Progression in Patients with Advanced Retinal Degenerative diseases (RDD) followed with Full-field Stimulus Threshold (FST) Testing

Abstract

Purpose: The FST has been established as a useful method for assessing visual function in patients with advanced retinal degeneration when other methods like visual fields and electroretinograms cannot be performed or have reached the limit of their sensitivity (Messias et al., Doc. Ophthalmol, 2013; Klein, Birch, Doc. Ophthalmol, 2009). Here we present prospective data from patients who had FST testing on two or more visits at least 12 months apart in order to determine whether progression can be detected as a difference in thresholds.

Methods: Included were 13 eyes from 13 patients. Each patient had advanced RDD, no detectable rod ERG and very small (<4.0 μV) to non-detectable (<0.1 μV) cone ERG responses and had performed the FST on two visits at least 12 months apart. FST was performed dilated and dark-adapted as described by Klein, Birch 2009 with 0 dB level set at 0.1 cd.s/m². The difference in thresholds (in log cd.s/m²) between the two visits was calculated. Based on repeat variability (Klein, Birch, Doc. Ophthalmol, 2009), differences of ≥ 0.3 log unit were considered significant.

Results: The average threshold on the initial visit was -2.6 ± 0.9 log cd.s/m², which is an elevation of 3.9 log units above mean normal (-6.5 log cd.s/m²). The average duration between visits was 29 months. Over this time, the average increase in threshold was 0.46 log units, or roughly 0.14 log unit per year. Of the 13 eyes, 7 eyes showed a significant (> 0.3 log unit) rise in threshold between visits with a mean duration between visits of 31.9 ± 12.5 months. Most patients with an elevation over 4.5 log units at the initial visit showed little change.

Conclusions: Progression can be followed by FST threshold in patients with minimally-detectable ERGs and visual fields. The average annual rate of change of 0.14 log unit is roughly consistent with the 10-20 % decline in visual field sensitivity reported in less advanced patients. Eyes with the greatest elevation in threshold at the initial visit showed no progression, suggesting a possible floor effect.

Keywords: 468 clinical research methodology • 696 retinal degenerations: hereditary • 584 low vision