Cystic Diseases of the Renal Medulla

Slide 1 · Cystic Diseases of the

Renal Medulla

Medullary sponge kidney | Nephronophthisis

Cortex

Corticomedullary junction

Medulla / papillary ducts

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Distinguish collecting-duct dilatation from progressive tubulointerstitial disease.

Identify the characteristic renal distribution and morphology of both entities.

Connect nephronophthisis-associated tubular damage with its clinical presentation.

Use clinical context and renal morphology to formulate a focused differential diagnosis.

Slide 3 · Two medullary cystic diseases, two very different courses

Feature

Medullary sponge kidney

Nephronophthisis

Underlying pattern

Collecting-duct dilatation

Progressive tubular injury and fibrosis

Usual setting

Adult; often radiographic discovery

Childhood or adolescence

Functional course

Renal function usually normal

Progressive renal insufficiency

Core pathology

Medullary papillary duct changes

Corticomedullary cysts + cortical injury

Slide 4 · A medullary cyst does not establish the prognosis

Both entities can contain cysts within the renal medulla.

In medullary sponge kidney, the characteristic finding is ductal dilatation.

In nephronophthisis, progressive cortical tubulointerstitial damage explains loss of function.

The tissue response around cysts and the clinical age pattern resolve the apparent similarity.

Slide 5 · Regional anatomy: Cortex to papillary ducts

Cortex

Corticomedullary junction

Medulla

Papillary ducts

Regional renal tubular anatomy

Cortex contains the parenchyma that can undergo tubulointerstitial atrophy.

Corticomedullary junction is the favored cyst location in nephronophthisis.

Medullary collecting/papillary ducts are dilated in medullary sponge kidney.

Slide 6 · The location of cysts supports the diagnosis

Cortex

Corticomedullary junction

Medulla

Papillary ducts

Dilated medullary collecting ducts

Medullary sponge kidney: Dilated papillary collecting ducts.

Nephronophthisis: Cysts often cluster at the corticomedullary junction.

Localization must be integrated with organ size, cortical injury and clinical context.

Slide 7 · Pattern recognition begins with the surrounding kidney

Observation

Likely interpretation

Medullary dilatation + preserved cortex

Favours medullary sponge kidney

Small kidney + corticomedullary cysts

Favours nephronophthisis

Cortical atrophy + fibrosis

Explains progressive insufficiency in nephronophthisis

Cysts without characteristic clinical context

Insufficient for definitive classification

Slide 8 · Medullary sponge kidney: Definition

Multiple cystic dilatations of collecting ducts within the renal medulla.

The terminology is restricted to this collecting-duct abnormality.

The condition is a relatively common structural change.

It is usually innocuous rather than a progressive parenchymal disorder.

Slide 9 · Medullary sponge kidney is a duct-centred lesion

Cortex

Corticomedullary junction

Medulla

Papillary ducts

Dilated medullary collecting ducts

Cystic structures arise from dilated collecting ducts.

Papillary/medullary location is central to the definition.

It is not synonymous with a kidney diffusely replaced by cysts.

Slide 10 · Collecting-duct dilatation: Structural concept

Usual duct

Dilated duct

Cuboidal epithelial lining around lumen

Dilation expands the ductal lumen while retaining an epithelial lining.

The lesion primarily reflects an anatomic alteration in medullary duct calibre.

Robbins does not establish a specific initiating molecular mechanism.

Simplified epithelial/tubular schematic — not histology

Slide 11 · Medullary sponge kidney: Gross morphology

Medullary papillary ducts are visibly dilated on gross examination.

Small cysts can be present within the medulla.

The diagnostic pattern is centred on the papillary collecting system.

The passage does not describe characteristic destructive cortical lesions.

Slide 12 · Cystic spaces retain an epithelial lining

Usual duct

Dilated duct

Cuboidal epithelial lining around lumen

Cuboidal epithelium is the usual cyst lining.

