Establishing Optic Nerve Diameter Threshold Sensitive and Specific for Optic Atrophy Diagnosis

Purpose: To determine a potential threshold optic nerve diameter (OND) that could reliably differentiate healthy nerves from those affected by optic atrophy (OA) and to determine correlations of OND in OA with retinal nerve fiber layer (RNFL) thickness, visual acuity (VA), and visual field mean deviation (VFMD). Methods: This was a retrospective case control study. Magnetic resonance (MR) images were reviewed from individuals with OA aged 18 years or older with vision loss for more than 6 months and an OA diagnosis established by a neuro-ophthalmologist. Individuals without OA who underwent MR imaging of the orbit for other purposes were also collected. OND was measured on coronal T2-weighted images in the midorbital section, 1cm posterior to the optic disc. Measurements of mean RNFL thickness, VA and VFMD were also collected. Results: In this study 47 OA subjects (63% women, 78 eyes) and 75 normal subjects (42.7% women, 127 eyes) were assessed. Healthy ONDs (mean 2.73 ± 0.24 mm) were significantly greater than OA nerve diameters (mean 1.94 ± 0.32 mm; P < 0.001). A threshold OND of ≤2.3 mm had a sensitivity of 0.92 and a specificity of 0.93 in predicting OA. Mean RNFL (r = 0.05, p = 0.68), VA (r = 0.17, p = 0.14), and VFMD (r = 0.18, p = 0.16) were not significantly associated with OND. Conclusion: ONDs are significantly reduced in patients with OA compared with healthy nerves. A threshold OND of ≤2.3 mm is highly sensitive and specific for a diagnosis of OA. OND was not significantly correlated with RNFL thickness, VA, or VFMD.