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PA27.{11,15} | Vascular Diseases & Thrombotic Microangiopathies — SDL Guide

Learning Objectives

Classify the vascular diseases of the kidney and outline the role of hypertension in benign and malignant nephrosclerosis.
Describe the pathological lesions of benign nephrosclerosis (hyaline arteriolosclerosis, fibroelastic hyperplasia) and their gross and microscopic consequences.
Distinguish the morphological hallmarks of malignant hypertension — fibrinoid necrosis, hyperplastic onion-skin arteriolitis, and the flea-bitten kidney.
Explain the mechanism of renovascular hypertension in renal artery stenosis and differentiate atherosclerotic from fibromuscular dysplasia causes.
Define thrombotic microangiopathy (TMA) and describe the shared pathological substrate of endothelial injury, microthrombi, MAHA, and thrombocytopenia.
Compare and contrast HUS (typical Shiga-toxin vs atypical complement-mediated) and TTP (ADAMTS13 deficiency) using clinical, laboratory, and morphological features.
Recognise the renal lesions of DIC, scleroderma renal crisis, cortical necrosis, and renal infarction.

INSTRUCTIONS

The kidney sits at the crossroads of systemic haemodynamics and microvascular biology. Sustained hypertension remodels renal vessels across a spectrum — from the silent scarring of benign nephrosclerosis to the catastrophic fibrinoid necrosis of malignant hypertension. At the other extreme, the thrombotic microangiopathies demonstrate how a single pathological theme — endothelial injury with microvascular thrombosis — can arise from a toxin (HUS), an enzyme deficiency (TTP), complement dysregulation, or immune activation, yet converge on the same triad of haemolytic anaemia, thrombocytopenia, and acute kidney injury. Mastering this module gives you a unifying vascular lens for interpreting renal biopsies, clinical syndromes, and laboratory results that will recur throughout medicine.

References

Robbins & Cotran Pathologic Basis of Disease, 10th ed., Ch 20 (The Kidney) (textbook)
Harsh Mohan: Textbook of Pathology, 8th ed., Ch 22 (textbook)

Version 2.0 | NMC CBUC 2024

CLINICAL SCENARIO

A 58-year-old man with a 15-year history of poorly controlled hypertension is found on autopsy to have bilaterally shrunken kidneys with finely granular surfaces. His 32-year-old niece is admitted with sudden headache, oliguria, microangiopathic haemolytic anaemia, and a platelet count of 18,000/μL — three days after a bloody diarrhoeal illness. Same organ, utterly different vascular catastrophes. By the end of this module, you will read both scenarios from the morphology up.

WHY THIS MATTERS

Hypertension affects 30% of Indian adults. Malignant hypertension — though less common — remains a preventable cause of acute renal failure. Thrombotic microangiopathies are under-recognised emergencies where early diagnosis (blood film, ADAMTS13 assay, Shiga-toxin PCR) is life-saving. In postgraduate entrance examinations, the onion-skin arteriolitis of malignant hypertension and the HUS/TTP distinction are perennially high-yield topics.

RECALL

Before proceeding, consolidate your Year-1 foundations:

Arterioles — structure: intima (endothelium + IEL), media (1-2 smooth muscle layers), adventitia.
Hyaline change in vessels — amorphous eosinophilic protein deposition (plasma protein insudation + basement membrane thickening) seen in diabetes and age.
Renin-angiotensin-aldosterone system (RAAS) — juxtaglomerular cells release renin → angiotensinogen → Ang I → ACE → Ang II → vasoconstriction + aldosterone → sodium retention → raised BP.
Haemolytic anaemia — extravascular (splenomegaly, elevated indirect bilirubin) vs intravascular (haemoglobinaemia, haemoglobinuria, low haptoglobin).
Schistocytes — fragmented red blood cells produced when RBCs are sheared by fibrin strands in small vessels.

If any of these are unclear, revisit them before continuing.

Overview: Vascular Diseases of the Kidney

A kidney vascular classification diagram links renal artery, interlobular artery, microvascular, and renal vein disease levels to renovascular hypertension, nephrosclerosis, thrombotic microangiopathy, and renal vein thrombosis. — **Panel A:** Coronal kidney overview showing renal artery, segmental artery, interlobar artery, arcuate artery, interlobular artery, afferent arteriole, glomerular capillaries, renal vein, cortex, medulla, and disease levels by vessel calibre.. **Panel B:** Large renal artery disease showing renal artery stenosis causing renovascular hypertension.. **Panel C:** Medium artery disease showing interlobular artery wall thickening, narrowed lumen, scarring, and benign/malignant nephrosclerosis.. **Panel D:** Small vessel disease showing arteriole and glomerular capillary thrombotic microangiopathy with microthrombi, endothelial injury, HUS, and TTP.. **Panel E:** Renal venous disease showing renal vein thrombosis as a complication of nephrotic syndrome..

