New Concepts in Diabetes and its Treatment

Introduction

Diabetes mellitus and its complications are clinical conditions of growing importance both from the clinical as well as epidemiological standpoint. The relevance of diabetes at clinical and individual level is given by its life- threatening acute complications and, especially, by its chronic complications affecting several organs and systems, with increased risk for ocular, renal, cardiac, cerebral, nervous and peripheral vascular diseases. The high preva- lence of diabetes in many developed countries or in special ethnic groups, entailing premature disability and mortality, points to its relevance at popula- tion level. It is, therefore, mandatory for both the specialist and the practitioner to be acquainted with the pathophysiological mechanisms, clinical manifesta- tions and, above all, therapy of diabetes mellitus.

Recent data showing that control of hyperglycemia may prevent the onset or slow down the progression of complications point to the importance of an appropriate and efficacious treatment. Indeed, the aim of this book is to serve as a tool to provide physicians with the latest views on diagnostic aspects and pathophysiological mechanisms as a premise to go deep into the various facets of the modern management of diabetes.

This book begins with introductory chapters on classification and clinical aspects, after which an account is given of insulin secretion as modulated by sulfonylureas and of insulin resistance (in its genetic and acquired components) as modified by diet and the new lipase-inhibitory drug or by metformin (and perhaps troglitazone agents). Insulin therapy of both type 1 and, when re- quired, type 2 diabetes is adequately covered. This is followed by an integrated view of metabolic control, including combined therapy and self-monitoring, in the light of the lesson from DCCT (Diabetes Control and Complications Trial) and UK-PDS (United Kingdom Prospective Diabetes Study).

The mechanisms of complications are treated as an introduction to the understanding of possible therapeutic strategies. Then retinopathy, nephrop- athy, hypertension and cardiovascular disease are considered in their clinical aspects and therapeutic interventions. Extensive space is devoted to the various neuropathic manifestations, including erectile dysfunction, as well as to the foot problems. Final chapters highlight the need for multifactorial treatment and the clinical and therapeutic problems of diabetic pregnancy.

The international panel of authors has made any effort to condense this rich content into a relatively short text and to present it in a clear and smooth-

to-read form. While more extensive information may be found in larger treatises (see Suggested Reading, below), we hope that this medium-size book will be useful to all physicians interested in the management of diabetic patients by providing them with a simple yet updated source of information concerning the New Concepts in Diabetes and Its Treatment.

Francesco Belfiore Carl Erik Mogensen

Suggested Reading

Alberti KGMM, Zimmet P, DeFronzo RA: International Textbook of Diabetes mellitus, ed 2. Chichester, Wiley, 1999. Belfiore F (ed): Frontiers in Diabetes. Basel, Karger, vol 8/1987, vol 9/1990, vol 10/1990, vol 11/1992, vol 12/1993, vol 14/1998. Bray G, Bouchard C, James WPT (eds): Handbook of Obesity. New York, Dekker, 1997. Kakn CR, Weir GC (eds): Joslin’s Diabetes mellitus, ed 13. Malvern, Lea & Febiger, 1994. Mogensen CE (ed): The Kidney and Hypertension in Diabetes mellitus, ed 5. Boston, Kluwer Academic,

2000. Pickup JC, Williams G (eds): Textbook of Diabetes, ed 2. Oxford, Blackwell, 1997. Porte D Jr, Sherwin RS (eds): Ellenberg and Rifkin’s Diabetes mellitus, ed 4, Amsterdam, Elsevier, 1990,

and ed 5, Old Tappan/NJ, Appleton & Lange, 1996.

Belfiore F, Mogensen CE (eds): New Concepts in Diabetes and Its Treatment. Basel, Karger, 2000, pp 3–19

Chapter I

Etiological Classification, Pathophysiology and Diagnosis

F. Belfiore, S. Iannello

Institute of Internal Medicine, University of Catania, Ospedale Garibaldi, Catania, Italy

Introduction

According to the classical definition, diabetes mellitus is a disorder re- sulting from both genetic predisposition and favoring environmental factors, and is characterized by alterations in the metabolism of carbohydrate, fat and protein, which are caused by a relative or absolute deficiency of insulin secretion and different levels of insulin resistance. In the patients with long-standing diabetes, late complications develop consisting of alterations and failure of various organs (especially the noninsulin-sensitive ones) including the eyes (retinopathy with vision loss), kidneys (nephropathy leading to renal failure), nerves (peripheral and autonomic neuropathy), heart and blood vessels (preco- cious and severe cardiovascular, cerebrovascular and peripheral vascular ath- erosclerosis). Diabetes mellitus includes etiologically and clinically different diseases that have hyperglycemia in common, representing a syndrome rather than a single disease.

Until 1997, the classification and diagnosis of diabetes were based on the criteria developed by an international work group, sponsored by the National Diabetes Data Group (NDDG) of the American National Institute of Health, and published in 1979. The World Health Organization (WHO) Expert Com- mittee on Diabetes in 1980 and the WHO Study Group on Diabetes mellitus in 1985 adopted the recommendations of the NDDG with slight alterations. In 1995, an International Expert Committee was established (sponsored by the American Diabetes Association) with the aim to review the scientific literature since 1979 and to decide the adequate changes in the classification and diagnostic criteria of diabetes. The committee work culminated in a document Until 1997, the classification and diagnosis of diabetes were based on the criteria developed by an international work group, sponsored by the National Diabetes Data Group (NDDG) of the American National Institute of Health, and published in 1979. The World Health Organization (WHO) Expert Com- mittee on Diabetes in 1980 and the WHO Study Group on Diabetes mellitus in 1985 adopted the recommendations of the NDDG with slight alterations. In 1995, an International Expert Committee was established (sponsored by the American Diabetes Association) with the aim to review the scientific literature since 1979 and to decide the adequate changes in the classification and diagnostic criteria of diabetes. The committee work culminated in a document

Definition and Description of Diabetes mellitus

The basis of the metabolic alterations in diabetes is the reduction (to a various degree) of insulin action on insulin-sensitive tissues, due to deficiency of insulin secretion or to insulin resistance or both. The majority of cases of diabetes mellitus falls into two major forms: type 1 and type 2 diabetes.

