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Diabetes Mellitus - Complete Robbins Review

Robbins, Cotran & Kumar - Pathologic Basis of Disease, Chapter 24, pp. 997-1010

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1. DEFINITION

• Leading cause of end-stage renal disease in the USA
• Leading cause of adult-onset blindness
• Leading cause of nontraumatic lower extremity amputations
• Affects >30 million in the USA (>11% of population); ~422 million worldwide

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2. DIAGNOSTIC CRITERIA (ADA / WHO)

All tests except the random glucose test in a patient with classic symptoms require confirmation on a separate day. When two assays are discordant, the result with the greater degree of abnormality is used.

Pre-Diabetes ("Dysglycemia")

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3. CLASSIFICATION

DIABETES MELLITUS
├── Type 1 (T1D) — ~5-10% of cases
│     └── Autoimmune β-cell destruction → ABSOLUTE insulin deficiency
│
├── Type 2 (T2D) — ~90-95% of cases
│     └── Peripheral insulin resistance + RELATIVE insulin deficiency
│
├── Monogenic Forms
│     ├── β-cell function defects (MODY 1-6, insulin gene mutations)
│     └── Tissue response to insulin defects
│           (Type A insulin resistance, leprechaunism,
│            Rabson-Mendenhall syndrome)
│
└── Secondary / Other
      ├── Gestational diabetes
      ├── Diseases of exocrine pancreas
      │     (pancreatitis, CF, hemochromatosis, neoplasia)
      ├── Drug-induced (glucocorticoids, thiazides, antiretrovirals)
      └── Endocrinopathies
            (Cushing's syndrome, acromegaly, pheochromocytoma)

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4. PATHOGENESIS

4A. Normal Insulin Physiology

Fed state:
  Blood glucose ↑ → β-cells sense glucose
     → Glycolysis → ATP production → K⁺ channel closure
     → Membrane depolarization → Ca²⁺ influx
     → Insulin granule exocytosis

Insulin actions:
  ├── Liver:  ↑ glycogen synthesis, ↓ gluconeogenesis
  ├── Muscle: ↑ GLUT4-mediated glucose uptake, ↑ glycogen
  └── Adipose: ↑ TG synthesis, ↓ lipolysis

Fasting state:
  Glucagon dominates → ↑ hepatic glucose output
  (glycogenolysis + gluconeogenesis)

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4B. Pathogenesis of Type 1 Diabetes

GENETIC SUSCEPTIBILITY
  ├── HLA-DR3, HLA-DR4 (present in ~95% of T1D patients)
  ├── HLA-DQ alleles (non-Asp at position 57 of DQβ → susceptibility)
  ├── Protective: HLA-DQB1*0602 (Asp at position 57)
  └── Non-HLA genes: PTPN22, CTLA4, INS gene region
         ↓
ENVIRONMENTAL TRIGGER
  ├── Viral infections (Coxsackievirus B, CMV, rubella, mumps)
  └── Molecular mimicry: viral antigen cross-reacts with β-cell antigen
         ↓
LOSS OF SELF-TOLERANCE
  └── Autoreactive CD4+ and CD8+ T cells escape thymic deletion
      Or regulatory T cells (Tregs) fail to suppress autoreactivity
         ↓
INSULITIS (lymphocytic infiltration of islets)
  ├── CD8+ cytotoxic T cells (dominant effectors)
  ├── CD4+ Th1 helper cells → activate macrophages
  └── B cells (produce autoantibodies — markers, not pathogenic)
         ↓
β-CELL DESTRUCTION (multiple mechanisms)
  ├── CD8+ CTLs: perforin/granzyme cytotoxicity
  ├── CD4+ Th1 cells: Fas-FasL killing
  ├── Cytokines: IL-1β, TNF-α, IFN-γ → β-cell apoptosis
  └── Autoantibodies (diagnostic markers):
        • Anti-insulin (IAA) — earliest in children
        • Anti-GAD65 (glutamic acid decarboxylase)
        • Anti-islet cell (ICA)
        • Anti-IA-2 (tyrosine phosphatase)
         ↓
PROGRESSIVE β-CELL LOSS
  (>90% must be destroyed before symptoms appear)
         ↓
ABSOLUTE INSULIN DEFICIENCY
  → Classic triad: polydipsia, polyuria, polyphagia
  → DKA (life-threatening)

