Objective: To evaluate directional diffusivities within the optic nerve in a first event of acute optic neuritis to determine whether decreased axial diffusivity (AD) would predict 6-month visual outcome and optic nerve integrity measures.
Design: Cohort study.
Setting: Academic multiple sclerosis center. Patients Referred sample of 25 individuals who presented within 31 days after acute visual symptoms consistent with optic neuritis. Visits were scheduled at baseline, 2 weeks, and 1, 3, 6, and 12 months.
Main outcome measures: Visual acuity, contrast sensitivity, visual evoked potentials (VEPs), and thickness of the retinal nerve fiber layer (RNFL).
Results: An incomplete 6-month visual recovery was associated with a lower baseline AD (1.50 μm(2)/ms [95% confidence interval {CI}, 1.36-1.64 μm(2)/ms for incomplete recovery vs 1.75 μm(2)/ms [95% CI, 1.67-1.83 μm(2)/ms] for complete recovery). Odds of complete recovery decreased by 53% (95% CI, 27%-70%) for every 0.1-unit decrease in baseline AD. A lower baseline AD correlated with worse 6-month visual outcomes in visual acuity (r = 0.40, P = .03), contrast sensitivity (r = 0.41, P = .02), VEP amplitude (r = 0.55, P < .01), VEP latency (r = -0.38, P = .04), and RNFL thickness (r = 0.53, P = .02). Radial diffusivity increased between months 1 and 3 to become higher in those with incomplete recovery at 12 months than in those with complete recovery (1.45 μm(2)/ms [95% CI, 1.31-1.59 μm(2)/ms] vs 1.19 μm(2)/ms [95% CI, 1.10-1.28 μm(2)/ms]).
Conclusions: Decreased AD in acute optic neuritis was associated with a worse 6-month visual outcome and correlated with VEP and RNFL measures of axon and myelin injury. Axial diffusivity may serve as a marker of axon injury in acute white matter injury.