A serial study of retinal changes following optic neuritis with sample size estimates for acute neuroprotection trials

Following an episode of optic neuritis, thinning of the retinal nerve fibre layer, which indicates axonal loss, is observed using optical coherence tomography. The longitudinal course of the retinal changes has not been well characterized. We performed a serial optical coherence tomography study in patients presenting with optic neuritis in order to define the temporal evolution of retinal nerve fibre layer changes and to estimate sample sizes for proof-of-concept trials of neuroprotection using retinal nerve fibre layer loss as the outcome measure. Twenty-three patients (7 male, 16 female, mean age 31 years) with acute clinically isolated unilateral optic neuritis were recruited to undergo optical coherence tomography, visual assessments and visual evoked potentials at presentation (median 16 days from onset of visual loss) and after 3, 6, 12 and 18 months. Compared with the clinically unaffected fellow eye, the retinal nerve fibre layer thickness of the affected eye was significantly increased at presentation and significantly reduced at all later time points. The evolution of retinal nerve fibre layer changes in the affected eye fitted well with an exponential model, with thinning appearing a mean of 1.6 months from symptom onset and the rate of ongoing retinal nerve fibre layer loss decreasing thereafter. At presentation, increased retinal nerve fibre layer thickness was associated with impaired visual acuity and prolonged visual evoked potential latency. Visual function after 12 months was not related to the extent of acute retinal nerve fibre layer swelling but was significantly associated with the extent of concurrent retinal nerve fibre layer loss. Sample size calculations for placebo-controlled trials of acute neuroprotection indicated that the numbers needed after 6 months of follow up are smaller than those after 3 months and similar to those after 12 months of follow-up. Study power was greater when investigating differences between clinically unaffected and affected eyes rather than retinal nerve fibre layer thickness of the affected eye alone. Inflammation in the optic nerve and impaired axonal transport (implied by retinal nerve fibre layer swelling) are associated with visual dysfunction and demyelination (long visual evoked potential latency) during acute optic neuritis. Retinal nerve fibre layer thinning is usually evident within 3 months. Optical coherence tomography-measured retinal nerve fibre layer loss after 6 months is a suitable outcome measure for proof-of-concept trials of acute neuroprotection in optic neuritis.