Abstract Background Use of photodynamic therapy (PDT) for the treatment of immune‐mediated keratitis (IMMK) in horses is becoming common. The safety and morphologic impact on the normal equine cornea have not been investigated, and the mechanism of its efficacy is unknown. Objective To investigate the morphologic effects and safety of in vivo PDT on healthy equine corneas. Study Design In vivo experiment. Methods Six university‐owned horses underwent unilateral corneal PDT with intrastromal infracyanine green (EmunDo®) and photoactivation with an 810 nm diode laser (500 mW for 2.5 min = 75 Joules). Complete ophthalmic examinations, clinical scoring, digital and infrared photography, ocular thermography, in vivo confocal microscopy (IVCM), and ultrasound biomicroscopy were performed pre‐treatment and post‐treatment on study days 1, 5, 15, 33, and 103. Results Post‐treatment, corneal ulceration occurred in all treated eyes. In all horses, IVCM identified destroyed keratocytes immediately post‐treatment followed by keratocyte depopulation on days 5 and 15. Slow keratocyte repopulation was evident on days 33 and 103. Stromal keratitis was seen on day 5 and persisted through day 103. EmunDo® remained visible in the cornea on study day 103 in 5/6 horses. No horses developed blinding complications. Main Limitations Limited sample size included horses of various ages. Conclusions Corneal ulceration, keratocyte depopulation, prolonged stromal keratitis, and prolonged corneal dye retention occurs with PDT in normal equine corneas. Corneal stromal cellular depopulation is proposed as the mechanism by which PDT is effective in the treatment of equine IMMK.
Rogers et al. (Sun,) studied this question.