Hydronephrosis / Ureterohydronephrosis

Definition

Congenital Causes

• UPJ obstruction is the most common cause of hydronephrosis in infants and children
• Incidence: approximately 1 in 500 live births screened by routine antenatal ultrasound
• More common in boys than girls when presenting early in life
• Bilateral in ~20% of early-presenting cases
• Agenesis of the contralateral kidney occurs in a minority of cases
• In adults, UPJ obstruction is more common in females and is most often unilateral

Mechanism of UPJ Obstruction

• Abnormal organization of smooth muscle bundles at the UPJ
• Excess stromal collagen deposition between smooth muscle bundles
• Rarely: extrinsic compression by aberrant renal vessels

Other congenital causes include:

Pathogenesis

1. Compresses the renal vasculature → arterial insufficiency and venous stasis
2. Exerts maximum pressure on the papillae (most vulnerable site)
3. Initial functional defect: impaired tubular concentrating ability
4. Later: glomerular filtration begins to fall
5. Interstitial inflammatory reaction → eventually fibrosis

Morphology (Gross and Microscopic)

FIG. 12.25 — Hydronephrosis of the kidney: marked dilation of the pelvis and calyces with thinning of renal parenchyma. (Robbins & Kumar Basic Pathology)

• Kidney may reach lengths of ~20 cm with subtotal or intermittent obstruction
• Organ consists almost entirely of the greatly distended pelvicalyceal system
• Renal parenchyma is compressed and atrophied
• Papillae are obliterated; pyramids are flattened
• If blockage is at or above the ureters: unilateral involvement
• If obstruction is below the ureters (e.g., bladder outlet): bilateral involvement
• With sudden complete obstruction: glomerular filtration fails early → renal function ceases with only mild dilation

• Early: tubular dilation and atrophy
• Later: loss of glomeruli
• End-stage: replacement of renal parenchyma by fibrous tissue
• Inflammatory reaction is minimal in uncomplicated cases
• Superimposed pyelonephritis is common

Ultrasound Grading

Fig. 5.6 — Mild (A), moderate (B), and severe (C) hydronephrosis on renal ultrasound. (Comprehensive Clinical Nephrology, 7th Ed.)

Clinical Features

• Bilateral hydronephrosis → anuria and renal failure
• Distal bladder obstruction → dominant symptoms are bladder distention
• Incomplete bilateral obstruction → paradoxically causes polyuria (due to tubular concentrating defect), which may obscure the diagnosis
• Unilateral hydronephrosis may remain clinically silent for long periods, especially if the contralateral kidney is functional — often discovered incidentally on physical exam or imaging
• Underlying causes (calculi, tumors) may produce symptoms that uncover the hydronephrosis
• Prognosis: removal of obstruction usually permits full functional recovery within weeks; however, longstanding obstruction produces irreversible changes

Fetal/Prenatal Hydronephrosis

• Increasingly detected prenatally by routine fetal ultrasonography
• A dilated collecting system may be the only indication of underlying congenital uropathy
• Two main mechanisms: VUR (reverse flow) or obstruction (impaired forward flow)
• Most common obstructive site: ureteropelvic junction
• Most cases are transient and resolve spontaneously
• Management protocol for confirmed fetal hydronephrosis:
  1. Postnatal renal ultrasound in first week of life
  2. If normal, repeat at 4 weeks and 1–2 years
  3. Urinalysis, urine culture, basic metabolic panel (if bilateral), VCUG (voiding cystourethrogram)
  4. If VCUG is normal → furosemide renography to assess obstruction degree

Ureteral Congenital Anomalies (Context)

• Ureteral duplication
• Vesicoureteral reflux
• Double and bifid ureters
• Ureteropelvic junction obstruction (most clinically significant)
• Ureteral diverticula

• Robbins & Kumar Basic Pathology — pp. 529–531
• Robbins, Cotran & Kumar Pathologic Basis of Disease — Congenital Anomalies, Ureters
• Comprehensive Clinical Nephrology, 7th Ed. — p. 81
• Textbook of Family Medicine, 9th Ed. — p. 1189
• Grainger & Allison's Diagnostic Radiology

Hydronephrosis & Ureterohydronephrosis

Congenital Anomalies of the Kidney and Urinary Tract

1. Definitions

• Hydronephrosis — dilation of the renal pelvis and calyces with accompanying progressive atrophy of the kidney parenchyma, caused by obstruction to the outflow of urine
• Ureterohydronephrosis (Hydroureteronephrosis) — combined dilation of the ureter (hydroureter) and the renal pelvis/calyces; results when obstruction lies at the ureterovesical junction (UVJ) or below, rather than at the UPJ

