ETDRS letter scoring method improves the accuracy of 1.25% low-contrast visual acuity measurement in optic neuritis secondary to MS

Objectives To examine whether the dynamic range of 1.25% low-contrast visual acuity (LCVA) measurement in MS patients with optic neuritis (ON) can be improved by using the Early Treatment Diabetic Retinopathy Study (ETDRS) letter scoring method. Methods LCVA was tested using 2.5% and 1.25% low contrast ETDRS-style 4m Sloan letter charts. When ≥20 letters were read correctly, the letter score was equal to letter count plus 30. If <20 letters were read correctly, the letter score was equal to letter count at 4m plus the total number of letters read correctly at 1m. Results 51 relapsing-remitting MS patients with unilateral ON were enrolled and 60.8% ON eyes had a 1.25% LCVA letter count worse than 1 line (5 letters). In ON eyes with <20 letter count, ETDRS letter score showed a significantly improved correlation with both macular GCIPL thickness (r=0.71, p<0.0001; vs r=0.31, p=0.08 for letter count) and VEP latency (r=-0.47, p=0.003; vs r=-0.15, p=0.4 for letter count). Discussion Given ON with VEP monitoring has been frequently used as a model in MS remyelination clinical trials, our proposed ETDRS-style LCVA letter scoring method may be considered to enhance the functional outcome measure, which is necessary for regulatory approval.