This study’s results indicated that the change in ICP was not correlated with the change in ONSD when patients changed from the supine position to the 30° head-up position. Additionally, in the dynamic test, only three of 16 patients showed good agreement between ICP and ONSD, indicating that ONSD was not an accurate tool to dynamically estimate ICP.
The ONSD measured by ultrasonography is an important noninvasive tool to estimate ICP. It can be used to screen patients with high ICP while avoiding the placement of intracranial probes and avoiding the risks of bleeding and infection(5). Previous studies have demonstrated the accuracy of ONSD for ICP measurement(4, 14). A meta-analysis performed by Robba et al. demonstrated that ultrasonographic ONSD could offer good accuracy for detecting intracranial hypertension(6). A recently published meta-analysis enrolled 779 patients from 22 studies and suggested that ONSD had high sensitivity and specificity to diagnose patients with high ICP(15). Consistent with these studies, this study revealed that the ONSD and ICP values obtained in the supine position on admission were strongly correlated (r = 0.799).
The head-up position has been used in the treatment of patients with high ICP for decades, as this position decreases ICP quickly(16). In this study, when patients changed position from supine to 30° head-up, the ICP and ONSD decreased by 4.27 ± 1.98 mmHg and 0.46 ± 0.34 mm, respectively. However, the change in ICP was not strongly correlated with the change in ONSD (r = 0.358). Kerscher et al. detected dynamic changes in ONSD and ICP after ICP decreasing therapy, and no correlation between ∆ICP and ∆ONSD was found(17). Kavi found that ONSD expansion could persist even after ICP control, so ONSD assessment throughout the acute phase of neurologic injury may not be a reliable method to monitor ICP(9). Bäuerle et al. reported similar findings; the ONSD remained expanded after normalization of ICP in 27 patients with subarachnoid hemorrhage after aneurysm rupture(18). This result was most likely related to the impairment of the retraction capability of the ONS(18). Zoerle et al. explored the changes in the ONSD at the time of fairly rapid changes in ICP after CSF drainage(5). Unfortunately, the changes in ONSD related poorly to the ICP alterations. The impairment of ONS elasticity after subarachnoid hemorrhage may have contributed to this result(5).
The impairment of ONS elasticity could be used to explain this study’s results. Usually, high ICP results in a shift of CSF into the ONS, causing the enlargement of the ONSD(6). When ICP decreases, the CSF will flow back into the intracranial subarachnoid space, and the ONS will reduce to its initial diameter. However, these changes require normal function of ONS elasticity(5). Hansen et al. isolated human optic nerve preparations to explore the elastic properties of the ONS(19). They found that ONSD reversibility may be impaired after episodes of prolonged intracranial hypertension(19). In this study, when the patients changed position from supine to 30° head-up, the ICP immediately decreased. However, intracranial hypertension impaired the ONS elasticity, so the ONSD did not correspondingly rapidly reduce. Therefore, ONSD might not be suitable to dynamically monitor ICP.
This study’s dynamic test further confirmed this hypothesis. From day one to day three, a good agreement between ICP and ONSD only existed in three (18.75%) patients. In addition, three patients had completely different profiles for ICP and ONSD. This result further demonstrated that ONSD might not be suitable to monitor ICP dynamically. However, the study conducted by Wand et al. indicated that ONSD measurements were a useful tool for dynamically evaluating ICP(11). They found that the changes in ICP and ONSD values were correlated (r = 0.669) from admission through follow-up. Wang et al. study had a long follow-up period (one month) and the maximum ICP value was only 400 mmH2O (29.41 mmHg)(11). The impairment of ONS elasticity may have recovered during the long follow-up period. Therefore, further studies are needed to clarify this issue.
The present study had several limitations. First, this study was limited by its small sample size, especially in the dynamic test (only 16 patients). Studies with more patients from different centers should be conducted to confirm this study’s findings. Second, the dynamic test was only conducted from day one to day three after admission. This monitoring time was too short. A study with a longer follow-up time should be performed to determine if ONSD can be used to dynamically monitor ICP. Furthermore, in this study, the mechanism of why ONSD was not suitable for ICP dynamic monitoring was not clarified. In addition, the impairment of ONS elasticity and whether it contributed to the findings should be investigated further. Finally, this study did not focus on the difference in ONSD measurements for each eye. The variance of ONSD measurements in the transverse and sagittal axes was not detected either. For ONSD measurement, a consensus was not reached regarding the axis to be measured and the eye to be used (20). These issues should be investigated in future studies.