Measurement of the retinal nerve fibre layer with scanning laser polarimetry in patients with previous demyelinating optic neuritis.

OBJECTIVES Subjective visual deficits are common after demyelinating optic neuritis despite the frequent return of normal visual acuity. Visual and electrodiagnostic tests have demonstrated evidence of these persisting functional abnormalities, which are thought to be secondary to demyelination and variable axonal loss in the optic nerve. Scanning laser polarimetry (SLP) is a new image analysis technique which uses the polarising properties of the retinal nerve fibre layer (RNFL) to produce a quantitative measure of its thickness. This study was carried out to assess the prevalence, extent, and pattern of RNFL loss after demyelinating optic neuritis using SLP. METHODS Twenty four patients with a history of previous demyelinating optic neuritis were re-examined. Examination included measurement of logmar visual acuity, Pelli-Robson contrast sensitivity, and the presence of a relative afferent pupil defect and optic atrophy. SLP was performed and a mean RNFL profile from a series of three images from each eye was constructed. This was compared with normative data from 20 age matched normal subjects. The lower 99.9% confidence limit of the normal data was calculated and used as the cut off criterion for abnormality. RESULTS There were a total of 31 eyes with a history of demyelinating optic neuritis and SLP disclosed an abnormality in 29 (94%) of these. Twenty three eyes recovered an acuity of 0.0 or better, 21 of which had evidence of RNFL loss on polarimetry. Scanning laser polarimetry was the only abnormality found in nine of the 31 eyes (29%). The pattern and extent of RNFL loss was very variable and there was no significant difference in these indices between patients with multiple sclerosis compared with those with isolated demyelinating optic neuritis. CONCLUSION Scanning laser polarimetry can provide a quantitative measure of RNFL loss after demyelinating optic neuritis, demonstrating its occurrence in a high percentage of patients recovering normal visual acuity.