Monitoring of Intracranial Pressure by CT-Defined Optic Nerve Sheath Diameter

BACKGROUND AND PURPOSE: Intracranial pressure (ICP) can be monitored by the optic nerve sheath diameter (ONSD) technique. We hypothesized that diameter of the optic canal (OC) can be a limiting factor for this technique. METHODS: In the prospective cohort study, we analyzed CT scans of 600 OCs of healthy adults and 54 canals of patients with ICP monitoring. The diameters were measured through its length and the narrowest one was chosen for further analysis. ONSD was measured at 3 and 10 mm from the anterior opening of the canal. The correlation analysis was performed between invasive and ONSD methods of ICP monitoring and OC diameters in pathological cases. RESULTS: The narrowest cross-sectional area of the normal OC was 13.85 +/- 2.89 mm(2) and varied from 25.5 to 6.6 mm(2). Apparently 9.17% OCs were narrow (<10.9 mm(2)). Correlations exist between the optic nerve sheath area at the 3-mm distance from the anterior opening of the canal and the area of the anterior opening itself (P =.012), and the sheath area 10 mm from the anterior opening and the narrowest part of the canal (P =.015). Cases with narrow canals provided false-negative readings via ONSD method if compared with invasive monitoring. CONCLUSION: In its narrowest part, the average OC is 11 to 16.75 mm(2) wide. We suggest measuring this area simultaneously with the ONSD during ICP monitoring. If the area of the narrowest lumen of the canal is less than 10 mm(2), ONSD technique for ICP monitoring should not be used.