Optic nerve diffusion changes and atrophy jointly predict visual dysfunction after optic neuritis.

Recently, there has been strong interest in the development of imaging techniques to quantify axonal and myelin pathology in patients with multiple sclerosis (MS). Optic neuritis, a condition characterised by inflammatory demyelination of the optic nerve, is one of the commonest sites of MS relapse, and exhibits similar pathological alterations to MS lesions elsewhere in the central nervous system (CNS). Unlike other regions of the CNS, however, the function of the optic nerve can be accurately assessed using clinical measures, as well as electrophysiological techniques such as visual evoked potential recordings. Therefore, optic neuritis is useful for investigating the relationship between abnormalities in optic nerve structure, assessed using magnetic resonance imaging (MRI), and visual dysfunction, assessed clinically and electrophysiologically. The aims of the present study were to assess optic nerve structural abnormalities in patients with a history of unilateral optic neuritis using MRI, and then to identify correlations between abnormalities in optic nerve MRI and visual dysfunction. Ten controls and sixteen patients underwent high resolution optic nerve diffusion tensor imaging (DTI), T2- and T1-weighted MRI. In addition, Snellen visual acuity and the latency and amplitude of multifocal visual evoked potentials (mfVEP) were tested in all patients. Diffusion and volumetric MRI indices were correlated to mfVEP functional indices. Significant abnormalities were detected in MRI and mfVEP measures in patients' affected nerves compared to unaffected optic nerves or optic nerves from healthy controls. Reduced mfVEP amplitude in the affected side significantly correlated with both affected optic nerve atrophy (R=0.58, p=0.02) and reduced fractional anisotropy (FA) (R=0.52, p=0.04). However, atrophy and reduced FA did not correlate with each other. To further investigate this disassociation, we used linear regression analysis with optic nerve atrophy and optic nerve FA as independent variables and mfVEP amplitude as the dependent variable. The resulting linear regression model was highly significant (R=0.819, p=0.001). These results show that, 4 years after unilateral optic neuritis, MRI-based measures of optic nerve structural abnormalities (decreased anisotropy and volume) independently predict visual dysfunction.