Objective: To establish normal value of optic nerve sheath complex on computed tomography.
Material and Method: This prospective study was approved by the institutional ethics committee. Measurement of the optic nerve sheath complex at retrobulbar and waist regions was done on volunteers who came for cranial computed tomography examination at Thammasat University Hospital during May 2013-November 2013. There were 138 volunteers (75 males and 63 females) with average age of 57 years. Volunteers were excluded if they have eye symptoms or known related disease that affected optic nerves such as endocrinopathy, or if they have metallic object or prostheses that might confound the measurement. Size of optic nerve sheath complex was measured at retrobulbar and waist regions on reconstructed orbital CT images in a plane parallel to the optic nerve sheath complex. Two measurements were performed separately at 1-month intervals to evaluate reliability and reproducibility of the measurement. Then normal size and range of the optic nerve sheath complex were evaluated. Correlation between age, gender and size of the optic nerve sheath complex were also evaluated as well as difference in size of the complex between both eyes.
Results: Normal value of the optic nerve sheath complex (mean + 2SDs) at retrobulbar and waist regions are 5.6+1.8 (3.8-7.4) mm and 3.7+0.8 (2.9-4.5) mm. There is no correlation between age, gender and size of the optic nerve sheath complex. No difference in size of the optic nerve sheath complex between both eyes was found.
Conclusion: Normal value of optic nerve sheath complex at retrobulbar and waist regions in Thai population was established. This information might be useful in diagnosis of some diseases or excluding some conditions.

Keywords: Optic nerve sheath complex, Optic nerve, Size, CT