The kidneys of people with nephrotic syndrome don't work properly, causing large amounts of protein to leak into their urine.
It can affect people of any age, but is often first seen in children aged between two and five years.
The loss of protein can cause a range of problems, including swelling of body tissues and a greater chance of catching infections.
Symptoms can usually be controlled by medication, with steroids often used.
Children with nephrotic syndrome have times when their symptoms are under control (remission), followed by times when symptoms return (relapses). Most will relapse less frequently as they get older, eventually "growing out of it" by their late teens.
Most children with nephrotic syndrome respond well to steroids and aren't at risk of kidney failure.
However, a small number of children have congenital (inherited) nephrotic syndrome and usually do less well. They may eventually have kidney failure and need a kidney transplant.
What problems can it cause?
Some of the main symptoms associated with nephrotic syndrome are:
Swelling
The low level of protein in the blood reduces the movement of water from surrounding tissues back into the blood vessels, leading to swelling (oedema).
Swelling is usually first noticed around the eyes, then around the lower legs and rest of the body.
Infections
Antibodies are a specialised group of proteins in the blood that help fight infection. When these are lost, children are much more likely to get infections and frequently experience fatigue, poor appetite and weakness.
Urine changes
Occasionally, the high levels of protein being passed into the urine can cause it to become "frothy". Some children with nephrotic syndrome may also pass less urine than usual during relapses.
Blood clots
Important proteins that help prevent the blood from clotting can be passed out in the urine of children with nephrotic syndrome. This can increase their risk of potentially seriousblood clots