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- Hydronephrosis refers to a structural finding—dilatation of the renal calyces and pelvis.
- May occur with urinary tract infection (UTI), vesicoureteric reflux (VUR), high urine output, or physiologic changes in pregnancy
- Sometimes accompanied with hydroureter
- Presentation varies from incidental finding to discovery during workup for UTI or for flank or abdominal pain.
- Hydronephrosis should not be used interchangeably with obstructive uropathy, which refers to the damage to renal parenchyma resulting from urinary tract obstruction (UTO).
- UTO is more common in men than women and in children than adults (congenital anomalies).
- Acute unilateral obstruction is more common than bilateral.
Etiology and Pathophysiology
- Hydronephrosis develops with increased pressure in the urinary collecting system.
- Increased pressure within the renal collecting system 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.
- UTO: may be acute/chronic, partial/complete, 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.
- UTO in transplanted kidneys is more common than native kidneys, specifically in immediate posttransplant period. It is due to ureteral strictures, lymphoceles (ureteral compression and bladder dysfunction).
- VUR resulting in varying degrees of hydroureteronephrosis
- Physiologic hydronephrosis of pregnancy
- 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
- 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.
- 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.