Equine recurrent uveitis (ERU) is often managed with topical corticosteroids to avoid unwanted consequences of systemic corticosteroids. A study evaluated penetration of two different steroid forms for topical use in the eye [Hermans, H.; van der Berg, E.M.H.; Slenter, I.J.M.; et al. Penetration of topically administered dexamethasone disodium phosphate and prednisolone acetate into the normal equine ocular fluids. Equine Veterinary Journal Oct 2021; DOI: 10.1111/evj.13526].
In the study, 21 Shetland ponies were treated that upon ocular examination were not suffering from anterior uveitis. Both eyes were treated every two hours for 24 hours to simulate an aggressive treatment protocol for acute ERU. One eye was treated with 0.15 mg dexamethasone disodium phosphate (0.1%); the other eye was treated with 1.5 mg prednisolone acetate (1%). Under general anesthesia, samples were taken from aqueous and vitreous humor at multiple time points following the last dose—5, 15, 30, 60, 90, 120 and 180 minutes.
In aqueous humor in the anterior chamber, dexamethasone reached a concentration of 32.4 ng/ml and prednisolone at 321.6 ng/ml.
The authors stated that penetration of dexamethasone and prednisolone were not statistically different considering that the prednisolone medication has a 10-times-higher concentration than dexamethasone—1% vs 0.1%. Also, prednisolone contains drug particles within its suspension, and it use causes ocular discomfort in horses; it is important to shake it vigorously prior to treatment. To exert anti-inflammatory effects, dexamethasone needs a minimum concentration of 20-25 ng/ml.
While therapeutic concentrations were achieved in the anterior chamber by both medications, concentration of both corticosteroids were below the limit of detection in the vitreous and in serum samples, thus making topical ophthalmic corticosteroid treatment unrealistic for cases of posterior uveitis.
To manage posterior uveitis, other routes of corticosteroid administration are necessary—subconjunctival, peribulbar injection or systemic administration.
