiology and natural history of NIDDM - lessons from the South Pacific. Diabetes Metab Res Rev 1990; 6:91-124.. People with type 2 diabetes may not show any symptoms for many years and the diagnosis is often made from associated complications or incidentally through an abnormal blood or urine glucose test.
Type 2 diabetes is often, but not always, associated with obesity, which itself can cause insulin resistance and lead to elevated blood sugar levels. It is strongly familial, but major susceptibility genes have not yet been identified. In contrast to type 1 diabetes, persons with type 2 diabetes are not dependent on exogenous insulin and are not ketosis-prone, but may require insulin for control of hyperglycaemia if this is not achieved with diet alone or with oral hypoglycaemic agents.
Type 2 diabetes constitutes about 85% to 95% of all diabetes in developed countries, and accounts for an even higher percentage in developing countries. It is now a common and serious global health problem, which, for most countries, has evolved in association with rapid cultural and social changes, ageing populations, increasing urbanization, dietary changes, reduced physical activity and other unhealthy lifestyle and behavioural patterns1World Health Organization. Prevention of diabetes mellitus. Technical Report Series no. 844. WHO, Geneva, 1994..
The figure below highlights the large range of type 2 diabetes prevalence even within the same or similar ethnic groups, when living under different conditions. Clearly, many of the differences between these rates reflect underlying behavioural, environmental and social risk factors, such as diet, level of obesity and physical activity. 
The most widely accepted definition of gestational diabetes mellitus (GDM) is "carbohydrate intolerance of varying degrees of severity with onset or first recognition during pregnancy"12National Diabetes Data Group. Classification and diagnosis of diabetes mellitus and other categories of glucose intolerance. Diabetes 1979; 28:1039-1057. 13Metzger BE, Coustan DR. Summary and recommendations of the Fourth International Workshop-Conference on Gestational Diabetes Mellitus. The Organizing Committee. Diabetes Care 1998; 21 Suppl 2: B161-167.. This definition applies regardless of whether insulin is used for treatment or the condition persists after pregnancy. It does not exclude the possibility that unrecognized glucose intolerance may have occurred before the pregnancy.
It is widely believed that differences in reported prevalence of GDM parallel the differences that have been found in the frequency of type 2 diabetes among different populations. Nonetheless GDM is increasing in prevalence in concert with the worldwide rise in type 2 diabetes. Studies currently in progress hold much hope of providing the data from which 'outcome based' diagnostic criteria and appropriate strategies for the detection of GDM can be developed.