Accuracy and safety of B-scan optic nerve ultrasonography to predict increased intracranial pressure in idiopathic intracranial hypertension

The usefulness of sonographic estimation of optic nerve sheath diameter (ONSD) as an indirect screening test, when raised intracranial pressure (ICP) is suspected and invasive ICP measurement has not been performed, has been proved in many studies.^2–4 In addition, it can guide further diagnostic testing and monitor the response of the patient to therapy.⁵

In response to the points raised by De Bernardo et al.,¹ we would like to clarify the following:

• Regarding the accuracy of B-scan ultrasound, earlier clinical studies^3,4 have described a good correlation between B-mode sonographic ONSD and invasive ICP measurements, which remains the gold standard. Raj et al.⁶ have stated that the ONSD measured by B-scan ultrasound was 88.5% sensitive (95% confidence interval (CI) 68% to 97%) and 90% specific (95% CI 55% to 99%) with computed tomography (CT) as the reference. Furthermore, compared to magnetic resonance imaging (MRI) transorbital B-mode sonography has exhibited reliable measurement accuracy.⁷ In addition, the B-scan technique which was extensively used to assess the ONSD in the case of increased ICP was chosen by our team in order to compare our results with the previously obtained ONSD cut-off points (i.e. we had to perform the same methodology).

• Regarding the safety issues, ocular sonography has been broadly and safely used for ocular evaluation for more than 20 years.⁸ Ophthalmic ultrasound is considered safe as long as Doppler frequency analysis is not used for a prolonged duration.⁹ B-mode ultrasound equipment is not capable of producing a harmful temperature rise.¹⁰ Nevertheless, in our study, the acoustic output of the ultrasound systems was adjusted according to the ALARA principle (‘as low as reasonably achievable’) in order to avoid any possible damage to the lens and retina.¹¹

• Another issue is the use of the probe with open eyelids; to the best of our knowledge B-scan is not routinely used with open eyelids. Alternatively, a thick layer of gel must be applied over the closed upper eyelid and the probe must be placed lightly on the gel in the temporal area of the eyelid (not on the eye itself) to prevent pressure being exerted on the eye.^6,12 The patients in our study were asked to gaze foreword and the placement of the probe was adjusted to allow a suitable angle for displaying the transverse view of the globe and the structures in the retrobulbar area.

• Regarding the statement ‘the optic nerve was visualized as a hypoechogenic structure beyond the retina surrounded by hyperechoic subarachnoid space and hypoechoic dura mater’, it is documented that the optic nerve appears as a sharply defined homogenous low-reflection band which extends posteriorly from the base of the bulb.¹² The optic nerve sheath is demonstrated as a bilateral thin hypoechoic line which is lateral and parallel to the nerve.¹³ On the other hand, the subarachnoid space holds approximately 0.1 ml of cerebrospinal fluid and represents a complex structure which consists of trabeculae, septa and stout pillars arranged between the arachnoid and the pia layers of the meninges of the nerve. This structure renders the subarachnoid space a heterogeneous and multi-chambered tubular system which ends blindly behind the globe.¹⁴ These anatomical features might explain the hyperechogenicity of this space, and this was consistent with previous studies which identified the subarachnoid space as hyperechoic.^15,16

Conflict of interest

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

Finance

None.