INTRODUCTION — Type 2 diabetes accounts for over 90 percent of cases of diabetes in the United States, Canada, and Europe; type 1 diabetes accounts for another 5 to 10 percent, with the remainder due to specific etiologic or pathophysiologic factors (table 1). Known monogenic causes of diabetes (eg, those causing maturity onset diabetes of the young or neonatal diabetes) represent a small fraction of cases. The genetic bases of common forms of type 1 and type 2 diabetes remain complex, with common variants of genes individually contributing only small degrees of risk or protection. A wide range of phenotypic forms of diabetes of uncertain pathophysiology that do not fit clearly into the categories of types 1 and 2 diabetes, collectively termed "atypical diabetes," is emerging. Furthermore, the worldwide epidemic of overweight and obesity has superimposed the pathophysiology of type 2 diabetes across all other types.
The classification of diabetes mellitus will be reviewed here, together with brief descriptions of some emerging forms of diabetes. The definition and diagnostic criteria for diabetes mellitus are discussed separately. (See "Clinical presentation, diagnosis, and initial evaluation of diabetes mellitus in adults".)
TYPE 1 DIABETES — Type 1 diabetes is characterized by destruction of the pancreatic beta cells, leading to absolute insulin deficiency. This is usually due to autoimmune destruction of the beta cells (type 1A). Testing for islet cell antibodies (ICA) or other islet autoantibodies (antibodies to glutamic acid decarboxylase 65 [GAD65]; insulin; the tyrosine phosphatases, insulinoma-associated protein 2 [IA-2] and IA-2 beta; and zinc transporter [ZnT8]) in serum is important; a positive result is generally indicative of autoimmune or type 1A diabetes [1]. However, some patients with absolute insulin deficiency have no evidence of autoimmunity and have no other known cause for beta cell destruction. The classification system of the American Diabetes Association (ADA) applies the term "idiopathic" or "type 1B" diabetes to refer to these patients; these terms could encompass an array of nonautoimmune pathophysiologic processes leading to near-complete loss of beta cell function. (See "Pathogenesis of type 1 diabetes mellitus".)
The ADA classification of diabetes mellitus does not reflect the clinical heterogeneity of patients with diabetes and the emergence of the concept that early beta cell dysfunction is likely to be a primary defect in the pathophysiology of diabetes, regardless of "type." Other classification schemes have been proposed, accounting for beta cell autoimmunity, beta cell function, clinical features, and body weight. The high prevalence of overweight/obesity in the population has further complicated classification systems with an added element of insulin resistance even in type 1 diabetes. (See "Syndromes of ketosis-prone diabetes mellitus", section on 'Classification of KPD'.)
The emergence of atypical phenotypes of both type 1 and type 2 diabetes emphasizes the need for a better classification of diabetes, ideally on an etiologic basis.
Latent autoimmune diabetes in adults (LADA) — Older studies in predominantly Scandinavian populations have suggested that as many as 7.5 to 10 percent of adults in populations with a high prevalence of type 1 diabetes and with apparent type 2 diabetes, based on older age of onset, may have circulating autoantibodies directed against pancreatic beta cell antigens (ICA or GAD65) [2-4]. The prevalence of LADA is lower in the more diverse United States population. These adults may not require insulin treatment at diagnosis but progress to insulin dependence after several months to years [5-7].
In genotyping analyses, LADA shares genetic features of both type 1 and type 2 diabetes [8-10]. As an example, in one analysis, patients with LADA shared an increased frequency of risk for an HLA-DQB1 genotype with patients with type 1 diabetes and for a variant in the transcription factor 7-like 2 (TCF7L2) gene with patients with type 2 diabetes [8]. The variant in TCF7L2 has been shown to increase the risk for type 2 diabetes in several populations, and the effect size was similar for LADA and type 2 diabetes [9]. (See "Pathogenesis of type 2 diabetes mellitus", section on 'Genetic susceptibility' and "Pathogenesis of type 1 diabetes mellitus".)
Patients with LADA are a heterogeneous group of patients with variable titers of antibodies, body mass index (BMI), and frequency of progression to insulin dependence [11]. Patients with high compared with low titers of GAD65 antibodies usually have a lower BMI, less endogenous insulin secretion (as measured by stimulated serum C-peptide concentrations), and progress more quickly to insulin dependence [11,12]. Thus, the presence and titers of anti-GAD antibodies (or ICA) can help to identify patients thought to have type 2 diabetes, who are likely to respond poorly to oral hypoglycemic drug therapy, require insulin, and to be at increased risk for developing ketoacidosis [4,5,11-14]. (See 'Distinguishing type 1 from type 2 diabetes' below.)
