What are the different types of Diabetes Mellitus (DM)?

Types of Diabetes Mellitus

Diabetes mellitus is classified into four main categories: Type 1 diabetes, Type 2 diabetes, Gestational diabetes mellitus (GDM), and Specific types of diabetes due to other causes. 1

Type 1 Diabetes

• Due to autoimmune β-cell destruction, usually leading to absolute insulin deficiency 1
• Typically presents with classic symptoms of polyuria, polydipsia, and weight loss
• Approximately half of children present with diabetic ketoacidosis (DKA) 1
• Features most useful in identifying Type 1 diabetes include:
  • Younger age at diagnosis (<35 years)
  • Lower BMI (<25 kg/m²)
  • Unintentional weight loss
  • Ketoacidosis
  • Glucose >360 mg/dL (20 mmol/L) at presentation 1
• Presence of islet autoantibodies (GAD, IA-2, ZnT8) can help confirm diagnosis 2

Type 2 Diabetes

• Characterized by progressive loss of adequate β-cell insulin secretion frequently on the background of insulin resistance 1
• Often associated with obesity (typically with abdominal distribution) and sedentary lifestyle 1
• Early stage characterized by impaired first-phase insulin secretion causing post-prandial hyperglycemia 1
• Later stages show deteriorating second-phase insulin response and persistent fasting hyperglycemia
• Comprises over 90% of adults with diabetes 1
• May occasionally present with DKA, particularly in ethnic and racial minorities 1

Gestational Diabetes Mellitus (GDM)

• Diabetes diagnosed in the second or third trimester of pregnancy that was not clearly overt diabetes prior to gestation 1
• Affects approximately 8.3% of pregnancies 2
• Approximately 70% of females with GDM will develop diabetes over time 1
• Screening typically occurs at 24-28 weeks of gestation 2

Specific Types of Diabetes Due to Other Causes

• Monogenic diabetes syndromes:
  • Neonatal diabetes
  • Maturity-onset diabetes of the young (MODY) 1
• Diseases of the exocrine pancreas:
  • Cystic fibrosis
  • Pancreatitis
  • Pancreatic cancer 1, 3
• Drug or chemical-induced diabetes:
  • Glucocorticoid use
  • Treatment of HIV/AIDS
  • After organ transplantation 1
  • Thiazides, anti-depressants, beta-blockers 1
• Genetic defects in insulin action 1
• Endocrinopathies 1
• Infections 1
• Haemochromatosis 3

Diagnostic Criteria

• Diabetes may be diagnosed based on:
  • A1C ≥6.5% (48 mmol/mol)
  • Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L)
  • 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during OGTT
  • Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with classic symptoms 2

Important Clinical Considerations

• The traditional paradigm that Type 1 diabetes occurs only in children and Type 2 only in adults is no longer accurate; both diseases occur across all age groups 1
• Classification is not always straightforward at diagnosis, and misdiagnosis is common, occurring in up to 40% of adults with new-onset Type 1 diabetes 2
• When diagnosis is unclear, testing for islet autoantibodies and C-peptide levels can help determine the type of diabetes 2, 4
• C-peptide values <1.5 ng/ml define a patient with absent pancreatic function 4
• Some patients cannot be clearly classified as having Type 1 or Type 2 diabetes at the time of diagnosis 1
• The clinical presentation and disease progression may vary considerably in both types of diabetes 1
• Regular reassessment of diabetes classification is important, especially when treatment response is suboptimal 2

Understanding the correct classification of diabetes is crucial for determining appropriate therapy and improving patient outcomes in terms of morbidity, mortality, and quality of life.