Compliance of the cerebrospinal space: comparison of three methods

Abstract

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.

Keywords: Cerebral arterial blood volume; Cerebrospinal compliance; Infusion test; Intracranial pressure; Pulse waveform.

Fig. 1

Illustrative example of signals recorded during the infusion test for a single patient. Grey vertical lines indicate the start and end of constant rate (1.5 ml/min) infusion. a Intracranial pressure (ICP). b Cerebral blood flow velocity (CBFV). c Arterial blood pressure (ABP)

Fig. 2

Illustrative examples of intracranial pressure (ICP) pulse waveforms from two different patients. Location of peaks P1 and P2 is indicated by cross (P1) and star (P2) signs. ICP signals are plotted with solid black lines. Additional signals used in the process of peak detection are plotted as dashed (arterial blood pressure (ABP)) and dotted (cerebral arterial blood volume (CaBV)) lines. All signals are normalized and aligned with regard to pulse onset location. Vertical lines indicate the correlation between the position of peaks P1 and P2 and the local maxima of the ABP (dashed line) and CaBV (dotted line) waveforms, respectively

Fig. 3

Illustrative example of time courses of amplitude of peaks P1 and P2 for a single patient. Full pulse-by-pulse time courses are presented as dots while 30-pulse moving averages are presented as solid lines. a Mean intracranial pressure (ICP). b Amplitude of peaks P1 (light grey symbols) and P2 (dark grey symbols) of the ICP pulse waveform

Fig. 4

Relationship between baseline P1/P2 ratio and change in P1/P2 ratio between baseline and plateau phases of infusion test. Values above the scatter plot indicate Spearman correlation coefficient and its p-value

Fig. 5

Illustrative examples of changes in intracranial pressure (ICP) pulse waveform between baseline and plateau phases of infusion test. Baseline and plateau phases are presented in the top and bottom plots, respectively. Location of peaks P1 and P2 is indicated by cross (P1) and star (P2) signs. a Patient with high baseline P1/P2 ratio. b Patient with low baseline P1/P2 ratio

Fig. 6

Illustrative example of time courses of compliance estimates for a single patient. Full pulse-by-pulse time courses are presented as grey lines, while 30-pulse moving averages are presented as black lines. a Mean intracranial pressure (ICP). b–d Compliance estimates obtained with b CSF dynamics model (C_CSF), c cerebral blood volume model (C_CaBV) and d P1/P2 peak ratio of ICP pulse waveform (C_P1/P2)