Why Intracranial Compliance Is Not Utilized as a Common Practical Tool in Clinical Practice

Abstract

Intracranial compliance (ICC) holds significant potential in neuromonitoring, serving as a diagnostic tool and contributing to the evaluation of treatment outcomes. Despite its comprehensive concept, which allows consideration of changes in both volume and intracranial pressure (ICP), ICC monitoring has not yet established itself as a standard component of medical care, unlike ICP monitoring. This review highlighted that the first challenge is the assessment of ICC values, because of the invasive nature of direct measurement, the time-consuming aspect of non-invasive calculation through computer simulations, and the inability to quantify ICC values in estimation methods. Addressing these challenges is crucial, and the development of a rapid, non-invasive computer simulation method could alleviate obstacles in quantifying ICC. Additionally, this review indicated the second challenge in the clinical application of ICC, which involves the dynamic and time-dependent nature of ICC. This was considered by introducing the concept of time elapsed (TE) in measuring the changes in volume or ICP in the ICC equation (volume change/ICP change). The choice of TE, whether short or long, directly influences the ICC values that must be considered in the clinical application of the ICC. Compensatory responses of the brain exhibit non-monotonic and variable changes in long TE assessments for certain disorders, contrasting with the mono-exponential pattern observed in short TE assessments. Furthermore, the recovery behavior of the brain undergoes changes during the treatment process of various brain disorders when exposed to short and long TE conditions. The review also highlighted differences in ICC values across brain disorders with various strain rates and loading durations on the brain, further emphasizing the dynamic nature of ICC for clinical application. The insight provided in this review may prove valuable to professionals in neurocritical care, neurology, and neurosurgery for standardizing ICC monitoring in practical application related to the diagnosis and evaluation of treatment outcomes in brain disorders.

Keywords: brain biomechanics; brain disorder; cerebrospinal fluid; clinical application; diagnostic tool; gradual onset brain disorders; intracranial compliance (ICC); intracranial pressure (ICP); neurosurgery; viscous component.

Figure 1

(a) Direct ICC Measurement: A schematic illustrating ICC measurement. For clarity, red color is used to represent the infused saline; in actual conditions, it is colorless, resembling CSF. The injection location in this figure is in the lateral ventricle, but it can also be in the subarachnoid space or lumbar space. (b) This panel displays ICP waveforms over time to estimate ICC based on ICP waveform morphology. The normal ICP waveform is synchronous with the arterial pulse. Abnormal A: In the early compensation stage, there is an increase in the peak interval between P1 and P2, signifying an increase in cerebral vasculature pulsations and ICC. Abnormal B: When ICP remains consistently high, this initial reaction is followed by a gradual decrease in the peak interval between P1 and P2. CSF: Cerebrospinal fluid; ICP: Intracranial pressure; ICC: Intracranial compliance.

Figure 2

Comparison of ICC concepts based on Equation (2) in a patient and a healthy subject. It should be noted that our previous studies indicated negligible volume change in the subarachnoid space, with the main volume change occurring in the ventricular system [26,28], as indicated in this figure. CSF: Cerebrospinal fluid; ICP: Intracranial pressure; ICC: Intracranial compliance.

Figure 3

Volume-ICP relationships in short and long TEs: (a) shows the monotonic trend of the volume-ICP (VI) curve in a short TE. Two compensatory reserve zones are shown. The first zone is the upper reserve zone (blue line). In this zone, the ICP remains relatively stable despite changes in volume. This is due to the brain’s ability to compensate by reducing the volume of CSF and increasing blood flow out of the brain. Another zone is the lower reserve zone (green line). In this zone, the brain’s compensatory mechanisms are exhausted, and further increases in volume lead to a rapid increase in ICP to reach a plateau ICP; (b) shows the non-monotonic trend of the volume-ICP curve in a long TE after treatment for a hydrocephalus patient. The compensatory response of the brain could somewhat recover at certain times in a long TE. This curve is not divisible into some specific compensatory reserve zones. ICP: Intracranial pressure, ICC: Intracranial compliance, TE: Time elapsed, CSF: Cerebrospinal fluid.

Figure 4

Changes in compensatory response of the brain in a long TE: (a) shows the changes in volume and ICP at different time points in a long TE. This shows that the parameter of time, in addition to volume and ICP, can affect the compensatory response of the brain in a long TE—contrasting with short TE; (b) shows changes in ICC with ICP at different time points. This shows that the ICC trend and the compensatory response of the brain can have non-monotonic and variable changes in a long TE. ICP: Intracranial pressure, ICC: Intracranial compliance, TE: Time elapsed.