Vascular diseases of the kidney can be classified by the calibre of vessels affected:

Vessel level	Disease	Prototype
Large (renal artery)	Renovascular hypertension	Renal artery stenosis
Medium (interlobular arteries)	Hypertensive nephropathy	Benign/malignant nephrosclerosis
Small (arterioles, glomerular capillaries)	Thrombotic microangiopathy	HUS, TTP
Renal vein	Renal vein thrombosis	Nephrotic syndrome complication

Nephrosclerosis refers to the hardening and scarring of the kidney as a consequence of vascular disease. Two distinct forms are recognised based on the severity and acuity of hypertension: benign and malignant (accelerated) nephrosclerosis.

PA27.11 covers both hypertensive forms and renovascular disease; PA27.15 covers thrombotic angiopathies.

Benign Nephrosclerosis: Pathogenesis and Morphology

A four-panel diagram summarizes benign nephrosclerosis pathogenesis, gross shrunken granular kidneys, arteriolar wall lesions, and microscopic glomerular, tubular, and interstitial scarring. — **Panel A:** Chronic moderately elevated blood pressure; hyaline arteriolosclerosis; fibroelastic intimal hyperplasia; narrowed lumen; patchy ischaemia; glomerular ischaemia; periglomerular fibrosis; tubular atrophy; interstitial fibrosis.. **Panel B:** Bilateral shrunken kidneys; finely granular cortical surface; surviving nephron granules; depressed scarred ischaemic areas; thinned cortex; preserved corticomedullary distinction early.. **Panel C:** Hyalinised afferent arteriole with PAS-positive homogeneous pink hyaline material, loss of smooth muscle cells, thickened wall, slit-like lumen; interlobular artery with fibroelastic intimal thickening, smooth muscle and collagen proliferation, narrowed lumen.. **Panel D:** Obsolescent hyalinised glomerulus; ghost glomerulus with washed-out appearance; periglomerular fibrosis; tubular atrophy and dropout; focal interstitial fibrosis; compensatory hypertrophy of surviving nephron with enlarged glomerulus and large tubules..

Benign nephrosclerosis is the renal manifestation of long-standing, moderately elevated blood pressure. It is the commonest form of hypertensive renal disease.

Pathogenesis:
Chronic pressure overload → two sequential arteriolar lesions:
1. Hyaline arteriolosclerosis — plasma proteins insudated into the arteriolar wall; PAS-positive homogeneous pink material replaces the media; smooth muscle cells are lost. Seen in afferent arterioles.
2. Fibroelastic hyperplasia (intimal thickening) — smooth muscle cells and collagen proliferate in the intima of interlobular arteries; lumen narrows progressively.

Both lesions narrow the lumen → patchy ischaemia → glomerular ischaemia → periglomerular fibrosis → tubular atrophy → interstitial fibrosis.

Gross morphology:
• Bilateral shrunken kidneys (each ~110-130 g vs normal 150 g)
• Finely granular cortical surface — each granule = a surviving nephron tuft surrounded by fibrosis; depressed areas = scarred, ischaemic nephrons
• Cortex thinned; corticomedullary distinction preserved early

Microscopic:
• Hyalinised afferent arterioles (eosinophilic, thickened wall, slit-like lumen)
• Obsolescent (hyalinised) glomeruli — ghost glomeruli with washed-out appearance
• Tubular atrophy and dropout
• Focal interstitial fibrosis
• Surviving nephrons undergo compensatory hypertrophy (large tubules, enlarged glomeruli)

Clinical: proteinuria is mild; renal failure is slow (decades); BP control slows progression.

A labeled histology diagram of benign nephrosclerosis showing hyaline arteriolosclerosis, an obsolescent glomerulus, tubular atrophy, and chronic ischemic scarring. — **Panel A:** 10x renal cortex overview showing hyaline arteriolosclerosis, obsolescent glomerulus, tubular atrophy, and patchy interstitial fibrosis.. **Panel B:** Magnified afferent arteriole showing homogeneous eosinophilic hyaline wall thickening and narrowed lumen.. **Panel C:** Magnified chronic ischemic renal damage showing obsolescent glomerulus, atrophic tubules, and interstitial fibrosis..

SELF-CHECK

Which microscopic lesion in benign nephrosclerosis directly explains the finely granular surface seen on gross examination?

A. Fibrinoid necrosis of arterioles causing acute haemorrhage

B. Deposition of immune complexes in glomerular capillary walls

C. Hyperplastic onion-skin intimal thickening

D. Focal ischaemic scarring alternating with surviving hypertrophied nephrons

Reveal Answer

Answer: D. Focal ischaemic scarring alternating with surviving hypertrophied nephrons

The finely granular surface of benign nephrosclerosis reflects patches of ischaemic scar (depressed granules) alternating with relatively preserved, compensatorily hypertrophied surviving nephrons (raised granules). The vessel lesions (hyaline arteriolosclerosis, fibroelastic hyperplasia) cause the ischaemia, but it is this focal pattern of dropout-and-survival that creates the macroscopic granularity. Fibrinoid necrosis and onion-skin changes are features of malignant hypertension.

