Comparison of accuracy of optic nerve ultrasound for the detection of intracranial hypertension in the setting of acutely fluctuating vs stable ICP: post-hoc analysis of data from a prospective, blinded single center study (Critical Care)

IntroductionOptic nerve sheath diameter (ONSD) measurement with bedside ultrasound has been shown in many studies to accurately detect high intracranial pressure (ICP). The accuracy of point-in-time ONSD measurement in the presence of ongoing fluctuation of ICP between high and normal is not known. Recent laboratory investigation suggests that reversal of optic nerve sheath distension may be impaired following bouts of intracranial hypertension. Our objective was to compare the accuracy of ONSD measurement in the setting of fluctuating versus stable ICP. Methods: Retrospective analysis of data from prospective study comparing ONSD to invasive ICP. Patients with invasive ICP monitors in the ICU underwent ONSD measurement with simultaneous blinded recording of ICP from the invasive monitor. 3 measurements were made in each eye. Significant acute ICP fluctuation (SAIF) was defined in 2 different ways; as the presence of ICP both above and below 20mmHg within a cluster of 6 measurements (Definition 1) and as magnitude of fluctuation >10mmHg within the cluster (Definition 2). The accuracy of point-in-time ONSD measurements for the detection of concurrent ICP>20mmHg within clusters fulfilling a specific definition of SAIF was compared to the accuracy of ONSD measurements within clusters not fulfilling the particular definition by comparison of independent receiver operating characteristic (ROC) curves. Results: 613 concurrent ONSD-ICP measurements in 109 clusters were made in 73 patients. 23 (21%) clusters fulfilled SAIF Definition 1 and 17 (16%) SAIF Definition 2. For Definition 1, the difference in the area under the curve (AUC) of ROC curves for groups with and without fluctuation was 0.10 (P=0.0001). There was a fall in the specificity from 98%(95%CI 96-99%) to 74%(63-83%) and in the positive predictive value from 89%(80-95%) to 76%(66-84%) with fluctuation. For Definition 2 also there was a significant difference between the AUC of ROC curves of groups with fluctuation-magnitude >10mmHg and those with fluctuation-magnitude 5-10mmHg (0.06, P=0.04) as well as 20mmHg are substantially less in patients demonstrating acute fluctuation of ICP between high and normal. This may be because of delayed reversal of nerve sheath distension.