Type 1 Diabetes Immune-Mediated Type 1 Diabetes

Type 1 diabetes (previously also named insulin-dependent diabetes mel- litus – IDDM – or juvenile-onset diabetes) is an immune-mediated form of diabetes, which accounts for approximately 5–10% of all diabetics in the West- ern world. It occurs mainly in healthy nonobese children or young adults but may also affect subjects at any age, and results from an absolute deficiency of insulin secretion (evidenced by low or undetectable levels of plasma C- peptide), caused by a cellular-mediated autoimmune destruction of pancreatic

b -cells. Although the affected subjects are usually nonobese, the presence of obesity is not incompatible with the diagnosis of type 1 diabetes. The course may be rapid in children and young adults, slower in older patients. Adult patients can retain for some time a residual b-cell function while children and adolescents often show early the effects of severe insulin lack, with a diabetes appearing abruptly over days or weeks and rapidly progressing to acute life- threatening complication (ketoacidotic coma), which may be the first mani- festation of the disease, particularly in presence of precipitating factors such as infections or other stress.

Genetic Predisposition. Type 1 diabetes is favored by a not yet fully under- stood genetic predisposition, linked to the HLA system. Pedigree studies of type 1 diabetes families have shown a low prevalence of direct vertical transmission. However, the risk to develop the disease for children who are first-degree relatives of type 1 diabetic patients is between 5 and 10%, the risk being increased when there is haploidentity with the affected sibling and even more when there is HLA identity. It has also been observed that the risk is 5-fold higher for children of a diabetic father compared to children of a diabetic mother (sexual imprinting). Candidate genes for type 1 diabetes have been Genetic Predisposition. Type 1 diabetes is favored by a not yet fully under- stood genetic predisposition, linked to the HLA system. Pedigree studies of type 1 diabetes families have shown a low prevalence of direct vertical transmission. However, the risk to develop the disease for children who are first-degree relatives of type 1 diabetic patients is between 5 and 10%, the risk being increased when there is haploidentity with the affected sibling and even more when there is HLA identity. It has also been observed that the risk is 5-fold higher for children of a diabetic father compared to children of a diabetic mother (sexual imprinting). Candidate genes for type 1 diabetes have been

Immunologic Mechanisms. Class II molecules are expressed by macro- phages, endothelial cells and lymphocytes, and are required for the presentation of an antigen to the regulatory T cells, which become activated, thus triggering the immune response. In other words, the favoring HLA haplotypes indicated above permit the interaction of environmental factors (such as certain viral infections or chemical agents) with specific cell membrane components (the HLA molecules), which results in the presentation of the antigen to the regu- latory T lymphocytes, thus triggering an autoimmune mechanism. Several viral infections have been suggested as favoring type 1 diabetes, including Coxsackievirus infections, infectious mononucleosis, mumps, congenital ru- bella, hepatitis and encephalomyocarditis. Some toxins have also been impli- cated. Consumption of cow’s milk during the early life may be an important environmental factor associated with type 1 diabetes development and, because the role of bovine albumin in the induction of b-cell autoimmunity have not been confirmed, b-casein has been suggested as the responsible protein. Virus, toxins, or other factors may directly damage b-cells or favor apoptosis (pro- grammed cell death), or may expose cryptic antigen to the immune system, or may act through molecular mimicry (exogenous molecules similar in amino acid sequence to some endogenous molecules), or they may induce expression of class II molecules in the b-cells (which therefore would become antigen- presenting cells, able to trigger the autoimmune response). An alternative hypothesis which does not rely on exogenous antigen postulates a defective removal of autoreactive T cells, which normally are destroyed in the thymus in the early life. In contrast to the most common form of type 1 diabetes, linked to environmental factors (formerly called type IA), in approximately 10% of all cases of type 1 diabetes (more frequently in females, with HLA- DR3, from 30 to50 years of age), the disease is a primary autoimmune disorder (previously called type IB) and is associated to other endocrine and nonendo- Immunologic Mechanisms. Class II molecules are expressed by macro- phages, endothelial cells and lymphocytes, and are required for the presentation of an antigen to the regulatory T cells, which become activated, thus triggering the immune response. In other words, the favoring HLA haplotypes indicated above permit the interaction of environmental factors (such as certain viral infections or chemical agents) with specific cell membrane components (the HLA molecules), which results in the presentation of the antigen to the regu- latory T lymphocytes, thus triggering an autoimmune mechanism. Several viral infections have been suggested as favoring type 1 diabetes, including Coxsackievirus infections, infectious mononucleosis, mumps, congenital ru- bella, hepatitis and encephalomyocarditis. Some toxins have also been impli- cated. Consumption of cow’s milk during the early life may be an important environmental factor associated with type 1 diabetes development and, because the role of bovine albumin in the induction of b-cell autoimmunity have not been confirmed, b-casein has been suggested as the responsible protein. Virus, toxins, or other factors may directly damage b-cells or favor apoptosis (pro- grammed cell death), or may expose cryptic antigen to the immune system, or may act through molecular mimicry (exogenous molecules similar in amino acid sequence to some endogenous molecules), or they may induce expression of class II molecules in the b-cells (which therefore would become antigen- presenting cells, able to trigger the autoimmune response). An alternative hypothesis which does not rely on exogenous antigen postulates a defective removal of autoreactive T cells, which normally are destroyed in the thymus in the early life. In contrast to the most common form of type 1 diabetes, linked to environmental factors (formerly called type IA), in approximately 10% of all cases of type 1 diabetes (more frequently in females, with HLA- DR3, from 30 to50 years of age), the disease is a primary autoimmune disorder (previously called type IB) and is associated to other endocrine and nonendo-

GAD 65 (autoantibodies to glutamic acid decarboxylase) and IA-2 or IA-2b (autoantibodies to tyrosine phosphatase). These autoantibodies disappear over the course of a few years in the majority of patients, and may be the result rather than the cause of the autoimmune process.

