LAUSR

Dear Editor, We were very interested in the noteworthy paper by Zoerle and colleagues, concerning the relationship between optic nerve sheath diameter (ONSD) and intracranial pressure in patients with subarachnoid hemorrhage [1]. They compared the ONSD measurements provided by magnetic resonance imaging (MRI) with the ultrasound findings, also analyzing the relationship between these ultrasound measurements and intracranial pressure values directly measured through external ventricular drainage [1]. We believe this is a really challenging study, but we would like to comment on some technical aspects related to ocular ultrasonography in gauging ONSD. To achieve the goals of their study, the authors utilized B-mode ultrasound on closed eyelids. Unluckily, this evaluation method is not the most accurate and objective to evaluate ONSD, as it has been previously discussed in the literature [2]. In fact, B-scan is affected by the “blooming effect,” which is related to the absence of a standard gain setting in performing this ultrasound examination. This means that, with a lower gain setting, the ONSD will show larger measurements compared to the ones got with a higher gain setting [3]. Unfortunately, the concordance they claimed between ultrasonography and MRI has not been proven, as the authors utilized a paired T test in only ten patients for such evaluation, making the statistical evaluation unmeaning. Furthermore, performing ocular ultrasonography with closed eyelids, it is not possible to clearly visualize the eye, making the probe orientation less more reliable, with potential errors in detecting gaze direction [4]. All these aspects could provide less trustworthy ultrasound measurements, potentially altering the results of the study. For this reason, to better assess ONSD, we would like to suggest the use of the standardized A-scan technique with open eyelids, utilizing anesthetic drops [5]. This blooming effect-free ultrasound technique displays easily noticeable hyperreflective spikes from the interface between arachnoid and subarachnoid fluid, providing exacter and more objective measurements [3]. Moreover, this ultrasound examination permits to perform the “30-degree test,” which allows us to distinguish between an ONSD increase caused by raised intracranial pressure related to increased subarachnoid fluid, and that one associated with other diseases [5]. This test could provide information similar to the ones obtained by the dynamic test performed by the authors in their study [1], although we are aware that, in unconscious patients, it would be advisable to use a forceps to move the eye in order to execute the “30-degree test.” In conclusion, the authors’ statement that ultrasound ONSD measurements “cannot be presumed to be related to simultaneous ICP values” [1] could be explained by the above-discussed B-scan limitations.