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

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.

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.

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.

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).

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.