TYPE 2 DIABETES — Type 2 diabetes is the most common phenotypic form of diabetes in adults and is characterized by hyperglycemia and variable degrees of insulin resistance and deficiency. Its prevalence rises markedly with increasing degrees of obesity. Insulin resistance and insulin deficiency can arise through genetic or environmental influences, making it difficult to determine the exact cause in an individual patient. In addition, hyperglycemia itself can impair pancreatic beta cell function and exacerbate insulin resistance, called "glucotoxicity." (See "Pathogenesis of type 2 diabetes mellitus".)
DKA in type 2 diabetes — Diabetic ketoacidosis (DKA) was thought to be rare in patients with an apparent phenotype of type 2 diabetes, but it can occur in specific situations:
●DKA may occur due to severe stresses that provoke secretion of counterregulatory hormones that worsen insulin resistance in the face of markedly impaired insulin secretion; these patients are unable to respond to the increased insulin demand, leading to DKA.
●DKA in the absence of a clinically apparent stressor may be the initial presentation of diabetes in patients with an apparent phenotype of type 2 diabetes who have a subtype of ketosis-prone diabetes. (See "Syndromes of ketosis-prone diabetes mellitus".)
DISTINGUISHING TYPE 1 FROM TYPE 2 DIABETES — With the explosion of diabetes worldwide, it has become increasingly difficult to distinguish type 1 from atypical presentations of type 2 diabetes. Patients with type 1 diabetes may have at presentation or develop over time an absolute requirement for insulin therapy. However, many patients with type 2 diabetes lose beta cell function over time and require insulin for glucose management. Thus, need for insulin per se does not distinguish between type 1 and type 2 diabetes. As noted above, diabetic ketoacidosis (DKA) cannot be relied upon as an absolute indicator that the patient has type 1 diabetes or that long-term insulin therapy will be required. (See "Syndromes of ketosis-prone diabetes mellitus".)
Patients with type 1 diabetes may coincidentally have pathophysiologic elements of type 2 diabetes. In the past, poor metabolic control of type 1 diabetes prevented most of these patients from gaining weight. Intensive therapy now commonly used to manage type 1 diabetes has resulted in an increasingly similar prevalence of overweight and obesity in the type 1 diabetes population as in the nondiabetic population. Insulin resistance and other features of type 2 diabetes may be exhibited in such patients with type 1 diabetes, especially those who also have a family history of type 2 diabetes [15].
●When to perform islet autoantibody testing – Given the overlap of type 1 and type 2 diabetes and the emergence of many atypical forms of diabetes, clinicians should have a low threshold for islet autoantibody testing in adults with any form of diabetes that is not easily classifiable.
We measure autoantibodies when the diagnosis of type 1 or type 2 diabetes is uncertain by clinical presentation:
•Thin patients with poor response to initial therapy with sulfonylureas or metformin
•Personal or family history of autoimmune disease
•Young adults with diabetes
Based upon a review of clinical features in 102 adult patients with diabetes who did not initially require insulin but who were positive for glutamic acid decarboxylase 65 (GAD65) autoantibodies, a screening tool was developed to identify adult patients with newly diagnosed diabetes who should be considered for antibody testing [16]. These features included: age of onset <50 years, acute symptoms, body mass index (BMI) <25 kg/m2, and personal or family history of autoimmune disease. The presence of two or more criteria had a 90 percent sensitivity and 71 percent specificity for identifying patients positive for anti-GAD antibodies. (See "Prediction of type 1 diabetes mellitus", section on 'Immunologic markers'.)
●Which islet autoantibodies should be measured? – Two (ie, islet cell antibodies [ICA] and GAD65) or a panel (insulin-associated antibodies [IAA], GAD65, insulinoma-associated protein 2 [IA-2], and zinc transporter [ZnT8]) antibodies can be measured. Measuring more than one antibody will increase the likelihood of a positive value, but it is also more costly.
Insulin antibodies should not be measured if the patient has received insulin therapy for ≥2 weeks, because this will generate insulin antibodies.
If one or more of the antibodies is present, and especially if two or more are positive, the patient should be presumed to have type 1 diabetes and should be treated promptly with insulin replacement therapy, as these patients respond poorly to diet and oral hypoglycemic drug therapy. In addition, during early stages in the development of type 1 diabetes, intensive insulin therapy prolongs beta cell function [17].
The presence of antibodies to GAD, islet cell, insulin, the tyrosine phosphatases (IA-2 and IA-2 beta), and ZnT8 in patients with presumed type 2 diabetes can identify patients who may have latent autoimmune diabetes in adults [LADA]) and are more likely to require insulin [4,5,12,13,18].
Given the risk of DKA, insulin should also be started in any patient (regardless of whether they are thought to have type 1 or type 2 diabetes) who is catabolic (weight loss or dehydration in the setting of hyperglycemia), or who has evidence of increased ketogenesis (ketonuria or acidosis). (See "Management of blood glucose in adults with type 1 diabetes mellitus" and "Insulin therapy in type 2 diabetes mellitus", section on 'Indications for insulin'.)