Transitional epithelium may occasionally line cysts.

Histologic identification requires attention to the dilated duct and its epithelial boundary.

Simplified epithelial/tubular schematic — not histology

Slide 13 · An epithelial lining does not imply malignancy

The term “cyst” here refers to dilatation of a pre-existing epithelial duct.

Cuboidal or occasional transitional epithelial lining is a described morphologic feature.

Medullary sponge kidney as benign, not as a neoplastic cystic process.

Do not interpret lining morphology alone without distribution and renal context.

Slide 14 · Medullary sponge kidney: Cortical sparing

Cortex

Corticomedullary junction

Medulla

Papillary ducts

Dilated medullary collecting ducts

Cortical scarring is absent in uncomplicated medullary sponge kidney.

A preserved cortex is compatible with usually normal renal function.

Cortical scars prompt consideration of superimposed pathology.

Slide 15 · Cortical scarring suggests a superimposed process

Robbins explicitly qualifies cortical sparing when pyelonephritis is present.

An accompanying infection can change the pathology of an otherwise benign lesion.

Do not attribute cortical scars to uncomplicated medullary sponge kidney.

Recognize concurrent disease before inferring intrinsic progression.

Slide 16 · Medullary sponge kidney: Typical clinical setting

Usually encountered in adults.

Often discovered during radiographic evaluation rather than through progressive kidney failure.

Renal function is typically normal.

The radiographic presentation must be linked back to medullary collecting-duct anatomy.

Slide 17 · Imaging can reveal a medullary duct abnormality

IMAGING PRINCIPLE

Radiographicevaluation

Medullary ductdilatations

Medullary spongekidney pattern

Excretory urography as a method showing medullary cysts. Imaging shows the pattern; Clinical assessment determines its significance.

Slide 18 · Preserved renal function is a discriminator

Renal function is usually normal in medullary sponge kidney.

Medullary ductal cysts therefore do not imply inevitable progression to ESKD.

Declining kidney function should not be attributed automatically to this finding.

Assess superimposed or alternative renal disease when cortical injury is present.

Slide 19 · Medullary sponge kidney: Reported clinical associations

Finding

Interpretive teaching point

Hematuria

A possible clinical clue; not specific to the lesion

Urinary tract infection

Consider an associated or superimposed process

Recurrent renal stones

Recognized association in the Robbins cystic disease table

Usually normal kidney function

Separates this disorder from progressive nephronophthisis

Slide 20 · Mechanistic uncertainty must be preserved

The pathogenesis of medullary sponge kidney is unknown.

The structural phenotype does not establish a molecular cause.

No specific genetic locus or obligate pathogenic pathway should be assigned on this source.

Its generally benign behaviour is an observation, not an explanation of origin.

Slide 21 · Medullary sponge kidney: The clinicopathologic pattern

Medullaryduct dilatation

Papillary cysticspaces

Often radiographicdiscovery

Usually preservedrenal function

Hematuria, urinary tract infection or stones may be associated; They do not change the central benign structural classification by themselves.

Slide 22 · Nephronophthisis: Definition and defining injury

A group of progressive renal disorders with cysts in the medulla.

Cysts are often most prominent at the corticomedullary junction.

Tubular atrophy and interstitial fibrosis involve both medulla and cortex.

Cortical tubulointerstitial injury ultimately underlies renal insufficiency.

Slide 23 · Nephronophthisis: Cysts favour the junction

Cortex

Corticomedullary junction

Medulla

Papillary ducts

Cyst concentration near junction

Variable numbers of medullary cysts occur.

The corticomedullary junction is a characteristic concentration site.

The cortex may also contain small cysts.

Distribution differs from papillary duct dilatation in medullary sponge kidney.

Slide 24 · Nephronophthisis: Small, contracted kidneys

The kidneys are small rather than massively enlarged.

The external surfaces appear contracted and granular.

Medullary cysts are present, often concentrated near the corticomedullary junction.

