Comment on: Optic nerve sheath diameter measurement by ultrasound and age group.

Abstract

To the Editor: We read with interest the article by Yilmaz et al.1 While the idea presented in the article is intriguing, we have some concerns to raise. The authors employed a 7.5 MHz linear B scan probe to perform ONSD measurement in longitudinal and coronal sections. The aforementioned method is extensively used in the identification of various ocular ailments, and in some cases, it has been shown to be superior to OCT in detecting ocular diseases such as ON drusen.2 ONSD measurement by US B-scan has been also widely used as a non-invasive method to detect increased intracranial pressure, but unfortunately its accuracy is limited by the presence of several artifacts.3-5 Among these artifacts there is the so called blooming effect.6-8 While this effect may not significantly affect the measurement of larger lesions, it may result in erroneous measurements when differences of less than 0.5 mm are critical, as it happens in the differential diagnosis of optic nerve lesions.9 The blooming effect could be responsible for variations in the thresholds reported across different studies. Furthermore, measuring ONSD in primary position is essential since altering eye position can affect the quantity of cerebrospinal fluid surrounding the optic nerve, resulting in an inaccurate ONSD measurement.10 The authors mention that they performed the examination with closed eyelids, which can make it challenging to determine gaze direction and decrease the image quality due to sound attenuation from the lids, thereby making the results less reliable.11,12 Moreover, the two groups of patients of this study have a very important difference in mean age. Even if there is no clear connection between ONSD variations and age, in future studies we suggest having two more homogenous groups for age, sex and previous disease to give more strength to the results of the study. To eliminate these anomalies and guarantee more accurate outcomes, we recommend the standardized A-scan techniques using anesthetic eye drops and in the case of the B scan, methylcellulose as a coupling agent between the probe and the eye be used in future studies.