Optic nerve atrophy: a comparison of two imaging modalities to evaluate their sensitivity for diagnostic purposes
Cheng, Anh-Dao M.
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PURPOSE: To evaluate the efficacy of MRI as a diagnostic tool by comparing it to OCT in patients with suspected optic nerve atrophy. Currently, MRI is an established noninvasive imaging modality for tumors and inflammatory tissues; however their use in optic nerve atrophy is limited to advanced cases. Our study investigates the use of OCT, a more sensitive imaging modality, compared to MRI as a potential adjunct to the clinical diagnosis of optic nerve atrophy. METHODS: This retrospective study analyzed 27 medical records (40 eyes) of patients with suspected optic nerve atrophy referred to the Neuro-ophthalmology Clinic of the Beth Israel Deaconess Medical Center (2009-2016) who had both MR imaging of the orbits and SD-OCT scans. Based on the RNFL thickness obtained from OCT scans, optic atrophy was defined as border, mild, moderate, or severe. MRIs were used to measure the optic nerve area, optic nerve diameter and sheath area of all eyes. From there, the ratio of optic nerve area to sheath area, percent difference in optic nerve diameters in a patient and percent difference in optic nerve areas in a patient were determined. RESULTS: As atrophy worsens, the optic nerve area and sheath area seem to steadily decline. The ratio between the two seems to remain constant (0.27) regardless of degree of atrophy. Focusing on unilateral patients, the percent difference in optic nerve area with mild optic atrophy seemed minimal (14%). It becomes more significant in moderate and severe atrophy cases (56.06% and 26.18% respectively). Overall, there does not seem to be a strong correlation between MRI measurements and OCT RNFL values. CONCLUSIONS: Overall, a strong correlation was not found between MRI measurements and OCT RNFL thickness values. While a general trend existed, there was too much variation to determine cut off points for atrophy based solely on the measurement of a single eye. MRI may be useful in identifying severe and moderate optic nerve atrophy especially in unilateral patients. Once the RNFL thins to about 70 μm, the difference in size is detectable on MRI. For all cases of mild optic atrophy and bilateral moderate atrophy, OCT remains a more reliable imaging diagnostic. Changes in nerve size appear minimal compared to a healthy human. The optic nerve sheath was also shown to decrease in size in cases of atrophy. Future studies with a larger sample size are needed to produce more conclusive results.