The device, known as brain4care or B4C, is built on a principle first reported in 2012. The skull is not as rigid as it appears. With every heartbeat, pulsations of intracranial pressure produce microscopic deformations of the cranial bones, and a highly sensitive mechanical extensometer placed on the temporal region can detect these nanometer-scale movements. The sensor transmits the resulting waveforms over Bluetooth to a mobile device, where cloud processing filters and analyzes the signals in real time, delivering a continuous report that clinicians can interpret at the bedside. Because the technique reads cranial pulsations synchronized with the cardiac cycle, it captures the same fundamental pressure wave that an invasive catheter records inside the head, without ever breaching the skull.

The physiological story centers on the shape of that wave. Each pulse contains characteristic peaks labeled P1, P2, and P3. P1 primarily reflects arterial pulsatility transmitted into the intracranial compartment, while P2 is shaped by the brain’s capacity to accommodate pulsatile volume changes. Intracranial compliance, the brain’s intrinsic ability to absorb volume shifts within the rigid confines of the skull without a dangerous rise in pressure, is the key variable. As compliance deteriorates, P2 grows progressively taller relative to P1, and in severe cases the dicrotic wave P3 may even exceed P1 in amplitude. A rising P2/P1 ratio therefore signals that the compensatory reserve is running out, often before the absolute pressure number crosses a treatment threshold.

To assess how well this noninvasive approach has actually been validated, an international team led by Juliana Caldas and Sérgio Brasil systematically reviewed the clinical literature following PRISMA-ScR guidelines, with the protocol prospectively registered in PROSPERO. The researchers searched MEDLINE, Embase, and the Scientific Electronic Library Online from 2016, when the first clinical study using the device appeared, through May 2025. Out of 986 records identified, 660 duplicates were removed, and after screening 326 articles, 134 abstracts met criteria for full analysis. Case reports, reviews, experimental studies, editorials, and studies in healthy subjects were excluded. Ultimately, 16 studies encompassing 826 patients with acute brain injury made the final cut, conducted across three institutions in Brazil, one in the United States, and one in Poland.

The headline finding is that the P2/P1 ratio, the most consistently evaluated metric, performed respectably as a detector of intracranial hypertension, defined in most studies as an invasive pressure above 20 mm Hg. Across the seven studies reporting diagnostic statistics, cutoff values for the ratio ranged from 1.06 to 1.4, with sensitivity estimates spanning 26.8 to 100 percent and specificity from 45.5 to 96.5 percent. Discriminative performance, quantified by the area under the receiver operating characteristic curve, fell between 0.71 and 0.83, which is generally regarded as moderate to good. Notably, one study found that higher P2/P1 values combined with elevated mean intracranial pressure were associated with early mortality, giving the waveform parameter prognostic as well as diagnostic weight.

The second derived parameter, time to peak, or TTP, measures the interval from the onset of the waveform to its highest point. Shorter values indicate preserved compliance and rapid buffering; prolonged values suggest a stiff, poorly compensating brain. Here the evidence was thinner and more heterogeneous. Reported sensitivities ranged from 33.3 to 85.7 percent with specificities between 50.0 and 73.7 percent, and area under the curve values hovered around a modest 0.69. Yet the largest study identified a TTP threshold of 0.3 as the optimal cutoff, achieving a striking 92 percent specificity, a figure supported by normative data showing that healthy individuals across age groups typically remain below that value. Every one of the 16 studies also demonstrated qualitative morphological concordance between the waveforms recorded noninvasively at the skull and those measured by invasive catheters.

Some of the most intriguing results came from studies that pushed the technology beyond simple threshold detection. One investigation tested a composite Intracranial Compliance Scale built from the P2/P1 ratio and TTP in 72 monitored patients, and the combined score outperformed either parameter alone, reaching an area under the curve of 0.83. A score of zero carried a 100 percent negative predictive value, while a score of three carried a 100 percent positive predictive value for intracranial hypertension. In another study, pairing the B4C waveform analysis with transcranial Doppler ultrasound produced the highest diagnostic accuracy of any combination tested, 0.802, for detecting intracranial hypertension. Machine learning has entered the picture as well: a histogram gradient boosting regression algorithm fed with B4C and Doppler inputs estimated invasive pressure with a strong Spearman correlation of 0.76 and a mean absolute error of just 3 mm Hg, while Doppler alone failed to achieve significant correlation.