Gross configuration reflects chronic parenchymal injury, not cyst volume alone.

Slide 25 · Nephronophthisis: Cyst epithelial lining

Flattened or cuboidal lining; Pericystic response

Medullary cysts are lined by flattened or cuboidal epithelium.

The cyst boundary may be accompanied by inflammatory cells or fibrous tissue.

The pericystic reaction is an important distinction from a simple anatomic dilatation.

Simplified epithelial/tubular schematic — not histology

Slide 26 · Pericystic inflammation and fibrosis

Inflammatory cells may surround cystic tubular structures.

Fibrous tissue may also surround cysts.

The surrounding interstitium therefore provides diagnostic information beyond the lumen.

A cyst should be interpreted within the broader tubulointerstitial pattern.

Slide 27 · Cortical tubules show chronic damage

Thickened tubular basement membranes with atrophy

Widespread tubular atrophy develops in the cortex.

Tubular basement membranes become thickened.

Interstitial fibrosis accompanies the tubular loss.

Together these changes account for substantial functional decline.

Simplified epithelial/tubular schematic — not histology

Slide 28 · Glomeruli can be relatively preserved

Generally preserved glomerular structure.

The dominant morphologic lesion is tubulointerstitial, not primarily glomerular.

Renal failure can occur even when glomerular architecture appears relatively intact.

Primary glomerular destruction should not be assumed from advanced renal dysfunction alone.

Slide 29 · Proposed initial lesion: Distal tubules

Robbins suggests that injury probably begins in distal tubules.

Tubular basement membrane disruption is an early proposed event.

The word “probably” is important: The initial lesion is not presented as proven in every case.

Later disease involves both medullary and cortical tubulointerstitium.

Slide 30 · Proposed injury sequence in nephronophthisis

Distal tubularinjury (probable)

Tubular basementmembrane disruption

Progressive tubularatrophy

Interstitialfibrosis

Progressive cortical involvement causes the renal insufficiency. This is a source-derived working sequence, not a complete molecular causal model.

Slide 31 · Why cortical fibrosis matters more than cyst count

Medullary cysts are a conspicuous feature but are not the only lesion.

Cortical tubular atrophy removes functioning renal parenchyma.

Interstitial fibrosis accompanies progressive tissue loss.

Cortical tubulointerstitial damage as the cause of eventual insufficiency.

Slide 32 · Recognized nephronophthisis clinical forms

Form

Characteristic context

Sporadic, nonfamilial

Disease without an established familial pattern

Familial juvenile

Most common recognized variant

Renal–retinal dysplasia

Renal disease accompanied by ocular lesions; ~15% in Robbins

Slide 33 · Familial juvenile nephronophthisis

The most common of the three clinical variants listed.

Familial forms are inherited as autosomal recessive traits.

They typically manifest during childhood or adolescence.

The resulting course is progressive, unlike uncomplicated medullary sponge kidney.

Slide 34 · Renal–retinal dysplasia: Systemic involvement

The renal disorder can coexist with ocular lesions.

Renal–retinal dysplasia as a distinct clinical variant.

The book quotes approximately 15% for this associated phenotype.

Extrarenal findings are diagnostically informative in a child with unexplained chronic kidney disease.

Slide 35 · Childhood or adolescent onset

Familial nephronophthisis commonly becomes manifest early in life.

The initial presentation may be tubular rather than a visible renal mass.

Polydipsia and polyuria may precede the recognition of advanced renal insufficiency.

Age at presentation distinguishes it from typical medullary sponge kidney.

Slide 36 · Autosomal recessive transmission

Carrier

Carrier

Affected child

Illustrative pedigree: Recessive transmission

The familial forms follow an autosomal recessive pattern.

An affected child need not have an affected parent.

A positive family history in a sibling can support clinical suspicion.

Inheritance helps distinguish this group from autosomal dominant tubulointerstitial disease.