Clinical Picture. Manifest type 1 diabetes is characterized by symptoms linked to the marked hyperglycemia, such as polyuria (due to the osmotic effect of glucose), polydipsia (to compensate for the water lost with polyuria), polyphagia (to compensate for the energetic substrate glucose lost in the urine), weight loss and fatigue (due to loss of glucose in urine and to dehydration), and blurred vision (due to lens osmotic disturbances). These patients are insulin- dependent for their survival and prone to ketosis; impairment of growth, susceptibility to certain infections, hypertension, lipoprotein metabolism al- terations, periodontal disease and psychosocial dysfunctions are frequent.

Idiopathic Type 1 Diabetes The idiopathic diabetes includes some forms of type 1 diabetes (common

in individuals of African and Asian origin) due to unknown etiology, with strong genetic inheritance (not HLA-associated), without markers of autoim- munity. There is severe deficit of insulin secretion and tendency to ketoacidosis, with absolute requirement of insulin therapy.

Pathophysiology of Type 1 Diabetes The pathophysiological changes occurring in type 1 diabetes as a con-

sequence of the severe insulin deficiency may be better understood by comparing the normal picture of the main metabolic pathways, as summarized in figure 1, with the abnormal situation present in type 1 diabetes, outlined in figure 2 (see also chapter III on Insulin Resistance). In type 1 diabetes, the deficit of insulin and the prevalence of counterregulatory hormones, primarily glucagon, leads to the activation of glycogenolysis and gluconeogenesis in liver, with ensuing enhanced hepatic glucose output (HGO). In addition, the deficiency in insulin action results in reduced glucose utilization in peripheral insulin sensitive tissues (primarily muscle) as well as in activation of lipolysis in the adipose tissue (insulin normally exerts an antilipolytic effect), with enhanced release of FFA. The latter, although they cannot be directly converted into glucose in man, favor gluconeo- genesis in the liver. Combination of enhanced HGO and reduced glucose utiliza-

Fig. 1. Scheme showing the main metabolic pathways of intermediate metabolism in the three insulin-sensitive tissues (liver, muscle and adipose tissue) participating in the metabolic homeostasis. Note that most metabolic pathways are opposed to each other to form couples composed of a ‘forward pathway’ and a ‘backward pathway’, thus allowing substrate cycling. Examples are: glycogen synthesis and glycogenolysis (steps 1 and 2 in liver, 11 and 12 in muscle), glycolysis and gluconeogenesis (steps 5 and 6), triglyceride synthesis and hydrolysis (lipolysis) (steps 17 and 18 in adipose tissue; 26 and 27 in liver), protein synthesis and proteolysis (steps 13 and 14), etc. Some cycles are ‘inter-tissular’, linking liver and muscle, such as the Cori cycle (expanded to include alanine in addition to lactate and pyruvate), composed of steps 10, 6, 3, 8 and 9, pertaining to carbohydrate metabolism, as well as the cycle linking liver and adipose tissue (steps 19, 22, 26, 28 and 29), pertaining to lipid metabolism. In the normal state, blood glucose is kept at the normal level through a balance between hepatic glucose production (step 3) and glucose utilization by peripheral tissues, mainly the muscle (step 8). VLDL and triglycerides are kept normal through a balance between hepatic production (step 28) and peripheral degradation by LPL, primarily at adipose tissue level (step 29). Ketones are not present because Ac-CoA is entirely oxidized

to CO 2 (or utilized for the synthesis of FFA – step 24).

tion results in hyperglycemia. In addition, FFA exert anti-insulin effects at the muscle level, through the mechanism of the glucose-FFA cycle (Randle’s cycle),

which may cause resistance to the therapeutically administered insulin (see the chapter on Insulin Resistance). It should also be considered that hyperglycemia itself favors glucose utilization (glucose effectiveness), perhaps by acting on non- insulin-dependent glucose transporters (GLUT1 in gut, GLUT2 in liver and GLUT3 in brain), and that in type 1 diabetes this glucose effect may be reduced,

i.e. there may be ‘glucose resistance’.

