The scale of the physiological transformation is striking. The most common technique, decompressive hemicraniectomy, involves removing a bone flap averaging 14 by 14 centimeters and opening the dura widely, which enlarges the cranial cavity by an estimated 120 to 200 milliliters. A meta-analysis cited in the viewpoint quantified the immediate effect: intracranial pressure drops by an average of 17.59 millimeters of mercury, a reduction sustained at 24 and 48 hours, while cerebral perfusion pressure rises by 7.37 millimeters of mercury. Cerebral blood flow and brain tissue oxygenation typically improve as well. But the surgery also produces a cascade of adverse changes: rapid outward brain expansion, impaired cerebrovascular autoregulation, disrupted cerebrospinal fluid dynamics that predispose patients to hydrocephalus, altered intracranial pressure waveform morphology, and even cardiovascular effects mediated through autonomic mechanisms.

The central paradox the authors highlight is that decompressive craniectomy is a temporizing hemodynamic intervention. It relieves the mechanical consequences of high intracranial pressure but does nothing to reverse the underlying structural brain injury. This means the rationale for continued invasive neuromonitoring remains as strong as ever, despite lingering skepticism in some centers that clinical assessment and neuroimaging suffice once the skull is open. Two major consensus statements support continued monitoring: the Milan Consensus Conference recommended intracranial pressure monitoring after secondary decompressive craniectomy to assess the effectiveness of the surgery and guide further therapy, and an international neurosurgical consensus stated that monitoring and treatment should continue until pressure values remain controlled and stable.

The empirical data make a compelling case. Observational studies show that intracranial hypertension persists or recurs in 36 to 76.5 percent of traumatic brain injury patients after decompressive craniectomy, with similar rates reported after surgery for malignant middle cerebral artery infarction. The rationale for continued monitoring includes persistence of the underlying pathophysiology, the risk of recurrent intracranial hypertension or de novo hematomas, and the need to rationalize therapies such as osmotherapy, sedation, and cerebrospinal fluid drainage. The authors also draw a critical distinction between primary decompressive craniectomy, performed as the first intervention, and secondary surgery after conventional pressure management fails. The greatest lesion evolution occurs in the first 8 to 12 hours after the initial insult, a window that coincides with the early postoperative period in primary cases where no prior monitoring was in place. Contralateral epidural hematoma, a well-recognized complication, occurs predominantly after primary surgery, providing an additional argument for sustained surveillance.

Where exactly to place the pressure sensor turns out to be a question with no evidence-based answer. Neither current consensus statements nor major decompressive craniectomy trials report data on sensor implantation site. The limited published series favor ipsilateral placement, on the reasoning that intracranial pressure is compartmentalized, interhemispheric pressure gradients may develop, and regional hypertension can persist or emerge despite decompression. The authors stress that this recommendation rests on pathophysiological reasoning rather than high-quality prospective evidence. Contralateral placement risks underestimating true pressure and brain tissue oxygenation, leaving focal hypoxia or hypertension undetected, though specific scenarios may warrant it: extensive hemispheric parenchymal destruction, large hemorrhages with marked midline shift, diffuse bilateral injury, a nonviable ipsilateral surgical trajectory, infection of pre-existing sensors, or post-primary surgery where no prior monitoring exists.

The choice of monitoring modality adds another layer of complexity. External ventricular drains offer both pressure measurement and therapeutic cerebrospinal fluid diversion but can be technically challenging after surgery because of ventricular distortion. Intraparenchymal probes are easier to deploy but provide only regional measurements without drainage capability. The authors suggest a staged approach, beginning with an ipsilateral parenchymal probe and adding a contralateral ventricular drain in selected patients. Meanwhile, the evidence base for noninvasive monitoring in the decompressed brain remains sparse and contradictory. Optic nerve sheath diameter ultrasonography has shown that postoperative reductions correlate with outcome in malignant stroke and traumatic brain injury, yet other studies report poor performance of both this technique and transcranial Doppler for estimating pressure and perfusion after surgery. The discrepancies likely reflect heterogeneous reference standards, variable timing of measurements, and the fact that optic nerve sheath measurements reflect global rather than regional pressure dynamics.

