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PA27.12-13 | Cystic Kidney Disease, Renal Stones & Obstructive Uropathy — SDL Guide

Learning Objectives

Classify cystic kidney diseases and distinguish ADPKD from ARPKD on the basis of genetics, morphology, and clinical course.
Explain the pathogenesis and extrarenal manifestations of autosomal dominant polycystic kidney disease (ADPKD).
Identify the four major types of renal stones, their composition, causative conditions, and radiological characteristics.
Describe the mechanism and morphological stages of hydronephrosis and the complications of obstructive uropathy.

INSTRUCTIONS

Cystic and obstructive diseases together account for a significant proportion of chronic kidney disease and transplant referrals in India. Understanding the genetic basis of PKD, the metabolic drivers of nephrolithiasis, and the pressure-atrophy cascade of obstruction provides the clinico-pathological framework you will use every time you interpret a renal biopsy report or counsel a patient with flank pain. Work through each section systematically; the micro-quizzes track your self-assessment at key branch points.

References

Robbins & Kumar: Basic Pathology, 11th ed., Chapter 14 — The Kidney (textbook)
Harsh Mohan: Textbook of Pathology, 8th ed., Chapter 22 — Kidney and Lower Urinary Tract (textbook)

Version 2.0 | NMC CBUC 2024

CLINICAL SCENARIO

A 38-year-old man presents to the nephrology OPD with poorly controlled hypertension, bilateral flank fullness, and a family history of 'kidney problems' — his father died on dialysis. Ultrasound shows both kidneys massively enlarged and studded with cysts of varying sizes. Blood urea and creatinine are elevated. What single genetic mutation could have set this cascade in motion thirty years before his kidneys began to fail?

WHY THIS MATTERS

Cystic kidney disease encompasses a spectrum from the devastating ADPKD to benign simple cysts found incidentally on imaging. Distinguishing them determines whether you order genetic testing, screen family members, initiate antihypertensives early, or simply reassure. Renal stones affect roughly 12% of the Indian population during their lifetime, and obstructive uropathy is the third commonest cause of renal failure in children. The pathological mechanisms you learn here directly inform surgical urgency and long-term renal preservation strategies.

RECALL

Before reading on, spend two minutes answering these silently:

What is the difference between autosomal dominant and autosomal recessive inheritance in terms of generations affected and carrier probability?
What are the normal functions of the renal tubular epithelium in concentrating urine?
Name two common causes of urinary tract obstruction you have encountered in surgery or medicine postings.

These anchor your Year-1 knowledge to the pathological detail ahead.

Classification of Cystic Kidney Disease

A multi-panel medical diagram classifies cystic kidney diseases by cyst origin, inheritance, age of onset, and characteristic renal or extrarenal features. — **Panel A:** Formation of a renal cyst from tubular or collecting duct epithelium, showing focal outpouching, loss of nephron connection, and fluid-filled cyst formation.. **Panel B:** ADPKD with PKD1/PKD2 mutation, autosomal dominant inheritance, adult onset, massively enlarged bilateral kidneys, and extrarenal cysts.. **Panel C:** ARPKD with PKHD1 mutation, autosomal recessive inheritance, infantile/perinatal onset, collecting duct cysts, and congenital hepatic fibrosis.. **Panel D:** Nephronophthisis with NPHP gene defects, autosomal recessive inheritance, childhood to adolescent onset, medullary cysts, tubular atrophy, and tubulointerstitial fibrosis.. **Panel E:** Medullary sponge kidney showing sporadic dilatation of papillary collecting ducts in renal pyramids with stones or nephrocalcinosis.. **Panel F:** Simple cortical cysts showing thin-walled cortical cysts with preserved surrounding renal parenchyma and usually incidental presentation..

