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What exactly is the condition normal-pressure hydrocephalus?

An estimated 375,000 Americans have a condition called normal-pressure hydrocephalus, or NPH, which is often misdiagnosed as either Alzheimer's disease because memory is impaired in both cases or as Parkinson's because gait is affected in both, said Dr. Peter Black, chief of neurosurgery at Brigham and Women's Hospital. Even CT and MRI scans often can't tell the difference between these conditions and NPH.

Many doctors don't know about it, but there is a way to tell the difference between Alzheimer's and Parkinson's, for which there is no curative treatment, and NPH, which can be reversed.

In NPH, cerebrospinal fluid fails to drain properly from spaces in the brain called ventricles. This excess fluid then pushes on brain regions that control walking, memory, and bladder control, creating the three hallmark symptoms of NPH: a shuffling gait in which the person feels as though his feet are glued to the floor, dementia, and incontinence.

If a doctor suspects NPH, he or she can place a tiny tube in the lower region of the back to drain fluid from the central nervous system. This drain is left in place for three days. If the symptoms, especially gait problems, get better, that suggests that the patient has NPH and that a shunt permanently placed in the brain to drain excess fluid will help. The shunt takes fluid from the ventricle and sends it through a tube into the abdomen, where the fluid is then absorbed. The entire system is under the skin.

Debbi Fields, executive director of the National Hydrocephalus Foundation, has had a shunt for 20 years and is grateful for it. Though shunts can sometimes get clogged or need to be reprogrammed to drain less or more fluid, they can return many people to normal lives, she said.

"The only time you realize you have a shunt," she added, "is when you wash your hair and can feel the reservoir for fluid that is under the skin in the scalp. And if you go to a new hairdresser, you have to warn them."

JUDY FOREMAN

E-mail health questions to foreman@globe.com.

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