Photodynamic therapy with infracyanine green induces keratocyte depopulation in the normal equine cornea.

Photodynamic therapy using infracyanine green causes a temporary loss of keratocytes (corneal cells) and prolonged stromal inflammation in healthy horse corneas, which helps explain its use in treating immune-mediated keratitis. This study assessed the safety and cellular effects of this treatment in normal equine eyes.

Study Overview and Objective

• Photodynamic therapy (PDT) with infracyanine green (EmunDo®) is increasingly used to treat immune-mediated keratitis (IMMK) in horses.
• The safety and cellular changes in normal horse corneas following PDT had not been previously studied.
• The study aimed to evaluate the morphologic effects and safety of in vivo PDT on healthy equine corneas to better understand its mechanism of action.

Methods

• Six healthy horses owned by the university participated in the study.
• Each horse received PDT in one eye involving:
  • Intrastromal injection of infracyanine green dye (EmunDo®).
  • Photoactivation with an 810 nm diode laser delivering 75 Joules over 2.5 minutes (500 mW power).
• Comprehensive ophthalmic exams and imaging studies were performed before treatment and on days 1, 5, 15, 33, and 103 after treatment including:
  • Clinical scoring and photography (digital and infrared).
  • Ocular thermography to assess temperature changes.
  • In vivo confocal microscopy (IVCM) to examine corneal cell morphology.
  • Ultrasound biomicroscopy to image the corneal layers.

Key Findings

• All treated eyes developed corneal ulcers following PDT.
• In vivo confocal microscopy revealed:
  • Immediate destruction of keratocytes (stromal corneal cells) right after treatment.
  • Continued keratocyte depopulation on days 5 and 15.
  • Slow repopulation of keratocytes starting at day 33 and continuing by day 103.
• Stromal keratitis (inflammation of the corneal stroma) was present by day 5 and persisted through day 103.
• Infracyanine green dye (EmunDo®) remained visible in the cornea at day 103 in 5 out of 6 horses, indicating prolonged dye retention.
• No horses experienced severe adverse effects like vision loss.

Conclusions and Implications

• The study indicates that PDT causes:
  • Corneal ulceration.
  • Keratocyte depopulation lasting several weeks.
  • Prolonged inflammation in the corneal stroma.
  • Retention of the dye within corneal tissue for months.
• The transient removal of keratocytes likely reduces immune cell activity in the stroma, which may explain how PDT helps treat immune-mediated keratitis.
• Despite the side effects seen, no blinding complications occurred, supporting relative safety in healthy eyes.
• Limitations include small sample size and inclusion of horses of different ages, which could affect healing rates.
• Overall, the findings provide insight into the cellular mechanisms of PDT and inform its use and safety considerations in equine ophthalmology.