Electrical stimulation of anterior visual pathways in retinitis pigmentosa

PURPOSE. To explore electrically induced phosphenes in blind patients with retinitis pigmentosa (RP) in comparison with healthy subjects and to develo p a screening test for candidates for an optic nerve visual prosthesis impl antation. METHODS. Phosphenes are obtained by charge balanced biphasic pulse stimulat ions through a surface cathode over the closed eyelids and an anode near th e opposite ear. The resulting strength-duration relationship for somatosens ory, phosphene, and pain threshold has been recorded in five RP patients as well as in 10 healthy volunteers. RESULTS. In sighted subjects, the average rheobase and chronaxy for phosphe ne perception are 0.28 mA and 3.07 msec, respectively. For pulse durations longer than 2 msec, phosphenes are usually obtained at current strengths be low the level giving rise to any other electrically generated sensation. In RP patients, however, phosphenes are not so easily obtained. One in five h ad no visual response at all. Another patient reported a flash perception f or the longest pulse durations only. Spontaneous phosphenes interfered heav ily with the stimulation in a third person. Finally, despite the higher thr eshold, two patients displayed normally shaped strength-duration curves. CONCLUSIONS. The surface stimulation has proven harmless, adequate, and ver y helpful to ascertain that the optic nerve can be electrically activated i n completely blind individuals. Long-duration stimulation pulses yield very low phosphene thresholds in healthy subjects. Anterior visual pathways act ivation requires higher currents in RP patients.