The Indirect Assessment of Intracranial Pressure by Measuring the Optic Nerve Sheath Diameter is Unreliable as Long as Interfering Factors are Disregarded

We read with interest the article by Parameswaran et al. regarding a prospective study comparing the optic nerve sheath diameter (ONSD) measured by transorbital ultrasound (US) and computed tomography (CT) in 140 patients admitted to the intensive care unit due to brain disease complicated by increased intracranial pressure (ICP). 1 The mean ONSD measured by US was 4.97 mm, and the mean ONSD measured by CT was 5.07 mm. 1 Based on CT, 65.7% of patients had elevated ICP; the sensitivity for predicting elevated ICP was 56%, and the specificity was 73%. 1 Based on US, elevated ICP was predicted with a sensitivity of 74% and a specificity of 56%. 1 There was no difference in the diagnostic performance of CT and US. 1 The study is interesting, but some uncertainties still need to be clarified.The first point is that, regardless of the measurement method used, ONSD depends not only on ICP but also on numerous other factors. 2These include the collagen and elastin content of the sclera, age, gender, posture, tissue stiffness, intraocular pressure (glaucoma), the filling status of the intraorbital veins, intraorbital pressure, intraorbital geometry, the morphology of the optic nerve, the morphology of the eyeball (tortuosity, radius, and scleral thickness), blood pressure, respiratory status, sea level, coughing, bending, and current medications. 3 Medications that increase ICP include vasodilators, opioids, vitamin A, tetracyclines, and lithium, while medications such as osmotic agents, diuretics, and some steroids lower ICP. 4 As long as these influencing factors are not taken into account and the cohort under investigation is not homogenized with regard to these confounding factors, the reliability of ONSD measurements remains low.The second point is that the interobserver variability (IOV) between the two researchers carrying out the study was not calculated. 1 Although the authors mention IOV measured in other studies in the methods section, their own IOV was not calculated. 1 Without knowing the IOV between the two researchers, the provided data become unreliable.The third point is that indirect assessment of ICP by ONSD measurement was not compared with direct measurements of ICP.Direct measurement of ICP is performed invasively using a microcatheter or microsensor through a burr hole drilled in the skull into the cerebrospinal fluid (CSF) spaces or onto the meninges. 5 The gold standard for measuring ICP is to measure ICP within the CSF spaces by connecting the probe to an external monitor to perform continuous pressure measurements. 5 As long as the assessment of orCid