Photodynamic therapy with infracyanine green induces keratocyte depopulation in the normal equine cornea.
Authors: Rogers Callie M, Ledbetter Eric C, Reid Abigail M, Scott Erin M, Knickelbein Kelly E
Journal: Equine veterinary journal
Summary
# Editorial Summary: Photodynamic Therapy and Corneal Safety in Horses Photodynamic therapy (PDT) using infracyanine green is increasingly used to treat immune-mediated keratitis in horses, yet its safety profile and mechanism of action in healthy corneal tissue remained poorly characterised. Rogers and colleagues conducted an in vivo study on six horses, applying intrastromal infracyanine green with 810 nm laser photoactivation (500 mW, 2.5 minutes) to one eye of each horse, then monitored corneal responses using clinical examination, in vivo confocal microscopy, ultrasound biomicroscopy, and thermal imaging over 103 days. All treated corneas developed ulceration post-treatment, with immediate keratocyte destruction evident on confocal microscopy, followed by significant keratocyte depopulation on days 5 and 15; whilst slow repopulation occurred by days 33 and 103, stromal keratitis persisted throughout the observation period and dye retention was visible in five of six horses at day 103. These findings suggest that PDT's therapeutic efficacy in immune-mediated keratitis operates through cytotoxic cellular depopulation of the corneal stroma, though practitioners should recognise that this mechanism necessitates substantial corneal inflammation and ulceration as treatment byproducts. The prolonged inflammatory response and slow cellular recovery documented here indicate that post-treatment management—including anti-inflammatory therapy, close monitoring for secondary complications, and realistic client expectations regarding recovery timelines—will be critical to optimising clinical outcomes in affected horses.
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Practical Takeaways
- •PDT induces significant corneal damage in normal equine corneas including ulceration and prolonged inflammation; use should be restricted to diseased corneas where benefits outweigh risks
- •Expect stromal keratitis and corneal ulceration as complications following PDT treatment; plan for prolonged healing and close monitoring through at least 15 days post-treatment
- •The mechanism of PDT efficacy in treating IMMK appears to be selective destruction and depopulation of keratocytes; this finding may help guide clinical application protocols
Key Findings
- •PDT with infracyanine green caused corneal ulceration in all 6 treated eyes post-treatment
- •IVCM revealed destroyed keratocytes immediately post-treatment with keratocyte depopulation on days 5-15
- •Stromal keratitis developed by day 5 and persisted through day 103 in all horses
- •Infracyanine green dye remained visible in corneas of 5/6 horses at day 103, with slow keratocyte repopulation evident by days 33-103