[Cortical response to age-related macular degeneration (Part II). Functional MRI study]

PURPOSE AND MATERIALS: To evaluate the cortical response to visual stimulation in patients with age-related macular degeneration (ARMD), we conducted a functional MRI study in ten patients presenting unilateral or bilateral ARMD and five age-matched controls, using white flashes during activation phases (see Part I).

After anatomical conformation, eight patients and four controls showed significant cortical hemodynamic response to monocular stimulations. Individual analysis was preferred to group evaluation, because of the differences in visual loss in a small number of patients. In controls, we observed cortical response in the primary visual cortex, especially at occipital poles corresponding to the macula. Patients showed a qualitative and quantitative restriction in cortical response and exclusion of occipital poles after stimulation of the affected eye, whereas activation was found in the peripheral striate and peristriate cortex. Cortical response showed hemispheric asymmetry in some patients.

Our study demonstrated an activation defect in the macular projected striate cortex, corresponding to visual impairment in ARMD patients. Nevertheless, at a given visual acuity, cortical response may vary among subjects. Patients' subjective apprehension may account for such variations, as well as objective visual capacity stemming from residual functional retinal areas within the affected macula. The hemispheric asymmetry in cortical activation may result from gaze deviation onto the new fixation area in the perimacular retina, thus altering the global visual field. Enhancement in the peripheral striate and peristriate areas suggests changes in cortical interactions, possibly by a lowering of the feedback from macular projected V1. Finally, cortical evaluations must take into account degenerative phenomena delaying the hemodynamic response in the elderly.

Aiming at a specific population of weakened patients with a serious visual impairment, we obtained significant results concerning cortical plasticity for visual perception in central vision deletion. Our preliminary findings must be confirmed in a larger population and correlated with other techniques exploring vision, in particular with multifocal electroretinography for retinal evaluation.