Normal Values of Optic Nerve Sheath Diameter on Computed Tomography and effect of Raised Intracranial Pressure on Head Injury Patients in North Central Nigeria

Aim: To establish a local nomogram of Computed Tomographic measurement of Optic Nerve Sheath Diameter and determine the values that can objectively predict raised intracranial pressure.

Study design: observational retrospective study

Place and duration of study: Department of Radiology of University of Abuja teaching hospital, Abuja from March 2016 to February 2020.

Methodology: We included images of 356 patients referred to the Radiology department for brain CT. Data was grouped into three:(A) 200 non head injury patients as control,(B) head injury patients without radioclinical signs of raised ICP ,(C) head injury patients with at least one radiologic sign of raised ICP.ONSD obtained in axial scans at 3mm retrobulbar region at constant window width and level of 250/50 were analysed using SAS software version 9.3 and statistical level of significance set at 0.05.

Result: Mean ONSD of 200 normal patients was 4.4mm± 0.5 with no significant correlation with age and side. Mean ONSD of 118 head injury patients with CT signs of raised ICP was 6.0mm±0.7 while mean ONSD of 38 head injury patients without CT signs of raised ICP was 4.3mm±0.7. ONSD was slightly higher in patients with significant midline shift compared with patients without shift but no correlation with degree of shift. The ONSD value above which raised ICP can be predicted with the highest sensitivity and specificity was >5.2mm with Receiver Operating Characteristic curve (ROC) demonstrating an area under the curve of 0.9796 (p-value < 0.0001) with specificity of 93.5% and sensitivity of 90.7%.

Conclusion: There is a positive correlation between ONSD measurement on CT and intracranial pressure. This measurement is therefore recommended as an additional indirect radiological marker of raised ICP. Normal mean ONSD in a North-Central Nigerian population is 4.4mm± 0.5 with 5.2mm proposed as the upper limit of normal.