a method of treating or preventing age - related retinal dysfunction involves administering to a patient a pharmaceutically effective amount of a synthetic retinal derivative repeatedly over a duration of at least about 3 months . effective synthetic retinal derivatives include 9 - cis - retinyl esters , 11 - cis - retinyl esters , derivatives and congeners thereof , and combinations thereof . suitable ester substituents include carboxylates of c 1 - monocarboxylic acids and c 2 - c 22 polycarboxylic acids .