Renal cysts arise from focal outpouchings of tubular or collecting duct epithelium that lose connection with the parent nephron. A practical classification groups them by genetic basis, age of onset, and renal consequence:

Disease	Inheritance	Onset	Key Feature
ADPKD	AD (PKD1/PKD2)	Adult	Bilateral massive enlargement, extrarenal cysts
ARPKD	AR (PKHD1)	Infantile/perinatal	Congenital hepatic fibrosis
Nephronophthisis	AR (NPHP genes)	Childhood–adolescence	Medullary cysts, tubulo-interstitial fibrosis
Medullary sponge kidney	Sporadic	Any	Dilated papillary collecting ducts, stones
Simple cortical cysts	Sporadic (somatic)	Middle age+	Single/few, benign, very common
Acquired cystic disease	Acquired	End-stage / dialysis	Multiple bilateral cysts, ↑ RCC risk
Multicystic dysplastic kidney	Developmental	Neonatal	Non-functional, unilateral, mass

Genetic (hereditary) cysts carry systemic implications; sporadic and acquired cysts are usually incidental findings.

Classification tree of cystic kidney diseases divided into hereditary and non-hereditary subtypes, with kidney icons showing cyst patterns plus age of onset and ESRD risk indicators. — **Panel A:** Root node 'Cystic Kidney Disease'; hereditary branch with ADPKD, ARPKD, nephronophthisis; non-hereditary branch with simple renal cysts, acquired cystic kidney disease, renal dysplasia / multicystic dysplastic kidney, and medullary sponge kidney; each subtype labeled with cyst distribution, age of onset, and ESRD risk..

ADPKD — Genetics and Molecular Pathogenesis

Diagram showing ADPKD pathogenesis through germline PKD1 or PKD2 mutation, somatic second-hit loss in one tubular epithelial cell, disrupted ciliary polycystin signaling, and cyst growth. — **Panel A:** Tubular epithelial cells, germline PKD1/PKD2 mutation in all cells, somatic second-hit loss in one cell, cyst initiation, normal non-cyst-forming cells, PKD1 chromosome 16p13.3, PKD2 chromosome 4q21.. **Panel B:** Primary cilium, tubular fluid flow, polycystin-1 receptor, polycystin-2 calcium channel, polycystin-1/2 complex, intracellular calcium influx, apical membrane, tubular epithelial cell.. **Panel C:** Loss of polycystin complex, reduced intracellular calcium, activated mTOR pathway, activated Ras/ERK pathway, impaired cell polarity, epithelial proliferation, fluid secretion, cyst expansion..

Autosomal dominant polycystic kidney disease (ADPKD) is the commonest inherited kidney disease, affecting approximately 1 in 1,000 individuals worldwide.

Genetic basis — two genes:
- PKD1 (chromosome 16p13.3) — encodes polycystin-1, a large transmembrane receptor. Mutated in ~85% of cases. Associated with earlier, more severe disease (ESRD by ~54 years).
- PKD2 (chromosome 4q21) — encodes polycystin-2, a calcium-channel protein. Mutated in ~15% of cases. Milder course (ESRD by ~74 years).

Polycystin-1 and polycystin-2 form a complex localised to primary cilia of tubular epithelial cells. Their function is to transduce mechanical flow-sensing signals into intracellular calcium flux. Loss of this complex activates proliferative pathways (mTOR, Ras/ERK) and impairs cell polarity, driving cyst initiation.

Two-hit model: although ADPKD is dominant (one germline mutation is sufficient to cause disease), somatic mutation of the second allele in individual tubular cells triggers cyst formation in those cells specifically — explaining why only a subset of nephrons cyst-form despite every cell carrying the germline mutation.

Diagram showing autosomal dominant polycystic kidney disease arising from germline PKD1 or PKD2 mutation plus somatic second-hit loss in one tubular epithelial cell, causing defective ciliary polycystin signalling and mTOR/ERK-driven cyst growth. — **Panel A:** Renal tubule lumen, tubular epithelial cells, nucleus, germline PKD1/PKD2 mutation in all cells, somatic second-hit loss in one cell, biallelic PKD inactivation, clonal epithelial expansion, early cyst outpouching.. **Panel B:** Primary cilium, tubular fluid, polycystin-1, polycystin-2 calcium channel complex, defective polycystin signalling, reduced Ca2+ entry.. **Panel C:** Decreased intracellular Ca2+, increased cAMP, mTOR activation, ERK activation, epithelial proliferation, fluid secretion, cyst enlargement..