• Reduced islet size and number
• Insulitis (lymphocytic infiltrate) in early/active disease
• β-cells selectively destroyed; α, δ, PP cells preserved

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4C. Pathogenesis of Type 2 Diabetes

GENETIC PREDISPOSITION (polygenic)
  ├── TCF7L2 (transcription factor - insulin secretion)
  ├── KCNJ11 (K⁺-ATP channel in β-cells)
  ├── PPARG (adipocyte differentiation)
  └── Many others (>100 susceptibility loci identified)

+

ENVIRONMENTAL FACTORS
  └── Obesity (especially visceral), sedentary lifestyle,
      high-caloric diet, aging
         ↓
VISCERAL ADIPOSE TISSUE EXCESS
  ├── ↑ Free Fatty Acids (FFAs) in portal circulation
  ├── ↑ Pro-inflammatory adipokines: TNF-α, IL-6, resistin
  └── ↓ Anti-inflammatory adipokines: adiponectin
         ↓
PERIPHERAL INSULIN RESISTANCE
  ├── Skeletal muscle: ↓ GLUT4 translocation → ↓ glucose uptake
  ├── Liver: failure to suppress gluconeogenesis → ↑ glucose output
  │           MASLD (Metabolic-Associated Steatotic Liver Disease)
  └── Adipose tissue: unrestrained lipolysis → ↑↑ FFAs (lipotoxicity)
         ↓
COMPENSATORY β-CELL HYPERFUNCTION
  └── Hyperinsulinemia to maintain euglycemia
         ↓
β-CELL EXHAUSTION / DYSFUNCTION (mechanisms):
  ├── Lipotoxicity (excess FFAs impair β-cell function)
  ├── Glucotoxicity (chronic hyperglycemia damages β-cells)
  ├── Abnormal incretin effect (↓ GIP, ↓ GLP-1 secretion)
  ├── Amyloid (IAPP/amylin) deposition in islets
  │     → Present in >90% of long-standing T2D islets
  └── Genetic defects in insulin secretion pathway
         ↓
RELATIVE INSULIN DEFICIENCY
         ↓
OVERT T2D → Hyperosmolar Hyperglycemic State (HHS), not DKA

• Amyloid deposition (IAPP) - characteristic finding
• Mild-to-moderate reduction in β-cell mass (~50%)
• No insulitis
• Possible mild fibrosis

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5. TYPE 1 vs TYPE 2 DM - COMPARISON TABLE

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6. MECHANISMS OF HYPERGLYCEMIA-INDUCED DAMAGE

HYPERGLYCEMIA (sustained, chronic)
         │
    ┌────┴────────────────────────────────────────┐
    │                                             │
    ▼                                             ▼
AGE PATHWAY                              POLYOL PATHWAY
(Advanced Glycation                      (Sorbitol accumulation)
 End-products)
                                         Glucose + Aldose reductase
Glucose + protein amino                  → Sorbitol (cannot exit cells)
groups → Schiff bases                    → Osmotic damage (lens, nerves)
→ Amadori products                       ↓ NADPH → ↓ Glutathione
→ Irreversible AGEs                      → Oxidative stress
                                         Contributes to: neuropathy,
├── Cross-link collagen                  cataracts
│   → BM thickening
├── Bind RAGE receptors               ──────────────────────────────
│   → Cytokines, ox. stress              PKC PATHWAY
│   → Inflammation                    (Protein Kinase C activation)
└── Trap LDL in vessel wall
    → Atherosclerosis                 Excess glucose → ↑ DAG
                                      → PKC activation
                                      → ↑ VEGF (neovascularization)
                                      → ↑ TGF-β (fibrosis)
                                      → ↑ Endothelin (vasoconstriction)
                                      → ↓ NO (endothelial dysfunction)

    HEXOSAMINE PATHWAY
    Glucose → UDP-GlcNAc
    → O-GlcNAc protein modification
    → ↑ TGF-β, ↑ PAI-1
    → Fibrosis + thrombosis

