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

Despite invasive methods are the gold standard for intracranial pressure (ICP) measurement, several non-invasive techniques (nICP) have been proposed as surrogate, although their use remains insufficiently recognized in clinical practice. These include transcranial Doppler blood flow velocity assessment (arterial or venous), optic nerve sheath diameter (ONSD), automated pupillometry, measurement of skull expansion and compliance, brain imaging, double-depth ophthalmic artery blood flow velocity, and ultrasound time-of-flight. The main limitations of all indirect methods are calibration and zeroing, which constrain the absolute accuracy of non-invasive ICP monitoring. For transcranial Doppler-based methods, the 95% limits of agreement are approximately ± 7-15 mmHg, while for ONSD-based techniques they range from ± 7-10 mmHg. Improved predictive accuracy may be achieved by combining different modalities and applying advanced signal analysis techniques. Importantly, in patients with acute brain injury, nICP can complement invasive monitoring by guiding patient selection for urgent monitoring, facilitating brain assessment in moderate traumatic brain injury, and assisting management in patients with coagulopathy. In the general intensive care population, nICP may provide valuable information after cardiac arrest, liver failure, and sepsis. In the emergency department, early detection of intracranial hypertension helps prevent missing the "golden hour" of brain care. Finally, nICP is particularly relevant in low-resource settings, where intensive care facilities are limited.