Description

• Hydronephrosis refers to a structural finding—dilatation of the renal calyces and pelvis.
  • May occur with urinary tract infection (UTI), vesicoureteral reflux (VUR), high urine output, or physiologic changes in pregnancy
  • Can be accompanied with hydroureter
  • Presentation varies from incidental finding to discovery during workup for UTI or for flank or abdominal pain, or for acute kidney injury.
• Hydronephrosis should not be used interchangeably with obstructive uropathy, which refers to the damage to renal parenchyma resulting from urinary tract obstruction (UTO).

Epidemiology

• The incidence is more common in men than women and in children than adults (congenital anomalies).
• Acute unilateral hydronephrosis is more common than bilateral.

Etiology and Pathophysiology

• Hydronephrosis develops with increased pressure in the urinary collecting system.
• Increased pressure can cause calyceal fornix rupture and urinary extravasation.
• Over time, pressures return to normal, but kidney function declines from intense renal vasoconstriction.
• With concomitant urinary infection, bacteria can enter the renal vasculature, resulting in sepsis.
• Hydronephrosis may be acute/chronic, partial/complete, and uni-/bilateral.
  • Intraluminal obstruction: calculi, sloughed renal papillae, blood clot, fungal ball
  • Intrinsic abnormality of the urinary collecting system: transitional cell carcinomas, benign prostatic hypertrophy, prostate cancer, congenital ureteropelvic junction (UPJ) obstruction, ureterocele, neurogenic bladder (functional obstruction), urethral stricture or tuberculosis (TB) (can cause ureteral narrowing)
  • Extrinsic compression of the urinary collecting system: extraurinary malignancy (lymphoma, colon, cervix), aortic/iliac aneurysm, retroperitoneal fibrosis, uterine prolapse (15% affected), endometriosis, ovarian vein syndrome, IgG4-related disease
  • Transplant hydronephrosis: Consider BK virus infection.
• Hydronephrosis in transplanted kidney is more common than native kidneys, specifically in immediate posttransplant period. It is due to ureteral strictures and lymphoceles (ureteral compression and bladder dysfunction).
• VUR resulting in varying degrees of hydroureteronephrosis
• Hydronephrosis due to high urine output (e.g., diabetes insipidus, psychogenic polydipsia)
• Hydronephrosis of infection: due to bacterial toxins inhibiting smooth muscle contraction of the renal pelvis and ureter

Pediatric Considerations

• Antenatal hydronephrosis is diagnosed in 1–5% of pregnancies, usually by US, as early as the 12th to 14th week of gestation.
• Children with antenatal hydronephrosis are at greater risk of postnatal pathology.
• Postnatal evaluation begins with US exam; further studies, such as voiding cystourethrogram (VCUG), based on the severity of postnatal hydronephrosis
• In neonates, it is the most common cause of abdominal mass.
• Common etiologies in children are VUR, congenital UPJ obstruction, neurogenic bladder, and posterior urethral valves.
• Pediatric diagnostic algorithm differs from adult due to different differential diagnosis necessitating age-appropriate testing.

Pregnancy Considerations

• Physiologic hydronephrosis in pregnancy is more prominent on the right than left and can be seen in up to 80% of pregnant women.
• Dilatation is caused by hormonal effects, external compression from expanding uterus, and intrinsic changes in the ureteral wall.
• Despite high incidence, most cases are asymptomatic.
• If symptomatic and refractory to medical management, ureteric calculus should be considered and urinary infection must be excluded.