Normative orbital measurements in an Australian cohort on computed tomography.

Purpose: To determine the normal diameters of the extraocular muscles (EOMs) and optic nerve sheath complex (ONSD) and correlate with patient demographics in an Australian cohort.

Methods: Consecutive patients who underwent contrast enhanced computed tomography (CT) orbits between December 2017 and March 2021 were included. Patients with bilateral disease, previous orbital surgery, or poor scan quality were excluded. Normal orbit was used in patients with unilateral orbital disease.

Results: Two hundred one orbits from 201 patients were included. Normal measurements (mean ± SD) were as follows: medial rectus (MR) 4.22 0.56 mm; inferior rectus (IR) 4.20 0.70 mm; lateral rectus (LR) 3.40 0.56 mm; superior muscle group (SMG) 4.13 0.72 mm; superior oblique (SO) 2.60 0.43 mm; inferior oblique (IO) on quasi-sagittal plane 2.19 0.42 mm, and the ONSD 5.62 0.82 mm. The mean diameters of the LR, SMG, IR, SO, and ONSD were significantly larger in male than female patients (p < .05). Statistically significant correlation was found between age and the diameters of the LR (r = 0.29, p < .01), SMG (r = 0.22, p < .01), IO on a coronal plane (r = -0.18, p < .01), and ONSD (r = 0.16, p = .02).

Conclusion: This normative data may be used to diagnose pathological enlargement of the optic nerve and extraocular muscles, including involvement of the oblique muscles.