Nephrotic syndrome may occur when the filtering units of the kidney are damaged. This damage allows protein normally kept in the plasma to leak into the urine in large amounts, which reduces the amount of protein in your blood. Since the protein in the blood helps keep fluid in the bloodstream, some of this fluid leaks out of the bloodstream into your tissues, causing swelling, called edema. The swelling may be most noticeable in your legs after you have been standing and in the tissue under your eyes when you first get up in the morning. Eventually, the swelling in your legs may be there all the time, and it may also occur in other parts of your body. You may notice that your urine foams more than usual because of the amount of protein in it.
How is nephrotic syndrome diagnosed?

Your doctor can detect protein in your urine with a routine urine analysis and get a rough idea about the amount of protein in your urine by a test that can be done in the office. Diagnosis of the nephrotic syndrome is made by collecting urine for 24 hours and measuring the amount of protein in it. Nephrotic syndrome may also cause an increase in fat in your blood. This can only be found by a blood test done by your doctor.
What causes nephrotic syndrome?

Nephrotic syndrome is not a specific kidney disease. It can occur in any kidney disease that damages the filtering units in a certain way that allows them to leak protein into the urine. Some of the diseases that cause nephrotic syndrome, such as nephritis, affect only the kidney. Other diseases that cause nephrotic syndrome, such as diabetes and lupus, affect other parts of the body as well.
How is nephrotic syndrome treated?

Some of the kidney diseases that cause nephrotic syndrome are treatable with medicine. Some may get better on their own, but others get worse and may lead to kidney failure no matter what treatment is used. Unfortunately, many diseases that cause nephrotic syndrome have no treatment. Only your doctor can find out what specific disease is causing you to have it. Diagnosis may require a kidney biopsy.
What happens if there is no treatment for my nephrotic syndrome?

If your nephrotic syndrome is caused by a disease that has no specific treatment, help may still be available. Reducing salt in your diet will help to control the edema. Your doctor may also prescribe diuretics (water pills) to help with the swelling. The doctor may also prescribe the use of certain medicines that can reduce the protein in your urine. Although the syndrome is caused by the loss of protein into your urine, eating a high-protein diet does not help and may actually make matters worse. If the level of fats in your blood is too high, your doctor may recommend treatment for the increased levels of fat in your blood.

Taken from the National Kidney Foundation.