Low-contrast visual acuity versus low-luminance visual acuity in choroideremia

Abstract

Clinical relevance Choroideremia is a progressive X‐linked inherited rod‐cone dystrophy. Patients present with nyctalopia and progressive visual field loss, but visual acuity remains well preserved early on. This study showed that low‐luminance visual acuity may be a useful clinical outcome measure during earlier disease stages. Background Choroideremia is a progressive X‐linked inherited rod‐cone dystrophy. Patients present with nyctalopia and progressive visual field loss. However, visual acuity remains well preserved until late in the disease process, limiting its usefulness as a clinical trial endpoint across the disease spectrum. Visual acuity measurements under low‐luminance and low‐contrast conditions may be affected sooner and have been suggested as early markers in other ocular diseases. This study assesses whether low‐luminance visual acuity and low‐contrast visual acuity provide useful endpoints in choroideremia clinical trials. Method Standard high‐contrast and low‐luminance visual acuity was obtained on 29 choroideremia subjects and 16 healthy controls, using a logMAR chart, set at four metres. Low‐luminance visual acuity was tested using a 2.0‐log unit neutral density filter, with the same chart set‐up, without formal dark adaptation. This was followed by low‐contrast visual acuity measured using 1.25 per cent and 2.5 per cent low‐contrast logMAR charts placed also at four metres. Data from the right eyes only were analysed using non‐parametric statistics. High‐contrast visual acuity minus low‐luminance and low‐contrast visual acuity provided the low‐luminance and low‐contrast difference scores. Results A higher number of choroideremia subjects were able to complete the low‐luminance test than the low‐contrast visual acuity tests. Choroideremia subjects had significantly higher low luminance, 2.5 per cent low‐contrast and 1.25 per cent low‐contrast difference scores compared with controls (p < 0.01, Mann–Whitney U‐test); 1.25 per cent low‐contrast visual acuity revealed the poorest performance. A strong positive correlation was found between low‐luminance and high‐contrast visual acuities (ρ = 0.818, p < 0.001) and 2.5 per cent low‐contrast and high‐contrast visual acuities (ρ = 0.671, p < 0.001). Conclusion The low‐luminance visual acuity test may be a useful additional clinical trial outcome measure for early‐to‐moderate disease, when high‐contrast visual acuity is preserved.