Comment on: B-waves in noninvasive capacitance signal correlate with B-waves in ICP

To the Editor

We read with great interest the recent technical report by Spiegelberg et al. [2], which demonstrates a strong correlation between intracranial B-waves observed via invasive ICP monitoring and oscillations in the noninvasive electric capacitance signal (W). The authors present an elegant signal acquisition setup, robust correlation analysis, and a compelling case for the future potential of noninvasive cerebral compliance monitoring.

The authors themselves acknowledge that using an invasive infusion protocol to provoke B-waves undermines the noninvasive premise of the W signal. Nevertheless, they proceed to suggest clinical applications for W in noninvasive triage without presenting any data obtained under spontaneous or physiologic conditions. This gap between the stated limitation and the scope of their conclusions warrants clarification.

While the temporal concordance is compelling, the comparison of ICP and W amplitudes appears to assume linearity, despite the exponential pressure-volume relationship formalized by Marmarou et al. [1], where intracranial compliance decreases with increasing pressure as $C=\frac{1}{\mathit{KP}}$. While it is possible that W exhibits an approximately linear relationship with ICP over small dynamic ranges, this assumption is not established, and the underlying pressure-volume mechanics suggest that such a correlation should be nonlinear-particularly under induced high-pressure states.

We suggest that future modeling efforts apply the inverse of this relation to transform ICP amplitudes into estimated volume fluctuations:

$$V=\frac{1}{\kappa }ln\left(\frac{P}{P_0}\right),$$

where $\kappa$ is the patient-specific compliance coefficient and $P_0$ the baseline pressure. This would permit a more physiologically grounded comparison between W (a volume-sensitive signal) and ICP (a pressure signal), potentially clarifying the observed amplitude mismatch.

To fully validate W as a noninvasive surrogate of intracranial compliance, we believe further studies are warranted under spontaneous physiologic conditions-such as REM sleep or postural modulation-where B-waves naturally emerge. We commend the authors for opening this promising domain and offer this commentary in support of continued refinement.

Author Contributions

S.B. and A.T. both wrote and reviewed the manuscript.

Funding

None.

Data Availability

No datasets were generated or analysed during the current study.

Declarations

Competing Interests

The authors declare no competing interests.