The review’s authors are careful about what these numbers mean clinically, and their interpretation is arguably the most interesting part of the story. A P2/P1 ratio of 0.8 or lower showed 92 percent sensitivity for ruling out intracranial hypertension, meaning a low reading could help clinicians avoid unnecessary invasive procedures in patients who are likely safe. Conversely, a ratio of 1.4 or higher strongly suggests dangerous pressure is present. The gray zone in between does not confirm or exclude anything, but it is valuable for tracking how compliance changes over time. Crucially, one study found that patients with pressures near conventional treatment thresholds, around 18 to 22 mm Hg, could still have favorable outcomes when their P2/P1 ratios stayed below 1.2, while similar pressure values paired with higher ratios predicted early death. The shape of the wave, in other words, adds context that a single pressure number cannot, supporting a shift toward individualized, physiology-driven precision medicine in neurocritical care.

None of this makes the device a replacement for the catheter, and the review is candid about the caveats. Methodological quality across the included studies was only moderate on the Newcastle–Ottawa Scale, with scores from 3 to 8 points. No study included a true healthy control group, few adjusted for confounders such as age, comorbidities, or injury severity, and most used cross-sectional or short-term designs that precluded long-term follow-up. The device itself is susceptible to motion artifacts, limiting its use in agitated or pediatric patients, and cardiac arrhythmias such as atrial fibrillation can disrupt the regular pulsatile signal and corrupt derived parameters. Accuracy drops in patients with compromised cranial integrity, such as those who have undergone decompressive craniectomy, and direct equivalence with invasive waveforms is not expected because the two methods measure at different locations with different physical properties. Regulatory approval currently extends only to Brazil and the United States, which constrains broader adoption.

Still, the overall picture is one of a technology that has quietly accumulated a coherent validation record. The B4C sensor was well tolerated in every study, with no reported discomfort or adverse effects, and its continuous, up-to-24-hour monitoring sessions offer far more than the snapshot provided by techniques like optic nerve sheath diameter ultrasound. The authors position it as an adjunct within multimodal neuromonitoring, consistent with the Brussels B-ICONIC consensus on noninvasive intracranial pressure assessment when invasive systems are unavailable, serving as a screening tool that guides escalation to invasive monitoring when compliance begins to fail. What is needed next, they argue, are adequately powered, prospective, multicenter trials with standardized protocols, homogeneous brain injury populations, and longitudinal outcomes to define exactly where the device performs best. If those trials succeed, the humble act of listening to the skull’s silent pulse could become a routine part of keeping dangerously swollen brains alive, in ICUs from São Paulo to settings where a neurosurgeon and a catheter are nowhere to be found.

Subject of Research: Noninvasive intracranial pressure and compliance monitoring using the brain4care skull deformation sensor in acute brain injury

Article Title: Intracranial Pressure Estimation Using B4C Device: A Systematic Review on the Validation Studies

Article References: Caldas, J., Alves, F., Rynkowski, C., Solla, D., Godoy, D. A., Robba, C., Taccone, F. S., & Brasil, S. (2026). Intracranial Pressure Estimation Using B4C Device: A Systematic Review on the Validation Studies. Neurocritical Care. https://doi.org/10.1007/s12028-026-02658-1

Image Credits: AI Generated

DOI: 10.1007/s12028-026-02658-1

Keywords: intracranial pressure, intracranial compliance, brain4care, noninvasive neuromonitoring, acute brain injury, neurocritical care, ICP waveform, P2/P1 ratio, systematic review, intracranial hypertension, skull deformation sensor, machine learning