Excimer laser calibration system.

BACKGROUND: Excimer laser photoablation for refractive and therapeutic keratectomies has been demonstrated to be feasible and practicable. However, corneal laser ablations are not without problems, including the delivery and maintenance of a homogeneous beam. We have developed an excimer laser calibration system capable of characterizing a laser ablation profile. METHODS: Beam homogeneity is determined by the analysis of a polymethylmethacrylate (PMMA)-based thin-film using video capture and image processing. The ablation profile is presented as a color-coded map. Interpolation of excimer calibration system analysis provides a three-dimensional representation of elevation profiles that correlates with two-dimensional scanning profilometry. Excimer calibration analysis was performed before treating a monkey undergoing phototherapeutic keratectomy and two human subjects undergoing myopic spherocylindrical photorefractive keratectomy. Excimer calibration analysis was performed before and after laser refurbishing. RESULTS: Laser ablation profiles in PMMA are resolved by the excimer calibration system to .006 microns/pulse. Correlations with ablative patterns in a monkey cornea were demonstrated with preoperative and postoperative keratometry using corneal topography, and two human subjects using video-keratography. Excimer calibration analysis predicted a central-steep-island ablative pattern with the VISX Twenty/Twenty laser, which was confirmed by corneal topography immediately postoperatively and at 1 week after reepithelialization in the monkey. Predicted central steep islands in the two human subjects were confirmed by video-keratography at 1 week and at 1 month. Subsequent technical refurbishing of the laser resulted in a beam with an overall increased ablation rate measured as microns/pulse with a donut ablation profile. A patient treated after repair of the laser electrodes demonstrated no central island. CONCLUSIONS: This excimer laser calibration system can precisely detect laser-beam ablation profiles. The calibration system correctly predicted central islands after excimer photoablation in a treated monkey cornea and in two treated human subjects. Detection of excimer-laser-beam ablation profiles may be useful for precise calibration of excimer lasers before human photorefractive and therapeutic surgery.

Photosensitization-induced retinopathy in the newborn beagle.

PURPOSE: Photosensitization is a mechanism by which oxygen and light may interact to generate free radicals capable of tissue injury. It has been proposed as a possible etiologic mechanism in the development of retinopathy of prematurity. The authors report the effects of light exposure and a photosensitizer, rose bengal (RB), on the developing retina of the beagle puppy. METHODS: Seven purebred beagle puppies (2 or 7 days old) were given RB (7.5 mg/ml) intravenously (0.9 ml bolus followed by a 74 microliters/min constant infusion), and one eye was exposed to filtered light delivered by a modified slit lamp at 30 mW/cm2 for 5, 15, 25, 35, and 45 minutes. The fellow eye was not irradiated and served as an RB-only control. Three beagles were exposed to light (one eye only) for 15 to 120 minutes in the absence of RB to provide light-only and no-treatment control eyes. Animals were followed clinically by indirect ophthalmoscopy and fluorescein angiography, and eyes were processed for light microscopy. RESULTS: Clinical and/or histologic abnormalities were found in all seven eyes exposed to light in the presence of rose bengal, as follows: vitreous hemorrhage in four eyes, incomplete peripheral retinal vascularization in four eyes, fibrovascular/fibrocellular proliferation with traction on the retina in two eyes (including preretinal neovascularization in one eye), total or partial retinal detachment in two eyes, dilated peripheral vessels compatible with shunt vessels in one eye, retinal dysplasia with loss of normal architecture and formation of rosettes in five eyes. All 13 control eyes showed normal, complete retinal vascularization. CONCLUSIONS: The failure to progress to the end-stage tractional retinal detachment seen in human infants is a shortcoming of existing animal models. Photosensitization injury to the developing retina in this animal model can produce a spectrum of retinal pathology that includes extraretinal vascularization and subsequent retinal detachment.

Light-induced deposits in Bruch's membrane of protoporphyric mice.

Photosensitization of choriocapillary endothelium with blood-borne photosensitizers, such as protoporphyrin IX, has been proposed as a mechanism for the choriocapillary sclerosis and Bruch's membrane deposits seen in age-related macular degeneration. Utilizing a mouse model of protoporphyria with approximately a 10-fold increase in protoporphyrin IX level and exposure to blue light (14 microW/cm2; bandwidth, 390 to 430 nm), a time- and light-dependent increase in choriocapillary and sub-retinal pigment epithelium basal laminar-like deposits could be demonstrated at 7 months by transmission electron microscopy. Thickening of the choriocapillary endothelial basement membrane with a homogeneous electron-dense material was first noted in protoporphyric mice exposed to blue light for 13 weeks. At 28 weeks the experimental animals exhibited a thick band of homogeneous deposits at the level of the choriocapillary basement membrane and electron-dense fibrillogranular deposits of varying sizes along the inner aspect of Bruch's membrane, with fibrils measuring up to 16 nm, with a periodicity of 13 nm. These deposits contributed to an overall thickening of Bruch's membrane with narrowing of the choriocapillaris. The morphologic appearance and localization of these deposits within Bruch's membrane of this animal model are similar to previously described deposits noted in the aging Bruch's membrane prior to the development of age-related macular degeneration.

Clinical follow-up of 193-nm ArF excimer laser photokeratectomy.

The excimer laser has been undergoing rapid development for clinical use since the early 1980s. The authors report 2-year follow-up results from studies in 31 eyes (15 women and 14 men) to evaluate the excimer laser in performing photokeratectomy. Patients were divided into two groups: 27 eyes (group 1) underwent phototherapeutic keratectomy, and 4 eyes (group 2) underwent photorefractive keratectomy. Visual function improved in 21 of 27 eyes in group 1 and in 2 of 4 eyes in group 2. Complications were minimal and manageable. The authors describe a procedure to minimize induced hyperopia in phototherapeutic keratectomy patients, and, although not statistically significant, less induced hyperopia was noted in these patients. Photokeratectomy may be an alternative to penetrating or lamellar keratoplasty and more invasive refractive procedures, but the long-term effects must be carefully observed.