Evaluating loss of visual function in multiple sclerosis as measured by low-contrast letter acuity

Disturbances in visual function are common in patients with multiple sclerosis (MS) and are often accompanied by substantial impairments in daily functioning and quality of life. Lesions associated with these impairments frequently involve the afferent visual pathway.

Because these impairments are often not readily apparent on commonly used high-contrast acuity tests, low-contrast charts (e.g., low-contrast Sloan letter charts) have gained validity in the assessment of visual dysfunction in patients with MS. Decrements in low-contrast letter acuity are associated with MS and correlate with increasing disability, MRI abnormalities, and reduced retinal nerve fiber layer (RNFL) thickness as measured by optical coherence tomography (OCT). These findings suggest that low-contrast letter acuity testing is a potentially useful addition to disability scales such as the Multiple Sclerosis Functional Composite, serving as another surrogate marker for MS disability. Assessment of RNFL thickness by OCT, which is also associated with visual impairment, also may be considered for inclusion in clinical trials evaluating treatments for MS.

The effects of disease-modifying therapies on visual dysfunction in patients with MS have been evaluated only recently. Two phase 3 studies of natalizumab showed that low-contrast letter acuity testing, included as an exploratory outcome, demonstrated treatment effects. Other ongoing studies have incorporated low-contrast acuity and OCT measures of RNFL thickness. The availability and wider use of low-contrast letter acuity tests, in combination with ocular imaging techniques, may improve assessment of treatment efficacy in patients with MS.