This study will examine whether the tendency to have thrombosis, or the formation of blood clots inside blood vessels, has a role in the development of pseudotumor cerebri (PTC). PTC causes symptoms and signs of isolated elevated blood pressure in the cranium, or covering of the brain. The disorder can lead to significant, negative effects on the visual system. Increased pressure of the cerebrospinal fluid, that is, fluid around the brain, is a factor, but the cause of the disorder is not clear. There has been documentation of clustering of PTC within families. It suggests that potential genetic polymorphisms-abilities to take on different forms-may become evident after exposure to conditions known to trigger PTC.

Thrombosis comes about by interactions between genetic and environmental or acquired factors, or both, resulting in a blood clot at a specific time and location. Because the disease occurs in episodes, the interaction of the genetic and nongenetic risk factors is important. Cystinosis is a recessive disorder caused by deposits of cystine within the lysosomes of cells-that is, sac-like cell parts that contain various enzymes. Involvement of the kidneys remains the primary characteristic, eventually leading to renal failure. Of all of the risk factors that make it easier for blood clotting, a high level of a substance called homocysteine is of particular interest. Too much homocysteine in blood plasma is a common finding in patients with kidney failure, and it has been recently identified as an independent risk factor for diseases of the blood vessels.

Participants of all ages who meet the Dandy criteria for PTC may be eligible for this study. Pregnant women will be excluded. There will also be a control group of nephropathic cystinosis patients who do not have PTC.

Participants will be asked to undergo the following tests and procedures:

• Medical history.
• Physical examination, to evaluate the eye and nervous systems.
• Collection of blood for DNA and other tests.
• Collection of cerebrospinal fluid, through a procedure called lumbar puncture or spinal tap.

The evaluation of patients will generally last 3 to 4 days. For the collection of cerebrospinal fluid, the patient's skin on the back will be numbed with a local anesthetic. A special needle will be inserted into the back, and a small amount of the fluid will be drawn through the needle. There will be pain for a minute, although there can be a headache lasting 24 hours. Also, there may be bruising, local pain, bleeding, or infection where the needle enters. Patients may also have a magnetic resonance imaging scan of their head. During the MRI scan, patients will lie still on a table that slides in and out of a metal cylinder surrounded by a strong magnetic field. Patients will be able to communicate with the MRI staff at all times and may ask to be moved out of the machine at any time.