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Cerebrospinal fluid absorption deficit in normal pressure hydrocephalus.

Five adults with signs of normal pressure hydrocephalus underwent ventriculolumbar perfusions. Average cerebrospinal fluid (CSF) formation rate, 0.313 ± 0.020 ml/min, was comparable with that for nonhydrocephalics, but due to the larger CSF compartment, turnover rate was approximately one third. Slow turnover was further aggravated by a CSF absorption deficiency. Normally, absorption begins at approximately 68 mm CSF pressure and increases linearly at a rate of 7.6μl/ min/mm CSF pressure. In four patients the rate was reduced to 2.5μl/min/mm CSF once a critical mean CSF pressure was reached. This absorption deficit, accompanied by elevated pressure plateaus and heightened pulse pressures may lead to ventricular enlargement and, secondarily, to symptoms associated with normal pressure hydrocephalus. Only two of these four patients showed good response to shunting. In two nonresponding patients, follow-up revealed evidence of complicating intrinsic brain disease.