Effects of visible-light irradiation on vitreous structure in the presence of a photosensitizer

Sensitized photo-induced changes of vitreous structure were investigated using both in vivo and in vitro model systems. In the former, rabbit eyes were injected with the photosensitizer riboflavin, and in the latter, calf vitreous samples were treated with riboflavin or Methylene Blue prior to irradiation with white light. The active species of oxygen, i.e. singlet oxygen, superoxide anion, hydroxyl radical and hydrogen peroxide, generated by the photodynamic action of the sensitizer, caused significant liquefaction of the calf vitreous in vitro. There was little liquefaction of the rabbit vitreous in vivo, suggesting the presence of a protective mechanism in vivo. Hyaluronidase induced significantly greater liquefaction in vitro than either Methylene Blue or riboflavin. This study suggests that loss of gel vitreous structure can result from extensive depolymerization of hyaluronate by hyaluronidase and less drastic conformation and molecular weight changes in the photosensitized reactions. Although light-induced liquefaction was less marked than enzyme-induced liquefaction, the mechanism of the former is more pertinent to age-related vitreous synchysis.