Fig. 2. Scheme of the main metabolic pathways (similar to that outlined in figure 1) and of their changes in activity rate occurring in states of severe insulin deficiency, such as decompensated type 1 diabetes (thick or thin arrows indicate increased or decreased activity, respectively). Note the prevalence of the catabolic pathways over the anabolic ones: glyco- genolysis over glycogen synthesis (steps 2 and 1 in liver, steps 12 and 11 in muscle), gluconeo- genesis over glycolysis (steps 6 and 5), triglyceride hydrolysis or lipolysis over triglyceride synthesis (steps 17 and 18), proteolysis over proteosynthesis (steps 14 and 13), etc. Concerning the ‘inter-tissural’ cycles, note the prevalence of hepatic glucose production (step 3) over glucose utilization (step 8), leading to glucose accumulation in blood (unnumbered arrow starting from glucose). The enhanced hepatic glucose production (step 3), effected by the enzyme glucose-6-Pase, utilizes glucose-6-P in part derived from glycogen (step 2) but mainly formed through the gluconeogenic process (step 6) which in turn utilizes the gluconeogenic precursors (pyruvate, lactate and alanine) coming from the muscle (step 10), where they are mainly produced from amino acids (step 15) derived from the enhanced proteolysis (step 14). Note the overall process of conversion of protein to glucose (steps 14, 15, 10, 6 and 3), and consider that some amount of the glucose-6-P formed through the gluconeogenic process may be converted into glycogen (this latter conversion being favored by cortisol). With regard to the FFA-VLDL cycle, linking liver and adipose tissue, note the enhanced FFA release from adipose tissue (step 19), the enhanced afflux of FFA to muscle (step 20), where they are oxidized (step 21) and oppose the oxidation of glucose-derived pyruvate (glucose-FFA cycle, see the text), thus inducing insulin resistance. Note also the hyperafflux of FFA to the liver, where they may be reesterified to triglycerides (step 26) or b-oxidized to Ac-CoA (step 23). The triglycerides so formed may be deposited in the hepatocytes (steatosis) or may

be incorporated into VLDL which are secreted into the circulation (step 28), leading to the marked hypertriglyceridemia of the decompensated diabetes. The large amount of Ac-CoA produced by b-oxidation of FFA cannot be entirely oxidized in the Krebs cycle (also for the relative deficiency of oxalacetate, which is diverted towards gluconeogenesis) and is converted into ketone bodies (step 25) leading the ketoacidosis. Thus, in the diabetic state, blood glucose is elevated because hepatic glucose production (step 3) prevails over glucose utilization

Type 2 Diabetes Type 2 diabetes (previously also named non-insulin-dependent diabetes

mellitus – NIDDM – or adult-onset diabetes) occurs in approximately 90–95% of diabetic people in the Western world, resulting from insulin resistance and insufficient compensatory insulin secretion. The disease has an insidious onset and remains asymptomatic and undiagnosed for a long period, even if the moderate hyperglycemia is able to induce severe diabetic late complications.

Type 2 diabetes is strongly favored by genetic predisposition. However, although it shows familial aggregation as well as a high concordance (80%) in monozygotic twins, its mode of inheritance is not fully understood. It may well be a polygenic disease. In any case, the risk of offspring and siblings of type 2 diabetic patients to develop the disease is relatively elevated.

In addition to the genetic predisposition, favoring environmental factors are involved, such as excessive caloric intake, obesity with increased body fat in the abdominal (visceral) site, sedentary habit, etc. The insulin levels may

be normal or even increased (especially in presence of obesity) for a long time, but may decrease in the late stage of the disease. The abnormal carbohydrate metabolism can be early identified measuring fasting glycemia (FPG) or per- forming an oral glucose tolerance test (OGTT). This type of diabetes is nonin- sulin-dependent for survival and is nonketosis prone. Hyperglycemia is usually improved or corrected by diet, weight loss and oral hypoglycemic drugs. In type 2 diabetics an acute life-threatening complication, the nonketotic hyperos- molar coma, can develop whereas ketoacidosis seldom occurs spontaneously, although it may arise during stress, infections or other illnesses.

Pathophysiology of Type 2 Diabetes This disease is due to a varying combination of insulin resistance and

reduction (especially in the late stage of the disease) in insulin secretion (see chapter II on Insulin Secretion and chapter III on Insulin Resistance). The metabolic alterations are less pronounced than those in type 1 diabetes, out- lined in figure 2 (see also chapter III on Insulin Resistance). Due to insulin resistance (and to enhanced counterregulatory hormones), there is increased HGO (which contributes primarily to fasting hyperglycemia) and reduced peripheral glucose utilization. There is also elevation of plasma FFA (resulting from activation of lipolysis and/or the often enhanced fat mass due to coexisting

by peripheral tissues, mainly the muscle (step 8). VLDL and triglycerides are increased because hepatic production (step 28) prevails over peripheral degradation by LPL, primarily at the adipose tissue level (step 29). Ketones are formed at high rate (step 25) because the

large amount of Ac-CoA cannot be entirely oxidized to CO 2 .

obesity), which in turn contributes to insulin resistance through the mechanism of the glucose-FFA cycle. As mentioned above (under Type 1 Diabetes), hyper- glycemia itself favors glucose utilization (glucose effectiveness). This mecha- nism may be impaired in type 2 diabetes, i.e. ‘glucose resistance’ may be present. It has been observed that in obesity and type 2 diabetes (as well as in acromegaly and Cushing’s disease), in the postabsorptive period, noninsulin- mediated glucose uptake is a major determinant of glucose disposal and is similar in the different pathologies studied. On the other hand, although absolute rates of basal insulin-mediated glucose uptake are reduced in insulin- resistant states, they do not achieve statistical value compared with control subjects because of compensatory hyperinsulinemia.

Other Specific Types of Diabetes Various, less common, types of diabetes are known to occur, in which the

secretory defect is based upon different mechanisms. Genetic Defects of b-Cell Function

The maturity-onset diabetes of the young (MODY) is a genetically hetero- geneous monogenic form of noninsulin-dependent diabetes, characterized by early onset, usually before 25 years of age and often in adolescence or child- hood, and by autosomal dominant inheritance. There is no HLA association nor evidence of cell-mediated autoimmunity. It has been estimated that 2–5% of patients with type 2 diabetes may have this form of diabetes mellitus. However, the frequency of MODY is probably underestimated. Clinical studies have shown that prediabetic MODY subjects have normal insulin sensitivity but suffer from a defect in glucose-stimulated insulin secretion, suggesting that pancreatic b-cell dysfunction, rather than insulin resistance, is the primary defect in this disorder. To date, three MODY genes have been identified.