●Cellular autoimmunity – The presence of islet autoantibodies serve as biomarkers for humoral autoimmune processes directed against the islets, but cellular islet autoimmunity is generally recognized to be the fundamental cause of autoimmune destruction of beta cells in type 1 diabetes. However, islet-specific T cell reactivity testing is a research tool and is not commercially available [19]. A significant proportion of patients with phenotypes of both type 1 and type 2 diabetes may have CD4+ T cells that display reactivity to a broad range of islet antigens, in the absence of the known circulating autoantibodies [19]. Thus, diabetes syndromes that overlap type 1 and type 2 diabetes may occur more frequently due to T cell reactivity to islet autoantigens than suggested by the limited frequency of LADA defined by islet autoantibodies alone.
GENETIC VARIANTS — As the human genome is further explored, it is likely that multiple genetic variations at different loci will be found that confer varying degrees of predisposition to type 1 and type 2 diabetes. Polymorphisms of multiple genes are reported to influence the risk of type 1A diabetes, including genes in both the major histocompatibility complex (MHC) and elsewhere in the genome, but only human leukocyte antigen (HLA) alleles have a large effect, followed by insulin gene polymorphisms, and PTPN22. (See "Pathogenesis of type 1 diabetes mellitus", section on 'Genetic susceptibility'.)
Numerous common polymorphisms (approximately 100 have been identified to date) weakly contribute to the risk for or protection from type 2 diabetes. The genes encode proteins that cause alterations in several pathways leading to diabetes, including pancreatic development; insulin synthesis, processing, and secretion; amyloid deposition in beta cells; cellular insulin resistance; and impaired regulation of gluconeogenesis. Monogenic causes of type 2 diabetes represent only a small fraction of cases and commonly inherited polymorphisms individually contribute only small degrees of risk for, or protection from, diabetes. Most of the genetic risk for type 2 diabetes results from complex polygenic risk factors. (See "Pathogenesis of type 2 diabetes mellitus", section on 'Genetic susceptibility'.)
Monogenic diabetes (formerly called maturity onset diabetes of the young) — Monogenic diabetes or maturity onset diabetes of the young (MODY) is a clinically heterogeneous disorder characterized by diabetes diagnosed at a young age (<25 years) with autosomal dominant transmission and lack of autoantibodies [20]. MODY is the most common form of monogenic diabetes, accounting for 2 to 5 percent of diabetes [21,22]. The population prevalence of MODY in the United Kingdom is estimated to be 68 to 108 cases per million [23]. These patients are quite heterogeneous, and clinical characteristics may not be reliable in predicting the underlying pathogenesis [24,25]. Many patients are misclassified as having either type 1 or 2 diabetes.
Several different genetic abnormalities have been identified, each leading to a different type of disease. The original MODY nomenclature ("MODY1," "MODY2," "MODY3," etc) has been superseded by the term "monogenic diabetes" with the name of the gene associated with the trait. Well-understood, known monogenic gene variants and the associated syndromes are described below and in the table (table 2). The genes involved control pancreatic beta cell development, function, and regulation, and the mutations in these genes cause impaired glucose sensing and insulin secretion with minimal or no defect in insulin action [26]. Mutations in hepatocyte nuclear factor-1-alpha (HNF1A) and the glucokinase (GCK) gene are most commonly identified, occurring in 52 to 65 and 15 to 32 percent of MODY cases, respectively [25,27]. Mutations in hepatocyte nuclear factor-4-alpha (HNF4A) account for approximately 10 percent of MODY cases. Some members of a family have the genetic defect but do not develop diabetes; the reason for this is unclear. Other patients may have the MODY phenotype but do not have an identifiable mutation in any of the known MODY genes [26].
Hepatocyte nuclear factor-4-alpha — Mutations in the HNF4A gene on chromosome 20 cause the condition formerly called MODY1 [28]. HNF4A is expressed both in the liver and in pancreatic beta cells. The precise mechanism by which a defect in HNF4A causes hyperglycemia is not clear, but it has been associated with reduced insulin secretory response to glucose, suggesting a primary genetic defect in insulin secretion [29-31]. The secretory defect is progressive, and patients typically present with hyperglycemia in adolescence or early childhood. Although the patients initially respond to sulfonylureas, they may require insulin as the secretory defect progresses. These patients are at risk for the microvascular and macrovascular complications of diabetes.
Although HNF4A plays a central role in the hepatic synthesis of lipoprotein and coagulation proteins, these functions are largely maintained in HNF4A diabetes, suggesting that this disorder is primarily one of impaired pancreatic beta cell function [30].
Glucokinase — More than a dozen mutations in the GCK gene on chromosome 7 have been described and were formerly called MODY2 [32]. Defects in the expression of GCK, which phosphorylates glucose to glucose-6-phosphate and acts as a glucose sensor, result in a higher threshold for glucose-stimulated insulin secretion. On occasion, the expressed enzyme is functional but unstable, leading to an insulin secretory deficit [33]. (See "Pancreatic beta cell function".)
