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PA31.{5,7} | Diabetes Mellitus & Pancreatic Cancer — SDL Guide (Part 3)

Chronic Complications — Pathological Basis of Microangiopathy

The chronic complications of diabetes arise from the damaging effects of prolonged hyperglycaemia on blood vessels and nerves. Microangiopathy — disease of small vessels (capillaries and arterioles) — is the unifying pathological mechanism for nephropathy, retinopathy, and neuropathy.

Biochemical mechanisms of hyperglycaemia-induced vascular damage (the four classical pathways):

1. Non-enzymatic glycation (NEG): Glucose reacts with proteins (collagen, fibronectin, IgG) → advanced glycation end-products (AGEs) → crosslinking of structural proteins → thickened, rigid basement membranes; also activate monocytes/macrophages to release TNF-α and IL-1

1. Polyol pathway (aldose reductase): Excess intracellular glucose → reduced by aldose reductase to sorbitol (accumulates in cells — osmotic damage) → then oxidised to fructose; NADPH is consumed → reduced glutathione → increased oxidative stress; particularly damaging in lens (cataracts), Schwann cells (neuropathy), pericytes of retinal vessels

1. Protein kinase C (PKC) activation: Excess diacylglycerol (DAG) from hyperglycaemia → PKC-β activation → increased VEGF (retinal neovascularisation), TGF-β (basement membrane thickening), and ET-1 (vasoconstriction)

1. Hexosamine pathway: Excess fructose-6-phosphate diverted → O-GlcNAc modification of proteins → altered gene expression → PAI-1↑, TGF-α↑

Morphological hallmark — basement membrane thickening:
- All small vessels in diabetes show diffuse thickening of the capillary basement membrane, detectable ultrastructurally
- Paradoxically, despite structural thickening, the membrane is more permeable than normal (to albumin and large molecules)
- The mechanism is AGE-mediated crosslinking + type IV collagen overproduction by endothelial cells

TEM Comparison: Normal vs. Diabetic Capillary Basement Membrane Thickening

Diabetic Nephropathy — Kimmelstiel-Wilson Lesion

Diabetic nephropathy (diabetic glomerulosclerosis) is the leading cause of end-stage renal disease (ESRD) in India and globally. It develops in approximately 30–40% of T1DM and 25% of T2DM patients.

Natural history:
1. Glomerular hyperfiltration (GFR elevated) — early; due to haemodynamic changes; no symptoms
2. Microalbuminuria (30–300 mg/day, detected by urinary ACR) — first clinical sign; marks glomerular injury
3. Macroalbuminuria / overt proteinuria (>300 mg/day) → nephrotic range proteinuria → GFR begins to fall
4. Declining GFR → chronic kidney disease → ESRD over 10–20 years from onset

Gross morphology: Kidneys initially enlarged (glomerular hypertrophy), later shrunken and scarred in ESRD

Microscopic morphology — the diagnostic hallmarks:

• Diffuse glomerulosclerosis — increase in mesangial matrix and basement membrane thickening throughout all glomeruli; most common change; not specific to DM

• Nodular glomerulosclerosis (Kimmelstiel-Wilson lesion) — the pathognomonic lesion:
• Kimmelstiel-Wilson (KW) nodules: rounded, laminated, PAS-positive mesangial deposits at the periphery of the glomerulus
• Composed of acellular matrix material (glycoprotein)
• Located at the periphery of lobules, not centrally (unlike amyloid)
• Capillary loops are displaced peripherally around the nodule
• Often seen alongside microaneurysms of the glomerular capillary loops

• Afferent and efferent arteriolar hyalinosis — thickening of arteriolar walls with PAS-positive hyaline deposits; efferent arteriolar hyalinosis is virtually pathognomonic of DM (other causes of hyalinosis affect only afferent)

• Capsular drop and fibrin cap — exudative lesions: accumulation of plasma proteins between parietal epithelium and Bowman's capsule (capsular drop) or filling a capillary loop (fibrin cap)

• Tubular changes: Glycogen vacuoles in proximal tubular epithelium (Armanni-Ebstein lesion) in severe uncontrolled DM

Kimmelstiel-Wilson Nodular Glomerulosclerosis — PAS vs H&E Histology

Diabetic Glomerulosclerosis: Diffuse vs Nodular (KW) Patterns

Diabetic Retinopathy, Neuropathy, and Macrovascular Disease

Diabetic retinopathy — the leading cause of blindness in working-age adults in India:

Non-proliferative (background) retinopathy:
- Retinal capillary basement membrane thickening → pericyte loss → capillary microaneurysms (first visible lesion on fundoscopy)
- Dot and blot haemorrhages (dot = microaneurysm rupture, blot = intraretinal haemorrhage)
- Hard exudates — lipid deposits from leaking vessels (cream-white deposits with sharp borders)
- Cotton-wool spots — microinfarcts of the nerve fibre layer (soft exudates, fluffy borders)
- Macular oedema — increased vascular permeability → fluid accumulation in the macula → visual impairment

Proliferative retinopathy:
- Retinal ischaemia → VEGF release → neovascularisation (new vessels grow on retinal surface / optic disc — fragile, bleed easily)
- Vitreous haemorrhage → sudden blindness
- Tractional retinal detachment → permanent visual loss

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Diabetic neuropathy — the most common complication (~50% of patients at 25 years of DM):
- Mechanism: polyol pathway (sorbitol in Schwann cells) + microangiopathy of vasa nervorum (endoneurial capillaries) → segmental demyelination + axonal loss
- Most common pattern: distal symmetrical sensorimotor polyneuropathy ("glove and stocking" distribution) — affects small fibres first (pain, temperature) then large fibres (vibration, proprioception)
- Autonomic neuropathy: gastroparesis, postural hypotension, neurogenic bladder, erectile dysfunction, silent MI (painless because cardiac autonomic neuropathy masks chest pain)

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Macrovascular disease — accelerated atherosclerosis:
- DM is a major independent risk factor for coronary artery disease (CAD), peripheral arterial disease (PAD), and stroke
- Mechanism: endothelial dysfunction (↓NO, ↑ET-1), dyslipidaemia (↑TG, ↑LDL, ↓HDL), prothrombotic state (↑PAI-1, ↑platelet aggregation), hypertension (common co-morbidity)
- Atherosclerosis in DM: more diffuse, more distal (involves tibial and popliteal arteries in PAD), more aggressive

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Diabetic foot — a convergence of neuropathy, ischaemia, and infection:
- Neuropathy → loss of protective pain sensation → unnoticed trauma → ulceration
- Ischaemia (PAD) → impaired wound healing
- Hyperglycaemia → impaired neutrophil function → infection → polymicrobial (including anaerobes) → osteomyelitis → amputation
- Charcot arthropathy: neuropathic joint destruction, typically midfoot, with bony fragmentation and collapse

Diabetic Retinopathy — NPDR vs PDR Fundus Comparison

Diabetic Neuropathic Foot Ulcer — Site and Characteristics