It can be surprising to realize that an organ as high-powered and sophisticated as the brain also has a plumbing system. And, as the case with a house's plumbing, the drainage side of the system can get gummed up. But the symptoms are different. When a home's drainage backs up, well...I won't go there. When the brain's drainage system backs up, the brain's owner can become confused, incontinent of urine and unsteady on his or her feet.

The plumbing system in question is that which produces and drains the cerebrospinal fluid (CSF). Normal CSF looks the same as water from a faucet, but is created from the bloodstream in the choroid plexus tissue within three of the brain's four inner chambers -- the right and left "lateral" ventricles and the midline "fourth" ventricle, but not the interposed, midline "third" ventricle. The CSF percolates through passageways from one ventricle to another, finally emerging through openings at the base of the brain to bathe the outer surfaces of the brain and spinal cord before getting reabsorbed into the bloodstream again. This re-absorption occurs in special collection-nodes in the membranes surrounding the brain. The entire CSF volume of about 150 milliliters or five ounces (about as much as a glass of wine) is produced and reabsorbed four times a day, so the fluid is constantly turning over.

But blockages along the way can interfere with the normal flow of the CSF. For example, when the passageway between the third and fourth ventricles becomes narrowed or choked with sludge, the CSF backs into the lateral and third ventricles. Those ventricles react to the increased pressure by becoming physically dilated or enlarged. In this case, a CT or MRI scan could reveal the location of the blockage by showing expansion of the two lateral and the single third ventricles, but a normal-sized fourth ventricle. Another example of a blockage and its consequences is when the collection-nodes responsible for CSF re-absorption in the brain's overlying membranes (meninges) become clogged. In this case, all four ventricles are upstream from the blockage, and all four of them expand. This, too, is visible on brain scans.

Both cases are examples of hydrocephalus, or water on the brain. The first case is one of "internal" or high-pressure hydrocephalus. The second is called "external" or normal-pressure hydrocephalus (NPH). In NPH the pressure is inexplicably normal much of the time, but the term is somewhat misleading because prolonged recordings with pressure-monitors do show intermittent periods of increased pressure.

Hydrocephalus of one kind or another is especially prevalent at the two extremes of the life cycle -- in the very young and the very old -- but can occur at any age. In infancy, hydrocephalus can be caused by malformed brain-tissue. In contrast, adults with hydrocephalus were usually born with normal brain anatomy, but acquired a blockage due to a tumor, injury, bleed or infection. However, many cases of hydrocephalus in adults occur without a history of these preceding illnesses.

CT and MRI scans are sensitive tools in detecting hydrocephalus, particularly when it's striking enough not be confused with ventricular enlargement due to gradual loss of surrounding brain tissue from aging. The main treatment of hydrocephalus is for a surgeon to insert a tube (shunt) into one of the swollen lateral ventricles and provide an alternative pathway for the backed-up CSF to drain. Once the shunt equipment is in place, a piece of hardware about the size of a large button sits outside the hole made in the skull (but inside the skin of the scalp) and redirects the excess CSF through another tube into either a jugular vein in the neck or into the abdominal cavity (peritoneum). Thus, the patient can receive either a "VJ" shunt or a "VP" shunt, with the letters designating the locations of the two ends of the shunt.

The success or failure of shunting depends not just on the skill of the surgeon, but also on the selection of appropriate patients. Sometimes hydrocephalus turns up unexpectedly on scans when doctors are looking for something else entirely. Although an unexpected finding like this should always cause the doctors to re-think the case, the point is that hydrocephalus doesn't always cause problems. Sometimes the hydrocephalus has been there for years and the brain has adjusted to it in a way that produces no symptoms. This is an example of a case that should not be shunted, though it would still be appropriate to monitor the patient and his or her scans over subsequent months and years.

Who, then, should receive a shunt? The answer, in short, is people for whom the benefits of the operation exceed its risks. Identifying them, however, is the tough part. And the task is made even more difficult by the lack of randomized, controlled trials in which a group of patients receiving treatment is compared to an equivalent group of patients not receiving treatment. Although similar reasoning applies to adults thought to have internal (high-pressure) hydrocephalus, I'll lay out the decision-tree as it applies to external (normal-pressure) hydrocephalus. Published observations imply that shunts are most likely to help NPH patients who have the following features:

• substantial enlargement of all four ventricles
• a full "triad" of symptoms, including confusion, urinary incontinence and altered walking
• poor walking as the first of the three symptoms
• temporary improvement of symptoms after drainage of 50-60 milliliters (2 ounces) of CSF by lumbar puncture (spinal tap)

So it's important for the doctor to determine if other diseases might be to blame for the very symptoms that seem, at first glance, to be from NPH. Assuming that NPH still seems likely, the next round of decision-making concerns the possibility that an operation will cause harm. Even a patient whose brain scan and symptoms are classic for NPH can develop serious complications from the operation. A particularly feared complication is bleeding into the space outside the brain, called a subdural hematoma. Older patients are also more likely to have other medical conditions that could compromise the safety of an operation, like coronary artery disease or emphysema.

Cases in which expected benefits of the operation are much greater than risks, or in which the risks are much greater than the expected benefits, are easy to make decisions about. But many other cases are in the gray zone in which potential benefits and risks are more evenly matched and the chances of doing harm with an operation come close to canceling out the chances of doing good.

(C) 2006 by Gary Cordingley