The resulting hyperglycemia is often stable, mild, and not associated with the vascular complications common in other types of diabetes [34]. Patients with a mutation in the GCK gene can often be controlled with diet alone.
Hepatocyte nuclear factor-1-alpha — One of several mutations in the HNF1A gene on chromosome 12 was formerly called MODY3 [35]. This form of diabetes is more common among Europeans [36,37]. HNF1A is a weak transactivator of the insulin gene in beta cells. Mutations of HNF1A can lead to abnormal insulin secretion; whether this or some other action is defective enough to cause diabetes mellitus is unclear [37]. Mutations also result in a low renal threshold for glucose. Thus, prior to onset of diabetes, mutation carriers have detectable glycosuria provoked by glucose loading [38]. Testing for glycosuria two hours after a glucose load could be used to screen children of mutation carriers and guide the need for further evaluation.
Patients with HNF1A diabetes have a similar clinical phenotype as patients with HNF4A diabetes, perhaps since HNF1A is regulated positively by HNF4A. Patients exhibit increased insulin sensitivity and marked sensitivity to the hypoglycemic effects of sulfonylureas compared with metformin and compared with patients with type 2 diabetes (3.9-fold greater reduction in fasting plasma glucose) [39]. Thus, patients with mutations in the HNF1A gene can be successfully treated with sulfonylurea monotherapy, and in one clinical study, approximately 70 percent of patients previously treated with insulin successfully switched to sulfonylureas once an HNF1A mutation was identified [40]. These patients are at risk for micro- and macrovascular complications of diabetes. In addition, patients with diabetes caused by a mutation in HNF1A appear to have an increased risk of cardiovascular mortality compared with unaffected family members [41].
Insulin promoter factor 1 — Mutations in the insulin promoter factor 1 (IPF1) gene can lead to what was called MODY4 by reduced binding of the protein to the insulin gene promoter [42,43] and perhaps by altering fibroblast growth factor signaling in beta cells [44]. Less severe mutations in IPF1 may predispose to late-onset type 2 diabetes [43,45]. In addition, carriers without diabetes have higher blood glucose concentrations and lower insulin-to-glucose ratios than family members without a mutation (and without diabetes) [46].
Hepatocyte nuclear factor-1-beta — Mutations in the hepatocyte nuclear factor-1-beta (HNF1B) gene produce a syndrome formerly called MODY5 [47-50]. Affected patients can develop a variety of manifestations in addition to early-onset diabetes. These include pancreatic atrophy (on computed tomography [CT] scan), abnormal renal development (renal dysplasia that can be detected on ultrasonography in the fetus, single or multiple renal cysts, glomerulocystic disease, oligomeganephronia [a form of renal hypoplasia]), slowly progressive renal insufficiency, hypomagnesemia, elevated serum aminotransferases, and genital abnormalities (epididymal cysts, atresia of vas deferens, and bicornuate uterus) [48]. (See "Renal cystic diseases in children", section on 'Glomerular renal cysts' and "Hypomagnesemia: Causes of hypomagnesemia", section on 'Hepatocyte nuclear factor-1-beta gene mutations' and "Hypomagnesemia: Causes of hypomagnesemia" and "Renal hypodysplasia", section on 'Genetic disorders'.)
In addition, some patients have a phenotype consistent with autosomal dominant tubulointerstitial kidney disease. (See "Autosomal dominant tubulointerstitial kidney disease", section on 'Other types of ADTKD'.)
One of the functions of HNF1B is the regulation of tissue-specific gene expression. In the kidney, the proximal promoter of the PKHD1 gene has a binding site for HNF1B. Mutations in HNF1B inhibit the expression of PKHD1 and lead to cyst formation [50]. This is not surprising, since mutations in PKHD1 are responsible for the autosomal recessive form of polycystic kidney disease. (See "Autosomal recessive polycystic kidney disease in children", section on 'Pathogenesis'.)
Neurogenic differentiation factor 1 — Mutations in the gene for neurogenic differentiation factor 1 (also called NEUROD1 or BETA2) can lead to what was called MODY6 [51,52]. NEUROD1 normally functions as a regulatory switch for endocrine pancreatic development.
Other genes — Mutations in carboxyl ester lipase (CEL) (see 'Genetically linked disorders of both exocrine and endocrine pancreatic lineages' below); insulin (INS); ATP-binding cassette, subfamily C, member 8 (ABCC8); potassium channel, inwardly rectifying, subfamily J, member 11 (KCNJ11); and paternal uniparental isodisomy of chromosome 6q24 (UPD6) genes have also been associated with the MODY phenotype [26]. Mutations in INS, ABCC8, and KCNJ11 are more commonly associated with neonatal diabetes mellitus. (See "Neonatal hyperglycemia", section on 'Neonatal diabetes mellitus'.)