Schematic pedigree — illustrative, not a patient family

Slide 37 · Nephronophthisis is genetically heterogeneous

Multiple associated NPHP and JBTS loci.

The named NPHP products include a protein family called nephrocystins.

Additional loci continue to be identified.

Gene lists must not be treated as a fixed or exhaustive 2026 diagnostic panel.

Slide 38 · Genes named in the source

The source enumerates NPHP1–NPHP11 among the nephronophthisis-associated loci.

It additionally lists JBTS2, JBTS3, JBTS9 and JBTS11.

This is the terminology reproduced in the supplied Robbins extract.

The enumeration reflects the textbook passage rather than independently verified current gene nomenclature.

Slide 39 · Nephrocystins: Cellular localization

The NPHP gene products are described as nephrocystins.

The implicated proteins localize to primary cilia, basal bodies or centrosomes.

These shared cellular compartments support the ciliopathy classification.

The passage does not specify a unique downstream pathway for each gene.

Slide 40 · Primary cilia and tubular epithelial biology

Primary cilium

Basal body

Renal tubular cell

The primary cilium is a cellular compartment implicated in these disorders.

Associated proteins occupy the cilium itself and related anchoring structures.

The structural localization helps explain the term ciliopathy.

This drawing is a compartment map, not proof of a specific signal-transduction defect.

Slide 41 · Basal body and centrosome: Linked compartments

Primary cilium

Basal body

Renal tubular cell

The basal body is associated with the base of the primary cilium.

The centrosome as the organelle from which basal bodies originate.

Nephronophthisis proteins may be found in each of these sites.

The localization is relevant to classification, not a stand-alone histologic test.

Slide 42 · Inversin is the NPHP2 gene product

NPHP2 as encoding inversin, also termed nephrocystin-2.

Inversin participates in embryonic left–right patterning.

This links at least one associated protein with development-related cellular biology.

The source does not justify assigning the same developmental role to all nephrocystins.

Slide 43 · Ciliopathy classification: What is established?

Multiple NPHP /JBTS loci

Protein localization tociliary apparatus

Nephronophthisisdisease group

The association and subcellular localization are supported by Robbins. A complete gene-by-gene mechanism of tubular injury is not supplied in this passage.

Slide 44 · Tubular concentrating defect explains early symptoms

Impaired urineconcentration

Increased urinevolume

Compensatorythirst / drinking

The textbook explicitly links polyuria and polydipsia to a marked renal tubular concentrating defect.

Slide 45 · Polyuria and polydipsia may be early clues

Children commonly first present with increased urine output.

Excessive thirst accompanies impaired renal concentrating ability.

The symptoms point toward a tubular defect rather than a cyst-related compressive syndrome.

Early symptoms can be more informative than the cyst burden seen on imaging.

Slide 46 · Sodium wasting is a major tubular manifestation

Sodium wasting as prominent.

This is consistent with the wider tubular functional injury in nephronophthisis.

Salt wasting supports a tubulointerstitial rather than purely structural diagnosis.

The supplied passage does not specify serum sodium thresholds or testing cutoffs.

Slide 47 · Tubular acidosis adds evidence of functional injury

Tubular acidosis is described as a prominent manifestation.

It coexists with an impaired concentrating mechanism and sodium wasting.

These findings show that dysfunction can be evident before cysts are demonstrable.

The passage does not define a specific renal tubular acidosis subtype.

Slide 48 · Nephronophthisis: Clinical features and pathology link

Clinical feature

Pathologic or functional connection

Polyuria and polydipsia

Severe renal concentrating defect

Sodium wasting

Associated tubular functional abnormality

Tubular acidosis

Associated defect of tubular handling

Growth retardation / anemia

Reported in Robbins comparative summary

Progressive kidney failure

Cortical tubular atrophy and interstitial fibrosis

Slide 49 · Nephronophthisis is a major genetic cause of pediatric ESKD

Robbins characterizes the nephronophthisis complex as the most common genetic cause of ESKD in children and young adults.

