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We read with great interest the article by Naranjo-Bonilla et al. concerning changes in retinal and choroidal thickness (ChT) in patients with obstructive sleep apnea (OSA) who underwent continuous positive airway pressure treatment (CPAP) [...]

Ultrasonographic appraisal of the optic nerve sheath diameter has become popular in recent years as a useful diagnostic tool to detect intracranial pressure variations. Intracranial hypertension is a life-threatening disease with possible poor clinical outcomes and can be caused by a variety of neurological and non-neurological conditions. Considering the latter, increases in intracranial pressure have also been described during several surgical procedures. Ocular ultrasonography might be utilized to identify intracranial pressure increases by evaluating optic nerve sheath diameter variations. The aim of this review is to provide a wide overview on the use of the optic nerve ultrasound evaluation to detect intracranial pressure changes during surgical procedures, also discussing the pitfalls of the B-scan technique, the most widely used for such a purpose. PubMed medical database, Web of Science and Scopus were used to carry out this review. The present review showed that ocular ultrasonography could be considered a valuable diagnostic tool in the surgical setting to indirectly assess intracranial pressure. However, the use of the B-scan ultrasound should always be coupled with the standardized A-scan technique for a more accurate, precise and trustworthy ultrasound assessment.

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To the Editor: We really appreciate the study from Copcuoglu et al 1 about the optic nerve sheath diameter (ONSD) in patients with chronic obstructive pulmonary disease.The authors made the measurements with ocular ultrasonography, which is a fast, non-invasive tool able to assess intracranial pressure during different medical conditions.We certainly agree with this but we would like to comment some points.The authors placed a linear probe of a standard USG machine, surrounded by a sterile glove, on the closed eyelids of the patients. In our opinion this is not the best way to perform such examination. Even if this method has been used by several authors in the international literature, placing the probe over closed eyelids makes very difficult to assess the gaze direction, resulting in a wrong probe orientation and a wrong ONSD measurement. 2,3 Besides, the obtained signal strength is reduced by the glove. For these reasons, we suggest performing ocular echography with open eyelids, using methylcellulose and anesthetic drops, to visualize the patient's gaze to correctly orient the probe and without any cover that may interfere with the ultrasound.Another point that we would like to discuss is the use of a B scan probe. This technique is very useful in differentiating papilledema from Drusen that can simulate a papilledema, but it is not so effective in ONSD measurements. 4 Among the reasons there is the socalled "blooming effect" because the gain of the scan, and therefore the brightness of the image, influences ONSD measurement. With high gain the ONSD will appear smaller due to the increased wall brightness, and vice versa with low gain. This effect is particularly important as the optic nerve is such a small structure and B-scan lacks a standard sensitivity setting. 5 For the above-mentioned reasons, we would like to suggest the use of Standardized A Scan: this method displays hyperreflective spikes from the interface between arachnoid and subarachnoid fluid overcoming the blooming effect and therefore providing standardized, objective, and reliable measurements. 6