Variability of optic nerve sheath diameter measurement on computed tomography; need for standardization

Context: Computed tomography measurement of optic nerve sheath diameter (ONSD) is established for the indirect evaluation of intracranial pressure in traumatic brain injury (TBI). There is however a significant difference in the reported values across different techniques with no clear-cut range of values agreed upon. Objective: To measure the ONSD of patients with TBI and those with normal brains using different window settings and locations and compare with published values. Methodology: Comparative evaluation of 300 ONSD measurements obtained from cranial CT examinations at the Radiology department of a tertiary institution. Seventy-five patients had radiologic signs of TBI while 75 had normal findings (controls). For each subset, ONSD was measured at 3mm and 8mm retrobulbar regions using spine window (ww/wl 250/50). ONSD was measured using additional settings (mediastinal ww/wl 450/60 and soft tissue 380/40) for controls. Correlation between age, gender, and side of the ONS complex at different lengths and contrast settings were also evaluated. Results: The age range of the patients was 1-78 years, mean age of 40.4 years ± 21.7 with male: female ratio of 3:1. The mean ONSD was statistically different (p<0.0001) using the different window settings with the highest value obtained with the mediastinal window. There was also a significant difference(p <0.0001) between the ONSD of TBI and control patients at 8mm and 3mm retrobulbar regions but no correlation with gender, age, and side. Conclusion: Significant variations in the CT measurements of ONSD with regards to retrobulbar locations and window settings highlight the need for standardization of protocols. Measurement taken at 3mm retrobulbar using a spine window setting shows greater distensibility and details of the optic nerve and allows for more reliable measurements.