The Prevalence Of Residual Visual Field Islands In Retinitis Pigmentosa

Abstract

Purpose: Accurate assessment of pre-implant residual visual function is critical for patient selection and determining the efficacy of treatment in retinal prostheses. However this is difficult at very low levels of vision as small areas of residual vision can be hard to detect. The purpose of this study was to determine the prevalence of small islands of peripheral visual field in a group of subjects with Retinitis Pigmentosa (RP).

Methods: 49 subjects with RP (55% male, mean age= 56 ±14 years, visual acuity= 6/6 to light perception) were recruited for our Bionic Eye study. Clinical examination was performed using a battery of tests including electroretinography (ERG) and perimetry. Retinal function was assessed using an ISCEV standard full-field ERG protocol. Goldmann kinetic perimetry was performed on both eyes using the smallest white light target size that the subject was able to detect. Visual field islands were defined as any residual areas of vision further than 10 degrees from fixation. The percentage of residual visual field (RVF) islands was estimated using ImageJ software.

Results: Four participants were excluded from analysis due to poor fixation (n=1) and intermittent, unreliable visual fields (n=3). Of the remaining 45 participants (90 eyes), 38 (84%, 95% CI= 71-94%) had some measurable visual field using at least the V4e target. Of these 38 participants (76 eyes), 25 eyes (33%, 95% CI= 23-45%) displayed peripheral islands of vision. 10 of these eyes (40%, 95% CI= 21-61%) had no remaining central vision and 15 eyes (60%, 95% CI= 39-79%) exhibited residual central vision with the addition of one or more peripheral islands. The mean size of the islands was 4.65 ±0.08% with a range of 0.06% to 26.82% of RVF. Clinically detectable ERG responses were observed in 6 eyes (24%, 95% CI= 9-45%) and the remaining 19 eyes had no detectable ERG response (76%, 95% CI= 55-91%), despite having residual islands of vision.

Conclusions: Measurement of visual function in patients with end stage RP is extremely difficult as remaining peripheral islands of vision can be missed if we rely only on central visual acuity or ERG. This study has shown that a significant proportion (76%) of patients with a clinically undetectable ERG response still have measurable peripheral visual field islands demonstrating the importance of Goldmann perimetry in the evaluation of residual visual function.

Keywords: 696 retinal degenerations: hereditary • 642 perimetry • 509 electroretinography: clinical