Type 2 Diabetes Mellitus

David W. Cooke

Type 2 diabetes mellitus is a disorder of glucose and lipid regulation caused by a combination of decreased insulin effectiveness at the cellular level and impaired insulin secretion. Because untreated type 2 diabetes does not generally deteriorate to ketoacidosis, it is a form of non–insulin-dependent diabetes mellitus (NIDDM). Until recently, type 2 diabetes was considered a disease of adulthood, being rarely diagnosed in children. In fact, it is a disease whose prevalence increases with age so that it was also uncommon in young adults. In recent decades, however, the prevalence of type 2 diabetes has been increasing in adults, and it has been identified in larger numbers of younger adults. This “epidemic” of type 2 diabetes has extended into childhood so that now significant numbers of children diagnosed with diabetes have type 2 diabetes, in contrast to the recent past, when type 1 diabetes would have been the only likely diagnosis.

EPIDEMIOLOGY

The recent diabetes epidemic has resulted in a 40% increase in the prevalence of diabetes in U.S. adults during the 1990s, with more than 6.5% of the U.S. population now having diabetes. Type 2 diabetes is the most common form of diabetes, accounting for 90% or more of these cases. Both genetic and environmental factors contribute to the risk of developing type 2 diabetes. Genetic factors explain the increased risk in individuals with a positive family history, as well as the variation in the prevalence of the disease across different racial and ethnic groups, with increased risk in African American, Hispanic, Asian, and Native American populations compared to Caucasians. The most significant environmental factor is the association of type 2 diabetes with obesity: whereas type 2 diabetes can occur in nonobese individuals, more than 90% of those who develop diabetes are obese. Criteria for screening children for type 2 diabetes are listed in Box 379.1.

Before the 1990s, fewer than 2% of children with diabetes were thought to have type 2 diabetes. During the 1990s, however, reports began to document a rising prevalence of type 2 diabetes in pediatric patients. The percentage of cases of diabetes in children and adolescents that are type 2 diabetes depends on the proportion that are adolescents, the prevalence and degree of obesity, and the racial and ethnic composition of the population, but now it may be as high as one-half of
newly diagnosed cases. Although there are not yet accurate population-wide prevalence estimates for type 2 diabetes in children and adolescents, the data indicate a rising prevalence that has yet to plateau. There has been a female predominance in diagnosed cases of type 2 diabetes, with a female-to-male ratio of approximately 2:1.

BOX 379.1 Criteria for Screening Children for Type 2 Diabetes

Age ≥10 years or children <10 years of age in whom puberty has begun

Plus

Overweight (body mass index [BMI] >85th percentile for age and sex, weight for height >85th percentile, or weight >120 percentile of ideal for height)

Plus

Any two of the following risk factors:

Family history of type 2 diabetes in firstor second-degree relatives
Of the following race/ethnicity: American Indian, African American, Hispanic, Asian/Pacific Islander
Signs of insulin resistance or conditions associated with insulin resistance (acanthosis nigricans, hypertension, dyslipidemia, polycystic ovary syndrome)

Footnote

Adapted from

American Diabetes Association. Type 2 diabetes in children and adolescents. Pediatrics 2000;105:671.

The majority of children presenting with type 2 diabetes are obese (body mass index [BMI] above the 95th percentile for age), with many being extremely obese. As in adults, however, a small percentage of children with type 2 diabetes are not obese. Most children will present with type 2 diabetes during puberty. The explanation for this is that in all children, puberty is associated with an approximately 30% decrease in insulin sensitivity; in the predisposed individual this additional challenge to glucose homeostasis may not be met, resulting in type 2 diabetes. Thus far, it remains relatively uncommon for children to present with type 2 diabetes before puberty, although that does occur.

Hypertension and a specific form of dyslipidemia (hypertriglyceridemia and decreased high-density lipoprotein [HDL] cholesterol level) associate together in obese adults in a syndrome referred to as the metabolic syndrome or syndrome X. Insulin resistance, one of the main defects leading to type 2 diabetes, is also a part of this syndrome, and adults with the metabolic syndrome are at increased risk of type 2 diabetes, so that hypertension and dyslipidemia may also indicate an increased risk of type 2 diabetes in children.

