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Figure 2.1. Disorders of glycaemia: aetiological types and stages. Reproduced from, Definition Diagnosis and Classification of Diabetes Mellitus and its Complications13.

Figure 2.1. Disorders of glycaemia: aetiological types and stages. Reproduced from, Definition Diagnosis and Classification of Diabetes Mellitus and its Complications13.

and may even revert from having diabetic glucose values to IGT or normo-glycaemia. Other individuals require insulin for adequate glycaemic control but can survive without it. These individuals, by definition, have some residual insulin secretion. Individuals with extensive beta-cell destruction, and therefore no residual insulin secretion, require insulin for survival which could result from any type of diabetes. Table 2.1 shows the classification of glycaemic disorders.

Gestational hyperglycaemia

Both the WHO and ADA committees agreed to adopt a common aetiological classification and share unified terminology. They suggested that the terms 'insulin-dependent diabetes mellitus' and 'non-insulin-dependent diabetes mellitus' and their acronyms 'IDDM' and 'NIDDM' should be eliminated, and that type 1 and type 2 be reintroduced with an emphasis on using arabic rather than roman numerals. The aetiological type named type 1 includes the majority of cases which are primarily due to pancreatic islet beta-cell destruction and are prone to ketoacidosis. This sub-type also includes cases attributable to an autoimmune process, as well as those prone to ketoacidosis in which beta-cell destruction is of uncertain aetiology (idiopathic). Forms of beta-cell destruction or failure where specific causes can be identified are now classified as Other Specific Types.

Table 2.1. Aetiological classification of disorders of glycaemia.

Type 1 (Beta cell destruction) Autoimmune Clinically rapidly progressive Clinically slowly progressive Idiopathic

Type 2 (Ranging from predominantly insulin resistance to a predominantly secretory defect with or without insulin resistance) Other specific types

• Genetic defects of beta cell function eg MODY 1-3, mtDNA

• Genetic defects in insulin action e.g. type A insulin resistance

• Diseases of the exocrine pancreas

• Endocrinopathies

• Drug- or chemical-induced

• Infectious e.g. congenital rubella

• Uncommon forms of immune-mediated diabetes e.g. "stiff man" syndrome

• Other genetic syndromes, sometimes associated with diabetes Gestational hyperglycaemia

Type 2 includes the common major form of diabetes which results from defects in insulin secretion, almost always with a major contribution from insulin resistance.

The former class of 'Malnutrition-related Diabetes Mellitus' (MRDM) was deleted, because the evidence that diabetes can be caused by malnutrition or protein deficiency per se was not convincing. Its subtype of Protein-deficient Pancreatic Diabetes (PDPD or PDDM) was considered as a malnutrition modulated or modified form of diabetes mellitus. The other former subtype of MRDM, Fibrocalculous Pancreatic Diabetes (FCPD), was classified as a disease of the exocrine pancreas, which may lead to diabetes mellitus and assigned to type 3 (Other Specific Types).

The class 'Impaired Glucose Tolerance' was classified as a stage of impaired glucose regulation, since it can be observed in any hyperglycaemic disorder, and is itself not diabetes.

A clinical stage of Impaired Fasting Glycaemia was introduced to classify individuals who have fasting glucose values above the normal range, but below those diagnostic of diabetes. Gestational Diabetes Mellitus (GDM) corresponding to diabetes type 4, was defined as carbohydrate intolerance of variable severity with onset or first recognition during pregnancy. The definition applies irrespective of whether or not insulin is used for treatment or whether the condition persists after pregnancy. It does not exclude the possibility that unrecognised glucose intolerance may have antedated or begun concomitantly with the pregnancy. After pregnancy ends, the woman has to be reclassified, either into diabetes mellitus, IGT or normal glucose tolerance. Unfortunately, the question of the best diagnostic criteria for GDM is the major area where the

ADA and WHO recommendations have not been able to come to a consensus. The WHO recommendations for GDM have remained the same since 1985. They suggest that an OGTT should be performed after overnight fasting (8-14 hours) using a 75 g glucose load, with plasma glucose measured at two hours (2 h). Pregnant women who meet WHO criteria for diabetes or IGT should be classified as having GDM. They suggest that at least six weeks post-pregnancy, a woman should be reclassified based on the results of 75 g load OGTT. Major issues such as the inclusion in the definition of GDM of women with glucose values which would have led to them being classified as IGT had they not been pregnant are not discussed in the WHO document. The controversies concerning the definition of Gestational Diabetes are discussed in full in Chapter 10.

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