By convention the ONSD is assessed 3 mm behind the papilla. In order to gauge the ONSD, the distance between the external borders of the hyperechoic area surrounding the optic nerve should be quantified (Fig. 1). Several studies reproduced a high intra- and interobserver reliability of the sonographic ONSD assessment [6], [7] and [8]. However, data on normal values
vary CHIR-99021 considerably, especially in former publications [9]. This may be explained by differing ultrasound equipments and their influence on sonographic findings and measurement criteria different from the ones stated above. Therefore, several authors emphasized the necessity of correctly used measuring points and clearly displayed optic nerve structures for reliable results
[10] and [11]. In our study on this topic, using above criteria, the mean ONSD was 5.4 ± 0.6 mm in healthy adults that matches closely with results derived from two MRI studies [7]. Rohr et al. found a value of 5.3 ± 0.6 mm in patients with mental disorders but without intracranial lesions or signs of elevated ICP [12]. Geeraerts et al. indicated a mean ONSD of 5.1 ± 0.5 mm in healthy volunteers [13]. Accordingly, a cadaver study illustrated a good correlation between the evaluation of the ONSD by MRI and transbulbar sonography. Despite the unfavorable angle between the course of the optic nerve and the insonation direction in transbulbar sonography Steinborn et al. observed an acceptable agreement between MRI and the sonographic approach [11]. These results have been verified in an investigation of sixty-five children, recently [10]. In comatose or sedated patients with intracranial BTK inhibitor bleeding and traumatic head injury sonographic ONSD evaluation has been proven to be feasible in predicting raised ICP [3], [14] and [15]. An MRI-based investigation confirmed this observation [13]. Geeraerts et al. found a mean ONSD of 6.3 ± 0.6 mm in brain injured adults using sonography [14]. By means of MRI they
indicated a mean ONSD of 6.3 ± 0.5 mm Flavopiridol (Alvocidib) [13]. The threshold of ONSD predicting an elevated ICP was proposed to be between 5.7 and 5.9 mm [3], [13], [14] and [15]. In a metaanalysis of six studies with data on a total of 231 patients with traumatic brain injury or intracranial hemorrhage the technique had a sensitivity of 90% and a specificity of 85% [16]. Furthermore, transbulbar ONSD assessment has been suggested for follow-up examinations of children with internal hydrocephalus and ventriculoperitoneal shunt systems [17]. Moreover, two sonographic investigations observed a correlation between the severity of acute mountain sickness and ONSD [18] and [19]. Only few results were published on the sonographic ONSD evaluation in idiopathic intracranial hypertension (IIH) [20]. One MRI based retrospective study described a mean ONSD of 6.5 ± 0.9 mm in patients suffering from IIH and quote a cut-off value for raised ICP of 5.8 mm [21].