Optic nerve ultrasonography to predict increased intracranial pressure in idiopathic intracranial hypertension

We were very interested in the article by Kishk et al. concerning the use of ocular ultrasonography to predict raised intracranial pressure (ICP) in idiopathic intracranial hypertension.¹

We congratulate the authors for their remarkable paper, but we would like to make some comments.

In the conclusions, they stated that ‘assessment of optic nerve diameter (OND) and optic nerve sheath diameter (ONSD) with ultrasound may be subject to errors of measurement related to artifact, shadowing and erroneous placement of the cursors’. This is particularly true because for measuring the ONSD they used the B-scan technique, an examination broadly utilised to diagnose ocular diseases that unfortunately is not sensitive enough in measuring all the orbital structures, as it is affected by the so-called blooming effect.^2,3 This one is related to the lack of a standard sensitivity setting in performing the B-scan and should not be confused with the Doppler associated blooming effect. In the case of the B-scan it means that if we measure ONSD with a lower sensitivity setting, this will give larger dimensions compared with those obtained with an increased sensitivity setting. This effect could be less important when we deal with large lesions, but it could be misleading if we suppose a difference inferior to 0.5 mm, as in the case of ONSD evaluation.^4–6

The blooming effect could also explain some strange outcomes Kishk et al. found; according to their results there was no difference in the OND/ONSD ratio between patients with increased ICP and the normal control group. This does not make too much sense because, as they correctly stated in the introduction: ‘increased ICP is transmitted to the subarachnoid space surrounding the optic nerve, leading to optic nerve sheath expansion’. The lack of a significant difference in the OND/ONSD ratio between patients and normal controls could be explained either by a wrong placement of the measurement markers that, as we previously discussed, is very difficult due to the B-scan limits, or by an increase in the optic nerve diameter itself that, to the best of our knowledge, has never been reported.

Furthermore, the authors reported that ‘the optic nerve was visualized as a hypoechogenic structure beyond the retina surrounded by hyperechoic subarachnoid space and hypoechoic dura mater’. In our opinion, this does not make too much sense because the subarachnoid space is filled by liquor, that is fluid, so it could not be hyperechoic, but it is usually hypoechoic/anechoic. Thus, we wonder whether this is a printing error or the authors have a different explanation about this finding.

However, due to the aforementioned limitations, for further studies we would like to suggest to utilise the standardised A-scan technique.^7,8 With this technique it is possible not only to measure with more precision the OND because the interface between the arachnoid and subarachnoidal fluid gives high reflective spikes, that makes such measurements objective, but it is also blooming effect free, as we had the opportunity to point out on different occasions.^9–13

Another issue that we would like to comment on is the use of the probe through the closed lids, making the gaze direction detection difficult, and consequently the exact probe position. In ophthalmology, during the ultrasound examination, the B-scan probe is routinely used with open lids, utilising methylcellulose and anaesthetic drops. This allows the eye to be visualised, making the probe orientation much more reliable.¹⁴

Moreover, we would like to comment on the possible damage to the eye in transorbital echography. Theoretically, this technique could damage the lens, for this reason utilising A-scan echography with the probe located temporally, the lens will be avoided, eliminating this risk. Nevertheless, ultrasonography is absolutely safe, as stated by the US Food and Drug Administration in a publication issued on 9 September 2008 (Information for manufacturers seeking marketing clearance of diagnostic ultrasound systems and transducers) that provides warnings not for B or A-scan modes, but for colour Doppler only.

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Finance

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.