This statement pertains to the disease group, not to any one clinical variant.

The characteristic loss of renal function is driven by progressive tubulointerstitial injury.

Consider this category when early-onset chronic kidney disease remains unexplained.

Slide 50 · Nephronophthisis: Progressive clinical course

Early clinical clues

Polyuria, polydipsia; Salt wasting / acidosis

Progressive injury

Tubular atrophy; Interstitial fibrosis

Advanced disease

End-stage kidney disease

A progression to ESKD over approximately 5–10 years; This is not a fixed outcome for every individual.

Slide 51 · Imaging may underestimate nephronophthisis

Medullary cysts can be too small to be recognized radiographically.

An apparently nondiagnostic scan does not exclude the disease.

Small kidneys and functional abnormalities may guide suspicion even without a convincing cyst pattern.

Histologic and familial evidence should be weighed with clinical findings.

Slide 52 · A clinical constellation supports consideration

Child or adolescent with otherwise unexplained chronic kidney failure.

Positive family history, where present.

Chronic tubulointerstitial nephritis on renal biopsy.

Robbins urges strong consideration of nephronophthisis when these features converge.

Slide 53 · Biopsy pattern: Chronic tubulointerstitial disease

Tubular loss + expanded fibrotic interstitium

Tubular atrophy and tubular basement membrane thickening in cortex.

Interstitial fibrosis with possible pericystic inflammatory cells.

Glomerular structure is generally preserved.

A biopsy pattern is not sufficient alone without age, inheritance and clinical correlation.

Simplified epithelial/tubular schematic — not histology

Slide 54 · Integrating gross and microscopic morphology

Compartment

Nephronophthisis finding

Kidney surface / size

Small kidney; contracted granular external surface

Medulla

Cysts, often centred at corticomedullary junction

Cortex

Small cysts; tubular atrophy; thickened basement membranes; fibrosis

Glomeruli

Generally preserved architecture

Cyst environs

Inflammatory cells or fibrous tissue

Slide 55 · Medullary sponge kidney versus nephronophthisis

Feature

Medullary sponge kidney

Nephronophthisis

Location

Medullary collecting ducts

Medulla, especially junction

Gross kidney

Dilated papillary ducts

Small, contracted, granular

Cortex

No scarring unless pyelonephritis

Atrophy and interstitial fibrosis

Usual age

Adult

Childhood / adolescence

Course

Usually benign

Progression to ESKD

Slide 56 · Nephronophthisis versus ADTKD: Nomenclature matters

Feature

Nephronophthisis

ADTKD

Inheritance

Familial forms autosomal recessive

Autosomal dominant

Usual presentation

Childhood / adolescence

Progressive renal failure in adult life

Term “medullary cystic”

Cysts often at corticomedullary junction

Formerly called medullary cystic kidney disease

Cysts

Characteristic but may be tiny

Variable; often not medullary

Slide 57 · ADTKD is a separate autosomal dominant disorder

Robbins reclassifies former “medullary cystic kidney disease” as ADTKD.

Listed genes include MUC1, UMOD, REN and HNF1B.

Its progressive adult-onset renal failure and dominant inheritance distinguish it from familial juvenile nephronophthisis.

Cysts in ADTKD may be absent or outside the medulla.

