Measuring optic nerve sheath diameter using ultrasonography for the detection of non invasive intracranial pressure: what it is and what it is not

Abstract

Measuring optic nerve sheath diameter using ultrasonography for the detection of non invasive intracranial pressure: what it is and what it is notMedição do diâmetro da bainha do nervo óptico por ultrassonografia para aferição não invasiva da pressão intracraniana: o que é e o que não é Chiara ROBBA 1,2 O ver the last decade, the study of optic nerve sheath diameter (ONSD) using ultrasonography has gained particular interest for the assessment of non-invasive intracranial pressure (nICP) 1 . ONSD ultrasonography is an elegant technique, which has the advantage of being easily available, repeatable at bedside, low cost and cheap, avoiding the risks of infection or hemorrhage related to intracranial catheters. The pathophysiology behind this technique is simple: the optic nerve is surrounded by the meninges and the subarachnoid space, and has elastic properties; therefore, when intracranial pressure (ICP) rises, the optic nerve sheath is distended and the ONSD increases.ONSD has been evaluated on Magnetic Resonance imaging and Computed Tomography studies 2,3 , demonstrating a good accuracy for the real time estimation of ICP. The possibility to evaluate ONSD as a surrogate of ICP using a safe method, which does not require the transfer of patients to the radiological suite and does not expose them to ionizing radiation has led to a high number of studies and publications on the Ultrasound based estimation of this tool.In general, observational studies and meta-analysis suggest promising results regarding the correlation between ONSD and nICP, showing moderate to high sensitivity for the detection of elevated ICP (86% to 97%) 4-6 .However, some striking points and issues have been described for this tool, especially regarding the lack of a universal methodology used in different studies. A vast heterogeneity is present in the literature regarding the transducers and frequencies adopted 4,5 by researchers, the measurements planes 4,5 , the number of measures from one or two ONSDs, the patient´s positioning 4,5 ; in addition, a non-universal definition of the quality of the images obtained and of the US visualization of ONSD exists, thus resulting in the measure in some cases of the ONS instead of the entire ONSD 4,5 .Open questions also remain, such as the ability of ONSD to return to its initial size after treatment or resolution of increased ICP, or in case of intracranial hypotension.In patients after subarachnoid hemorrhage (SAH), for instance, dramatic increases of ICP consequent to aneurysm rupture can lead to a disruption of the elastic properties of the ONS membrane, thus resulting in enlarged ONSD even without the presence of intracranial hypertension 7 .In addition, a major issue remains the large variations in ONSD cutoffs evaluated to estimate the critical threshold of ICP> 20 mmHg, ranging from 4.2 to 6.5 mm with wide confidence intervals according to literature 8,9 .Systematic reviews and meta-analysis 5,9 , suggest the best cut-off value of ONSD of 5.1 and 5.8 mm, with the area under the hierarchica...