Kidney Hydronephrosis: What It Is, Causes, and Why It Is Dangerous

Hydronephrosis is dilation of the renal pelvis and calyces due to impaired urine outflow. It is not an independent disease but a consequence of another process, so the term in an ultrasound report should be followed by a search for the cause.

What kidney hydronephrosis is

Hydronephrosis (synonym — dropsy of the kidney) is defined as dilation of the renal pelvis and calyx due to impaired urine outflow at the ureteropelvic junction; the process can lead to gradual atrophy of the renal parenchyma. Urine accumulates in the pelvis, and the kidney swells and enlarges.

The frequency depends on age and ranges from 0.6–4.5% of the population. Congenital and acquired, aseptic and infected hydronephrosis are distinguished; antenatal hydronephrosis — detected in the fetus — is classified separately.

Calyces and pelvis in hydronephrosis

The calyces are cavities into which urine flows from the parenchyma and from which it passes into the pelvis. An ultrasound report sometimes contains the entry "hydrocalycosis"; dilation of the calyces is part of the picture of hydronephrosis: at stage III, the pelvis and calyces are severely dilated, and the kidney is a multicystic cavity.

Along with pyelectasis, congenital hydronephrosis is often detected on fetal ultrasound; the degree is determined by the size of the pelvis on ultrasound.

Why hydronephrosis develops

The underlying cause is an obstruction to urine outflow from the kidneys to the bladder, reflux of urine from the bladder into the ureters, or obstruction of the urethra. The causes are varied: kidney stones, pregnancy, an enlarged prostate, urinary tract infections, trauma or surgery, a blood clot, a tumor, scars, congenital anomalies of the urinary system, reflux, and urethral stricture.

Congenital hydronephrosis is associated with stenosis of the ureteropelvic junction, a valve of the ureteral mucosa, and compression by an accessory vessel. Acquired hydronephrosis is associated with urolithiasis, tumors, and damage to the urinary tract. In the fetus, dilation is explained by increased urine production in late pregnancy; it usually resolves on its own.

How hydronephrosis manifests

Symptoms: pain in the lower back, side, or abdomen, frequent urination, difficulty urinating, cloudy urine, blood in the urine, high temperature, a feeling of heat, chills, or shivering. Flank pain, nausea and vomiting, urinary tract infection, and painful urination are possible.

In children, the first signs are often a tumor-like mass and abdominal enlargement, dyspepsia, vomiting, restlessness, and symptoms of infection; in a bilateral process, developmental delay is possible. Some cases are asymptomatic and are detected on imaging. Antenatal hydronephrosis causes no symptoms in either the mother or the child.

Why hydronephrosis is dangerous for the kidneys

With timely treatment, long-term consequences usually do not occur. The danger is associated with advanced and severe forms: loss of kidney function, infections and pain, and subsequently renal failure requiring dialysis or kidney transplantation. With a single functioning kidney, failure develops predictably.

• Stage I — only the renal pelvis is dilated; kidney function is unchanged or mildly impaired.
• Stage II — the size of the kidney is increased by 15–20%, the pelvis is significantly dilated, its wall is thinned, and function is reduced by 20–40%.
• Stage III — the size of the kidney is increased 1.5–2 times, the calyces are severely dilated, and function is reduced by 60–80% or absent.

How hydronephrosis is detected and when to see a urologist

Examination includes blood and urine tests to rule out infection; in men, a rectal examination to assess the prostate. Imaging: ultrasound of the kidneys, abdomen, or pelvis, radiography, CT, MRI, intravenous urography, radionuclide scanning; excretory urography and renography are also used. In the fetus, dilation is usually found on screening ultrasound at 20 weeks.

Patients consult a urologist for persistent or severe flank pain, elevated temperature, and any of the described symptoms: these are checked to establish the cause. Patients with a single kidney, weakened immunity (diabetes, HIV), and after kidney transplantation receive care immediately.

Hydronephrosis by itself is not a reason to call an ambulance; 103/112 — if a high temperature with chills or fever appears, especially with a single kidney, weakened immunity, or after kidney transplantation.

This material is for reference purposes and does not replace consultation with a doctor.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer