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Hydronephrosis is a condition in which specific parts of the kidney(s) become stretched, swollen, and widened with urine. The kidneys are two organs located on each side of the spine, behind the stomach. The kidneys filter (remove) wastes from the blood.


The parts of the kidney(s) that are widened in hydronephrosis are the renal calyces and renal pelvis. The word "renal" refers to the kidneys. The renal calyces are narrow tubes that carry urine from the kidneys to the renal pelvis (both of which are shown below). The renal pelvis is a funnel-shaped opening that drains urine from the renal calyces to the ureters. The ureters are two thin, tube shaped structures that connect to the kidneys and carry urine to the bladder. One ureter is attached to each kidney. The bladder is a stretchable structure in the body that holds urine.
Urine travels from the kidneys to the renal calyces, then to the renal pelvis, then to the ureters, then to the bladder, and then to the urethra where it is drained outside of the body. The urethra is a tube shaped structure in the body that drains urine from the bladder. The kidneys, ureters, bladder, and urethra make up the urinary tract, which is shown to the right. The widening in hydronepohrosis is most common at the point where the renal pelvis connects with the ureters.


Hydronephrosis is not a disease. Rather, hydronephrosis is a physical consequence of a disease (see below) that causes the urine to have difficulty draining out of the kidneys, ureters, and bladder.


Unilateral hydronephrosis is hydronephrosis that affect one kidney. Bilateral hydronephrosis is hydronephrosis that affects both kidneys.


Signs and symptoms of hydronephrosis can vary, depending on whether the condition occurs suddenly or develops slowly over a period of time. If hydronephrosis occurs suddenly, the person may experience severe back pain.If hydronephrosis develops slowly over a period of time, it may not cause any signs or symptoms until the ureter is completely blocked and kidney failure results. If the kidney becomes infected, it can become filled with pus and lead to a fever. Pus is a yellow or green creamy substance sometimes found at the site of infections.
General signs and symptoms of hydronephrosis include pain in the sides, a mass in the belly, very high fever, blood in the urine, nausea, vomiting, and a high number of white blood cells in the urine. White blood cells help protect the body against diseases and fight infections. In the newborn, an infection of the urinary tract is a sign that some type of blockage in the kidney is present that may lead to hydronephrosis. In older children, repeated infections of the urinary tract may reflect a blockage of the kidney.

Feeling a frequent urge to pee and peeing very often are other signs and symptoms of hydronephrosis. Painful and burning sensation while peeing is another symptom of hydronephrosis that is usually caused by an infection of the urinary tract. It should be noted that in many people, there are no signs or symptoms of a blockage in the urinary tract. This is especially true for children with mild hydronephrosis.


Neonatal hydropnephrosis (also known as congenital hydronephrosis) is hydronephrosis that occurs in newborns. Antenatal hydronephrosis is hydronephrosis that is detected before the birth of the child. There is no evidence that congenital hydronephrosis is due to anything that the parents did wrong during pregnancy.


In hydronephrosis, the kidneys become widened with urine (pee) that cannot pass by a blockage or narrowing. The blockage or narrowing is usually in the ureter or urethra, but can also be in the bladder. Blockages can occur at the uretovesical junction, which is where the ureter meets the bladder. Since the urine has difficulty getting past the blockage, it backs up into the kidney and becomes trapped. This causes areas inside the kidney to stretch. The blockages can occur suddenly or slowly over time. In infants with this condition, the areas inside of the kidney may appear very wide because the infant's tissues are stretchier than adults

There are many possible reasons why the structures mentioned above can become blocked or narrowed. One reason is the formation of stones in the kidney (known as kidney stones). Another reason is the formation of scar tissue in the ureter or urethra. A blockage where the bladder connects to the urethra can lead to hydronephrosis. This type of blockage is known as bladder outlet obstruction.

A tumor in the kidney or other parts of the urinary tract can cause blockage of the ureter(s). Tumors are abnormal masses of tissue that form when cells in a certain area of the body reproduce at an increased rate. Tumors outside or in back of the peritoneum can lead to hydronephrosis. The peritoneum is a layer of tissue that lines the belly and covers most of the organs in the belly.

As was mentioned earlier, the widening in hydronepohrosis is most common at the point where the renal pelvis connects with the ureters (known as the uretopelvic junction). A blockage in the area where the renal pelvis connects with the ureters can cause widening in this area to occur. A blockage in this area is known as a uretopelvic junction obstruction. A uretopelvic junction obstruction usually develops in the first four months that a mother is pregnant, in children born with this type of blockage.

The presence of abnormal folds in a specific part of the urethra can lead to hydronephrosis. This condition is known as posterior urethral valves. Also, fibrosis of structures behind the peritoneum and of connective tissue that often makes up the ureters can lead to hydronephrosis because it can cause the ureters to be blocked. Fibrosis is an overgrowth of scar tissue or connective tissue. Connective tissue is any tissue that surrounds and supports specialized structures in the body.

Displacement of the kidneys from its normal position can lead to hydronephrosis as can pressure from adhesions outside of the urinary tract. Adhesions are fibrous types of tissue that join body parts that are normally unconnected. Fibrous tissue is a type of connective tissue.

Sometimes, the kidney may contain many cysts, which can lead to hydronephrosis. A cyst is an abnormal lump, swelling, or sac that contains fluid, a part solid material, or a gas, and is covered with a membrane. A blood clot can also lead to a blockage of the ureters and cause hydronephrosis. A blood clot is a collection of a mass of blood.

Pressure from abnormal arteries in the kidneys can lead to hydronephrosis. An artery is a blood vessel that carries blood away from the heart. Aneurysms of the arteries of the kidney are one type of abnormality that can lead to hydrnephrosis. An aneursym is a weakening of the wall of a blood vessel, causing it to expand like a balloon, sometimes leading it to burst.

Neurogenic bladder can lead to hydronephrosis. Neurogenic bladder is dysfunction of the bladder that is caused by damage in the nervous system (the system of nerves in the body). Neurogenic bladder is characterized by retaining or emptying too much urine.

Another cause of hydronephrosis is an enlarged prostate gland. The prostate is a organ near the bladder that produces a reproductive fluid.  If the prostate is enlarged, the fluid that it produces can be blocked, which would cause the urine in the bladder to be under pressure.

Cancer of the prostate gland can cause blockage that leads to hydronephrosis. Cancer is an abnormal growth of new tissue characterized by uncontrolled growth of abnormally structured cells that have a more primitive form. One type of cancer in the prostate that can lead to hydronephrosis is a carcinoma.

A carcinoma is any malignant cancer that arises from cells in the covering surface layer or membrane (outer covering) that lines an organ of the body. When describing cancer, malignant means that the newly formed tissues are made of abnormally structured and primitive-looking cells that grow uncontrollably, spread throughout the body, and invade surrounding tissues.

Vesicouretal reflux is another possible cause of hydronephrosis. Vesicouretal reflux is a backward flow of urine from the bladder into the ureter, when the bladder fills or empties. The urine can also flow black into the renal pelvis. This condition is caused when the valve between the ureter and bladder does not function. A valve is a natural structure or man-made device in a passageway, tube, vessel, or hollow organ that allows fluid or partly fluid contents to travel in one direction, but closes to prevent the flow of those contents in the opposite direction.

About 75% or people with vesicouretal reflux outgrow this condition during childhood. However, many children will need to be treated with antibiotics to prevent kidney damage before outgrowing the condition.

Edema that is caused by an infection of the urinary tract can lead to hydronephrosis. Edema is an abnormal buildup of too much watery fluid in the cells, tissues, and or openings inside the body. Pregnancy can cause changes in the urinary tract that lead urine to be backed up, which can cause hydronephrosis.

A condition known as prune-belly syndrome is associated with hydronephrosis. In prune belly syndrome, certain muscles in the belly are missing, the kidneys are small and abnormally developed, the testicles do not drop down, and the ureters and bladder are usually greatly enlarged. The condition is called prune-belly syndrome because the belly sticks out and is wrinkly like a prune.

There are several disorders of the muscles, brain, and/or spinal cord that can lead to hydronephrosis, such as paraplegia, tabes dorsalis, spina bifida, and multiple sclerosis. Paraplegia is a loss of the ability to move and/or feel both legs and generally, the lower trunk (stomach area and lower back). Tabes dorsalis is damage to the spinal cord due to infection from the sexually transmitted disease, syphilis.

Spina bifida is a birth defect in which part of the bones that surround the spinal cord do not form properly, leaving part of the spinal cord exposed. Multiple sclerosis (MS) is a condition in which multiple areas of abnormal patches (known as plaques) develop in the brain and/or spinal cord (depending on the stage of the illness).

Sometimes, in adults and children, the ureters can become narrowed at the area of the renal pelvis with no apparent cause. In some children with hydronephrosis, there is no blockage that is apparent. Some of these cases may be due to abnormal smooth muscle in the renal pelvis or ureter, which causes these areas to widen.

A rare cause of hydronephrosis can occur when there are two ureters collecting urine from a single kidney. In 99% of people, there is one ureter that collects urine from each kidney. In the 1% of people that have 2 ureters for one kidney, a uterocele may be present which causes hydronephrosis. A uterocele is a balloon-like blockage at the end of one of the ureters.


It is estimated that one in 100 people have hydronephrosis affecting one kidney. Less people have hydronephrosis affecting both kidneys. In approximately 2% of all ultrasounds performed before the child is born, hydronephrosis is detected. The cases detected in this manner are usually males.


Hydronephrosis is diagnosed with an ultrasound of the kidneys or belly, especially in unborn children. An ultrasound is a procedure that uses types of sound waves to produce images of the body. The ultrasound used to diagnose hydronephrosis will produce a picture of the kidneys and the ureters. An ultrasound can detect hydronephrosis in an unborn child as early as the first three months that the mother is pregnant. It will be able to show blockages that are causing hydronephrosis as well as many of the other causes described above, such as uretopelvic junction obstruction, posterior urethral valves, and prune belly syndrome.

Another technique used to diagnose hydronephrosis is an intravenous pyelogram. An intravenous pyelogram (commonly abbreviated as IVP) is a technique in which x-rays are used to take pictures of the urinary tract, after a liquid substance called contrast is injected into a vein (a blood vessel that caries blood to the heart). X-rays are a type of radiation, which is a form of energy. To better distinguish between organs, a liquid substance called contrast is used that x-rays cannot pass through. Contrast helps to form an artificial distinction between organs in the body so that the doctor can tell them apart.

Other techniques that can be used to diagnose hydronephrosis are retrograde pyelography and cystoscopy. A retrograde pyelography is another visualization technique that uses x-rays to examine structures in the kidneys after a contrast material has been injected into the ureters and renal calyces. A cystoscopy is a direct examination of the urinary tract by inserting a viewing tube into the ureter.

A renal (kidney) scan is also used to diagnose hydronephrosis in some cases. A renal scan (also known as a flow and function scan) is a technique in which detailed pictures are produced after a radioactive substance is injected into the body, which provides information about kidney functioning. A radioactive substance is a substance that sends out radiation (a type of energy). The kidney filters this radioactive substance, which is detected by the scanning procedure.

The renal scan will be able to detect differences in functioning between the right and left kidney. It can estimate the size of a blockage that is present and how well the urine is draining out of the kidney. A small tube is inserted in the bladder to keep it empty. Renal scans are usually not performed until one month after birth for the most accurate results.

A CT scan of the kidneys or belly can be used to diagnose hydronephrosis. CT (computerized tomography) scanning is an advanced imaging technique that uses x-rays and computer technology to produce more clear and detailed pictures than a traditional x-ray. An MRI (Magnetic Resonance Imaging) scan can also be used to diagnose hydronephrosis. MRI scans produce extremely detailed pictures of the inside of the body by using very powerful magnets and computer technology. MRI scans are more detailed, but more expensive than CT scans.

A cystourethogram can be used to help determine if the cause of hydronephrosis is vesicourethral reflux. A cystourethogram is an examination of the urethra and bladder after a contrast substance is injected inside the body. Remember that contrast is a liquid substance that helps to form an artificial distinction between organs in the body so that the doctor can tell them apart. The contrast is passed into the child's bladder through a small tube inserted in the child's urethra.


Usually, the delivery of the baby will not be affected in cases of infants with hydronephrosis. In unusual cases, in which the kidneys are very large, a cesarean-section (c-section) may be required. A c-section is when the belly and uterus are cut open so that the baby can be delivered through the belly. The uterus is a hollow organ in a female's body where the egg is implanted and the baby develops.

There is currently no accepted method for how all children with hydronephrosis should be treated. This is because there is presently no way to predict which cases will go away on its own and which will require surgery to remove the blockage. Thus, doctors may differ in their opinion as to whether surgery should be performed or whether the child should be monitored over time. Most doctors today, however, are choosing to observe the child over time (even in moderate to severe cases) before opting for surgery.

In most cases, no specific changes need to be made in caring for the unborn child that is diagnosed with hydronephrosis. Rather, the hydronephrosis and kidney growth of the unborn child will need to be monitored closely as it develops. Another ultrasound is usually done the third day after the child is born to monitor the situation. Repeat ultrasounds may be performed every three months. The condition will likely continue to be monitored (without surgery) if it seems to be improving and if no blockages are seen.
The cause of hydronephrosis is what is treated. Treatment will differ depending on the cause. If the ultrasound (see earlier section) reveals a blockage and the kidney is at least somewhat healthy, the blockage may be removed or decreased by surgery. After surgery, the kidney resumes normal functioning shortly.

The surgery mentioned above usually involves removing the blocked part of the ureter and reconnecting the healthy part of the ureter to the kidney. This type of surgery is known as a pyeloplasty. After the surgery, the child is in the hospital for 3 to 5 days. This type of surgery is successful in about 90 to 95% of the cases. Thus, the surgery is generally safe and effective.

In some patients, a rod-like or thread-like device known as a stent is placed in the ureter. The stent will widen the ureter and help urine get through any blockages that are there. In other patients with a blockage in the ureter, a nephrostomy tube can be used. A nephrostomy tube is a tube that is placed in the kidney, which bypasses (goes around) the ureter, and drains it through the back.

However, some doctors prefer to observe the child for a given period of time and hold off on surgery, since the condition may go away by itself without the risks of surgery. Doctors are more likely to hold off on surgery in cases where it does not appear kidney functioning is declining and that the kidneys are growing normally

Antibiotics are used to prevent and treat urinary tract infections. If the patient is known to have recurring infections of the urinary tract, he/she may be given antibiotics to prevent further infections. Patients with vesicoureteral reflux (see causes section) are often treated with antibiotics, follow-up ultrasounds, and follow-up cystourethograms (see diagnosis section).

If the kidney is severely damaged, it may need to be removed. Removal of the kidney is known as a nephrectomy. If a kidney needs to be removed, the other kidney takes over for the functioning of the removed kidney. Some doctors recommend removing a kidney with many cysts on it because it does not work. Other doctors do not recommend removing a kidney with many cysts on it unless its large size causes problems or if there is something present that is blocking the urine.

For some unborn children with severe hydronephrosis, some hospitals are experimenting with surgeries to decrease blockages or pressure in the unborn child when it is still in the uterus. The uterus is a hollow organ in a female's body where the egg is implanted and the baby develops. This type of surgery is usually only done when the kidneys are very abnormal and their lungs are usually not well developed.


The prognosis for people with hydronephrosis generally depends on how severe the condition is. Mild cases generally have a better prognosis than moderate and severe conditions. The severity of the condition is determined by how big the blockage is and how much the kidney has been stretched. In children with hydronephrosis, the condition may disappear by the first year of life. In fact, in about half of the cases in which hydronephrosis is detected with an ultrasound before the child is born, the condition goes away by itself by the time the child is born.

Overall, about 93% of hydronephrosis cases go away without treatment. The cases that persist are usually those in which the case is more severe. In these cases, the condition usually worsens over time, kidney function decreases, and surgery is required. Kidney functioning is usually not affected after surgery.

In children with moderate hydronephrosis, kidney functioning usually does not decrease, growth of the kidneys remains normal, and the condition usually does not worsen. The kidney usually compensates for the hydronephrosis in moderate conditions, and often finds a way to function normally. Some children with moderate hydronephrosis may even get better on their own. In very severe cases of hydronephrosis, kidney damage may occur.

If a blockage is detected in the unborn child and surgery is used to correct it shortly after birth, kidney function often improves. Early diagnosis of a blockage that leads to hydronephrosis improves outcome, because treatment can be implemented earlier. If hydronephrosis is not treated, it can lead to severe kidney damage and the kidney can wear away.

If both kidneys are affected, kidney failure can result. Kidney failure rarely occurs when one kidney is affected because the other kidney can usually compensate for the bad one. If the patient only has one kidney to begin with, however, kidney failure will occur if the condition goes untreated.

In cases of severe kidney abnormalities, the kidneys usually will not function properly regardless of treatment. In severe cases where an experimental surgery is done while the unborn child is still inside the mother, the outcome of these children has not yet been improved to date. Follow-up studies in the future should lead to a better understanding of the prognosis of unborn children that undergo such a surgery.


Hydronephrosis is also known as pelvocaliectasis and pyeloureterectasis.


Hydronephrosis comes from the Greek word "hydor" meaning "water," the Greek word "nephros" meaning "kidney," and the Greek word "osis" meaning "condition." Put the words together and you get "water kidney condition."