Hydronephrosis is a medical condition where one or both kidneys become swollen due to a buildup of urine. This swelling happens when urine can’t drain properly from the kidney to the bladder, causing it to back up into the kidney.

To understand this, it helps to first understand the normal anatomy of the urinary tract. It is a one-way drainage system:

• The kidneys filter blood and produce urine continuously.
• Urine collects in a central, funnel-shaped part of the kidney called the renal pelvis.
• From the renal pelvis, the urine flows down a long, thin tube called the ureter.
• Each kidney has a ureter that carries urine to the bladder, a muscular sac that stores it.
• When the bladder is full, urine is expelled from the body through a final tube called the urethra.

Hydronephrosis is what happens when a blockage or obstruction occurs anywhere along this pathway. The obstruction prevents the normal outflow of urine, causing it to back up. This fluid backup increases pressure inside the ureter and the renal pelvis, causing them to stretch and swell.

It is helpful to think of the urinary system as the plumbing of a house. The kidneys are two sinks that are constantly producing water (urine). The ureters are the thin drain pipes from each sink, leading to a holding tank (the bladder). If a clog develops in one of the pipes or in the main drain, the water has nowhere to go. It backs up, filling the pipe and eventually the sink bowl itself, causing it to swell under pressure. This swelling of the “sink” (the renal pelvis) is hydronephrosis.

This condition can be:

• Unilateral: Affecting only one kidney. This is more common.
• Bilateral: Affecting both kidneys simultaneously. This is more serious as it can affect total kidney function.
• Acute: Occurring suddenly, often with severe pain.
• Chronic: Developing slowly over weeks, months, or even years, sometimes with no symptoms at all.

In my experience, hydronephrosis is often picked up incidentally on imaging, especially in patients with kidney stones, prostate enlargement, or pregnancy.

Hydronephrosis is caused by impaired urine drainage. This impairment is typically the result of one of two main problems: an obstruction in the urinary tract or vesicoureteral reflux.

• Obstruction: This is the most common cause. There is a physical blockage somewhere downstream of the kidney preventing normal flow of urine. The blockage can be partial or complete, and it can occur anywhere from the renal pelvis down to the urethra.
• Vesicoureteral Reflux (VUR): This is a condition, most common in infants and young children, where the one-way valve mechanism at the junction of the ureter and the bladder is faulty. This allows urine to flow backward from the bladder up the ureter to the kidney.

Patients often ask if hydronephrosis is permanent, I explain that it’s reversible in many cases if the underlying cause is promptly addressed.

Hydronephrosis can develop suddenly (acute) or progress slowly over time (chronic), depending on the cause. The specific causes often differ depending on the person’s age.

In Fetuses and Newborns (Congenital Causes)

Hydronephrosis is one of the most common abnormalities detected on prenatal ultrasounds. Congenital causes include:

• Ureteropelvic Junction (UPJ) Obstruction: This is the most common cause of hydronephrosis in infants. It is a blockage at the point where the kidney’s renal pelvis joins the ureter.
• Vesicoureteral Reflux (VUR): As described above, the faulty valve between the ureter and bladder is a common cause in this age group.
• Posterior Urethral Valves (PUV): This is a condition that occurs only in boys, where a flap of tissue in the urethra blocks the outflow of urine from the bladder, causing a backup that affects both kidneys.
• Ureterocele: A ballooning of the end of the ureter inside the bladder, which can cause an obstruction.

In Adults

In adults, hydronephrosis is typically an acquired condition. Common causes include:

• Kidney Stones: This is a very frequent cause of acute, unilateral hydronephrosis. A stone that has passed out of the kidney can become lodged in the narrow ureter, creating a sudden and often very painful blockage.
• Benign Prostatic Hyperplasia (BPH): In older men, an enlarged prostate gland is a leading cause of chronic, bilateral hydronephrosis. The enlarged prostate squeezes the urethra, obstructing the outflow of urine from the bladder and causing pressure to back up to both kidneys.
• Tumors and Cancers: A tumor can cause an obstruction by growing inside the urinary tract or by pressing on it from the outside. This can include cancers of the kidney, ureter, bladder, prostate, cervix, uterus, or colon.
• Ureteral Strictures: This is a narrowing of the ureter caused by scar tissue. Scarring can result from previous surgeries, radiation therapy, the passage of a kidney stone, or chronic inflammation.
• Pregnancy: Mild, temporary hydronephrosis is very common during pregnancy. The growing uterus can compress the ureters, and pregnancy hormones can cause a slight decrease in the ureters’ muscular contractions. This is considered physiological and usually resolves after delivery.
• Neurogenic Bladder: Nerve damage from conditions like a spinal cord injury, multiple sclerosis, or diabetes can prevent the bladder from emptying properly, leading to a backup of urine.

Clinically, I often find that adult cases are secondary to stones or BPH, while pediatric cases are more commonly due to congenital abnormalities.

Symptoms vary based on how quickly the condition develops, how severe the obstruction is, and whether one or both kidneys are affected.

