In this study, the findings suggest that both ONSDI (Optic Nerve Sheath Diameter Internal) and ONSASW (Optic Nerve Subarachnoid Space Width) can be utilized to evaluate increased intracranial pressure (ICP) after brain injury. The ONSASW, defined as ONSDI minus OND (Optic Nerve Diameter), demonstrated better diagnostic value for increased ICP compared to ONSDI alone. Two different measurement methods of ONSASW were considered, and it was found that ONSASW defined by ONSDI minus OND has superior diagnostic value than ONSDE minus OND. This suggests that ONSASW, specifically defined as ONSDI minus OND, could be a novel and potentially more accurate indicator for non-invasive ICP measurement.
In orbital ultrasound images (Fig. 1), the low-echo optic nerve is enveloped by three distinguishable layers (from the inner to the outer layer): the high-echo space signifies the subarachnoid space, while the outer low-echo area signifies the dura mater and the outer high-echo space represents the retrobulbar fat tissue as described previously (16). Anatomically, OND signifies the distance within the pia mater, and ONSD represents the distance within the dura mater (corresponding to ONSDI in our study). However, in some studies (10–12), ONSD is quantified by measuring the distance between the outer walls of the internal high-echo area (corresponding to ONSDE in our study). In our study, these two methods of ONSD measurement were utilized, expressed as ONSDI and ONSDE, respectively. Presently, only a limited number of studies (17–20) have concurrently evaluated ONSDI and ONSDE, comparing their association with increased ICP. A previous study (20) suggested that there was no significant difference in AUC for predicting elevated ICP between ONSDI and ONSDE. Another study (19) compared their relationship with ICP and found that the predictive value of ONSDI was superior to ONSDE, consistent with our results. In our observations, the outer line of the subarachnoid space was more likely to appear indistinct on ultrasonography compared with the outer line of the dura mater. This might explain why ONSDE showed no difference between the two groups and lacked correlation with ICP in this study. In fact, ONSD has not been widely applied in clinical practice, and a crucial reason is the heterogeneity of results due to different measurement methods. Therefore, a crucial aspect of future research is standardized measurement methods, providing an essential foundation for ensuring result accuracy.
The space between the optic nerve sheath and the optic nerve, commonly referred to as the ONSAS in previous studies (21, 22), is connected to the cranial subarachnoid space. As the ONSAS is affected by any change in ICP and dilates with increased ICP, patients with increased ICP exhibit a wider ONSAS than normal individuals, and vice versa (17). As is known, cerebrospinal fluid (CSF) can pass through the optic foramen into the ONSAS, contributing to its expansion when ICP increases. However, a potential oversight is that ONSAS expansion involves not only the outward expansion of the ONSD but also potential optic nerve compression, contributing to the narrowing of the OND. Additionally, individual variations in OND were disregarded when directly substituting subarachnoid space with ONSD. Consequently, compared to utilizing ONSD for assessing ICP levels, the outcomes of ONSASW may be more direct, reliable, and possess higher diagnostic value. This study revealed a positive correlation between ONSDI and ONSASW with ICP, whereas OND exhibited a negative correlation trend with ICP, though statistically insignificant. Furthermore, this study observed that the diagnostic value of ONSASW surpassed that of ONSDI. We observed that ONSASW, defined as ONSDI-OND, exhibits significantly better diagnostic value for increased ICP compared to ONSDI and ONSDE-OND. This outcome implied that ONSASW was associated with ICP and might serve as a viable substitute for ICP evaluation. However, further investigation with a larger sample size is necessary to validate this result.
In this study, it was observed that the ONSDI/EDT exhibited no statistical difference between the normal and increased ICP groups, and its diagnostic value for increased ICP was found to be limited. In prior retrospective studies (23, 24), findings demonstrated that the ONSD/ETD ratio based on CT possessed a robust predictive value for elevated ICP, surpassing even the predictive value of ONSD. Another prospective study suggested that the ONSD/ETD ratio, whether based on ultrasound or CT, holds equivalent diagnostic value for increased ICP (25). It is speculated that the varying conclusions in comparison with previous studies may arise from a limited sample size and disparities in research subjects. Indeed, this study discovered that the ONSDI/EDT ratio was associated with the ICP, despite the lack of significant diagnostic value for increased ICP. Consequently, further study is necessary before the ONSD/ETD ratio can be used in making clinical decisions.
Our study possesses several limitations. Firstly, the study population was limited, with a relatively small number of patients exhibiting increased ICP. Secondly, individuals with severe ICP among brain-injured patients were excluded. Thirdly, the validation of the relationship between ICP and dynamic changes in ONSASW was not conducted. The extrapolation of findings from this study should be interpreted with caution due to these limitations.