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If initial brain magnetic resonance imaging (MRI) findings are normal, the odds of developing multiple sclerosis (MS) after an acute episode of optic neuritis are only 25 per cent, results of a 15-year, prospective follow-up study indicate. The risk is lowest if features of optic neuritis are atypical, the investigators report in the June Archives of Neurology, suggesting that prophylactic treatment can safely be withheld in the absence of other neurologic symptoms.

Robin L. Gal, at the Jaeb Center for Health Research in Tampa, Florida, and members of the Optic Neuritis Study Group enrolled 389 patients with acute unilateral optic neuritis between 1988 and 1991. The number of white matter lesions at least three mm in diameter was determined on unenhanced MRI of the brain at baseline.

The overall cumulative probability of developing MS after onset of optic neuritis during the ensuing 15 years was 50 per cent. The presence of brain MRI lesions was the strongest predictor of risk - 72 per cent of patients with at least one lesion developed MS compared with 25 per cent of those with no brain lesions at baseline.

Among patients without MS at the ten-year examination, the likelihood of developing MS within the next five years was 32 per cent among those with brain lesions at baseline, the authors report. Only one patient without lesions developed new-onset MS after ten years.

"Patients with abnormal brain MRI findings already have morphologic evidence of disseminated disease and could be considered to have MS at the time of the optic neuritis episode," Gal and associates advise. They recommend additional ancillary testing to determine if prophylactic treatment for these patients is appropriate.

Risk factors for progression to MS among those without brain lesions were female sex and retrobulbar neuritis.

By contrast, the investigators note, "both sexes had a low risk when atypical features of the optic neuritis were present, namely, no light perception in the affected eye, absence of periocular pain, and ophthalmoscopic findings of severe optic disc swelling, peripapillary hemorrhages, or retinal exudates."

For such patients, "the results certainly justify withholding treatment (specifically for MS) because many may never develop MS."

Arch Neurol 2008;65:727-732