Diagnosis — A suspected diagnosis of monogenic diabetes should be confirmed by diagnostic genetic testing. Laboratories in several countries offer clinical testing, primarily for mutations in HNF4A, HNF1A, and GCK. A list of laboratories that provide genetic testing is available at Genetic Testing Registry. Only CLIA (Clinical Laboratory Improvement Amendments)-certified labs should be used. Genetic testing should only be performed after informed consent and genetic counseling. (See "Genetic testing", section on 'Practical issues'.)
It is important to distinguish MODY from type 1 and type 2 diabetes because the optimal treatment and risk for diabetes complications varies with the underlying genetic defect. As an example, patients with MODY due to HNF1A or HNF4A mutations are frequently misdiagnosed as having insulin-requiring type 1 diabetes because they present at an early age and are not obese. However, many of these patients can be successfully managed with sulfonylurea monotherapy. In addition, distinguishing MODY from type 1 and type 2 diabetes allows earlier identification of at-risk family members.
Indications for genetic testing — In all patients, it is important to obtain a detailed history of diabetes at diagnosis, including age, body mass index (BMI), and presenting symptoms [53]. It is also important to ascertain insulin dependency and the presence or absence of family history of diabetes. Genetic testing for monogenic diabetes should be performed when there is a high index of suspicion, as indicated by any of the following [23,26,54,55]:
●Multigenerational family history of diabetes (eg, ≥3 generations) with other clinical characteristics described above and in the table (table 3) and negative autoantibodies to glutamic acid decarboxylase 65 (GAD65), insulinoma-associated protein 2 (IA-2), and zinc transporter (ZnT8). In a patient with presumed type 1 diabetes, measurement of serum autoantibodies (islet cell antibodies [ICA], GAD65, insulin, tyrosine phosphatases IA-2 and IA-2 beta) should be performed prior to consideration of genetic testing for MODY. The presence of autoantibodies makes MODY very unlikely [22].
●A high probability of MODY (eg, >25 percent in people not treated with insulin) using the MODY Clinical Risk Calculator.
●Preserved fasting C-peptide >0.6 ng/mL with glucose >72 mg/dL three to five years after presentation.
It is more difficult to differentiate between MODY and other atypical forms of diabetes or type 2 diabetes. For patients with presumed type 2 diabetes, the presence of a simple (non-multigenerational) family history does not discriminate between MODY and type 2 diabetes. Insulin resistance is not a common feature of MODY. Thus, diabetes in the absence of obesity is suspicious for MODY, particularly in adolescents with presumed type 2 diabetes. However, the absence of obesity or surrogate markers of insulin resistance is, in general, a poor discriminator of MODY and type 2 diabetes in adults [23,53]. There are no biochemical tests that reliably differentiate between the two diseases.
For family members of mutation carriers, biochemical testing to confirm diabetes should be performed before genetic testing is considered [54]. If the biochemical tests are consistent with a diagnosis of diabetes, genetic testing can be performed to confirm the diagnosis of a MODY mutation. (See "Clinical presentation, diagnosis, and initial evaluation of diabetes mellitus in adults", section on 'Diagnostic criteria'.)
Other beta cell gene defects — There are other rare genetic defects in beta cell function that are not considered part of the MODY spectrum. One type results from a dominantly inherited missense mutation in the sulfonylurea 1 receptor subunit (SUR1) that causes hyperinsulinemia in childhood, but beta cell dysfunction and diabetes in adulthood [56,57] (see "Pathogenesis, clinical presentation, and diagnosis of congenital hyperinsulinism"). Other examples include point mutations in mitochondrial DNA [58], genetic abnormalities that result in the inability to convert proinsulin to insulin [59], and production of mutant insulin molecules [60].
Genetic defects in insulin action — There are rare abnormalities in the insulin receptor (due to a genetic defect or polycystic ovary syndrome) or in the structure of insulin itself. (See "Insulin resistance: Definition and clinical spectrum".)
Genetic defects in mitochondrial DNA — Maternally inherited diabetes and deafness (MIDD) is a rare mitochondrial disorder caused predominantly by a genetic mutation at position 3243 in the transfer RNA for leucine [61,62]. Although phenotypic expression is variable, subjects universally have both a defect in insulin secretion, which progresses to insulin dependence, and sensorineural hearing loss. The mean age of onset of diabetes and hearing loss is between the ages of 30 and 40 [62]. Other abnormalities seen include cardiac conduction defects, gestational diabetes, proteinuria, and neuropathy.
Although subjects may be treated with insulin secretagogues until insulin dependence develops, metformin is less effective and carries a higher risk of lactic acidosis in this population [62,63]. Supplementation with CoQ10 may be of some benefit. (See "Metformin in the treatment of adults with type 2 diabetes mellitus", section on 'Lactic acidosis'.)
