Ocular distribution and toxicity of intravitreal injection of triamcinolone acetonide in normal equine eyes.

The research article is about a study conducted to understand the effects and distribution of a single, intravitreal injection of triamcinolone acetonide (a corticosteroid) on the eyes of normal horses. The experiment, mainly aimed at detecting any possible toxicity, confirmed that the administered drug was found only in the parts of the eye and not in the plasma, showing no overt toxicity, but suggested increased risk of bacterial infections.

Research Methodology

• Six adult horses were selected for this study, which was carried out at North Carolina State University. The horses were injected intravitreally (into the eye’s vitreous body) with varying doses of triamcinolone acetonide (TA) – 10mg, 20mg, or 40mg (two horses for each dosage). The opposite eye of each horse was injected with a balanced salt solution (BSS).
• Observations were made for any ocular toxicity through several methods – biomicroscopy (magnified examination of the eye), tonometry (evaluation of the eye’s internal pressure), indirect ophthalmoscopy (examination of the back of the eye), and electroretinogram (ERG – measures the electrical responses of cells in the eyes).
• Aqueous humour (fluid in front of the eye lens), vitreous humour (gel-like substance filling the back of the eye), and plasma samples were collected from the horses. Seven or twenty-one days after injection, the horses were euthanized, their eyes were enucleated (removed) for further study.
• High-Performance Liquid Chromatography (HPLC) was used to measure the concentrations of TA in the aqueous humour, vitreous humour, and plasma samples.

Results and Findings

• Inflammation was observed in three control eyes (injected with BSS) and one eye injected with TA, following injection or collection of aqueous humor. Bacterial cultures showed presence of bacteria such as Corynebacterium spp., Staphylococcus spp., and Streptococcus spp.
• No substantial changes were observed apart from transient corneal edema (swelling) in TA injected eyes, which resolved within a week after injection. Visible TA crystals were located within the vitreous body of the eyes.
• There was no evidence of any toxic effect from TA found on histopathology (study of changes in tissues caused by disease).
• TA was detected in every aqueous and vitreous humour samples from the injected eyes, but not in plasma.

Conclusion

• The study did not find overt toxicity from intravitreal TA injection in the normal horses, and TA ocular levels remained consistent for 21 days following a single injection.
• However, the significant risk highlighted is bacterial infections arising from intravitreal injection or anterior chamber aspirations in horses, which is high. The study thus recommends the use of topical and systemic antibiotics after the injection procedure.