---

Diabetes Mellitus - Complete Lecture Notes

PA 31.5: Classification, Epidemiology, Etiology, Pathogenesis, Pathology, Clinical/Laboratory Features, Complications, and Progression

---

1. DEFINITION

Harrison's Principles of Internal Medicine 22E, p. 3292

---

2. CLASSIFICATION

• Latent autoimmune diabetes in adults (LADA): A slow-onset autoimmune form misclassified as T2DM in adults; positive for GAD65 antibodies.
• Maturity-onset diabetes of the young (MODY): Monogenic, autosomal dominant, presents in youth; does NOT require insulin.
• Ketosis-prone T2DM: Found in some African-origin populations; episodic DKA but with preserved beta-cell function between episodes.

Textbook of Family Medicine 9e, p. 964

---

3. EPIDEMIOLOGY

• Global burden: The WHO estimates over 347 million people worldwide have diabetes; 90% involve T2DM.
• Projected to rise to 439 million (7.7% of global population) by 2030.
• T1DM accounts for ~5-10% of all DM cases; peak onset is childhood/adolescence but can occur at any age.
• T2DM prevalence is rising due to the global increase in obesity, physical inactivity, and aging populations.
• GDM affects ~7% of all pregnancies; women with GDM have a 35-60% risk of developing T2DM within 10 years.
• Diabetes is a leading cause of blindness, renal failure, lower-limb amputation, and cardiovascular mortality worldwide.
• Sex differences: Women with DM have a sixfold greater risk of dying from cardiovascular disease (CVD) compared to women without DM. Premenopausal women with DM lose their natural cardioprotective advantage.

Textbook of Family Medicine 9e, p. 964; Harrison's 22E, p. 3292

---

4. ETIOLOGY

Type 1 DM

• Autoimmune destruction of pancreatic beta-cells, mediated by CD8+ and CD4+ T lymphocytes and autoantibodies:
  • Islet cell antibodies (ICA)
  • Anti-glutamic acid decarboxylase (GAD65) antibodies
  • Insulin autoantibodies (IAA)
  • Anti-IA-2 (tyrosine phosphatase) antibodies
• Genetic susceptibility: Strongly associated with HLA-DR3 and HLA-DR4 alleles (HLA region accounts for ~40% of genetic risk). Non-HLA genes (INS, CTLA4, PTPN22) also contribute.
• Environmental triggers: Viral infections (enterovirus, Coxsackie B4), dietary factors (bovine milk protein exposure in early life), gut microbiome alterations.
• The process is gradual - most beta-cells are lost before clinical symptoms appear (~80-90% destruction required).

Type 2 DM

• Polygenic disorder with strong environmental interaction.
• Two core defects:
  1. Peripheral insulin resistance (skeletal muscle, liver, adipose tissue) - the first detectable abnormality
  2. Progressive beta-cell failure - compensatory hyperinsulinemia eventually gives way to insufficient insulin secretion
• Genetic factors: >400 genetic variants identified; major risk genes include TCF7L2, KCNJ11, PPARG, SLC30A8.
• Environmental activators (from Table 34-4 of Family Medicine textbook):
  • Advanced age (DNA methylation changes, chronic inflammation)
  • Obesity - especially visceral/central adiposity
  • Physical inactivity
  • Obstructive sleep apnea and sleep disorders (circadian misalignment affects glucose-insulin metabolism)
  • Mental illness (depression, schizophrenia) - linked to inflammation, inactivity, and dopaminergic pathways
  • Second-hand smoke exposure (induces adipocyte hypertrophy, insulin resistance, chronic pancreatic inflammation)
  • History of physical/sexual abuse (Nurses' Health Study II - moderate physical abuse: 26% higher diabetes risk)
  • Late chronotype (skipping breakfast, high caloric intake at dinner)

Textbook of Family Medicine 9e, p. 968

---

5. PATHOGENESIS

Normal Glucose Homeostasis

• Insulin: lowers glucose by stimulating uptake in skeletal muscle/fat, suppressing hepatic glucose output, limiting lipolysis
• Glucagon: raises glucose via hepatic glycogenolysis and gluconeogenesis
• Counterregulatory hormones (catecholamines, cortisol, growth hormone): activated when glucose drops >20 mg/dL
• The body stores ~450 g glucose; brain requires 125 g/day; liver contributes 50% via glycogenolysis and 30% via gluconeogenesis