Wolfram syndrome — Another example of a rare genetic syndrome associated with diabetes is the Wolfram or DIDMOAD (diabetes insipidus, diabetes mellitus, optic atrophy, and deafness) syndrome [64,65]. This disorder is inherited as an autosomal recessive trait with incomplete penetrance.
The gene responsible for Wolfram syndrome, named WFS1, encodes an endoplasmic reticulum membrane-embedded protein called wolframin that is expressed in pancreatic beta cells and neurons [66-68]. The estimated prevalence of Wolfram syndrome is 1 in 770,000, and it is believed to occur in 1 of 150 patients with a phenotype of type 1 diabetes [68].
Affected patients usually develop insulin-requiring diabetes and optic atrophy in early childhood and diabetes insipidus as teenagers or young adults [69]. Diabetes insipidus is due to loss of vasopressin-secreting neurons in the supraoptic nucleus and impaired processing of vasopressin precursors [70] (see "Clinical manifestations and causes of central diabetes insipidus"). Anterior pituitary dysfunction has also been reported [68].
Other manifestations of Wolfram syndrome include progressive sensorineural deafness, hydronephrosis (due in part to the high urine flow in diabetes insipidus), and neurologic dysfunction [71]. Why severe insulin-requiring diabetes develops is not known; immunologic factors do not appear to be important [65].
Fulminant diabetes — This distinctive form of diabetes, termed a "subtype" of type 1 diabetes, was originally described in Japan and has been reported predominantly in a series of case reports in adults of Far Eastern descent [72]. It is characterized by presentation in diabetic ketoacidosis (DKA) of persons with no prior history of diabetes, who remain completely insulin-dependent following this abrupt onset. Acute beta cell destruction is the cause, presumably due to either a hitherto uncharacterized viral infection or a novel form of islet autoimmunity against a background of genetic susceptibility. The Japan Diabetes Society has proposed the following diagnostic criteria [73]:
●Occurrence of diabetic ketosis or DKA within approximately seven days after onset of hyperglycemic symptoms
●Plasma glucose >288 mg/dL and glycated hemoglobin (A1C) <8.5 percent
●Fasting serum C‐peptide <0.3 ng/mL (or <0.5 ng/mL after glucagon stimulation)
Intensive insulin therapy is required lifelong following recovery from DKA.
Other emerging forms of diabetes with acute, severe presentation include checkpoint inhibitor-associated diabetes and SARS-CoV-2 (COVID-19) associated diabetes. (See "Pathogenesis of type 2 diabetes mellitus", section on 'Drug-induced hyperglycemia' and "COVID-19: Issues related to diabetes mellitus in adults", section on 'Clinical presentations'.)
DISEASES OF THE EXOCRINE PANCREAS — Any disease that damages the pancreas, or surgical removal of pancreatic tissue (eg, for treatment of congenital hyperinsulinism), can result in diabetes. There is wide variability in the frequency with which this occurs, primarily determined by the degree of pancreatic insufficiency (table 4) [74]. Among patients with pancreatic exocrine disease, diabetes is more likely to occur in those with a family history of type 1 or type 2 diabetes. This observation suggests a role for an underlying decrease in pancreatic reserve or in insulin responsiveness that makes overt diabetes more likely in patients with pancreatic insufficiency.
Diabetes that occurs in patients with pancreatic disease is usually insulin requiring. However, it is different from type 1 diabetes in that the glucagon-producing alpha cells are also affected. As a result, there is an increased risk of insulin-treated hypoglycemia.
Cystic fibrosis — The mechanisms of cystic fibrosis-related diabetes are unique and share features with both type 1 and type 2 diabetes, with both decreased insulin production and insulin resistance [75,76]. Patients with no pancreatic exocrine deficiency have normal insulin secretion and responsiveness. In comparison, patients with exocrine deficiency have decreased insulin secretion, but often glucose tolerance remains normal despite increased hepatic glucose production [77] because increased energy expenditure (glucose utilization) occurs concomitantly. Patients with exocrine deficiency and either impaired glucose tolerance or overt diabetes have reductions in both peripheral glucose utilization and hepatic insulin sensitivity. (See "Cystic fibrosis: Nutritional issues", section on 'Cystic fibrosis-related diabetes mellitus' and "Cystic fibrosis: Overview of gastrointestinal disease", section on 'Cystic fibrosis-related diabetes'.)
Hereditary hemochromatosis — Diabetes is common in patients with hereditary hemochromatosis, being present at diagnosis in up to 50 percent of symptomatic patients. (See "Clinical manifestations and diagnosis of hereditary hemochromatosis".)
Chronic pancreatitis — Glucose intolerance occurs with some frequency in chronic pancreatitis, but overt diabetes mellitus usually occurs late in the course of disease. (See "Chronic pancreatitis: Clinical manifestations and diagnosis in adults" and "Clinical manifestations and diagnosis of chronic and acute recurrent pancreatitis in children".)
