Objective: An index of cerebrospinal compensatory reserve (RAP) has been introduced as a potential descriptor of neurological deterioration after head trauma. It is numerically computed as a linear correlation coefficient between the mean intracranial pressure and the pulse amplitude of the pressure waveform. We explore how RAP varies with different forms of physiological or nonphysiological intracranial volume loads in adult hydrocephalus, with and without a functioning cerebrospinal fluid (CSF) shunt.
Methods: A database of intracranial pressure recordings during CSF infusion studies and overnight monitoring in hydrocephalic patients was reviewed for clinical comparison of homogeneous subgroups of patients with hypothetical differences of pressure-volume compensatory reserve. The database includes 980 patients of mixed etiology: idiopathic normal pressure hydrocephalus (NPH), 47%; postsubarachnoid hemorrhage NPH, 12%; noncommunicating hydrocephalus, 22%; others, 19%. All CSF compensatory parameters were calculated by using intracranial pressure waveforms.
Results: In NPH, RAP correlated strongly with the resistance to CSF outflow (r(s) = 0.35; P = 0.045), but weakly correlated with ventriculomegaly (r(s) = 0.13; P = 0.41). In idiopathic nonshunted NPH patients, RAP did not correlate significantly with elasticity calculated from the CSF infusion test (r(s) = 0.11; P = 0.21). During infusion studies, RAP increased in comparison to values recorded at baseline (from a median of 0.45-0.86, P = 0.14 * 10(-8)), indicating a narrowing of the volume-pressure compensatory reserve. During B-waves associated with the REM (rapid eye movement) phase of sleep, RAP increased from a median of 0.53 to 0.89; P = 1.2 * 10(-5). After shunting, RAP decreased (median before shunting, 0.59; median after shunting, 0.34; P = 0.0001). RAP also showed the ability to reflect the functional state of the shunt (patent shunt median, 0.36; blocked shunt median, 0.84; P = 0.0002).
Conclusion: RAP appears to characterize pressure-volume compensatory reserve in patients with hydrocephalus.