Testing macular letter recognition - reliability and influence of refraction errors.

Purpose: The goal was the validation of the Macular Mapping Test (MMT) for clinical use. We studied its susceptibility to blur caused by refractive errors and its test-retest reliability.

Methods: We tested letter recognition in 33 target locations in the central visual field (10° radius) at two contrast levels, 10 and 100 per cent. Healthy subjects were either young (mean: 25.7 years) or elderly (mean: 67.0 years). A third group (patients with age-related macular degeneration, mean age: 76.4 years) were tested with their habitual optical correction and subsequently with optimal correction. The influence of refractive errors on performance was measured only for the healthy subgroups. All visual acuities were measured at 6.0 and 0.4 metres. Outcome measure was the 'general field score' (GFS), a single number expressing the overall level of letter recognition performance.

Results: The general field score versus refractive error showed a mean loss of 3.9 points per dioptre and 5.9 points per dioptre in young subjects at 100 and 10 per cent contrast, respectively. In elderly subjects, mean losses were 2.6 points per dioptre and 5.5 points per dioptre for 100 and 10 per cent contrast, respectively. The general field score ratio (GFS at 10 per cent divided by GFS at 100 per cent) shows a decrease of eight per cent for the young group and 11 per cent for the elderly group. Performance increased after improvement of the refractive status in the AMD group: At 100 per cent contrast, the general field score increase from 21.5 to 24.1 (p < 0.008) was significant but not at 10 per cent contrast. Mean increase of acuity with the optimal correction was 0.32 to 0.46 decimal for distance (p = 0.018) and 0.28 to 0.44 decimal near (p = 0.018). Test-retest reliability was good but dependent on contrast.

Conclusions: At both contrasts, performance declined with increasing blur caused by refractive errors. The slope of this decline is more pronounced at the lower contrast. In healthy subjects, optical blur beyond 1.00 D can cause significant performance losses.