CSF dynamics and pressure-volume relationships in communicating hydrocephalus

Twenty-nine patients consecutively admitted with a diagnosis of communicating hydrocephalus underwent continuous intracranial pressure (ICP) monitoring; pressure-volume studies; and measurement of resistance to outflow of cerebrospinal fluid (Rout). The two latter calculations were made by the bolus injection and pressure-volume index (PVI) techniques. In 19 patients mean ICP never exceeded 15 mm Hg. In the other 10 patients varying degrees of mildly raised ICP was noted. The frequency of waves at 1/2 to 2/min varied from 3% to 58%. The ICP pulse amplitude ranged from 0.5 to 10 mm Hg, and PVI from 4.6 to 18.2 ml. The Rout ranged from 2.5 to 31.4 mm Hg/ml/min, and was linearly correlated to the ICP. Thus, patients with a higher Rout also had a higher ICP as compared with patients with lower Rout, yet ICP could still be within limits considered normal. The cerebrospinal fluid dynamics (formation rate X resistance) contributed much more to the ICP than in normal individuals. It is postulated that communicating hydrocephalus represents one endpoint of a continuum, where the preceding phase is high-pressure and high-resistance hydrocephalus as, for instance, is seen after subarachnoid hemorrhage. In some patients, there is a possibility of cerebral atrophy accompanied by otherwise insignificant increased Rout. In this study, the PVI technique proved to be a fast and safe method of measuring Rout.