# Obstructive Uropathy and Hydronephrosis

> Obstructive uropathy and hydronephrosis for NEET-PG Medicine: causes, post-obstructive diuresis, imaging choices and relief by stent or nephrostomy.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/obstructive-uropathy-and-hydronephrosis
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Campbell-Walsh Urology; Harrison's Principles of Internal Medicine; Brenner and Rector's The Kidney
- How to cite: "Obstructive Uropathy and Hydronephrosis", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/obstructive-uropathy-and-hydronephrosis

## Direct answer

Obstructive uropathy is kidney damage caused by an obstruction to urine flow anywhere between the calyces and the urethra, and hydronephrosis is the dilatation of the renal pelvis and calyces that results. The commonest causes are renal calculi, benign prostatic hyperplasia and malignancy. Bilateral obstruction, or obstruction of a solitary kidney, causes acute kidney injury and needs urgent decompression.

## What you must remember

- **Intrinsic causes:** calculi are the commonest, followed by strictures, papillary necrosis and urothelial tumours.
- **Extrinsic causes:** benign prostatic hyperplasia, pelvic and cervical malignancy, and retroperitoneal fibrosis compressing the ureters from outside.
- **Pelvi-ureteric junction obstruction:** the commonest congenital cause of hydronephrosis in children, often presenting in early life.
- **Post-obstructive diuresis:** large urine output after relief of bilateral or solitary obstruction, caused by osmotic diuresis and tubular dysfunction.
- **Imaging first:** ultrasound shows dilatation, and non-contrast CT detects calculi with the highest sensitivity.
- **Functional assessment:** a renogram such as MAG3 shows drainage and function, and a diuretic challenge separates true obstruction from a dilated but unobstructed system.
- **Relief options:** urethral or suprapubic catheter for bladder outlet obstruction, ureteric stent, or percutaneous nephrostomy.
- **Pregnancy:** physiological hydronephrosis is common, more often on the right side, because of progesterone and uterine compression.

## A pathway from suspected obstruction to relief

Obstructive uropathy is an emergency when it is bilateral, affects a solitary kidney, or coexists with infection. The approach below puts relief of obstruction ahead of a complete diagnosis, because function lost to obstruction does not always return.

1. **Confirm the dilatation:** Renal ultrasound shows hydronephrosis, and a dilated system in a non-pregnant adult is treated as obstruction until the cause is found.
2. **Define the level:** Non-contrast CT detects calculi with high sensitivity, while CT urography or MR urography locates strictures and tumours.
3. **Assess function:** A renogram measures drainage and function, and a diuretic challenge helps to separate true obstruction from a dilated system.
4. **Check the other kidney and the bladder:** Bilateral obstruction, a solitary kidney or a raised post-void residual makes the case more urgent, because creatinine rises quickly.
5. **Relieve the obstruction:** A catheter relieves bladder outlet obstruction, while a ureteric stent or percutaneous nephrostomy relieves upper tract obstruction.
6. **Watch for post-obstructive diuresis:** After relief of bilateral or solitary obstruction, urine output can be very large, so volume, sodium and potassium are monitored closely.

Early relief protects the nephrons, and the outcome is worse when obstruction persists for days. Infection behind an obstruction should prompt drainage without delay.

## Where the examiner tries to catch you

Obstruction questions often test whether the student recognises a normal-looking kidney on one side and a physiological cause of dilatation.

- **Trap:** attributing hydronephrosis in pregnancy to obstruction without further thought. — **Correct:** progesterone and uterine compression cause physiological dilatation, most often on the right, and dilatation alone does not prove obstruction.
- **Trap:** assuming unilateral obstruction always raises creatinine. — **Correct:** a normal contralateral kidney compensates, so creatinine may stay normal, though the obstructed kidney is still at risk of progressive loss.
- **Trap:** using a normal ultrasound to exclude a ureteric stone. — **Correct:** ultrasound misses many ureteric stones, and non-contrast CT is the preferred test in suspected colic.

## Frequently asked questions

### What is the commonest cause of bladder outlet obstruction in older men?

Benign prostatic hyperplasia is the commonest cause of bladder outlet obstruction in older men, and untreated obstruction can lead to bilateral hydronephrosis. Urinary retention with a raised post-void residual is the usual clue, and catheterisation followed by definitive treatment relieves it.

### What is post-obstructive diuresis and why does it matter?

Post-obstructive diuresis is a large urine output after relief of bilateral or solitary kidney obstruction, caused by retained urea, sodium and tubular dysfunction. It risks hypovolaemia, hypokalaemia and hyponatraemia, so urine output, electrolytes and fluid replacement are monitored closely.

### Which imaging test detects ureteric calculi most sensitively?

Non-contrast computed tomography is the most sensitive imaging test for ureteric calculi, and it does not require contrast. Ultrasound is useful for showing hydronephrosis but misses many small ureteric stones.

### Why is hydronephrosis common in pregnancy?

Hydronephrosis is common in pregnancy because progesterone relaxes ureteric smooth muscle and the enlarging uterus compresses the ureters. It is more common on the right side, and dilatation alone does not prove obstruction in a pregnant woman.

### How does retroperitoneal fibrosis cause obstruction?

Retroperitoneal fibrosis is a fibro-inflammatory process that encases the ureters and causes unilateral or bilateral obstruction. Corticosteroids treat the inflammatory form, and ureteric stenting relieves the obstruction while the disease is controlled.