Diabetes Mellitus - A Comprehensive Overview

Definition

Classification (ADA)

Note: The older terms "insulin-dependent" (IDDM) and "non-insulin-dependent" (NIDDM) were abolished by the 1997 ADA reclassification. - Tietz Textbook of Laboratory Medicine, 7th Ed.

Epidemiology

• T2DM accounts for 90-95% of all diabetes cases; T1DM and other causes make up the remaining 5-10%. - Washington Manual of Medical Therapeutics
• T1DM accounts for approximately 5-10% of cases; peak incidence is in childhood and adolescence, with ~75% diagnosed before age 18. - Tietz Textbook of Laboratory Medicine, 7th Ed.
• T2DM is increasing in prevalence worldwide, largely driven by rising obesity rates. It is now also appearing in children and adolescents.
• T2DM is accompanied by hypertension in ~75% and hyperlipidemia in >50% of adult patients and is considered a cardiac risk equivalent. - Washington Manual of Medical Therapeutics

Pathophysiology

Type 1 DM

• A chronic autoimmune disease in which cellular-mediated destruction of pancreatic beta-cells leads to absolute insulin deficiency.
• Markers include: islet cell autoantibodies, anti-insulin antibodies, anti-GAD65 antibodies, anti-tyrosine phosphatase (IA-2) antibodies.
• Occurs in genetically susceptible individuals with an environmental trigger (e.g., enteroviruses); >60 genetic loci identified.
• Three stages:
  • Stage 1: Normoglycemic, multiple islet autoantibodies present
  • Stage 2: Dysglycemia (IFG/IGT) with autoantibodies
  • Stage 3: Overt hyperglycemia with clinical symptoms

Type 2 DM

• Core defects: insulin resistance (especially in muscle, liver, and adipose tissue) + progressive beta-cell failure.
• Obesity is the major modifiable risk factor; weight loss alone can improve hyperglycemia.
• Patients are not prone to ketosis and are not initially dependent on insulin to prevent ketonuria.

Diagnostic Criteria (ADA)

• A random plasma glucose ≥200 mg/dL with classic symptoms (polyuria, polydipsia, unexplained weight loss) also confirms diabetes. - Henry's Clinical Diagnosis and Management by Laboratory Methods
• Progression from prediabetes to T2DM occurs at 2-22% per year (average ~12%). - Washington Manual of Medical Therapeutics

Clinical Features

Complications

Microvascular

• Diabetic retinopathy - leading cause of blindness in working-age adults
• Diabetic nephropathy - leading cause of end-stage renal disease
• Diabetic neuropathy - peripheral, autonomic

Macrovascular

• Accelerated atherosclerosis: coronary artery disease, stroke, peripheral arterial disease

Management

Monitoring

• HbA1c reflects average blood glucose over the preceding 2-3 months. Target is generally <7% (individualized). Measure every 3 months if not at goal, twice yearly when well-controlled.
• SMBG: Recommended for all insulin-using patients; CGM reduces HbA1c and hypoglycemia in adults on intensive insulin therapy.
• Urine/blood ketones: T1DM patients should test during febrile illness, sustained BG >300 mg/dL, or signs of DKA.

Non-pharmacologic Therapy

• Lifestyle modification: First-line for T2DM and prediabetes
• Caloric restriction: 1000-1800 kcal/day (individualized); goal weight loss ≥5-7%
• Aerobic exercise: ≥150 min/week of moderate-to-vigorous activity + resistance training 2-3x/week
• Dietary patterns: Mediterranean, DASH, plant-based, low-carbohydrate diets are all acceptable.

Pharmacologic Therapy

• Insulin is mandatory (multiple daily injections or insulin pump).

• Bariatric surgery is an option for patients with severe obesity and poorly controlled T2DM. - Guyton and Hall Textbook of Medical Physiology

Acute Complications

• Diabetic Ketoacidosis (DKA): Glucose >250 mg/dL + arterial pH <7.30 + bicarbonate <15 mEq/L + ketonemia/ketonuria (mainly T1DM)
• Hyperosmolar Hyperglycemic State (HHS): Glucose ≥600 mg/dL + serum osmolality >320 mOsm/kg + impaired consciousness (mainly T2DM)
• Hypoglycemia: Requires hospitalization if sulfonylurea-induced, deliberate overdose, or results in coma/seizure.

Inpatient Glycemic Targets

• Critical care: 140-180 mg/dL (7.8-10.0 mmol/L)
• Non-critical care: <140 mg/dL (7.8 mmol/L)
• Washington Manual of Medical Therapeutics

Gestational Diabetes (GDM)

• Diagnosed at 24-28 weeks of gestation in women without prior overt diabetes.
• One-step IADPSG: 75g OGTT; diagnose if fasting ≥92, 1h ≥180, or 2h ≥153 mg/dL.
• Two-step (US): 50g screen at 1h, then 100g OGTT if positive.
• ~50% of women with GDM develop T2DM within 5-10 years.
• All women with GDM should undergo postpartum glucose testing at 4-12 weeks and every 1-3 years thereafter. - Washington Manual of Medical Therapeutics

Recent Evidence (PubMed, 2024-2026)

• GLP-1 receptor agonists: A 2025 systematic review (PMID: 40499738) confirms significant GI adverse events (nausea, vomiting, diarrhea) that must be weighed against cardiovascular benefits.
• Digital self-management: A 2024 meta-analysis (PMID: 39037772) supports digital interventions for improving T2DM self-management outcomes.
• Triglyceride-glucose index: A 2024 meta-analysis (PMID: 39180024) validates the TyG index as a useful surrogate marker for insulin resistance and cardiovascular risk in DM.