ADPKD — Morphology and Extrarenal Manifestations

Diagram showing massively enlarged polycystic kidneys in ADPKD, comparison with a normal kidney, microscopic cyst wall changes, and extrarenal manifestations including hepatic cysts, berry aneurysm, and mitral valve prolapse. — **Panel A:** Bilateral massively enlarged ADPKD kidneys, innumerable renal cysts of variable size, compressed residual renal parenchyma, haemorrhagic cysts, normal kidney outline, scale ruler, and microscopic inset showing flattened-cuboidal epithelium, tubular atrophy, interstitial fibrosis, and neutrophils in infected cysts.. **Panel B:** Normal kidney silhouette compared with markedly enlarged ADPKD kidney silhouette showing cystic replacement and obliteration of renal parenchyma.. **Panel C:** Extrarenal manifestations of ADPKD: hepatic cysts, berry aneurysm at the Circle of Willis, and mitral valve prolapse..

Gross morphology: Both kidneys are massively enlarged — combined weight may exceed 4 kg (normal combined ~300 g). The external surface is replaced by hundreds of cysts ranging from a few millimetres to several centimetres, filled with clear, serous, or haemorrhagic fluid. Normal parenchyma is progressively obliterated.

Microscopic morphology: Cyst walls are lined by a flattened to cuboidal epithelium. The residual nephrons show tubular atrophy and interstitial fibrosis as compression progresses. Secondary infection may cause neutrophilic infiltration (infected cysts).

Medical diagram showing massively enlarged bilateral ADPKD kidneys filled with numerous cysts, a normal kidney outline for scale comparison, and key extrarenal manifestations. — **Panel A:** Bilateral massively enlarged ADPKD kidneys, innumerable renal cysts of varying size, compressed residual renal parenchyma, ruler for scale, dotted normal kidney outline.. **Panel B:** Normal kidney silhouette compared with enlarged ADPKD kidney silhouette showing cystic replacement of renal parenchyma.. **Panel C:** Extrarenal manifestations including hepatic cysts, berry aneurysm at the Circle of Willis, and mitral valve prolapse..

Extrarenal (systemic) manifestations — critical for exams and clinical practice:

Hepatic cysts — in ~75% of patients; usually non-functional; the most common extrarenal manifestation.
Berry (saccular) aneurysms — in ~10–15%; arise at branch points of the Circle of Willis; rupture causes subarachnoid haemorrhage (SAH) — the most lethal extrarenal complication.
Mitral valve prolapse (MVP) — in ~25%; generally haemodynamically insignificant.
Intracranial cysts, pancreatic cysts, seminal vesicle cysts (less common).

Clinical course: Patients are typically asymptomatic until the 4th–5th decade. Presenting features include hypertension (renin–angiotensin activation as cysts compress intrarenal vessels), gross haematuria (cyst rupture), flank pain, and palpable kidneys. Proteinuria is mild. Progression to end-stage renal disease (ESRD) occurs in ~50% by age 60.

SELF-CHECK

A 42-year-old man with ADPKD presents with the sudden onset of the worst headache of his life. CT shows blood in the subarachnoid space. The responsible extrarenal lesion is:

A. Hepatic cyst rupture

B. Berry aneurysm rupture at the Circle of Willis

C. Mitral valve vegetations embolising to cerebral vessels

D. Renal vein thrombosis with paradoxical embolism

Reveal Answer

Answer: B. Berry aneurysm rupture at the Circle of Willis

Berry (saccular) aneurysms at Circle of Willis branch-points are the most lethal extrarenal manifestation of ADPKD. Their rupture produces classic thunderclap headache and subarachnoid haemorrhage. Hepatic cysts are common but do not cause intracranial bleeding. MVP in ADPKD is haemodynamically benign. Renal vein thrombosis does not directly cause SAH.