Compliance of the cerebrospinal space: comparison of three methods

BACKGROUND: Cerebrospinal compliance describes the ability of the cerebrospinal space to buffer changes in volume. Diminished compliance is associated with increased risk of potentially threatening increases in intracranial pressure (ICP) when changes in cerebrospinal volume occur. However, despite various methods of estimation proposed so far, compliance is seldom used in clinical practice. This study aimed to compare three measures of cerebrospinal compliance. METHODS: ICP recordings from 36 normal-pressure hydrocephalus patients who underwent infusion tests with parallel recording of transcranial Doppler blood flow velocity were retrospectively analysed. Three methods were used to calculate compliance estimates during changes in the mean ICP induced by infusion of fluid into the cerebrospinal fluid space: (a) based on Marmarou’s model of cerebrospinal fluid dynamics (C(CSF)), (b) based on the evaluation of changes in cerebral arterial blood volume (C(CaBV)), and (c) based on the amplitudes of peaks P1 and P2 of ICP pulse waveform (C(P1/P2)). RESULTS: Increase in ICP caused a significant decrease in all compliance estimates (p < 0.0001). Time courses of compliance estimators were strongly positively correlated with each other (group-averaged Spearman correlation coefficients: 0.94 [0.88–0.97] for C(CSF) vs. C(CaBV), 0.77 [0.63–0.91] for C(CSF) vs. C(P1/P2), and 0.68 [0.48–0.91] for C(CaBV) vs. C(P1/P2)). CONCLUSIONS: Indirect methods, C(CaBV) and C(P1/P2), allow for the assessment of relative changes in cerebrospinal compliance and produce results exhibiting good correlation with the direct method of volumetric manipulation. This opens the possibility of monitoring relative changes in compliance continuously.