Sebastián Vásquez-García1,2,3,4, Andrés M. Rubiano2,5,6
Intracranial hypertension is a major determinant of secondary brain injury in patients with traumatic brain injury (TBI) and is associated with increased morbidity and mortality. While invasive monitoring remains the recommended practice parameter for intracranial pressure (ICP) assessment, its availability is limited in low-resource settings, and certain clinical conditions may preclude its use. Non-invasive ICP (nICP) monitoring techniques have emerged as viable alternatives when invasive methods are not accessible. The Brussels consensus (B-ICONIC) provides critical recommendations for the application of nICP monitoring tools, including transcranial Doppler/color-coded Duplex (TCD/TCCD) pulsatility index (PI) estimation, optic nerve sheath diameter (ONSD) measurement, and automated pupillometry via the neurological pupil index (NPi). This summary discusses the clinical thresholds for these methods and their implications for neurotrauma and neurocritical care.
TCD/TCCD for Non-Invasive ICP Estimation TCD/TCCD allows the assessment of cerebral hemodynamics and provides surrogate markers for elevated ICP. The B-ICONIC consensus identifies a PI threshold of 1.3, coupled with a diastolic flow velocity (FVd) of less than 20 cm/s, as indicative of intracranial hypertension. Additionally, a non-invasive ICP estimation formula using TCD/TCCD-derived parameters (nICP = Mean Arterial Pressure – [MAP * FVd/FVmean] + 14 mmHg) suggests that nICP values exceeding 20-22 mmHg should raise suspicion of elevated ICP. For follow-up studies, a change of at least 0.5 in the PI is considered significant, highlighting the importance of serial measurements to track ICP trends. TCD/TCCD is widely used in neurocritical care for assessing cerebral autoregulation, detecting vasospasm, and estimating cerebral perfusion pressure. The combination of PI and diastolic velocity measurements enhances the accuracy of ICP estimation, allowing for early detection of worsening cerebral compliance. In conditions where invasive ICP monitoring is contraindicated due to coagulopathy or other medical factors, TCD/TCCD serves as a vital tool to guide therapeutic interventions. Moreover, the ability of TCD/TCCD to provide real-time assessment at the bedside makes it particularly advantageous in dynamic neurocritical care environments. Despite its advantages, the interpretation of TCD/TCCD findings requires adequate training. Operator-dependent factors such as insonation angle and probe placement can influence accuracy. Standardized protocols have been developed to mitigate these limitations and improve
reproducibility across different clinical settings. Future developments, including automated TCD/TCCD analysis using artificial intelligence, hold promise for further refining ICP estimation.
Optic Nerve Sheath Diameter as a Marker of Elevated ICP Ultrasound assessment of ONSD is a rapid bedside technique with growing evidence supporting its correlation with ICP dynamics. The consensus defines a threshold of 6 mm as indicative of raised ICP, with a change of 0.5 mm in serial measurements being clinically meaningful. Given that the optic nerve sheath distends in response to elevated ICP, serial assessments provide valuable insights into disease progression. However, it is crucial to ensure optimal imaging quality and operator expertise to mitigate measurement variability. ONSD measurement has gained traction due to its ease of use and strong correlation with invasive ICP readings. Studies have demonstrated that ONSD changes precede clinical signs of herniation, making it an invaluable tool for early intervention. Unlike invasive methods, ONSD assessment does not require specialized surgical expertise and can be performed in emergency settings with portable ultrasound devices. The incorporation of standardized protocols for ONSD measurement has improved reproducibility, making it a reliable adjunct in multimodal neuromonitoring strategies. Furthermore, ONSD assessment has potential applications beyond TBI. It has been used in conditions such as hydrocephalus, intracranial infections, and stroke, where ICP fluctuations impact clinical outcomes. The ongoing refinement of ONSD cutoff values across diverse patient populations will enhance its utility as a broadly applicable neuromonitoring tool.
