Cataract from infrared lasers: Evidence for photochemical mechanisms

Two theories for cataract formation by IR radiation are reviewed: Goldmann's hypothesis that cataracts result from heat transferred to the lens from the IR absorbed by the iris is contrasted with Vogt's hypothesis in which the IR energy is absorbed directly by the lens. Data show that cataracts can be formed by both mechanisms and that cataracts resulting from long-term exposures are most likely in accordance with Vogt's hypothesis. Data from laser and IR heat lamp exposures to in vivo lenses and incubated whole lenses and various extracted lens crystallins, as well as electrophoresis of lens crystallins on polyacrylamide gels and other types of substrates, show conversion of soluble α and β(H) crystallins into insoluble protein factions with high molecular weight as a result of exposure to broad band heat or narrow band IR laser radiation. Sub-threshold laser exposure levels produce a markedly decreased mobility of the β crystallins in the lens. Reciprocity between exposure power level and duration is obeyed over many orders of magnitude. This constant energy/dose-related requirement for cataract formation suggests major contributions from photochemical rather than thermal mechanisms, while the failure of this relation for longer exposures possibly indicates a partial repair process.