Even electroencephalography becomes harder to interpret in these patients. The skull defect produces the so-called breach rhythm, high-voltage physiological waveforms that can mimic epileptiform discharges, while magnetoencephalography is less affected by the missing bone. Near-infrared spectroscopy demonstrated significant bilateral improvement in cerebral oxygen saturation after surgery for malignant infarction, though only contralateral changes correlated with 30-day outcome. Novel noninvasive devices that detect cranial vault micro-expansions to reconstruct pressure waveforms are particularly controversial in the absence of a bone flap, and an artificial intelligence-based pulse shape index for classifying intracranial compliance requires especially cautious interpretation in this population. A systematic review and meta-analysis concluded that invasive and noninvasive methods show similar effectiveness after decompressive craniectomy, and a prospective study by Alhamdan and colleagues demonstrated that invasive monitoring provides significant prognostic information after malignant infarction, with autonomic indices such as heart rate variability, baroreflex sensitivity, and signal complexity potentially complementing standard pressure data.

Perhaps the most provocative section of the viewpoint concerns treatment thresholds. Current Brain Trauma Foundation and Seattle International consensus guidelines recommend initiating treatment for intracranial hypertension at pressures above 22 millimeters of mercury, recommendations based on low-level evidence and extrapolated to decompressed patients despite fundamental physiological differences. Multiple observational studies now challenge this. A large prospective analysis identified 19 millimeters of mercury as the value most strongly associated with outcome, a finding maintained in patients undergoing decompressive craniectomy. After surgery for malignant infarction, Hernández-Durán and colleagues found 10 millimeters of mercury as the threshold most strongly associated with mortality, and in a comparable population pressures above 15 millimeters of mercury correlated with unfavorable outcomes, as did a pressure reactivity index near zero or perfusion pressure below 80 millimeters of mercury. A retrospective traumatic brain injury analysis using a 15-millimeter treatment threshold found Glasgow Outcome Scale Extended scores of 1 versus 5 depending on whether pressures exceeded 25 millimeters, and identified 40 millimeters of mercury as the functional lower limit of cerebral perfusion pressure.

A German retrospective cohort that monitored pressures for seven days after surgery adds further weight. Favorable outcomes corresponded to pressures that never exceeded 14 millimeters of mercury, with a mean of 11.5, while unfavorable outcomes corresponded to a mean of 17.5 millimeters, with statistical significance for both traumatic brain injury and malignant infarction independently. Kaplan-Meier analysis showed 75 percent versus 40 percent survival probability respectively. The authors are careful to note that all these data are exclusively retrospective, small-sample, and heterogeneous, and must be considered hypothesis-generating rather than practice-defining. Still, the pattern is consistent: clinically relevant deterioration appears to occur at substantially lower pressures in the decompressed brain than conventional guidelines assume.

The viewpoint distills the problem into six key physiological differences that undermine extrapolation from intact-skull physiology: abolition of the closed-box condition negates the Monro-Kellie doctrine and profoundly alters the pressure-volume relationship; cerebrospinal fluid dynamics are disrupted with predisposition to hydrocephalus; pressure waveform morphology and pulse amplitude characteristics are fundamentally changed; interhemispheric pressure gradients may render single-site measurements unrepresentative; autoregulatory indices require reinterpretation in the altered compliance environment; and noninvasive tools relying on acoustic or mechanical coupling through an intact skull lose reliability without a bone flap. The authors call for prospective, standardized research to fill these gaps and suggest that, in the interim, a structured consensus-based approach could harmonize clinical practice and identify research priorities. For a procedure performed thousands of times each year worldwide, the message is sobering: the brain without its skull is, in monitoring terms, an almost entirely uncharted organ.

Subject of Research: Neuromonitoring physiology and intracranial pressure thresholds after decompressive craniectomy

Article Title: Neuromonitoring in the Decompressed Brain: Rethinking Physiology, Monitoring, and Thresholds After Decompressive Craniectomy

Article References: Godoy, D. A., Paiva, W. S., de Amorim, R. L. O., Rovegno, M., & Rubiano, A. M. (2026). Neuromonitoring in the Decompressed Brain: Rethinking Physiology, Monitoring, and Thresholds After Decompressive Craniectomy. Neurocritical Care. https://doi.org/10.1007/s12028-026-02622-z

Image Credits: AI Generated

DOI: 10.1007/s12028-026-02622-z

Keywords: decompressive craniectomy, intracranial pressure, neuromonitoring, cerebral perfusion pressure, traumatic brain injury, malignant cerebral infarction, neurocritical care, ICP thresholds, sensor placement, noninvasive monitoring, cerebral autoregulation, Monro-Kellie doctrine