REPRODUCIBILITY OF VIDEOKERATOGRAPHIC DIGITAL SUBTRACTION MAPS AFTER EXCIMER-LASER PHOTOREFRACTIVE KERATECTOMY

Background: Digital subtraction photokeratography can best identify to pographic changes after excimer laser photorefractive keratectomy (PRK ). To evaluate the reproducibility of these topographic maps, the auth ors used a topographic modeling system to generate multiple subtractio n maps from different combinations of technically acceptable preoperat ive and postoperative maps for eyes that underwent PRK. The assigned p atterns for each patient then were evaluated for consistency. Methods: Seven hundred twenty-two individual subtraction maps were generated f or 64 eyes that underwent PRK. A mean of 11.3 maps were generated for each eye. The topography of each map was individually classified as no rmal, central island, peninsula, or asymmetric. All maps within a set (consisting of examinations for 1 patient at a single postoperative in terval) then were examined as a unit to determine the overall topograp hic classification for that set of maps. Each set in which each consti tuent map had the same topographic assignment as the set was considere d ''nonvariant,'' whereas those sets in which one or more individual s ubtraction maps had different topographic assignments were considered ''variant.'' Results: Of the 64 sets, 33 (52%) were variant and 31 (48 %) were nonvariant. Conclusions. Any one subtraction map produced by t he topographic modeling system may not be a reliable indicator of the excimer effect.