Pathogenesis of Type 1 DM

Genetic predisposition (HLA + non-HLA genes)
        ↓
Environmental trigger (virus, dietary antigen)
        ↓
Immune activation → T-cell-mediated insulitis (CD8+ cytotoxic T cells destroy beta cells)
        ↓
Progressive beta-cell loss (asymptomatic for years)
        ↓
~80-90% beta-cell destruction → clinical onset (hyperglycemia, polyuria/polydipsia)
        ↓
Absolute insulin deficiency → uninhibited lipolysis → FFA → ketogenesis → DKA risk

• Insulin deficiency leads to unrestrained hepatic gluconeogenesis and glycogenolysis
• Loss of insulin causes lipolysis → free fatty acids → hepatic ketone body production (acetoacetate, beta-hydroxybutyrate, acetone)
• This is the metabolic basis of diabetic ketoacidosis (DKA)

Pathogenesis of Type 2 DM

Genetic susceptibility + Obesity/Lifestyle factors
        ↓
Peripheral insulin resistance (skeletal muscle, liver, adipose)
        ↓
Compensatory hyperinsulinemia (beta-cells work harder)
        ↓
Glucotoxicity + Lipotoxicity → progressive beta-cell exhaustion and apoptosis
        ↓
Relative then absolute insulin insufficiency → frank hyperglycemia
        ↓
"Glucose toxicity" cycle: hyperglycemia further impairs insulin secretion

• Increased circulating free fatty acids (from adipose tissue) impair insulin signaling via protein kinase C and IRS-1 phosphorylation
• Adipokine imbalance: decreased adiponectin, increased TNF-alpha, IL-6, resistin
• Ectopic fat deposition in liver (hepatic steatosis) and muscle

• GLP-1 (glucagon-like peptide-1) is normally released by intestinal L-cells in response to meals and potentiates insulin secretion
• In T2DM, both the secretion of GLP-1 and the beta-cell response to GLP-1 are impaired ("incretin defect")

• GLUT4 (insulin-sensitive glucose transporter in muscle and fat) is impaired at rest in T2DM but is enhanced by muscle contractions during exercise - this is the basis for exercise as therapy

Textbook of Family Medicine 9e, pp. 968-970

---

6. PATHOLOGY

Pancreatic Histopathology

• Insulitis: In T1DM, islets show dense lymphocytic infiltration predominantly by CD8+ T cells; seen early in the disease course; in established T1DM the islets are atrophic and devoid of beta-cells
• IAPP (amylin) deposition: In T2DM, amyloid derived from IAPP co-secreted with insulin accumulates in islets; this contributes to progressive beta-cell dysfunction and apoptosis

Vascular Pathology (basis of chronic complications)

1. Microvascular: Thickening of capillary basement membranes - the hallmark of diabetic microangiopathy. Seen in retina, kidney glomeruli, and peripheral nerves.
2. Macrovascular: Accelerated atherosclerosis due to:
   • Advanced glycation end-products (AGEs)
   • Oxidative stress
   • Endothelial dysfunction
   • Dyslipidemia (high TG, low HDL)
   • Hypertension
3. Diabetic nephropathy histology:
   • Diffuse glomerulosclerosis (most common) - diffuse increase in mesangial matrix
   • Nodular glomerulosclerosis - Kimmelstiel-Wilson nodules (PAS-positive nodular deposits in the mesangium) - pathognomonic
   • Arteriolar hyalinosis (afferent > efferent)
   • Tubular atrophy, interstitial fibrosis in advanced disease

---

7. CLINICAL FEATURES

Symptoms of Acute Hyperglycemia (the "classic triad")

• Polyuria - osmotic diuresis from glucosuria (when glucose exceeds renal threshold ~180 mg/dL)
• Polydipsia - compensatory response to osmotic diuresis and dehydration
• Polyphagia - cellular "starvation" despite hyperglycemia (in T1DM especially)
• Weight loss - urinary caloric loss + muscle protein catabolism (T1DM > T2DM)
• Fatigue and weakness
• Blurred vision - from osmotic changes in lens water content (reversible with glycemic control)
• Frequent infections - vaginitis, fungal skin infections, recurrent UTIs, boils, impaired wound healing

Type 1 vs Type 2 Clinical Presentation

---

8. LABORATORY FEATURES AND DIAGNOSIS

ADA Diagnostic Criteria (any one criterion is sufficient)

