PURPOSE: To evaluate the effect of treatment with interferon β-1a (Avonex) initiated at the time of a first episode of optic neuritis in patients at high risk for multiple sclerosis (MS).

DESIGN: Randomized clinical trial

METHODS:

Prospective.

SETTING: Fifty clinical centers throughout the US and Canada.

STUDY POPULATION: After the onset of a first episode of optic neuritis treated with intravenous and oral corticosteroids, 192 patients with brain magnetic resonance imaging (MRI) evidence of subclinical demyelination were randomly assigned to receive weekly intramuscular injections of 30 μg interferon β-1a or placebo.

MAIN OUTCOME MEASURE: The study outcomes were the development of clinically definite MS within 3 years of follow-up and brain MRI changes at 6, 12, and 18 months.

RESULTS: The rates of clinically definite MS and of a combined MS/MRI outcome were lower in the interferon β-1a group than in the placebo group (adjusted rate ratios 0.58, 95% confidence interval 0.34 to 1.00; and 0.50, 95% confidence interval 0.34 to 0.73, respectively). Compared with the placebo group, on the 18-month brain MRI the interferon β-1a group had a smaller change from baseline in T2 lesion volume (P = .02), fewer new or enlarging T2 lesions (P < .001), and a lower frequency of Gd-enhancing lesions (P < .001).

CONCLUSION: The clinical and brain MRI results of this trial support initiating interferon β-1a treatment at the time of a first episode of optic neuritis occurring in patients at high risk for MS based on the presence of subclinical brain MRI lesions.