Retention time of gentamicin in the vitreous of horses after an intravitreal injection

An intravitreal gentamicin injection (IVGI) was first described as an alternative treatment option for equine recurrent uveitis (ERU) in 2005. The aim of the present study is to determine the retention time of gentamicin in the vitreous of horses after an IVGI. Accordingly, an IVGI was performed on a total of 15 horses and vitreous samples were taken from 3 horses each after 0, 7, 10, 14 and 21 days and tested for their gentamicin concentration. The measurements showed that gentamicin can be detected in all horses sampled with concentrations above the minimal inhibitory concentration in the vitreous up to 10 days after an IVGI. After a sampling period of 14 days, only one of the three horses still showed a detectable concentration of gentamicin in the vitreous (2.87 mu g/ml). No gentamicin above the detection limit of 0.4 mu g/ml could be measured in any of the three vitreous humours 21 days after an IVGI. Blood samples taken one hour after an IVGI from three healthy horses and three horses suffering from ERU showed no measurable gentamicin concentrations. Because of this, a systemic effect with potential nephro- and ototoxicity of gentamicin after an IVGI of 4 mg can be neglected. A gentamicin concentration of 138.0-147.0 mu g/ml was measured in vitreous samples taken at the end of a vitrectomy in three additional horses. The measurements of three additional vitreous samples without a prior IVGI as a control group showed no evidence of gentamicin. Further research in this area is necessary in the future to clarify the long-lasting efficacy of gentamicin against recurrences of ERU with a relatively short retention time in the vitreous.