On the effects of different laser energy sources upon the iris of the pigmented and the albino rabbit.

The irides of pigmented and albino rabbits have been irradiated by a) a CW argon laser beam (exposure duration up to 1 s), b) a 1 ms pulses Nd:YAG laser and c) a 30 ns pulses Q-switched ruby laser. The immediate and long-term pathologies were analysed by scanning and transmission electron microscopy over a period of 13 months. At both the gross and ultrastructural levels, damage configuration may differ considerably, depending on the three modes of irradiation. For each source there are both thermal and mechanical damage components and the significance of mechanical effects increases with decreasing pulse duration for a constant pulse energy. In the argon experiments, tissue destruction is predominantly a consequence of heat, resulting from conduction and convection. The subsequent regeneration of tissue after such heat-induced trauma is fast. The effects of the Nd:YAG laser, at the irradiance levels used in the present study, are again predominantly of a thermal nature and are caused by heating and local evaporation. The pigmented and the nonpigmented iris epithelium are destroyed and widespread decay of the stroma occurs over some months. Such damage never results in full repair. The most prominent feature of Q-switched ruby laser irradiations is their independence of the iris pigment content. In contrast, at the energy levels studied, the argon laser is entirely ineffective, whilst the effect of the Nd:YAG laser is much reduced in the absence of pigment. The consequences of these findings for the clinical applications of such lasers are discussed.