BOX 379.2 Genetic Factors as a Cause of Type 2 Diabetes

Virtually all children identified with type 2 diabetes will have a family history of type 2 diabetes, and most will have a parent with type 2 diabetes. Type 2 diabetes is felt to be a multigenic disorder; that is, it is caused by the combined effects of a number of genes (and the effects of environmental factors) rather than being the result of a mutation of a single gene. Although a number of genes have now been implicated as potential contributors to the genetic risk of type 2 diabetes (including the genes for the peroxisome proliferator-activated receptor-gamma [PPAR-gamma]; the beta-3-adrenergic receptor; calpain-10; and insulin receptor substrate 1 [IRS-1]), current knowledge of the identity of type 2 diabetes genes is very incomplete. Notably, many genes in the glucose metabolism, insulin secretion, and insulin signaling pathways have been investigated and excluded as significant type 2 diabetes genes. Individuals with homozygous or compound heterozygous mutations of the insulin receptor do have syndromes with extreme insulin resistance, but these are distinct from type 2 diabetes. The phenotype associated with these mutations varies from relatively normal glucose control (rarely) or diabetes identified in adolescence (type A insulin resistance) to more severe phenotypes evident in infancy (Rabson-Mendenhall syndrome and leprechaunism, which is usually fatal in infancy).

There is a type of non–insulin-dependent diabetes for which the genetic cause is known. These are the single gene mutations that lead to maturity-onset diabetes in youth (MODY), inherited in an autosomal dominant manner and resulting in diabetes that usually has its onset before age 25 years. Six separate MODY types have been identified. These are due to mutations in the genes for glucokinase (MODY2) and for transcription factors: hepatocyte nuclear factor (HNF)-4-alpha (MODY1); HNF-1-alpha (MODY3); insulin promoter factor-1 (IPF1; MODY4); HNF-1-beta (MODY5); and neurogenic differentiation 1/beta-cell E-box transactivator 2 (NeuroD1/BETA2; MODY6). These genes are all expressed in the pancreatic beta cell, and heterozygous mutations cause a deficiency in insulin secretion. Because insulin resistance is not part of the pathophysiology, obesity is not a feature of MODY. The severity of the insulin deficiency varies for the different MODY types, and consequently the severity of the untreated diabetes can vary from mild, sometimes unrecognized hyperglycemia with little risk of long-term microvascular diabetic complications to severe hyperglycemia with a very high risk of microvascular complications. However, in contrast to the ultimately total insulin deficiency of type 1 diabetes, there is generally sufficient insulin secretion in MODY to prevent ketosis. It is necessary to emphasize that the recent increased prevalence of non–insulin-dependent diabetes in children is not due to children with MODY. MODY is a relatively uncommon cause of diabetes, responsible for less than 5% of adult cases of diabetes. As in adults, the vast majority of children and adolescents with non–insulin-dependent diabetes have type 2 diabetes.

Genetic and environmental factors affecting fetal growth may also alter the risk for the development of type 2 diabetes. Intrauterine growth restriction (IUGR) is associated with an increased risk of type 2 diabetes in adulthood, and children with a history of IUGR have decreased insulin sensitivity. As other environmental factors increase the risk of diabetes, the increased insulin resistance in these children is likely to further increase their risk of developing type 2 diabetes during childhood. At the opposite extreme, overnutrition of the fetus, as occurs in infants born to diabetic mothers, also increases the risk of type 2 diabetes for the offspring. From a public health perspective, this is very concerning, given the increasing prevalence of diabetes in younger individuals, the result of which potentially will lead to more pregnancies in women with diabetes, setting the stage for an explosion of the type 2 diabetes epidemic.

Adults who have blood glucose levels that fall within a “prediabetes” range of impaired glucose regulation are at very high risk of developing type 2 diabetes within several years—up to 40% in 5 to 10 years. However, there are not yet data available to demonstrate that impaired fasting glucose (IFG) or impaired glucose tolerance (IGT; see later) in children or adolescents indicates the same increased risk for the development of type 2 diabetes as in adults. One possible confounder that could alter this relationship of glucose levels with later risk of diabetes is the insulin resistance of puberty that resolves at the end of puberty.

There is a strong genetic component to the risk of type 2 diabetes (Box 379.2).

PATHOGENESIS

Because the emergence of type 2 diabetes as a significant pediatric disease has been a recent development, there may be aspects of the disease specific to pediatric patients that are not yet known. Current knowledge, however, has indicated that
the pathophysiology of type 2 diabetes in children mirrors that of the adult disease, although physiologic changes of puberty contribute a unique aspect to the pathophysiology.

Two defects are present in patients with type 2 diabetes: insulin resistance and defective insulin secretion. Insulin resistance refers to a decreased effectiveness of insulin in activating signals distal to binding of insulin to the insulin receptor. Insulin resistance itself, except in the most extreme situation, will not lead to diabetes, as normal metabolic control can be maintained by a compensatory increase in insulin secretion. When a second defect results in an inability to respond to the requirement for increased insulin secretion imposed by insulin resistance, type 2 diabetes mellitus occurs. There is evidence, however, that at least in some of the at-risk ethnic populations, the increased risk of diabetes is related to increased insulin resistance. The data supporting this are strongest for African Americans and Native Americans, and the increased insulin resistance in these populations is almost certainly genetically based. Additional information about the role of insulin resistance and insulin secretion in the pathogenesis of type 2 diabetes is presented in Box 379.3.

