PRK as a possible risk factor in cataractogenesis.

Short-term oxidative status of lens and aqueous humor after excimer laser photorefractive keratec-tomy. J Refract Surg 1999;15:673-679). Since their results partially disagree with our previous findings , it is important to comment on the methodolog-ical differences between our and their experiments and secondly to single out certain discrepancies in their results. In 1994, we published the first report on the possible cataractogenic effect of PRK. 1 Further studies of ours (at various follow-up periods) allowed confirmation of our preliminary results and led us to try to ascertain the pathogenesis of lens damage. 2 In our experiments, photoablative treatments were performed using an ArF 193-nm excimer laser (Meditec Aesculap, Heroldsberg, Germany). The laser pulse rate was fixed at 20 Hz with the fluence adjusted to 250 mJ/cm 2 per pulse, equivalent to an ablation depth of 0.5 µm. In the first experiment, the number of pulses delivered ranged between 3,000 and 12,000, corresponding to a cumulative UV dose delivered by the excimer laser that ranged between 750 and 3,000 J/cm 2 , approximately equivalent to the dose employed to reduce 6 to 24 diopters of myopia in humans. The irradiation field was 5 x 10 mm (50.0 mm 2); the scanning mode (split beam profile 5 x 1 mm) and the mask system were used. The treated area was equivalent to one-third of the irradiated area (due to the presence of the mask). Thus, the true treated area was 16.6 mm 2. This reduced the amount of active energy by a factor of 50 (between 15 and 63 J/cm 2). However, this value was further diminished by the presence of the scanning slit mode by a factor of one-third of 5.0 mm (in length) and by an additional factor of two, due to the overlap of about 50%. Thus, the final amount of energy at the level of the anterior surface of the cornea ranged between 0.45 and 1.80 J/cm 2 , equivalent to an energy at the level of the anterior surface of the lens ranging between 45 and 180 µJ/cm 2. In the article by Giasson and colleagues, the cumulative UV dose was always 120 J/cm 2. At this presumably lowest UV exposure, these authors did not find any variation in H 2 O 2 levels in aqueous humor, nor in total and oxidized glutathione and MDA concentration in the lens between treated and untreated rabbits, 10 minutes after …