To the Editor:
We read with great consideration the valuable and interesting case report by Wipplinger et al concerning the use of ultrasound as a diagnostic tool to identify acute high altitude illness. 1 Particularly, we focused on the optic nerve sheath diameter (ONSD) ultrasound evaluation, and we would like to point out some considerations on this helpful method.
The authors used B-scan ultrasonography to assess the ONSD increase between Day 7 and Day 8, as shown in Figures 3 and 4 of their paper. 1 However, these 2 figures seem not to be comparable with each other. Regardless of a small methodological bias related to the difference of 0.01 cm in the ONSD measurement distance between the 2 figures, 1 which would not significantly alter the ONSD results, the optic nerve sections evaluated and depicted in the 2 figures are not the same. Figure 4 seems to show the optic nerve insertion, but the same could not be said for Figure 3, where some scleral tissue appears to be present between the optic nerve and the retinal margin. This is due to the nonperpendicularity of the ultrasound beam to the optic nerve head in this scan, which could provide misleading data.
Furthermore, in this kind of measurement, to provide information regarding the gain used, the probe positioning on the eye and the echographic plane are required for repeatable and reliable results. Unfortunately, this information is missing from the presented case. Moreover, trustworthy and precise caliper positioning in ONSD measurements with B-scan ultrasonography is very difficult because it has been widely demonstrated that this technique is affected by the “blooming” effect, which makes such measurements inaccurate and unreliable. 2 -5
For all the aforesaid limitations, it is advisable to use the standardized A-scan technique proposed in the 1970s, 6 which is able to provide more objective measurements. This technique is free of the “blooming” effect to allow more precise optic nerve identification and evaluation through the visualization of easily discernible high reflective spikes from the interface between arachnoid and subarachnoidal fluid.7,8 Another important advantage of this ultrasound technique is the ability to perform the “30 degrees” test, a very useful test to discriminate between an ONSD increase related to raised intracranial pressure and an ONSD increase due to a solid thickening of the optic nerve.9,10
In conclusion, we would like to suggest the use of the standardized A-scan technique, instead of B-scan ultrasonography, to obtain a more complete, accurate, and reliable optic nerve evaluation.