Longitudinal analysis of contrast acuity in Friedreich ataxia

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Abstract

Objective

To determine the natural history of contrast acuity in Friedreich ataxia.

Methods

In the Friedreich Ataxia–Clinical Outcome Measures Study, participants (n = 764) underwent binocular high- and low-contrast visual acuity testing at annual study visits. Mixed-effects linear regression was used to model visual acuity as a function of time, with random intercepts and slopes to account for intraindividual correlation of repeated measurements. A time-varying covariate was used to adjust for diabetes, and interaction terms were used to assess for effect modification by GAA repeat length, disease duration, and other variables.

Results

Across a median of 4.4 years of follow-up, visual acuity decreased significantly at 100% contrast (−0.37 letters/y, 95% confidence interval [CI]: −0.52 to −0.21), 2.5% contrast (−0.81 letters/year, 95% CI: −0.99 to −0.65), and 1.25% contrast (−1.12 letters/y, 95% CI: −1.29 to −0.96 letters/year). There was a significant interaction between time and GAA repeat length such that the rate of decrease in visual acuity was greater for patients with higher GAA repeat lengths at 2.5% contrast (p = 0.018) and 1.25% contrast (p = 0.043) but not 100% contrast. There was no effect modification by age at onset after adjusting for GAA repeat length.

Conclusions

Low-contrast visual acuity decreases linearly over time in Friedreich ataxia, and the rate of decrease is greater at higher GAA repeat lengths. Contrast sensitivity has the potential to serve as a biomarker and surrogate outcome in future studies of Friedreich ataxia.

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