What is hydronephrosis?

What is Hydronephrosis

Hydronephrosis is unilateral or bilateral urine-filled aseptic dilation of the renal pelvis and calyces, generally occurring as a response to urinary obstruction or reflux. 1

Definition and Pathophysiology

• Hydronephrosis specifically describes the structural dilation and swelling of the kidney's collecting system when urine cannot drain normally from the kidney to the bladder 1, 2
• This condition represents a critical threat to kidney function because progressive dilation of the upper urinary tract can lead to acute kidney injury and, if not corrected, permanent nephron loss 1
• The dilation is typically aseptic (without infection initially), though infection can develop as a complication of the obstruction 1

Clinical Significance and Detection

• Hydronephrosis is increasingly diagnosed due to widespread availability of CT and ultrasound imaging 3
• It may be identified incidentally through imaging requested for unrelated intra-abdominal symptoms, or in symptomatic patients presenting with flank or groin pain, infection, nausea, or urinary urgency 1
• Upper urinary tract deterioration in adults is often clinically silent and detected incidentally with nonspecific symptoms 1, 4
• An elevated serum creatinine may exist with obstructive hydronephrosis, although it can remain normal in unilateral cases due to contralateral kidney compensation 1

Classification Systems

Hydronephrosis can be classified by several parameters 1:

• Laterality: Unilateral versus bilateral involvement
• Cause: Obstructive versus non-obstructive (reflux)
• Level of obstruction: Supravesical, bladder, or urethral
• Completeness: Complete versus partial obstruction
• Location: Intrinsic versus extrinsic obstruction

Common Etiologies

The causes of hydronephrosis are diverse and include 1:

• Urolithiasis (kidney or ureteral stones) - the most common cause in adults 5
• Malignant obstruction from pelvic or retroperitoneal tumors
• Strictures of the ureter or ureteropelvic junction
• Bladder outlet obstruction from prostatic hyperplasia or urethral stricture
• Mechanical compression from enlarged uterus, pelvic organ prolapse, or endometriosis
• Retroperitoneal fibrosis
• Congenital abnormalities including ureteropelvic junction obstruction and posterior urethral valves
• Drug effects from cyclophosphamide or ketamine
• Upper urinary tract infection, traumatic or ischemic injury, and post-radiation changes

Special Population: Pregnancy

• Asymptomatic hydronephrosis occurs in an estimated 70% to 90% of pregnant patients, typically more prominent on the right side 1
• This physiologic hydronephrosis results from mechanical obstruction by the enlarged uterus combined with collecting system smooth muscle relaxation from progesterone 1
• Approximately 0.2% to 4.7% of pregnant patients experience symptomatic hydronephrosis, with higher prevalence in advancing trimesters and multiparous patients 1
• When left untreated, symptomatic hydronephrosis may lead to preterm labor or maternal and/or fetal death 1

Critical Clinical Pitfall

Bilateral hydronephrosis requires urgent evaluation regardless of normal kidney function and urinalysis, because both kidneys are at simultaneous risk without contralateral functional reserve, and progressive obstruction can cause irreversible nephron loss even when initially asymptomatic 4. Serum creatinine can remain deceptively normal in unilateral obstruction due to compensatory function from the contralateral kidney, but this protective mechanism is absent when both kidneys are affected 4.