Pre-Diabetes Thresholds

Key Laboratory Tests in Management

---

9. COMPLICATIONS

Acute Complications

A. Diabetic Ketoacidosis (DKA) - primarily T1DM

• Precipitants: Infection (most common), insulin omission, new-onset T1DM, surgery/trauma, MI
• Pathophysiology: Absolute insulin deficiency → unrestrained lipolysis → FFA → hepatic ketogenesis (acetoacetate, beta-hydroxybutyrate) → anion gap metabolic acidosis
• Clinical triad: Hyperglycemia + ketonemia + metabolic acidosis
• Symptoms: Nausea, vomiting, abdominal pain, Kussmaul respiration (deep rapid breathing), fruity breath (acetone), altered consciousness
• Lab: Glucose >250 mg/dL, pH <7.3, bicarbonate <18 mEq/L, elevated ketones, anion gap >12
• Treatment: IV fluids, IV insulin infusion, potassium replacement (critical - hypokalemia kills)

B. Hyperosmolar Hyperglycemic State (HHS) - primarily T2DM (elderly)

• Glucose often >600 mg/dL, severe dehydration, no/minimal ketosis (enough insulin to prevent ketogenesis)
• Hyperosmolality → obtundation, seizures
• High mortality (~15%)

C. Hypoglycemia

• Most common acute complication, especially with insulin or sulfonylurea therapy
• Glucose <70 mg/dL (<54 mg/dL for Level 2 hypoglycemia)
• Symptoms: tremor, sweating, palpitations, confusion, seizures, coma
• In T1DM with "hypoglycemia unawareness," counterregulatory response is blunted - silent and dangerous

---

Chronic Complications

Microvascular Complications (due to hyperglycemia-driven basement membrane thickening)

• Leading cause of end-stage renal disease (ESRD) in developed countries
• Stages: Hyperfiltration (increased GFR) → microalbuminuria → macroalbuminuria → declining GFR → ESRD
• Key histologic lesion: Kimmelstiel-Wilson nodular glomerulosclerosis
• Screening: Annual UACR and eGFR
• Treatment: ACE inhibitors/ARBs (reduce proteinuria and slow progression); SGLT-2 inhibitors (renoprotective)

• Most common cause of new-onset blindness in working-age adults
• Stages:
  • Non-proliferative (NPDR): Microaneurysms, hemorrhages, hard exudates, cotton-wool spots, venous beading
  • Proliferative (PDR): Neovascularization (new fragile vessels that bleed) → vitreous hemorrhage, traction retinal detachment → blindness
  • Diabetic macular edema (DME): Can occur at any stage; causes central vision loss
• Screening: Annual dilated fundus exam
• Treatment: Laser photocoagulation (PDR), anti-VEGF injections (DME)

• Most common complication overall
• Types:
  • Distal symmetric polyneuropathy (DSP): "Glove-and-stocking" distribution; burning, tingling, numbness; loss of vibration/position sense; impaired ankle reflexes
  • Autonomic neuropathy: Gastroparesis (delayed gastric emptying → nausea, postprandial hypoglycemia, erratic glucose control), orthostatic hypotension, cardiac denervation (resting tachycardia, silent MI), erectile dysfunction, neurogenic bladder, sudomotor dysfunction (anhidrosis)
  • Focal/multifocal neuropathies: Cranial nerve palsies (CN III most common - pupil-sparing), diabetic amyotrophy (lumbosacral plexopathy)

Macrovascular Complications (accelerated atherosclerosis)

• Leading cause of death in diabetic patients
• T2DM patients have 2-4x increased risk of coronary artery disease (CAD), MI, stroke
• Women with DM lose their natural sex-related cardioprotection; have 6x greater CVD death risk
• Hypoglycemia in patients with ASCVD triggers proinflammatory, procoagulant, and proatherothrombotic responses (increases platelet aggregation, PAI-1, intravascular neutrophil activation)
• GLP-1 receptor agonists and SGLT-2 inhibitors have demonstrated direct cardiovascular benefit beyond glucose lowering

• PAD + neuropathy + infection = the triad driving lower limb amputations
• Diabetic foot: Charcot neuroarthropathy, non-healing ulcers, osteomyelitis

• Ischemic stroke risk 2-4x higher in DM
• Lacunar infarcts more common due to small vessel disease

Other Complications

• Infections: Impaired neutrophil function, poor microvascular perfusion; susceptibility to TB, fungal infections (mucormycosis in DKA), emphysematous pyelonephritis
• Cataracts: Osmotic lens changes; occur earlier and more frequently
• Glaucoma: Increased risk
• Non-alcoholic fatty liver disease (NAFLD/MASH): Closely linked to T2DM and insulin resistance

