INTRODUCTION: Blunt head trauma is a common cause of increased intracranial pressure (ICP). Ultrasound measurement of the optic nerve sheath diameter (OSND) is an accurate and non-invasive way to detect increased ICP. Blunt trauma patients are often immobilized in a rigid cervical spine collar. Our objective was to describe the changes in ONSD following the placement of a c-collar and determine if any changes were time-dependent. METHODS: We performed a prospective cohort study measuring the ONSD of healthy volunteers before and after placement of a c-collar. Two physicians obtained the measurements. Each eye was scanned twice using a standardized technique. This was done before c-collar placement, 5min after placement and 20min after placement. A mean of both eyes was calculated and analyzed using descriptive statistics. An intraclass correlation coefficient (ICC) was used to assess inter-rater reliability. RESULTS: Twenty study participants with a mean age of 37.1years old were enrolled. The mean baseline ONSD was 3.77mm (95% CI 3.48-4.07). The mean ONSD 5 min after the c-collar was placed was 4.47 (95% CI 4.17-4.78). The mean ONSD at 20min after c-collar placement was 4.53 (95% CI 4.13-4.92). These changes were statistically significant (p=0.003 and <0.001). Reliability was relatively strong overall (ICC=0.74; 95% CI: 0.65, 0.81). CONCLUSION: The placement of a cervical collar increased the ONSD at 5min and this change remained increased at 20min. Future study should assess whether similar results are found in patients with blunt head trauma.