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Alexander Ho, Travis Smith, Amirhossein Hariri, Elvira Chegarnov, Frederick L Ferris, Paul Van Veldhuisen, Srinivas R Sadda, Richard G Weleber, David G Birch, VPA Clinical Trial Group, EZ Area Working Group; Area Measurement of the Ellipsoid Zone (EZ) by SD-OCT and its Correlation with Visual Field Identifies a Potential Anatomical Endpoint for Clinical Trials in Retinitis Pigmentosa (RP). Invest. Ophthalmol. Vis. Sci. 2014;55(13):3380.
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Photoreceptor loss is characteristic of RP. Traditional visual function measures have limited sensitivity at quantifying this process, at least over the short term, which impacts initiation of clinical treatment trials. Birch et al. (JAMA Ophthalmol. 2013 Sep;131(9):1143-50) reported that the perifoveal ellipsoid zone (IS/OS boundary), which is measurable with repeatability and precision, may be a useful outcome measure for prospective RP trials. The purpose of this study was to assess the utility of EZ area in an ongoing clinical trial by correlation with visual field (VF).
Both eyes of 24 participants in the ongoing “Trial of Oral Valproic Acid for Retinitis Pigmentosa” (VPA, NCT01233609), for whom baseline and end-of-study (week 52) SD-OCTs were available, were studied. All analyses were conducted masked to treatment assignment. GCP-compliant semi-automated EZ area measurement protocols were developed. Full-field static perimetric VF studies (164 loci, GATEi test strategy, target size V, performed in replicate) from these same patients, that passed quality assessment, were included in the analyses. Correlations at the baseline visit and changes over 12 months were estimated between EZ area and a variety of volumetric visual field parameters including total hill of vision, central 30° hill of vision, and sensitivity loss in decibels below normal.
The EZ area could be measured at baseline in ~50% of VPA cases. Exclusions were mostly because the EZ extended beyond the area scanned. The correlation of EZ area between eyes was high (mean=2.6mm2, r≥0.96, p<0.001). At baseline, the size of the EZ area fit best, on average, with a 4-5 dB sensitivity loss in visual fields, as compared to normals.
Although our results are preliminary, the EZ area is correlated with the hill of vision. Our future plans in developing this potential outcome variable are to conduct prospective evaluation of the EZ area and hill of vision change and to explore OCT protocols that capture larger EZ areas in earlier stage disease.
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