Fibrocalculous pancreatic diabetes — Fibrocalculous pancreatic diabetes is a unique form of diabetes secondary to tropical pancreatitis that is endemic in certain parts of the world (eg, southern India). In a prospective evaluation of 370 patients, all of the macro- and microvascular complications typically associated with diabetes were found. Pancreatic cancer and complications of chronic pancreatitis also contribute to the mortality associated with this disease [78]. (See "Etiology and pathogenesis of chronic pancreatitis in adults".)
Genetically linked disorders of both exocrine and endocrine pancreatic lineages — Both exocrine and endocrine pancreatic cells originate from the same endodermal pool. A genetic factor common to both endocrine and exocrine pancreatic development may account for some cases of pancreatic exocrine dysfunction with diabetes. Norwegian kindreds with an autosomal dominant inheritance pattern for diabetes and exocrine pancreatic dysfunction have been identified with a single-base deletion in the carboxyl ester lipase (CEL) gene [79]. The mechanism by which carboxyl ester lipase deficiency in pancreatic acinar cells is linked to beta cell failure is not known [80].
ENDOCRINOPATHIES — Several hormones, such as epinephrine, glucagon, cortisol, and growth hormone, antagonize the action of insulin. Increased release of these hormones constitutes the protective counterregulatory response to hypoglycemia. On the other hand, primary oversecretion of these hormones can result in impaired fasting glucose or overt diabetes. (See "Physiologic response to hypoglycemia in healthy individuals and patients with diabetes mellitus".)
Endocrine abnormalities that can lead to abnormalities in glucose regulation include:
●Cushing's syndrome, due to pituitary or adrenal disease or to exogenous glucocorticoid administration. (See "Epidemiology and clinical manifestations of Cushing's syndrome".)
●Acromegaly. (See "Causes and clinical manifestations of acromegaly".)
●Catecholamine excess in pheochromocytoma. (See "Clinical presentation and diagnosis of pheochromocytoma".)
●Glucagon-secreting tumors (glucagonomas), associated with an unusual constellation of other clinical features, including skin rash, weight loss, anemia, and thromboembolic problems. (See "Glucagonoma and the glucagonoma syndrome".)
●Somatostatin-secreting tumors (somatostatinomas), typically associated with the triad of diabetes mellitus, cholelithiasis, and diarrhea with steatorrhea. (See "Somatostatinoma: Clinical manifestations, diagnosis, and management".)
●Hyperthyroidism, which can interfere with glucose metabolism, although overt diabetes is unusual. (See "Overview of the clinical manifestations of hyperthyroidism in adults".)
DRUG-INDUCED DIABETES — A large number of drugs can impair glucose tolerance; they act by decreasing insulin secretion, increasing hepatic glucose production, or causing resistance to the action of insulin (table 5). This topic is discussed separately. (See "Pathogenesis of type 2 diabetes mellitus", section on 'Drug-induced hyperglycemia'.)
VIRAL INFECTIONS — Certain viruses (eg, Coxsackie, mumps, SARS-CoV-2) can cause diabetes, either through direct beta cell destruction or, hypothetically, by inducing autoimmune damage (see "Pathogenesis of type 1 diabetes mellitus"). Chronic hepatitis C virus infection has been associated with an increased incidence of diabetes, but it is uncertain if there is a cause-and-effect relationship. (See "Extrahepatic manifestations of hepatitis C virus infection", section on 'Diabetes mellitus'.)
COVID-19 associated diabetes — SARS-CoV-2 (COVID-19) appears to precipitate severe manifestations of diabetes in patients with or without a prior history of diabetes, including diabetic ketoacidosis (DKA), hyperosmolar hyperglycemic state (HHS), and severe insulin resistance. This topic is reviewed in more detail elsewhere. (See "COVID-19: Issues related to diabetes mellitus in adults", section on 'Clinical presentations'.)
GESTATIONAL DIABETES MELLITUS — Gestational diabetes occurs when a woman's insulin secretory capacity is not sufficient to overcome both the insulin resistance created by the anti-insulin hormones secreted by the placenta during pregnancy (eg, estrogen, prolactin, human placental lactogen, cortisol, and progesterone) and the increased fuel consumption necessary to provide for the growing mother and fetus. It is estimated to occur in approximately 2.1 percent of pregnant women in the United States, usually developing in the second or third trimester. (See "Gestational diabetes mellitus: Screening, diagnosis, and prevention".)
UNCOMMON IMMUNE-MEDIATED DIABETES — Several uncommon forms of immune-mediated diabetes have been identified.
Stiff-person syndrome — The stiff-person syndrome (formerly called stiff-man syndrome) is an autoimmune disorder of the central nervous system, which is characterized by progressive muscle stiffness, rigidity, and spasm involving the axial muscles, with severe impairment of ambulation. Patients usually have high titers of anti-glutamic acid decarboxylase (GAD) antibodies, and diabetes occurs in approximately one-third of cases. (See "Stiff-person syndrome".)