MODY-1. Studies in an affected family showed that the gene responsible for MODY-1 is tightly linked to the adenosine deaminase gene on chromosome

20q. Further research has shown that responsible for MODY-1 is a mutation in the gene-encoding hepatocyte nuclear factor (HNF)-4a, a member of the steroid/thyroid hormone receptor superfamily and an upstream regulator of HNF-1a expression.

MODY-2. This form is due to mutations in glucokinase (GK – see chapter

II for the functional meaning of GK in b-cells) and is associated with defects in insulin secretion, reduction in hepatic glycogen synthesis and in the net accumulation of hepatic glycogen as well as increased hepatic gluconeogenesis following meals, resulting in impaired glucose tolerance or diabetes mellitus II for the functional meaning of GK in b-cells) and is associated with defects in insulin secretion, reduction in hepatic glycogen synthesis and in the net accumulation of hepatic glycogen as well as increased hepatic gluconeogenesis following meals, resulting in impaired glucose tolerance or diabetes mellitus

MODY-3. In several families, this form of MODY was found to be linked with microsatellite markers on chromosome 12q. The disease was estimated to

be linked to this chromosome region in approximately 50% of families in a heter- ogeneity analysis. It is the most common form of MODY. Affected patients ex-

hibit major hyperglycemia with a severe insulin secretory defect, suggesting that the causal gene is implicated in pancreatic b-cell function. MODY-3 was further shown to be due to mutations in the gene-encoding HNF-1a (which is encoded by the gene TCF1). HNF-1a is a transcription factor that helps in the tissue- specific regulation of the expression of several liver genes and also functions as

a weak transactivator of the rat insulin-I gene. Familial Hyperinsulinemia. The high-affinity sulfonylurea receptor, a novel

member of the ATP-binding cassette superfamily, is one component of the ATP-sensitive K + channel. The protein is critical for regulation of insulin secretion from pancreatic b-cells, and mutations in the receptor (or in the K ATP channels) have been linked to familial hyperinsulinemia, a disorder character- ized by unregulated insulin release despite severe hypoglycemia. Other forms may be due to mutation in the GK gene, leading to a hyperresponsive enzyme.

Other. In addition, a diabetes type associated with deafness may be linked to point mutations in mitochondrial DNA, and still other forms with less clearly defined defects are known to occur. In about 50% of cases of MODY, the genetic background is uncertain. It should be stressed that the role of the above genes (responsible for b-cell dysfunction) in the susceptibility to the more common late-onset form of type 2 diabetes remains uncertain. Genetic studies seem to exclude any function as major susceptibility genes, although they might play a minor role in a polygenic context or a major role in particular populations.

Rare Genetic Defects of Insulin Action These are a heterogeneous group of rare conditions which includes: (a) syn-

dromes associated with acanthosis nigricans, which is a brown to almost black hyperpigmentation of the skin, most often located in the neck, axilla, groin or other areas, less rare in Blacks or in subjects of Hispanic origin. The affected patients show high insulin levels. Some cases are due to mutation in the insulin receptor resulting in diminished tyrosine-kinase activity (type A syndrome). Others are due to antibodies to the insulin receptors which prevent insulin dromes associated with acanthosis nigricans, which is a brown to almost black hyperpigmentation of the skin, most often located in the neck, axilla, groin or other areas, less rare in Blacks or in subjects of Hispanic origin. The affected patients show high insulin levels. Some cases are due to mutation in the insulin receptor resulting in diminished tyrosine-kinase activity (type A syndrome). Others are due to antibodies to the insulin receptors which prevent insulin

mia. (b) Generalized or partial (face and trunk) lipodystrophies, which may

be congenital or acquired, are characterized by fat depletion, and result from decrease in the number or affinity of the receptor for insulin or from postrecep- tor defects. Patients show high insulin levels, hyperglycemia (without ketoac- idosis for the scarcity of fat), hypertriglyceridemia (with eruptive xanthomas), enlargement of liver, spleen, heart, and hypertrophy of external genitalia.

Lymphadenopathy and hirsutism may also occur as well as varicose veins, mental retardation and kidney involvement. In the congenital form, there is also muscle hypertrophy. (c) Leprechaunism syndrome, due to mutation in insulin receptors (which may be altered in both the a and b subunits and whose expression in the cell membrane is markedly reduced), and consisting of insulin resistance associated with severe growth retardation, elfin appearance of the face, hirsutism, absence of subcutaneous fat and thickened skin. (d) Other rare conditions such as the Werner’s syndrome, the Alstro¨m syn- drome, the Rabson-Mendenhall syndrome (which may be associated with acanthosis nigricans), the pineal hypertrophy syndrome, and the ataxia telan- giectasia syndrome.

Diseases of the Exocrine Pancreas Any disease process affecting the pancreas may involve the islets and pro-

duce diabetes (table 1). May we recall the fibrocalculous pancreatopathy, that occurs in India, Africa and West Indies with a frequency similar to that of type

2 diabetes. This form involves young people with malnutrition and pancreatic calculi, and is characterized by severe hyperglycemia and insulin dependence but not by proneness to ketosis, as a moderate insulin secretion is retained.