2. Causes of Obstruction

Fig. 20.43 — Obstructive lesions of the urinary tract by anatomical level. (Robbins, Cotran & Kumar Pathologic Basis of Disease)

Congenital Causes (most relevant here)

Acquired Causes (for comparison)

• Calculi, sloughed necrotic papillae
• Benign prostatic hyperplasia, carcinoma of the prostate
• Bladder tumors, retroperitoneal lymphoma, cervical/uterine carcinoma
• Retroperitoneal fibrosis
• Neurogenic bladder (spinal cord damage)
• Pregnancy (mild, physiologic)

3. UPJ Obstruction — The Dominant Congenital Form

Figure 37-9 — Left: UPJ obstruction producing hydronephrosis only (ureter not dilated). Right: UVJ obstruction producing ureterohydronephrosis (hydroureter + hydronephrosis + blunted calyces). (Smith and Tanagho's General Urology, 19th Ed.)

• Incidence ~1 in 500 live births on routine antenatal ultrasound
• Preferentially affects males when presenting early in life
• Bilateral in ~20% of early-presenting cases
• Contralateral renal agenesis in a minority
• In adults, more common in females, usually unilateral

• Abnormal organization of smooth muscle bundles at the UPJ
• Excess stromal collagen deposition between smooth muscle bundles → impaired peristalsis
• Thin-walled or hypoplastic proximal ureter segment
• Kinking/angulation at the junction of dilated pelvis and ureter
• Extrinsic compression by a lower pole renal artery (particularly in horseshoe kidney)
• Decreased BMP4 signaling → disruption of smooth muscle investment of the ureter
• Overexpression of extracellular matrix + depleted nerve supply

4. Pathogenesis of Renal Damage

1. Compresses renal vasculature → arterial insufficiency + venous stasis; medullary inner blood flow is diminished
2. Maximum pressure at the papillae → papillary necrosis is the earliest structural lesion
3. Initial tubular dysfunction → impaired urinary concentrating ability (earliest functional sign)
4. Later: GFR begins to fall
5. Interstitial inflammatory reaction → progressive interstitial fibrosis

5. Morphology

Gross Pathology

FIG. 12.25 — Hydronephrosis: marked dilation of the renal pelvis and calyces, thinning of renal parenchyma. (Robbins & Kumar Basic Pathology)

• In ureterohydronephrosis: one or both ureters are also dilated (hydroureter), depending on the level of obstruction
• Bilateral hydronephrosis: obstruction must be below the level of the ureters (e.g., bladder neck, urethra, PUV)
• Unilateral: obstruction at or above the ureter

Microscopy

6. Clinical Features

Symptoms by Pattern of Obstruction

Prenatal / Fetal Hydronephrosis

• Increasingly diagnosed on routine fetal ultrasonography (dilated collecting system may be the only sign)
• Two mechanisms: VUR (reverse flow) or obstruction (impaired forward flow)
• Most common site: ureteropelvic junction
• Majority are transient and resolve spontaneously
• UPJ cases tend to be intrinsic; many resolve without surgery
• Massive hydronephrosis, diminished renal function, infection, or stones → early surgical intervention recommended

7. Diagnosis

1. Renal ultrasound in first week of life
2. If normal, repeat at 4 weeks and 1–2 years
3. Urinalysis, urine culture, basic metabolic panel (if bilateral), VCUG
4. If VCUG normal → furosemide renography to quantify obstruction

8. Treatment

• Relief of obstruction — within the first weeks, allows full recovery of renal function
• Longstanding obstruction → irreversible changes; prognosis for recovery is poor
• UPJ obstruction (symptomatic) → surgical repair (pyeloplasty):
  • Dismembered pyeloureteroplasty (Anderson-Hynes) — most popular; mandatory when obstruction is caused by a dyskinetic proximal ureteral segment or when ureter hooks over a lower pole vessel
  • Foley Y-V plasty — used when there is a dilated extrarenal pelvis
  • Culp-De Weerd pelvic flap — suited when UPJ remains dependent despite pelvic dilation; preserves ureteral blood supply
• Asymptomatic / mild congenital hydronephrosis: nonoperative surveillance is safe; ~25% ultimately require surgery for pain, UTI, or reduced function on nuclear scan
• Post-obstruction diuresis after relief of complete bilateral obstruction — may be massive; requires careful fluid and electrolyte management

• Robbins & Kumar Basic Pathology — pp. 529–531
• Robbins, Cotran & Kumar Pathologic Basis of Disease — pp. 876–878
• Smith and Tanagho's General Urology, 19th Ed. — pp. 589–591
• Textbook of Family Medicine, 9th Ed. — p. 1189
• Grainger & Allison's Diagnostic Radiology
• Comprehensive Clinical Nephrology, 7th Ed. — p. 81