Malignant Hypertension / Accelerated Nephrosclerosis

Four-panel diagram of malignant hypertension showing flea-bitten kidney, normal artery, onion-skin hyperplastic arteriolitis, and fibrinoid necrosis with thrombosis. — **Panel A:** Gross kidney showing petechial cortical hemorrhages, normal or slightly enlarged kidney size, and cortical ischemia from thrombosed arterioles.. **Panel B:** Normal interlobular artery cross-section showing open lumen, thin intima, smooth muscle media, and adventitia.. **Panel C:** Hyperplastic arteriolitis showing concentric intimal hyperplasia, onion-skin lamination, smooth muscle cells and fibroblasts, collagen layers, and residual narrowed lumen.. **Panel D:** Fibrinoid necrosis of an arteriole showing destroyed arteriolar wall, eosinophilic fibrin deposition, acute thrombus, fragmented nuclei, and adjacent cortical ischemia..

Malignant hypertension (diastolic BP >120 mmHg, rising acutely, with end-organ damage) produces a distinct and rapidly destructive renal syndrome — malignant nephrosclerosis (also called accelerated nephrosclerosis).

Key vessel lesions (the two diagnostic hallmarks):

Fibrinoid necrosis of arterioles — the acutely injured arteriole undergoes necrosis; plasma proteins (including fibrin) flood the wall → intensely eosinophilic, granular, smudgy appearance on H&E. The entire wall is destroyed. Associated with acute thrombosis → sudden cortical ischaemia.

Hyperplastic arteriolitis (onion-skin lesion) — in interlobular arteries, smooth muscle cells and fibroblasts proliferate concentrically in the intima; each successive layer of cells and collagen resembles the layers of an onion in cross-section. Lumen is severely reduced. This is the classic histological hallmark of malignant hypertension.

Medical diagram showing malignant hypertension with onion-skin hyperplastic arteriolitis, narrowed residual lumen, flea-bitten kidney petechiae, and comparison with benign nephrosclerosis. — **Panel A:** 20x renal photomicrograph-style view showing interlobular artery with concentric intimal hyperplasia, near-obliterated residual lumen, smooth muscle cells, surrounding renal tubules, and adjacent glomerular tissue.. **Panel B:** Magnified schematic of onion-skin arteriolitis showing layered intimal proliferation, narrowed residual lumen, circumferential smooth muscle cells, and fibrinoid material.. **Panel C:** Gross malignant nephrosclerosis showing normal-sized kidney with multiple cortical petechiae producing a flea-bitten appearance.. **Panel D:** Side-by-side comparison of benign nephrosclerosis with finely granular shrunken kidney and hyaline arteriolosclerosis versus malignant nephrosclerosis with flea-bitten petechiae, fibrinoid necrosis, and onion-skin arteriolitis..

Gross morphology — the flea-bitten kidney:
• Kidneys may be normal size or slightly enlarged (acute, not yet atrophic)
• Multiple petechial haemorrhages on the cortical surface → flea-bitten appearance (haemorrhages from necrotic arterioles and damaged glomerular capillaries)

Microscopic:
• Fibrinoid necrosis of afferent arterioles
• Onion-skin arteriolitis of interlobular arteries
• Necrotising glomerulitis — mesangiolysis, capillary wall necrosis, crescent formation (in severe cases)
• Interstitial haemorrhage

Clinical: acute renal failure, haematuria, red cell casts, heavy proteinuria, hypertensive encephalopathy, papilloedema. Medical emergency.

Side-by-side diagram comparing benign nephrosclerosis with a shrunken finely granular kidney and hyaline arteriolosclerosis versus malignant nephrosclerosis with flea-bitten petechiae and fibrinoid necrosis with onion-skin arteriolitis. — **Panel A:** Benign nephrosclerosis gross kidney: shrunken kidney, cortical loss, finely granular surface, chronic ischemic scarring.. **Panel B:** Benign nephrosclerosis vessel lesion: hyaline arteriolosclerosis, homogeneous pink hyaline wall thickening, narrowed arteriole lumen.. **Panel C:** Malignant nephrosclerosis gross kidney: relatively normal-sized kidney, flea-bitten petechiae, pinpoint cortical hemorrhages.. **Panel D:** Malignant nephrosclerosis vessel lesion: fibrinoid necrosis, onion-skin arteriolitis, concentric intimal proliferation, severe luminal narrowing..

CLINICAL PEARL

The flea-bitten kidney is not finely granular — this distinction catches candidates out. Benign nephrosclerosis = shrunken, finely granular (chronic ischaemia, cortical loss). Malignant hypertension = petechial haemorrhages on a relatively normal-sized kidney (acute arteriolar blowouts). The granules in benign disease are structural (hypertrophied nephrons); the haemorrhages in malignant disease are vascular (fibrinoid necrosis). Get these two gross appearances locked before the exam.