Gestational Diabetes mellitus (GDM) GDM is defined as any degree of glucose intolerance with onset during

pregnancy. It should be distinguished by the mild deterioration of glucose toler- ance which may occur also during normal pregnancy (particularly in the 3rd trimester). The prevalence of GDM can range from 2 to 3% of pregnancies, depending on the different racial/ethnic subpopulations studied. A known dia- betic woman who becomes pregnant is not classified as GDM. The GDM is a serious problem and its recognition is important to prevent the associated peri- natal morbidity or mortality and the maternal complications (cesarean delivery and chronic hypertension). GDM usually returns to a normal glucose tolerance state after delivery, but 60% of affected women can develop diabetes within 15

Table 1. Etiologic classification of diabetes mellitus 1. Type 1 diabetes

A. Immune-mediated B. Idiopathic

2. Type 2 diabetes 3. Other specific types

A. Genetic defects of b-cell function (MODY-1, MODY-2, MODY-3, mitochondrial DNA, and others) B. Genetic defects in insulin action (type A insulin resistance, leprechaunism, Rabson-Men- denhall syndrome, lipoatrophic diabetes, and others) C. Diseases of the exocrine pancreas (pancreatitis, pancreatectomy, trauma, neoplasia, cystic fibrosis, hemochromatosis, fibrocalculous pancreatopathy, and others) D. Endocrinopathies (acromegaly, Cushing’s syndrome, glucagonoma, pheochromocy- toma, hyperthyroidism, somatostatinoma, aldosteronoma, and others) E. Drug- or chemical-induced diabetes (vacor, pentamidine, nicotinic acid, glucocorticoids, thyroid hormone, diazoxide, b-adrenergic agonists, thiazides, dilantin, a-interferon, and others)

F. Infections (congenital rubella, cytomegalovirus, and others) G. Uncommon forms of immune-mediated diabetes (‘stiff-man’ syndrome, anti-insulin re-

ceptor antibodies, and others) H. Other genetic syndromes sometimes associated with diabetes (Down’s syndrome, Kline- felter’s syndrome, Turner’s syndrome, Wolfram’s syndrome, Friedreich’s ataxia, Hun- tington’s chorea, Lawrence-Moon-Biedl syndrome, myotonic dystrophy, porphyria, Prader-Willi syndrome, and others)

4. Gestational diabetes mellitus (GDM)

years after parturition. About 6 weeks after the delivery, the GDM woman should be reclassified as diabetic or glucose intolerant or normoglycemic.

Comment

In the previous NDDG/WHO classification, diabetes mellitus was divided into 5 distinct types: IDDM, NIDDM, GDM (gestational diabetes), malnutri- tion-related diabetes and other types, and the category of IGT (impaired glucose tolerance) was included, in which plasma glycemia during an OGTT was above normal but not diabetic. The 1997 Expert Committee changed the NDDG/ WHO classification, including only 4 clinical classes: (1) type 1 diabetes, (2) type

2 diabetes, (3) other specific types and (4) GDM (table 1). The most important changes introduced include the following: (a) Elimination of the terms ‘insulin- dependent’ or ‘noninsulin-dependent’ diabetes mellitus and ‘IDDM’ or

‘NIDDM’ (which are confusing as they classified the patient according to treat- ment rather than etiology). (b) Preservation of the terms ‘type 1’ or ‘type 2’ diabetes (with Arabic numerals) and elimination of the confusing terms ‘type I’ or ‘type II’ diabetes (with Roman numerals); patients with no evidence of autoimmunity are classified as being affected by type 1 idiopathic diabetes. (c) Type 1 diabetes does not include those forms of b-cell destruction due to nonautoimmune-specific causes. (d) Type 2 diabetes includes the most common form characterized by insulin resistance and insulin secretory defect. (e) The class previously named malnutrition-related diabetes mellitus has been elimi- nated. (f ) The IGT stage has been retained, and the stage of IFG was added. (g) GDM, as defined by WHO and NDDG, was retained.

Diagnostic Criteria for Diabetes mellitus

A precocious diagnosis of diabetes is important to prevent or attenuate late diabetic complication, and depends upon the adequate use and interpreta- tion of laboratory tests (especially in absence of specific symptoms). Many different diagnostic schemes have been in use. Recently, on the basis of the available data, the diagnostic criteria previously recommended by NDDG or WHO were modified. According to the revised criteria by the Expert Commit- tee [1997], the ‘normal values’ and the ‘diagnostic values’ for diabetes (which do not coincide with the goals of therapy) are as follows (values given in the text refer to venous plasma glucose which is the preferred measurement; equivalents for whole blood and capillary glucose estimations, according to the IDF guidelines [1999] to type 2 diabetes, are indicated in footnotes).

Normal Values. The upper limit of normal venous plasma values has been set at 110 mg/dl (6.1 mmol/l) for FPG and at 140 mg/dl (7.8 mmol/l) for the 2-hour value after glucose load (OGTT).

Diagnostic Values. (a) FPG q126 mg/dl (or 7.0 mmol/l) 1 after a fasting of at least 8 h, confirmed on a subsequent day, to rule out a labeling or technical error; (b) 2-hour value during OGTT q200 mg/dl (or q11.1 mmol/l) 2 , con- firmed in a repeated test to make the final diagnosis; (c) symptoms of diabetes and a casual value q200 mg/dl (or 11.1 mmol/l) at any time of day.