Slide 58 · Nephronophthisis versus childhood polycystic kidney disease

Feature

Nephronophthisis

Autosomal recessive PKD

Typical organ size

Small, contracted kidneys

Enlarged cystic kidneys at birth

Principal pattern

Tubulointerstitial injury; junctional cysts

Collecting tubule dilatation

Extrarenal association

Ocular lesions in renal–retinal form

Hepatic fibrosis

Clinical framing

Progressive juvenile tubular disease

Neonatal / childhood cystic disease

Slide 59 · Nephronophthisis versus multicystic renal dysplasia

Feature

Nephronophthisis

Multicystic renal dysplasia

Typical architecture

Small shrunken kidney with junctional cysts

Enlarged, highly irregular multicystic kidney

Microscopic clue

Cortical atrophy, fibrosis, thickened TBM

Primitive mesenchyme ± cartilage; immature ducts

Clinical context

Childhood progressive tubular disorder

Often associated urinary tract malformations

Pathologic category

Tubulointerstitial disease

Renal developmental dysplasia

Slide 60 · Nephronophthisis versus acquired cystic disease

Feature

Nephronophthisis

Acquired cystic disease

Context

Inherited/juvenile or sporadic complex

Long-standing ESKD with dialysis

Interpretation of cysts

Cysts accompany a disease causing failure

Cysts develop in an already failed kidney

Distribution

Especially corticomedullary junction

Cortical and medullary cysts

Additional concern

Progressive tubulointerstitial failure

Renal-cell carcinoma association in Robbins

Slide 61 · A phenotype-first diagnostic decision

Age + course

Gross/scan distribution

Tubulointerstitial injury

Inheritance +associated findings

Juvenile polyuria, tubular dysfunction, small scarred kidneys and familial recessive history favour nephronophthisis; Adult radiographic medullary duct dilatation with preserved function favours medullary sponge kidney.

Slide 62 · Case 1: Incidental medullary cystic finding

APPLICATION — ILLUSTRATIVE CLINICAL VIGNETTE

An adult undergoing urinary tract imaging has medullary papillary duct dilatations. Renal function is preserved. No cortical scarring is described.

Which diagnosis best fits this combination, and what would alter the interpretation?

Slide 63 · Case 1: Interpretation

Medullary sponge kidney best matches adult discovery with papillary duct dilatation.

Usual preservation of renal function and absent cortical scarring support the diagnosis.

Concurrent pyelonephritis may produce cortical scarring and modify the clinical picture.

Associated stones, urinary infection or hematuria may be present without redefining the core lesion.

Slide 64 · Case 2: Unexplained progressive juvenile disease

APPLICATION — ILLUSTRATIVE CLINICAL VIGNETTE

An adolescent develops persistent polyuria, polydipsia and progressive renal impairment. Imaging identifies few convincing cysts. A sibling has a similar illness.

What diagnosis should be considered despite nondiagnostic imaging?

Slide 65 · Case 2: Interpretation

Nephronophthisis should be strongly considered.

Tubular concentrating failure explains the early polyuria and polydipsia.

An affected sibling is consistent with a familial autosomal recessive disorder.

Cysts may be too small to image; Chronic tubulointerstitial injury is more decisive than visible cyst count.

Slide 66 · Diagnostic pitfall: A “normal” cyst search does not exclude disease

Nephronophthisis may be radiographically subtle because cysts are small.

Diagnosis requires attention to renal function, tubular manifestations and family history.

Cortical atrophy/fibrosis can be informative even when cysts are sparse.

A radiographic cyst burden must never be used as a stand-alone severity surrogate.

Slide 67 · Final synthesis: Recognize the lesion, then the consequence

Question

Medullary sponge kidney

Nephronophthisis

What changes?

Medullary collecting-duct calibre

Tubules and interstitium progressively degenerate

Where is the clue?

Papillary medulla

Corticomedullary cysts + injured cortex

What happens to function?

Usually maintained

Progressive loss, ESKD

Who presents?

Usually adult

Often child / adolescent

What prevents error?

Check for associated infection / stones

Do not rely on visible cysts alone

Slide 68 · Reference and image provenance

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026), Chapter 20, pp. 850, 858–860.

Primary focus: “Cystic Diseases of the Renal Medulla,” including “Medullary Sponge Kidney” and “Nephronophthisis.”

All anatomical, cellular, pedigree and morphology diagrams in this presentation are original editable schematics.

No textbook photomicrographs, gross specimens or diagnostic imaging were reproduced.