When acute hydronephrosis develops suddenly, the symptoms are usually dramatic:

• Severe Pain: Often described as a sharp, intense pain in the flank (the side between the ribs and the hip) or the back. The pain may come in waves and can radiate down to the groin.
• Nausea and Vomiting: Often accompanies severe pain.
• Painful Urination (Dysuria).
• Blood in the urine (Hematuria).
• Fever and Chills: This is a sign that a urinary tract infection (UTI) has developed behind the blockage, which is a medical emergency.

When chronic hydronephrosis does cause symptoms, they are often much more vague:

• A dull ache in the flank.
• Symptoms related to the underlying cause, such as a weak urine stream or difficulty urinating in a man with an enlarged prostate.
• Symptoms of a UTI, like a frequent urge to urinate.
• In advanced, untreated bilateral hydronephrosis, symptoms of kidney failure like fatigue and swelling may develop.

I’ve seen that many patients only become symptomatic when the kidney starts swelling significantly or if infection sets in, it’s a silent problem until it isn’t.

The first step in diagnosis is usually an imaging test to confirm the presence of a swollen kidney. Subsequent investigation is focused on finding the exact location and cause of the problem.

• Ultrasound: This is the primary diagnostic tool. It is a non-invasive, safe, and readily available test that uses sound waves to create images of the kidneys. An ultrasound can easily show the dilation of the renal pelvis, confirming the presence of hydronephrosis.
• Urinalysis and Blood Tests: A urine sample will be analyzed to check for signs of infection or blood. Blood tests are done to assess kidney function by measuring levels of creatinine and blood urea nitrogen (BUN).
• Computed Tomography (CT) Scan: A CT scan of the abdomen and pelvis provides highly detailed images of the entire urinary tract. It is excellent for identifying the precise location and cause of an obstruction.
• Voiding Cystourethrogram (VCUG): This test is often used in children to diagnose vesicoureteral reflux. A catheter is used to fill the bladder with a special contrast dye, and X-ray images are taken as the bladder fills and as the child urinates.
• Nuclear Medicine Scan (MAG3 or DMSA Scan): These specialized scans involve injecting a small amount of a radioactive tracer into the bloodstream. A gamma camera then tracks how the tracer is taken up and excreted by the kidneys. These tests are very effective at assessing the function of each individual kidney and determining the severity of a blockage.

In my clinical experience, a simple ultrasound can often give us enough information to start managing hydronephrosis quickly and safely.

Treatment focuses on relieving the obstruction and preserving kidney function. The approach depends on the cause, severity, and whether the patient is symptomatic.

1. Draining the Kidney (Urgent Intervention)

If the blockage is severe, causing significant pain, an infection, or a decline in kidney function, the first and most urgent step is to drain the backed-up urine. This can be done in two main ways:

• Ureteral Stent: A urologist passes a thin, flexible plastic tube called a stent through the bladder and up into the ureter. The stent runs past the point of obstruction, allowing urine to drain from the kidney down to the bladder internally.
• Percutaneous Nephrostomy Tube: In this procedure, a doctor uses ultrasound or X-ray guidance to insert a tube directly through the skin of the back into the kidney’s collecting system. This tube externally drains urine into a collection bag.

These procedures relieve the immediate pressure and allow the kidney to recover.

2. Treating the Underlying Cause

Once the patient is stable, treatment focuses on definitively fixing the root problem.

• Kidney Stones may be broken up with shock waves (lithotripsy) or removed with endoscopic surgery.
• An Enlarged Prostate (BPH) can be treated with medications or with a surgical procedure to remove the obstructing prostate tissue.
• A Ureteropelvic Junction (UPJ) Obstruction is corrected with a surgical procedure called a pyeloplasty, where the narrowed, blocked segment is removed, and the ureter is reattached to the kidney.
• Tumors or Cancers will require specific treatment depending on their type and stage, which may include surgery, chemotherapy, or radiation.

In cases of mild hydronephrosis, such as that seen during a normal pregnancy or some mild congenital forms in infants, the doctor may recommend a course of observation with regular follow-up ultrasounds to ensure the condition does not worsen and resolves on its own. I always emphasize early intervention, once kidney tissue is damaged by long-standing pressure, it may not recover even after fixing the blockage.

Hydronephrosis, or a swollen kidney, is a critical sign that there is a problem with the urinary tract’s plumbing system. It is a warning that urine is not draining properly, leading to a backup of pressure that can damage the kidney over time. While the condition can be “silent” and discovered by accident, it can also cause sudden, severe pain. The causes are vast, ranging from kidney stones and an enlarged prostate in adults to congenital blockages in infants. The key to a successful outcome is a prompt and accurate diagnosis to identify the underlying cause. Patients are usually relieved to know that hydronephrosis isn’t always permanent, but it is urgent. Timely care leads to excellent outcomes in most cases.

National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). (2018). Hydronephrosis. Retrieved from https://www.niddk.nih.gov/health-information/urologic-diseases/hydronephrosis

Mayo Clinic. (2023). Hydronephrosis. Retrieved from https://www.mayoclinic.org/diseases-conditions/hydronephrosis/symptoms-causes/syc-20355555

Urology Care Foundation. (n.d.). What is hydronephrosis? Retrieved from https://www.urologyhealth.org/urology-a-z/h/hydronephrosis