For epidemiological studies, diabetes prevalence and incidence should be estimated by a FPG q126 mg/dl. The value of FPG was changed from the

1 Same value for capillary plasma glucose; q110 mg/dl (?6.0 mmol/l) for venous or capillary whole blood glucose.

2 q 220 mg/dl (q12.2 mmol/l) for capillary plasma glucose; q180 mg/dl (q10.0 mmol/l) for venous whole blood glucose; q200 mg/dl (q11.0 mmol/l) for capillary whole blood glucose.

previous value (q140 mg/dl) to current value (q126 mg/dl), because (1) the cutpoint of FPG q140 mg/dl defines a greater degree of hyperglycemia than did the cutpoint of the 2-hour value q200 mg/dl, and (2) this degree of hyperglycemia usually reflects a serious abnormality associated with serious chronic diabetic complications. The 2-hour value q200 mg/dl has been re- tained for the diagnosis of diabetes because it was well accepted, and enormous clinical and epidemiological data are based on this cutpoint value. The criteria for diagnosis of diabetes in an asymptomatic child should be stricter than those for the adults to avoid overdiagnosis of diabetes, and it should be considered that normal children commonly present OGTT values lower than adults. The diagnostic values for GDM as proposed by O’Sullivan and Mahan [1993], revised by NDDG and adopted by ADA and the American College of Obstetricians and Gynecologists (ACOG), are set lower than those for nonpregnant adults. A screening test is indicated between 24 and 28 weeks of gestation in asymptomatic female patients at risk, and a value 1 h after a 50 g of glucose load q140 mg/dl (or 7.8 mmol/l) can identify the individuals at risk for GDM in whom a full diagnostic 3-hour OGTT with 100 g of glucose should be performed. GDM occurs with an FPG q105 mg/dl (or 5.8 mmol/l) and a 2-hour value during OGTT q165 mg/dl (or 9.2 mmol/l).

An intermediate metabolic state was introduced, which is characterized by glucose levels above those considered as normal but below those accepted for the diagnosis of diabetes mellitus. Referring to the fasting state, this condition was named impaired fasting glycemia or IFG (FPG q110 but

p 126 mg/dl or q6.0 but p7.0 mmol/l ) 3 . Referring to the postload state, it was named impaired glucose tolerance or IGT (2-hour postload value in OGTT q140 mg/dl but p200 mg/dl or q7.8 but p11.1 mmol/l) 4 , without spontaneous hyperglycemia). IFG or IGT are not clinical entities but rather risk factors for future type 2 diabetes and cardiovascular disease, being associated with the metabolic syndrome or insulin resistance syndrome, charac- terized by abdominal or visceral obesity, hypertension, dyslipidemia (hypertri- glyceridemia and low HDL value) and hyperuricemia. Conversion of IGT to type 2 diabetes takes years or decades and occurs in about 10–50% of IGT patients. Thus, IGT may not progress to overt diabetes and may revert to normoglycemia, especially in obese patients after dietary treatment and weight reduction.

3 Same value for capillary plasma glucose; q100 but p110 mg/dl (q5.5 but p6.0 mmol/l) for venous or capillary whole blood glucose.

4 q 160 but p220 mg/dl (q8.9 but p12.2 mmol/l) for capillary plasma glucose; q120 but p180 mg/dl (q6.7 but p10.0 mmol/l) for venous whole blood glucose; q140 but

p 200 mg/dl (q7.8 but p11.1 mmol/l) for capillary whole blood glucose.

Table 2. Subjects in whom OGTT should be performed First-degree relative of type 2 diabetic patients (especially if monozygotic twin of a diabetic

patient or offspring of two diabetic parents) Subjects with abnormal or borderline glycemic values (FPG q110 mg/dl but p126 mg/dl) during screening test for diabetes

Pregnant women with suspected GDM Obese subjects (especially when a family history of diabetes is present) Individuals with a family history of MODY Members of racial or ethnic groups with high prevalence of diabetes (American Indians or

Pacific Islanders, African-Americans, Hispanics, etc.) Patients with unexplained neuropathy or coronary disease or peripheral vascular disease or retinopathy or nephropathy (especially under 50 years of age) Patients with hyperglycemia or glycosuria found during acute illness, stress situations, surgical procedures, steroid administration, etc.

Oral Glucose Tolerance Test The OGTT is not recommended for routine clinical use (being a nonspe-

cific test) and should be standardized for both procedure and interpretation, while the use of FPG is encouraged as a simple, convenient, accurate, acceptable to patients and low cost test for diagnosing diabetes. FPG and 2-hour OGTT values are equivalent for the diagnosis of diabetes (even if not perfectly corre- lated with each other), and actually the FPG alone is preferable for its better reproducibility (6% variation) whereas OGTT, repeated in adults during a 2- to 6-week interval, presents an intraindividual coefficient of variation of 17% for the 2-hour value. OGTT remains, however, the most sensitive and practical test for the early recognition of asymptomatic diabetes without high FPG value, and it is an invaluable tool in research studies. If the OGTT is used, the test procedures recommended are that of WHO. The indications of OGTT are outlined in table 2.

The following variables may affect the OGTT results: Technical Variables. Venous versus capillary blood: In adults venous

blood from an antecubital vein is usually employed, obtained with minimum stasis. In the capillary blood, glucose approximates that of arterial blood, and is higher than in venous blood by 2–3 mg/dl in the fasting state and by 20–70 mg/dl during OGTT.

Plasma or serum versus whole blood : Plasma or serum is generally em- ployed, providing more stable values. In these materials glucose concentration is 15% higher than in whole blood. The blood sample should be immediately refrigerated to prevent glycolysis of glucose by blood cells (fluoride cannot be Plasma or serum versus whole blood : Plasma or serum is generally em- ployed, providing more stable values. In these materials glucose concentration is 15% higher than in whole blood. The blood sample should be immediately refrigerated to prevent glycolysis of glucose by blood cells (fluoride cannot be

Methods for determining glycemia: The most commonly used methods are the glucose-specific enzymatic methods. The use of strips, read with glucose reflectance meter, is not recommended for diagnostic purpose (for its great variability) whereas it is useful for blood glucose self-monitoring during dia- betes treatment.

Glucose dose and concentration: In the past, glucose doses for OGTT varied from 50 to 100 g. To avoid nausea and to achieve a better standardization the use of an oral flavored solution of 75 g glucose dissolved in 300 ml of water for adults is now recommended. In children, 1.75 g/kg ideal body weight (up to a maximum of 75 g) should be used. During pregnancy, the OGTT is performed utilizing 100 g of glucose. The glucose solution should be consumed over 5 min.