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7. MICROVASCULAR vs MACROVASCULAR COMPLICATIONS

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7A. MICROVASCULAR COMPLICATIONS

i. Diabetic Nephropathy

PROGRESSION OF DIABETIC NEPHROPATHY
  
Early:
  Glomerular hyperfiltration (↑ GFR)
  Glomerular hypertrophy
       ↓
  Microalbuminuria (30-300 mg/day) ← EARLIEST CLINICAL SIGN
       ↓
  Macroalbuminuria (>300 mg/day) / Overt proteinuria
       ↓
  Nephrotic syndrome (heavy proteinuria)
       ↓
  Progressive GFR decline
       ↓
  END-STAGE RENAL DISEASE (ESRD)

MORPHOLOGICAL CHANGES:
  Glomerular:
  ├── Diffuse glomerulosclerosis (MOST COMMON)
  │     - Mesangial matrix expansion
  │     - GBM thickening
  └── Nodular glomerulosclerosis (KIMMELSTIEL-WILSON LESION)
        - PATHOGNOMONIC of diabetic nephropathy
        - PAS-positive ovoid/spherical nodules in mesangium
        - "Capsular drop" and "fibrin cap" lesions
  
  Vascular:
  └── Hyaline arteriolosclerosis
        - Afferent AND efferent arterioles affected
        - Efferent arteriolar hyalinosis = VIRTUALLY PATHOGNOMONIC of DM
          (hypertension causes only afferent hyalinosis)
  
  Tubular:
  └── Armanni-Ebstein lesion
        - Glycogen accumulation in tubular cells
        - Seen in poorly controlled T1D

ii. Diabetic Retinopathy

DIABETIC RETINOPATHY PROGRESSION

NON-PROLIFERATIVE (Background) Retinopathy:
  ├── Microaneurysms ← EARLIEST lesion (focal capillary bulges)
  ├── Dot-blot hemorrhages (intraretinal)
  ├── Hard exudates (lipid/protein leakage from leaky vessels)
  ├── Soft exudates / cotton-wool spots (retinal nerve fiber infarcts)
  ├── Venous beading and dilation
  └── IRMA (intraretinal microvascular abnormalities)
         ↓ (without treatment / with poor control)
PROLIFERATIVE Retinopathy:
  ├── Neovascularization (VEGF-driven new vessel formation)
  │     - On retinal surface, optic disc, iris (rubeosis)
  ├── Vitreous hemorrhage (from fragile new vessels)
  ├── Fibrous traction bands → Tractional retinal detachment
  └── BLINDNESS
  
+ Diabetic Macular Edema (can occur at any stage → central vision loss)

iii. Diabetic Neuropathy

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7B. MACROVASCULAR COMPLICATIONS

ACCELERATED ATHEROSCLEROSIS IN DM

Mechanisms:
  ├── AGE-modified LDL trapped in arterial intima
  ├── Endothelial dysfunction (↓ NO, ↑ oxidative stress)
  ├── Platelet hyperaggregability
  ├── Dyslipidemia (↑ TG, ↓ HDL, ↑ small dense LDL)
  ├── Insulin resistance → ↑ PAI-1 → pro-thrombotic state
  └── Hypertension (co-existent, accelerates damage)
         │
    ┌────┴──────────────────────────────┐
    ▼                                   ▼                    ▼
CORONARY ARTERY              CEREBROVASCULAR          PERIPHERAL ARTERY
DISEASE                      DISEASE                  DISEASE (PAD)
  │                            │                          │
Leading cause of death       2-4x ↑ stroke risk        Claudication,
in T2D                       Lacunar infarcts           rest pain,
Silent MIs common            TIA                        gangrene,
(autonomic neuropathy)       Vascular dementia          amputation

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8. COMPLETE COMPLICATIONS OVERVIEW

