Hydronephrosis

Overview

Pelvis+calyces dilatation from partial/intermittent outflow obstruction. Requires INCOMPETENT pelviureteric sphincter(s) — else obstruction → bladder dilatation/hypertrophy instead, no hydronephrosis. Hydroureter almost always accompanies. Uni- or bilateral.

Unilateral (PUJ-level obstruction)

1. Intraluminal — calculus (ureter/pelvis)
2. Intramural — congenital PUJ obstruction, ureteral atresia, stricture, trauma, neoplasm
3. Extramural — renal artery/vein compression, external pressure (cervical/prostatic/rectal/colonic/caecal carcinoma, retroperitoneal fibrosis)

Bilateral (usually URETHRAL obstruction, or bilateral versions of above)

1. Congenital — urethral meatal atresia, posterior urethral valve
2. Acquired — bladder tumour (both orifices), prostatic enlargement/carcinoma/prostatitis, bladder neck stenosis, urethral stricture, phimosis

Morphology

Depends on sudden/complete vs (commoner) incomplete/intermittent obstruction.

Gross — staged: Extrarenal hydronephrosis (early, pelvis dilates medially as sac) → progressive dilatation + pressure atrophy → Intrarenal hydronephrosis (advanced, dilated system extends deep into cortex, thin stretched rim, lobulated external surface).

KEY DISTINCTION from polycystic kidney: hydronephrosis cysts DIRECTLY CONTINUOUS with pelvis (opposite of ADPKD, where cysts DON’T communicate).

Micro: sac wall thickened (fibrous scarring+chronic infiltrate). Progressive tubular+glomerular atrophy+interstitial fibrosis. Stasis → infection (pyelitis) → pus-filled sac = PYONEPHROSIS.

Clinical relevance

Sphincter incompetence requirement (not obstruction alone) = key mechanistic point — without it, obstruction → bladder hypertrophy instead; same obstructive process, different structural consequence depending on where the “weak point” lies. 3-tier causation (intraluminal/intramural/extramural) = practical bedside framework — imaging finding (calculus/stricture/external mass) maps to one tier, guides diagnosis+referral. Continuity vs non-continuity (hydronephrosis vs ADPKD cysts) = single most useful imaging/gross discriminator — both look grossly cystic, only one traces back to common dilated pelvis. Stasis→infection link (pyelitis→pyonephrosis) = general urinary tract principle — stagnant urine anywhere predisposes to bacterial colonisation, relieving obstruction = infection prevention too.

Overview

Hydronephrosis is dilatation of the renal pelvis and calyces from partial or intermittent urinary outflow obstruction. It specifically requires incompetence of one or both pelviureteric sphincters — if the sphincters remain competent, obstruction instead produces bladder dilatation/hypertrophy without hydronephrosis. Hydroureter almost always accompanies it. May be unilateral or bilateral.

Unilateral Hydronephrosis

From ureteral obstruction at the pelviureteric junction (PUJ) level:

1. Intraluminal — calculus in ureter/renal pelvis.
2. Intramural — congenital PUJ obstruction, ureteral atresia, inflammatory stricture, trauma, ureteral/bladder neoplasm.
3. Extramural — compression by an inferior renal artery/vein, or external pressure (cervical/prostatic/rectal/colonic/caecal carcinoma, retroperitoneal fibrosis).

Bilateral Hydronephrosis

Generally from urethral obstruction, or from bilateral involvement of the causes above:

1. Congenital — urethral meatal atresia, congenital posterior urethral valve.
2. Acquired — bladder tumour involving both ureteric orifices, prostatic enlargement/carcinoma/prostatitis, bladder neck stenosis, inflammatory/traumatic urethral stricture, phimosis.

Morphology

Findings depend on whether obstruction is sudden/complete or (more commonly) incomplete/intermittent.

Gross — staged progression:

• Extrarenal hydronephrosis (early) — renal pelvis dilates medially as a sac.
• As obstruction persists, pelvis/calyces progressively dilate with pressure atrophy of the parenchyma.
• Intrarenal hydronephrosis (advanced) — the dilated pelvicalyceal system extends deep into the cortex, leaving only a thin stretched cortical rim; external surface becomes lobulated.

Key distinguishing point from polycystic kidney disease: in hydronephrosis, the dilated cystic spaces (calyces) are directly continuous with the renal pelvis — the opposite of ADPKD, where cysts do not communicate with the pelvis (see Polycystic Kidney Disease).

Micro: hydronephrotic sac wall thickened by fibrous scarring and chronic inflammatory infiltrate; progressive tubular and glomerular atrophy with interstitial fibrosis. Urinary stasis predisposes to infection (pyelitis), and if the sac fills with pus, the condition becomes pyonephrosis.

Clinical Relevance

• Recognising hydronephrosis as requiring sphincter incompetence — not just obstruction alone — is a genuinely important mechanistic point: without it, obstruction manifests instead as bladder wall hypertrophy, meaning the same underlying obstructive process can produce very different structural consequences depending on where along the urinary tract the “weak point” for pressure transmission actually lies.
• The three-tier causation scheme (intraluminal/intramural/extramural for unilateral disease) is a practical framework directly applicable at the bedside — imaging findings of a calculus, a stricture, or an external mass each map onto one tier, guiding both diagnosis and the appropriate specialty referral.
• The continuity-versus-non-continuity distinction between hydronephrotic cysts and ADPKD cysts is the single most useful discriminator on cross-sectional imaging or gross pathology between the two — both present as a grossly cystic kidney, but only one has cysts that trace back to a common dilated pelvis.
• Recognising urinary stasis as the mechanistic link between hydronephrosis and secondary infection (pyelitis, progressing to pyonephrosis) reinforces a general principle seen across the urinary tract — stagnant urine anywhere along the system predisposes to bacterial colonisation, which is why relieving obstruction is as much an infection-prevention measure as a mechanical one.

• Staged progression from extrarenal to intrarenal hydronephrosis
• Hydronephrosis vs polycystic kidney disease: cyst-pelvis continuity as the key discriminator