Intracranial Pressure Threshold Heuristics in Traumatic Brain Injury: One, None, Many!

Abstract

Critical care of the patient with severe traumatic brain injury (TBI) revolves around strategies to address intracranial hypertension (IHT), and optimizing cerebral perfusion. Strong evidence associates IHT (especially refractory IHT) with mortality, and unfavorable clinical outcomes. However, this association may merely indicate that IHT is a surrogate for severity of injury and not necessarily a modifiable, outcome-altering, treatment target, i.e., some patients die with IHT, not from IHT. Another reason for intracranial pressure (ICP), and derived cerebral perfusion pressure (CPP), to be central tenets of bedside management is the fact that are relatively easily monitored as opposed to other secondary brain injury (SBI) mechanisms such as metabolic and energy crisis, cortical spreading ischemia, diffusion microvascular hypoxia, and mitochondrial failure. Consequently, we may be misled targeting what we can measure, not necessarily what matters. A further concern is that by solely focusing on one pathophysiological aspect we may ignore complex multidimensional cascades leading to SBI, and our treatment interventions run the risk of being counterproductive and harmful. Despite limitations, it is unlikely that ICP-guided management should or will entirely go away in the near future. One telling indication is the concerns, and responses, generated by the Benchmark Evidence from South American Trials: Treatment of Intracranial Pressure (BEST TRIP) trial [1]. This is the only level-1 evidence-producing randomized clinical trial (RCT) on the topic, finding no primary outcome

“Take a Number”—Precision Monitoring Directs Precision Therapy

“…However, any therapeutic regimen developed to target brain swelling in TBI relies largely on the vast experience obtained from ICP-directed therapy [4,29]. In agreement with Lazaridis et al [1], rather than take steps backward, and abandon ICP monitoring, we believe that it is important to build on the information that ICP monitoring provides-even with decades of heuristic data. Also, greater integration of advanced imaging into clinical decision making is further supported given the fact that in the Best Trip study, more rather than less therapy, in general, was used in the group managed without ICP monitoring [27].…”

“…In the current era, all facets of TBI management are being challenged. As mentioned, Lazaridis et al [1] suggest that the association between raised ICP and both mortality and unfavorable outcome may simply reflect its association with severity of injury. That may, in some cases be true.…”

“…In this issue of Neurocritical Care, Lazaridis et al [1] present a timely and provocative "View Point" that addresses the heuristic nature of ICP thresholds. They raise appropriate concern about "unidimensional excursions, " the fact that an association between an ICP threshold and outcome does not imply that a therapy targeting values about that threshold will improve outcome, and suggest that heuristic thresholds may simply exist because of the association between ICP and injury severity.…”

“…It is unfortunate and surprising that decompressive craniectomy-taking ICP concerns out of the secondary injury equation-although life-saving in some cases, appears to have some unwanted effects possibly related to issues such as brain deformation, alterations in CBF regulation, disturbed glymphatic flux, persistent alterations in brain compliance, spreading depression, rapid deescalation of otherwise protective facets of neurocritical care, or others yet to be characterized [25,26] that may blunt its efficacy. And, as pointed out by Lazaridis et al [1], how data from patients with decompression should influence ICP or CPP thresholds remains a conundrum, given that data from them are often included in studies addressing these issues, but they are clearly a group distinct from patients with an intact skull.…”

“…An ICP threshold represents a heuristic starting point in the management of severe TBI-and across other relevant diseases in neurocritical care-while recognizing the need for integrating multi-modal monitoring in a temporal, potentially regional, more nuanced, and cybernetic manner to guide therapy. Lazaridis et al [1] also suggest that in some patients it may be very difficult to generate an optimal ICP threshold.…”