Timing of samples for OGTT: Blood samples are obtained in the fasting state and after 30, 60, 90, 120 min according to NDDG for testing individual patients. According to WHO, only 0- and 120-min samples should be used, which makes the test more suitable for testing large population groups or for epidemiological studies. During pregnancy, a 180-min sample should also be obtained. For the diagnosis of reactive hypoglycemia, the OGTT should be prolonged to 5 h.

Time of day: There is a diurnal variation in glucose tolerance (which deteriorates in the afternoon); thus, a standard OGTT should be obtained in the morning, after a fasting of 10–14 h.

Host Variables. Preceding diet: A diet containing 250 g of carbohydrate is recommended for at least 3 days before the test. In subjects on reduced diets, a diet containing at least 200 g of carbohydrates should be taken for 1 week before the OGTT. Coffee or smoking are avoided before and during the test.

Physical activity: OGTT should not be performed in patients at bed rest, hospitalized or immobilized (conditions which may reduce glucose tolerance).

A moderate walking during the test is permitted, but physical exercise should

be avoided. Acute or chronic illness: OGTT should not be performed in patients affected by acute infections, acute cardiovascular and cerebrovascular diseases, active endocrinopathies, hepatic or renal diseases, or in subjects under stress or treated with some drugs such as glucocorticoids, estrogens, salicylates, thiazides, nicotinic acid, dilantin, etc.

Age: Glucose tolerance deteriorates with advancing age (because of de- creased or delayed insulin secretion, reduced insulin sensitivity, increase of insulin antagonists, physical inactivity, obesity and other associated diseases, etc.). In the elderly, the 2-hour glycemic level would increase by 10 mg/dl for each decade over 50 years.

Other Tests Oral cortisone-glucose tolerance test is not a diagnostic test but is used

for research purpose. The intravenous glucose tolerance test (or IVGTT) should be used as a diagnostic test only for patients with gastrointestinal disorders interfering with absorption of glucose. It is less physiological than OGTT, bypassing the effects of several relevant gastrointestinal hormones active with oral glucose load. Glucose (25 g as 50% solution) is infused over 3 min and samples are obtained every 10 min for 1 h. Through a formula, the K coefficient can be calculated, whose normal value is between 1.2 and 2.2; values =1 indicate diabetes, values between 1 and 1.2 are regarded as borderline.

Determination of insulin during OGTT is not recommended for routine diagnostic purpose (because of extreme variability in fasting state and after glucose load), although it can be of prognostic value. Values are elevated in subjects with insulin resistance.

HbA 1c measurement is not currently used for diagnosis of diabetes whereas it is useful in monitoring the metabolic control. Normal values of HbA 1c range from 4.0–4.5 to 6.0–6.4% of total hemoglobin, although differences exist among values depending on laboratories and/or methods. According to the

IDF guidelines [1999] to type 2 diabetes, HbA 1c can be useful for the diagnosis provided that confirmatory venous plasma glucose estimations are obtained, the assay is DCCT standardized, an HPLC chromatogram is reviewed for presence of abnormal hemoglobins, and erythrocyte turnover is not abnormal.

Approximately: HbA 1c ? 7.5% B fasting plasma glucose q7.0 mmol/l (?125 mg/dl), and HbA 1c ? 6.5% B fasting plasma glucose ?6.0 mmol/l (q110 mg/dl). Glycosuria is not useful for the diagnosis, being present only when glyce- mia is higher than the renal threshold for glucose. It may be useful for a coarse monitoring of diabetic control. The aged people may have a higher than normal renal threshold for glucose (having glycosuria only at elevated glucose levels), whereas pregnant women often have a lowered glucose threshold (show- ing glycosuria even with normal glycemia).

Testing for Diabetes mellitus

Type 1 Diabetes. In type 1 diabetes, routine testing for immune markers (outside of clinical trials or research studies) is not recommend for many reasons, including: (a) cut-off values have not been completely established for clinical settings; (b) there is no consensus on proven measures that can prevent Type 1 Diabetes. In type 1 diabetes, routine testing for immune markers (outside of clinical trials or research studies) is not recommend for many reasons, including: (a) cut-off values have not been completely established for clinical settings; (b) there is no consensus on proven measures that can prevent

toantibody tests, however, may be useful to detect which newly diagnosed patients have immune-mediated type 1 diabetes.

Type 2 Diabetes. Type 2 diabetes is commonly undiagnosed in about 50% of affected subjects. On the other hand, retinopathy may develop early, even

7 years before the diagnosis of overt diabetes. Thus, the unapparent hyperglyce- mia can cause microvascular complications and favor macrovascular disease. Therefore, the undiagnosed diabetes is a serious problem. Early detection and treatment are indispensable to reduce the late complications of type 2 diabetes. Thus, testing for diabetes (especially with FPG) should be recommended in the clinical setting and in high-risk subjects.

In asymptomatic and undiagnosed individuals, testing for type 2 diabetes by FPG should be performed in: (a) all individuals at age 45 and above, repeated at 3-year intervals if results are normal; (b) individuals at younger age if at risk (obese subjects, first-degree relatives of diabetic patients, compo- nents of high-risk ethnic populations, women with GDM, mothers of obese baby ?9 lb or 4 kg, etc.); (c) hypertensive subjects with low HDL cholesterol (p35 mg/dl) or high triglycerides (q250 mg/dl); (d) individuals with IGT or IFG on previous testing.

Suggested Reading

Expert Committee on the Diagnosis and Classification of Diabetes mellitus: Report of the Expert Committee