Sir,
We are writing to congratulate Sujata et al. for their original and interesting case report on optic nerve sheath diameter (ONSD) guided extubation plan in obese patients undergoing robotic pelvic surgery in steep Trendelenburg position.[1] We consider it a really challenging article, but we would like to comment on some aspects of optic nerve sheath diameter assessment with ultrasound.
For their evaluation, the authors utilised ultrasound B scan technique, which has been employed for more than 50 years to identify several ocular and orbital diseases. Unluckily, it has been demonstrated to be quite untrustworthy for measurements of small structures such as ONSD, due to the so-called blooming effect.[2] It is caused by the absence of a standard sensitivity setting in performing this examination and this means that, when decreasing the gain, the ONSD will show larger measurements compared to the ones acquired with an increased gain.
However, this effect could be unimportant when large lesions need to be evaluated, but will definitely be very significant in case of lesions less than 0.5 mm, where also few microns could influence the interpretation of the ONSD.
For this reason, to overcome the aforesaid pitfalls, we would like to advise to use the Standardised A Scan. It is a blooming effect-free ultrasound technique that displays easily noticeable hyperreflective spikes from the interface between arachnoid and subarachnoidal fluid, making these measurements objective and exact, thereby providing more accurate and universal reference range values.[3]
Moreover, A scan examination permits to perform the ’30 degrees test’, which allows us to distinguish between an ONSD increase caused by raised intracranial pressure related to increased subarachnoidal fluid, and that one associated with other diseases, such as optic neuritis or optic nerve meningioma.[4,5]
Lastly, we would like to suggest performing ocular ultrasonography with open eyelids, using methylcellulose and anaesthetic drops, to clearly visualise the eye, making the probe orientation much more accurate, which would help in avoiding errors in detecting gaze direction.[6]
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
REFERENCES
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