Non-invasive intracranial pressure estimation in the intensive care unit: narrative review of methods and clinical applications

Intracranial hypertension (IH) after acute brain injury (ABI) is an important cause of secondary brain damage and remains a matter of continuous investigation [ 1 ]. It is widely recognized that intracranial pressure (ICP) is ‘more than the number’; therefore, its continuous monitoring should be considered as a mandatory tool to avoid additional brain insults, resulting from a hyperdynamic course of ICP (plateau waves, sudden refractory hypertension, changes in arterial partial pressure of carbon dioxide (PaCO 2 ), etc.) [ 1 ]. Despite the lack of a positive Level-I evidence on outcome in traumatic brain injury (TBI), ICP monitoring is generally considered of paramount importance in the neuro-intensive care setting [ 2 , 3 , 4 , 5 ]. Invasive methods (i.e., intraparenchymal and external ventricular drain (EVD) systems) are considered the gold standard for ICP measurement and treatment [ 6 ]. However, in some settings (e.g., low-income countries, austere environments, etc.), these systems are not immediately and easily available. Moreover, patients may have contraindications for invasive tools (i.e., infections, coagulopathy) or may not completely meet indications for insertion. This has induced many authors to focus their efforts, over the last decades, on the study of non-invasive ICP (nICP) estimators [ 7 , 8 ]. A variety of non-invasive tools have been proposed, each with different levels of accuracy, availability, and feasibility (Table 1 ) [ 8 ]. Although none of these modalities has demonstrated sufficient accuracy to replace invasive ICP monitoring (mainly due to lack of calibration and zeroing), several have shown potential to complement invasive measurements. Rather than serving as substitutes, non-invasive tools provide additional, clinically relevant information regarding the risk and likelihood of IH. Importantly, they may also add value even when invasive ICP monitoring is in place, by contributing complementary physiological insights that support clinical decision-making across a range of settings [ 9 ]. Importantly, high sensitivity and high specificity are generally mutually exclusive. For example, a highly sensitive approach will result in many patients undergoing invasive monitoring unnecessarily, whereas a highly specific strategy risks missing patients with elevated ICP. This review aims to provide clinicians involved in the care of critical care patients [e.g., emergency physicians, general intensive care unit (ICU) physicians, neurosurgeons, etc.] with an updated overview on the methodology and clinical applications of nICP monitoring tools.