CLINICAL MANIFESTATIONS AND COMPLICATIONS

Many children with type 2 diabetes present with the same classic symptoms of diabetes mellitus as children with type 1 diabetes: polyuria, polydipsia, and polyphagia (in contrast to type 1 diabetes, significant weight loss is less likely to have occurred with type 2 diabetes). However, in contrast to children presenting with type 1 diabetes, where the symptoms are typically present for only a few weeks, children with type 2 diabetes may have had these symptoms for many months. Many children with type 2 diabetes will not have any specific symptoms, and diabetes will be diagnosed based on screening tests obtained due to the presence of risk factors for type 2 diabetes or because a urinalysis obtained for other reasons indicates glucosuria. Finally, although diabetic ketoacidosis (DKA) is much less common in patients with type 2 diabetes than in patients with type 1 diabetes, a significant number of children with type 2 diabetes will present in DKA. (See Chapter 378 for discussion of the presenting features of DKA.)

Patients with diabetes mellitus are at risk for both acute and chronic complications. The acute complications (hypoglycemia, DKA, and nonketotic hyperosmolar coma) can occur at any time after the diagnosis of diabetes, while the chronic complications, including macrovascular and microvascular complications, develop over many years. A more detailed discussion of these complications is presented in Chapter 378.

Diabetic ketoacidosis

In established type 1 diabetes, without treatment with insulin, the patient will quickly develop DKA. In contrast, patients with type 2 diabetes generally have sufficient insulin action to restrain lipolysis, limiting free fatty acid delivery to the liver and subsequent ketoacid production. Because of this, patients with type 2 diabetes are at much lower risk of developing DKA, even without treatment for their diabetes. However, significant numbers of children who have ultimately been determined to have type 2 diabetes have presented with DKA. While these children require initial treatment with insulin, many can ultimately be managed with oral medications. It is likely that in these children, hyperglycemia has induced sufficient glucotoxicity on the beta cell to diminish insulin secretion to such a degree that DKA can occur. Once this toxicity is relieved by appropriate glycemic control, sufficient endogenous insulin secretion is restored to allow the discontinuation of exogenous insulin treatment. It is not yet clear why children with type 2 diabetes appear to have a greater risk of DKA than is seen in adults. Nonetheless, although DKA is uncommon in children with type 2 diabetes, the child and family should be taught the skills necessary to detect developing ketosis and prevent deterioration into DKA, just as is done for the child with type 1 diabetes. The child with type 2 diabetes who presents in DKA (or has an episode of DKA subsequent to the diagnosis of type 2 diabetes) should be considered at higher risk for subsequent episodes of DKA.

Nonketotic Hyperosmolar Coma

The classic hyperglycemic crisis in patients with type 2 diabetes is nonketotic hyperosmolar coma. This is similar to DKA, in that stress hormones (epinephrine, cortisol, growth hormone) are increased due to an intercurrent illness and antagonize insulin action, inducing an acute worsening of hyperglycemia. The hyperglycemia induces an osmotic diuresis, and if not compensated for with increased fluid intake, the dehydration compounds the hyperglycemia by impairing glucose clearance by the kidneys. Marked hyperglycemia, hyperosmolarity, and dehydration result. In contrast to DKA, at most only moderate levels of ketones are present in the blood. Because nonketotic hyperosmolar coma is due to an inability to maintain hydration in the face of ongoing fluid loss from the osmotic diuresis and develops insidiously over a period of a number of days, it typically only develops in debilitated patients with type 2 diabetes. It therefore is not a common feature in type 2 diabetes in children.

Hypoglycemia

Treatment of diabetes with insulin seeks to match the dose of insulin given to the current insulin requirement. If more insulin is given than is needed, there is a risk of producing hypoglycemia. This is true whether insulin is used to treat type 1 or type 2 diabetes, although hypoglycemia is generally less common in insulin-treated patients with type 2 diabetes than in patients with type 1 diabetes.

The insulin secretagogues, which include the sulfonylureas and the newer meglitinide analogues, act by stimulating endogenous insulin secretion. Treatment with these medications carries the risk that they will stimulate more insulin secretion than is needed to meet the current insulin requirement and induce hypoglycemia. This risk in the typical child with type 2 diabetes is quite low. The meglitinide analogue secretagogues are very short acting, intended to be given at mealtime. As such, they have an even lower risk of hypoglycemia than the longer-acting sulfonylureas, where there is some risk of hypoglycemia if meals are missed. The signs, symptoms, and treatment of hypoglycemia in type 2 diabetes, whether induced by insulin or insulin secretagogue treatment, is the same as in type 1 diabetes; see Chapter 378 for a further discussion of hypoglycemia in diabetes.

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