CHRONIC COMPLICATIONS OF DIABETES
│
├── MACROVASCULAR (accelerated atherosclerosis in large vessels)
│   ├── Ischemic Heart Disease → MI ← LEADING CAUSE OF DEATH in DM
│   ├── Cerebrovascular Disease → Stroke, TIA, vascular dementia
│   └── Peripheral Arterial Disease → Claudication → Gangrene
│
├── MICROVASCULAR (BM thickening, pericyte loss in capillaries)
│   ├── RETINOPATHY → BLINDNESS (leading cause in adults)
│   │   ├── Non-proliferative: microaneurysms, hemorrhages, exudates
│   │   └── Proliferative: neovascularization, vitreous bleed, detachment
│   │
│   ├── NEPHROPATHY → ESRD (leading cause of dialysis in USA)
│   │   ├── Kimmelstiel-Wilson nodules (pathognomonic)
│   │   ├── Diffuse glomerulosclerosis
│   │   └── Efferent arteriolar hyalinosis
│   │
│   └── NEUROPATHY → Ulcers, Autonomic dysfunction
│       ├── Distal symmetric polyneuropathy (most common)
│       ├── Autonomic neuropathy (gastroparesis, ED, OH)
│       └── Mononeuropathy (CN III palsy)
│
├── DIABETIC FOOT
│   └── Neuropathy + PAD + Infection → Ulceration → Amputation
│       (leading cause of nontraumatic lower limb amputation)
│
├── INCREASED INFECTION SUSCEPTIBILITY
│   ├── Impaired neutrophil function, chemotaxis, phagocytosis
│   ├── Mucocutaneous candidiasis (oral, vaginal)
│   ├── TB, recurrent UTI, pyelonephritis
│   ├── Malignant (necrotizing) otitis externa (Pseudomonas aeruginosa)
│   └── Rhinocerebral mucormycosis (especially during DKA)
│
└── ACUTE METABOLIC COMPLICATIONS
    ├── DKA (predominantly T1D)
    ├── Hyperosmolar Hyperglycemic State (T2D elderly)
    └── Hypoglycemia (insulin/sulfonylurea treatment)

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9. ACUTE METABOLIC COMPLICATIONS - COMPARISON

DKA Pathophysiology

ABSOLUTE INSULIN DEFICIENCY
  (trigger: infection, omission of insulin, new-onset T1D)
           │
    ┌──────┴──────────────────────────────────────┐
    ▼                                             ▼
↑ Gluconeogenesis &                        ↑ Hormone-sensitive lipase
  Glycogenolysis                             (adipose tissue lipolysis)
    └── HYPERGLYCEMIA                            │
          └── Osmotic diuresis               ↑ Free Fatty Acids
                └── Polyuria,                  (enter hepatic mitochondria)
                    dehydration                    │
                    electrolyte loss           β-oxidation → Acetyl-CoA
                                                   │
                                         KETONE BODY SYNTHESIS
                                         (acetoacetate, β-hydroxybutyrate)
                                                   │
                                         Ketonemia + Ketonuria
                                         → METABOLIC ACIDOSIS
                                         → Kussmaul breathing (deep, labored)
                                         → Fruity odor (acetone)
                                         → Nausea/vomiting, abdominal pain
                                         → Depressed consciousness → COMA

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10. ISLET MORPHOLOGY SUMMARY

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IMAGES

T2D: Obesity-Driven Pathophysiology to Vascular Complications

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T1D: Autoimmune β-cell Destruction

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Diabetic Retinopathy - Non-Proliferative Fundus

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KEY ROBBINS MEMORY POINTS

1. T1D = HLA-DR3/DR4 + autoimmune insulitis + absolute insulin deficiency + DKA
2. T2D = obesity + insulin resistance + islet amyloid (IAPP) + relative insulin deficiency + HHS
3. Kimmelstiel-Wilson nodules = pathognomonic of diabetic nephropathy
4. Efferent arteriolar hyalinosis = virtually pathognomonic of DM (distinguishes from hypertension which affects only afferent)
5. Microaneurysms = earliest lesion in diabetic retinopathy
6. VEGF drives neovascularization in proliferative retinopathy
7. AGEs + PKC activation + polyol pathway + hexosamine pathway = four key mechanisms of microvascular damage
8. Amyloid (IAPP) in >90% of long-standing T2D islets
9. >90% β-cell destruction required before T1D becomes symptomatic
10. Efferent arteriolar hyalinosis = DM | Afferent only = hypertension