Noninvasive Intracranial Pressure Monitoring: Are We There Yet?

Abstract

There is an urgent unmet need for a reliable noninvasive tool to detect elevations in intracranial pressure (ICP) above guideline-recommended thresholds for treatment. Gold standard invasive ICP monitoring is unavailable in many settings, including resource-limited environments, and in situations such as liver failure in which coagulopathy increases the risk of invasive monitoring. Although a large number of noninvasive techniques have been evaluated, this article reviews the potential clinical role, if any, of the techniques that have undergone the most extensive evaluation and are already in clinical use. Elevations in ICP transmitted through the subarachnoid space result in distension of the optic nerve sheath. The optic nerve sheath diameter (ONSD) can be measured with ultrasound, and an ONSD threshold can be used to detect elevated ICP. Although many studies suggest this technique accurately detects elevated ICP, there is concern for risk of bias and variations in ONSD thresholds across studies that preclude routine use of this technique in clinical practice. Multiple transcranial Doppler techniques have been used to assess ICP, but the best studied are the pulsatility index and the Czosnyka method to estimate cerebral perfusion pressure and ICP. Although there is inconsistency in the literature, recent prospective studies, including an international multicenter study, suggest the estimated ICP technique has a high negative predictive value (> 95%) but a poor positive predictive value (≤ 30%). Quantitative pupillometry is a sensitive and objective method to assess pupillary size and reactivity. Proprietary indices have been developed to quantify the pupillary light response. Limited data suggest these quantitative measurements may be useful for the early detection of ICP elevation. No current noninvasive technology can replace invasive ICP monitoring. Where ICP monitoring is unavailable, multimodal noninvasive assessment may be useful. Further innovation and research are required to develop a reliable, continuous technique of noninvasive ICP assessment.

Keywords: Acute brain injuries; Intracranial pressure; Optic nerve; Pupillary reflex; Transcranial Doppler ultrasonography.

Fig. 1

Sonographic measurement of optic nerve sheath diameter (ONSD). Transverse view of the eyeball, with zoomed view of the posterior globe. The globe and retina are visible, with the optic nerve (OpN) visible as a linear densely hypoechoic structure posterior to the globe. The contents of the optic nerve sheath (ONS) are visible on either side of the optic nerve. Caliper A identifies a point 3 mm posterior to the retina, whereas caliper B measures the ONSD at this point at 0.64 cm

Fig. 2

Potential for error in measurement of the optic nerve sheath diameter (ONSD). In panel a, the ONSD is measured at 0.461 cm. However, this is an erroneous measurement from a poor-quality image: the optic nerve and the margins of the optic nerve sheath on either side are not clearly visible. Minimal tilting of the transducer brings these structures into clear focus in panel b, where the ONSD is now measured correctly at 0.567 cm

Fig. 3

Transcranial color-coded sonography (TCCS) for estimation of intracranial pressure (ICP) in a patient with severe traumatic brain injury. The time-averaged peak velocity (TAP) in the right middle cerebral artery (MCA) is measured at 109.6 cm/s following correction for an angle of insonation of 12°. The end diastolic velocity (EDV) is 66.3 cm/s. Simultaneously measured intraarterial mean arterial pressure (MAP) is 101 mm Hg. Using the Czosnyka formula, the estimated ICP (ICPe) is calculated at 26 mm Hg. The simultaneously measured ICP from the invasive monitor is 22 mm Hg

Fig. 4

Quantitative pupillometry. The image on the left shows use of the pupillometer, which automatically detects the pupil with image analysis. The image on the right shows measurements of the bilateral neurological pupil index (NPi; NeurOptics, Irvine, CA) and pupillary size with side-to-side difference