---

10. PROGRESSION

Natural History of Type 1 DM

Pre-diabetes stage (months to years):
  ↓ Silent autoimmune destruction
  ↓ Positive autoantibodies (GAD, ICA, IAA)
  ↓ Normal glucose → impaired first-phase insulin secretion
  ↓
Clinical onset (~80-90% beta-cell loss):
  ↓ Polyuria, polydipsia, weight loss, DKA
  ↓
"Honeymoon period" (weeks to months):
  ↓ Partial beta-cell recovery; reduced insulin requirements
  ↓
Established T1DM:
  ↓ Complete insulin dependence
  ↓ Lifelong risk of hypoglycemia and DKA
  ↓
Chronic complications (typically after 10+ years of hyperglycemia):
  Retinopathy → Nephropathy → Neuropathy → CVD

Natural History of Type 2 DM

Insulin resistance (years before diagnosis):
  ↓ Compensatory hyperinsulinemia - euglycemia maintained
  ↓ Beta-cell "overstress" begins
  ↓
Pre-diabetes (IFG and/or IGT):
  ↓ 5-10% per year progress to frank T2DM
  ↓ High risk of CVD even at this stage
  ↓
Early T2DM:
  ↓ Post-prandial hyperglycemia (first-phase insulin secretion lost)
  ↓ Metformin + lifestyle usually sufficient
  ↓
Progressive T2DM:
  ↓ Fasting hyperglycemia (basal insulin secretion impaired)
  ↓ Multiple oral agents required
  ↓ Incretin therapy (GLP-1 RAs) / SGLT-2 inhibitors
  ↓
Advanced T2DM:
  ↓ Near-complete beta-cell failure → insulin therapy required
  ↓ Chronic complications established
  ↓
End-stage complications:
  ESRD (dialysis/transplant), blindness, amputations, ASCVD events

• Microvascular complications by 37%
• MI risk by 14%
• Stroke risk by 12%

---

Summary Table: Key Distinguishing Features

---

• Harrison's Principles of Internal Medicine, 22nd Edition (2025) - Chapters 415-417, pp. 3227-3292
• Textbook of Family Medicine, 9th Edition - Chapter 34, pp. 964-974
• Recent reviews: Aamodt & Powers, Diabetes Obes Metab (2025, PMID 40734585) on T1DM pathophysiology and classification

---

What Was Added

PART 1 - PATHOLOGY

• Pancreatic histopathology for both T1DM and T2DM side-by-side - insulitis, IAPP amyloid, beta-cell mass changes
• 4 molecular mechanisms of vascular damage: AGEs, polyol pathway (aldose reductase), PKC activation, hexosamine pathway - these are the "why" behind every complication
• Diabetic nephropathy in depth: GBM changes, Kimmelstiel-Wilson nodules (PAS+, pathognomonic), diffuse vs nodular glomerulosclerosis, immunofluorescence findings, the SGLT/RAAS glomerular hemodynamic model, and the full natural history timeline
• Diabetic retinopathy staging: the full 5-stage classification table (mild NPDR → PDR) with the "4-2-1 rule" for severe NPDR
• Diabetic neuropathy histology: segmental demyelination, axonal loss, vasa nervorum microangiopathy

PART 2 - MEDICINE

• ADA diagnostic criteria table with pre-diabetes thresholds
• Full clinical evaluation (history + physical exam findings including acanthosis nigricans, necrobiosis lipoidica)
• Monitoring schedule with targets (HbA1c, UACR, eGFR, lipids, foot exam)
• DKA management - the 4 pillars, resolution criteria, why K⁺ must be replicated before insulin
• HHS vs DKA comparison
• Hypoglycemia classification and the cardiovascular danger of hypoglycemia
• Chronic complication management: nephropathy (ACEi/ARB, SGLT-2i, finerenone), retinopathy (PRP laser vs anti-VEGF), painful neuropathy drugs, autonomic neuropathy by system, CV benefit drugs, Charcot foot

PART 3 - PHARMACOLOGY

• Every drug class: mechanism, HbA1c reduction, weight effect, hypoglycemia risk, adverse effects, contraindications
• Insulin preparation table with onset/peak/duration for each type and clinical rationale
• Euglycemic DKA from SGLT-2 inhibitors explained (easily missed!)
• Landmark trials referenced (EMPA-REG, CREDENCE, LEADER, UKPDS, DCCT)
• ADA 2024 stepped management algorithm with priority-driver decision tree