ONSD measurement by ultrasound:
A review article by Rosenberg et al. concluded ONSD and Doppler flow as the best noninvasive modalities for determining raised ICP (3). Ultrasonographic measurement of ONSD at a fixed distance from retina has been defined as a standard method of measuring the presence of raised ICP (14). The method of measuring ONSD by sonography using B mode was standardized by Hansel et al. in 1994 (12). It’s a bed side procedure which can be easily done by 5 to 10 MHz linear ultrasound probe (3). It takes just 4 minutes to measure ONSD in both eyes (12). Measurement of ONSD requires trained personnel and sound experience to currently predict raised ICP (8).
A study conducted by Rajajee et al. (4) found ONSD measurement by USG to be an accurate noninvasive method of detecting raised ICP of more than 20mm of Hg. Rajajee et al.(4) had conducted a prospective blinded observational study in which 536 ONSD measurements were performed on 65 patients with invasive ICP measurement side by side on these patients. Correlation between each ONSD measurement by USG and ICP measurement at the same time period was done in this study (4). This study also concluded that ONSD of 04.8mm as a cut off for raised ICP. Sensitivity with 4.8mm cut off value for ONSD was 96% and specificity was 94% for detecting ICP of more than 20cm of water. However, authors suggested that the ONSD values have to be validated for it to become true. They suggested discarding the risk of invasive monitoring and high cost technique of CT or MRI, and including noninvasive method of ultrasound as an alternative tool for detecting raised ICP. It is more cost-effective and less tedious (4).
A study performed by Kimberly et al. (15) identified the correlation of ONSD with direct measurement of raised ICP. They conducted a prospective observational study in which invasive intracranial monitors were placed and ocular ultrasound was performed by 10 to 5 MHz linear probe among 15 individual patients. This study found a significant correlation between ONSD and ICP which was 0.59 calculated by spearman rank correlation coefficient test (15). The authors performed Receiver Operating Characteristic (ROC) curve test to assess the ability of ONSD to detect raised ICP. Authors of this study claim ONSD of greater than 5mm for detecting ICP of more than 20cm of water. Furthermore, the authors demonstrated ROC curve detecting sensitivity of 88% and specificity of 93% (15). They further suggested USG guided ONSD can be used as an noninvasive tool for measuring raised ICP (15).
A systematic literature review and meta-analysis conducted by Ohle et al. (12) concluded both USG of ONSD and CT scan were equally effective in detecting raised ICP. The authors performed a full review on 45 articles out of 1214 available article searches until August 2013 and included 12 studies meeting inclusion criteria with a total of 478 participants. Quality assessment of papers was done by Quality assessment of diagnostic accuracy studies tool (12). Their analysis shows sonography of ONSD was 95.6% sensitive with 95% confidence interval (CI) of 87.7-98.5%. Also, it had specificity of 92.3% (95% CI of 77.9%- 98.4%) and positive plus negative likelihood ratio comparable to that of computed tomography. They concluded USG as a good diagnostic test accuracy compared to CT with high sensitivity for ruling out raised ICP especially in low risk group. Similarly, it also had high specificity than CT scan (12). Authors of this paper concluded that USG as a better modality as it is noninvasive, can be performed at bedside and saves time as there is no need to transport the patient to CT machine. This method provides more economic value and centers without CT machines could largely benefit through this technique (12).
Newman et al. (16) conducted a study among shunted hydrocephalus children to evaluate the utility of ulrasonographic measurement of ONSD in children suspected of having raised ICP. In their study, 23 children who were shunted for hydrocephalus were included who gave clinical history suggestive of raised intracranial hypertension. ONSD of 102 control patient were also measured. The authors of this study (16) found that children with functioning VP shunt had lesser diameter of ONSD compared to those with non-functional shunt. This study showed the upper limit of ONSD was 4.5mm for children above 1 years of age. Similarly, children with functional VP shunt had mean ONSD of 2.9 (SD 0.5) mm and those with non-functional VP shunt had mean ONSD of 5.6 (SD 0.6mm) with p