Automated Pupillometry and the Neurological Pupil Index Pupillary examination is a fundamental component of neurological assessment, and automated pupillometry offers objective and reproducible data. The B-ICONIC consensus recommends an NPi value of ≤3 as suggestive of increased ICP, with an NPi ≤2 conferring a higher predictive value for significant intracranial hypertension. Changes in NPi of at least 1 point from baseline are also considered clinically relevant, reinforcing the importance of continuous monitoring in critically ill patients. Traditional pupillary assessment is subject to interobserver variability, but automated pupillometry eliminates this limitation by providing quantitative measurements. The ability of pupillometry to detect subtle changes in pupillary reactivity before overt clinical deterioration makes it a powerful screening tool for intracranial hypertension. Additionally, the use of pupillometry in combination with other non-invasive modalities strengthens its diagnostic utility. This technique has also been employed to monitor the response to therapeutic interventions, such as osmotherapy and decompressive craniectomy. Automated pupillometry also enables precise tracking of neurological deterioration in real- time. By integrating NPi with other neuromonitoring data, clinicians can establish trends in ICP changes and make timely interventions. Technological advancements in pupillometry are expected to further enhance its role in TBI management, with newer devices offering increased sensitivity and specificity in detecting subtle neurophysiological changes.
Integration of Non-Invasive ICP Monitoring in Clinical Practice The B-ICONIC consensus underscores that no single nICP tool should be used in isolation; rather, a multimodal approach incorporating at least two techniques is advised to enhance accuracy. nICP monitoring should be integrated with clinical assessment and neuroimaging when available. These methods are particularly valuable in settings where invasive monitoring is not feasible, allowing for early detection and timely therapeutic interventions. The integration of non-invasive techniques into existing neurocritical care protocols has significantly impacted the management of TBI patients. Combining TCD/TCCD, ONSD, and pupillometry allows for a comprehensive evaluation of intracranial dynamics, facilitating early detection of rising ICP. The use of these tools also enhances decision-making regarding the need for additional interventions, such as cerebrospinal fluid drainage or hyperosmolar therapy, but also for considering therapy de-escalation (see figures 3 and 4 below, extracted from original paper). Furthermore, the ability to perform serial measurements provides a dynamic view of ICP trends, which is essential for titrating treatment strategies.
Clinical Scenarios and Practical Considerations Several clinical scenarios benefit from the implementation of non-invasive ICP monitoring. In patients with moderate TBI (GCS 9-12) and evolving intracranial pathology, serial assessments using TCD/TCCD, ONSD, and pupillometry help guide escalation or de-escalation of therapy. Similarly, in cases where invasive ICP monitoring is contraindicated, non-invasive methods serve as essential tools for ongoing surveillance. Moreover, non-invasive methods play a crucial role in resource-limited settings where access to advanced neuromonitoring is restricted. The ability to obtain real-time data at the bedside allows for early therapeutic interventions, reducing the risk of delayed treatment and subsequent neurological deterioration. Training programs focused on standardizing nICP measurement techniques have improved the adoption of these methods in clinical practice, ensuring their effective implementation across diverse healthcare settings.
Conclusion The B-ICONIC consensus provides a structured approach to non-invasive ICP monitoring in TBI, delineating evidence-based thresholds for TCD/TCCD, ONSD, and pupillometry. While these techniques cannot replace invasive ICP monitoring, they serve as critical adjuncts in guiding clinical decision-making. Further studies are required to validate these recommendations and optimize their implementation in neurocritical care settings. By refining non-invasive monitoring strategies, we can enhance the care of TBI patients worldwide, particularly in resource-limited environments. The future of non-invasive neuromonitoring lies in the development of advanced algorithms that integrate multiple data sources to improve predictive accuracy. Ongoing research is focused on refining existing modalities and exploring novel approaches, such as machine learning-based ICP estimation models. As technology continues to evolve, the role of non-invasive ICP monitoring will expand, bridging the gap between traditional neuromonitoring methods and cutting-edge innovation in neurocritical care.
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