Visual acuity and fixation characteristics in age-related macular degeneration

To compare "single letter" (SL) acuity, "crowded letter" (CL) acuity, and "repeated letter" (RL) acuity for patients with age-related macular degeneration (AMD) and investigate if differences between these visual acuities are associated with fixation characteristics.

A total of 243 patients with AMD had their best-corrected visual acuity measured on an ETDRS chart. SL, CL, and RL acuities were measured using Landolt C targets on a monitor. Fifty-degree-field red-free fundus photographs were taken and a static target was used to calculate the Preferred Retinal Locus (PRL) distance and direction from the fovea. Quality of fixation (consistency and oculomotor response) was also assessed using a fundus camera and a dynamic target.

RL acuity was almost always better than CL acuity and SL acuity was almost always better than CL acuity. The mean (+/-SD) RL-CL and SL-CL acuity differences were -0.13 (+/-0.15) logMAR and -0.11 (+/-0.13) logMAR respectively. The median PRL distance was 3.73 degrees and the preferred retinal areas for the location of the PRL were the left (left quadrant of visual field; 39.5% of cases) and superior (inferior quadrant of visual field; 25.4%). Visual acuity was significantly associated with PRL distance but PRL distance only explained 10% of the variation in visual acuity. PRL distance was found to be a significant but weak predictor of the SL-CL acuity difference but fixation quality was not a good predictor of the RL-CL acuity difference.

Although the acuity measured under different stimulus conditions varies, the absolute differences are small. This suggests that these techniques would not be helpful in determining fixation characteristics, or predicting the outcome of rehabilitation in individual patients with AMD.