Anti-insulin receptor antibodies — Anti-insulin receptor antibodies can bind to insulin receptors and either act as an agonist, leading to hypoglycemia, or block the binding of insulin and cause diabetes [81].
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Diabetes mellitus in adults" and "Society guideline links: Diabetes mellitus in children".)
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topics (see "Patient education: Type 1 diabetes (The Basics)" and "Patient education: Type 2 diabetes (The Basics)")
●Beyond the Basics topics (see "Patient education: Type 1 diabetes: Overview (Beyond the Basics)" and "Patient education: Type 2 diabetes: Overview (Beyond the Basics)")
SUMMARY AND RECOMMENDATIONS
●Type 1 diabetes – Type 1 diabetes is characterized by destruction of the pancreatic beta cells, leading to absolute insulin deficiency. This is usually due to autoimmune destruction of the pancreatic beta cells. (See 'Type 1 diabetes' above.)
●Type 2 diabetes – Type 2 diabetes is by far the most common type of diabetes and is characterized by variable degrees of insulin resistance and deficiency. (See 'Type 2 diabetes' above.)
●Difficulty distinguishing between type 1 diabetes and atypical presentation of type 2 diabetes – When the diagnosis of type 1 or type 2 diabetes is uncertain by clinical presentation (ie, thin patient with poor response to initial therapy with sulfonylureas or metformin, personal or family history of autoimmune disease), we measure two or three autoantibodies (glutamic acid decarboxylase 65 [GAD65], insulin, tyrosine phosphatases [insulinoma-associated protein 2 (IA-2) and IA-2 beta], islet cell, or zinc transporter [ZnT8]). If one or more of the antibodies is present (and especially if two or more are positive), the patient should be presumed to have type 1 diabetes and should be treated with insulin replacement therapy, as these patients respond poorly to diet and oral hypoglycemic drug therapy. (See 'Distinguishing type 1 from type 2 diabetes' above.)
●Genetic variants – As the human genome is further explored, it is likely that multiple genetic variants at different loci will be found that confer varying degrees of predisposition to type 1 and type 2 diabetes. Monogenic causes of type 2 diabetes represent only a small fraction of cases and commonly inherited polymorphisms individually contribute only small degrees of risk for, or protection from, diabetes. Most of the genetic risk for type 2 diabetes results from complex polygenic risk factors. (See 'Genetic variants' above and "Pathogenesis of type 2 diabetes mellitus", section on 'Genetic susceptibility'.)
●Monogenic diabetes – Monogenic diabetes, formerly referred to as maturity onset diabetes of the young (MODY), is a clinically heterogeneous disorder characterized by onset of diabetes at a young age (<25 years) with autosomal dominant transmission and lack of autoantibodies. It is classified by the underlying genetic defect (table 2). Many patients with monogenic diabetes are misclassified as having either type 1 or 2 diabetes (table 3). (See 'Monogenic diabetes (formerly called maturity onset diabetes of the young)' above.)
•Diagnosis – The diagnosis of monogenic diabetes (MODY) is made by performing diagnostic genetic testing by direct sequencing of the gene. Laboratories in several countries offer clinical testing, including panels for the most common mutations: hepatocyte nuclear factor-4-alpha (HNF4A), hepatocyte nuclear factor-1-alpha (HNF1A), and glucokinase (GCK). (See 'Diagnosis' above.)
We typically perform genetic testing for monogenic diabetes (MODY) when there is a high index of suspicion (familial diabetes with autosomal dominant pattern of inheritance [≥3 generations], onset <25 years, nonobese, negative islet autoantibodies) (table 3). For family members of mutation carriers, biochemical testing to confirm diabetes should be performed before genetic testing is considered. If the biochemical tests are consistent with a diagnosis of diabetes, genetic testing can be performed to confirm the diagnosis of a MODY mutation. (See 'Indications for genetic testing' above.)
●Diseases of the exocrine pancreas – Diseases that damage the pancreas, or surgical removal of pancreatic tissue, can result in diabetes. There is wide variability in the frequency with which this occurs, primarily determined by the degree of pancreatic insufficiency (table 4). (See 'Diseases of the exocrine pancreas' above.)
●Endocrinopathies – Several endocrinopathies, including Cushing's syndrome, acromegaly, and pheochromocytoma, can lead to abnormalities in glucose regulation. (See 'Endocrinopathies' above.)
●Drug-induced diabetes – A large number of drugs can impair glucose tolerance; they act by decreasing insulin secretion, increasing hepatic glucose production, or causing resistance to the action of insulin (table 5). (See "Pathogenesis of type 2 diabetes mellitus", section on 'Drug-induced hyperglycemia'.)
ACKNOWLEDGMENT — The editorial staff at UpToDate acknowledges David McCulloch, MD, who contributed to an earlier version of this topic review.
