Toxicity of thimerosal on the corneas of adult pigmented rabbits

Thimerosal is a common preservative agent used in cleaning and disinfecting solutions for hard and soft contact lenses. This agent's toxicity is commonly assessed by the Draize test in which drops of thimerosal are applied to the rabbit eye and reaction of the eye evaluated. Thimerosal is known to cause adverse ocular reaction in some contact lens patients but it is not known whether these reactions are chemotoxic or immunologic in origin. The primary aim of the study was to compare the tissue thresholds to the effects of thimerosal when applied as drops and in a soft contact lens. A secondary aim was to establish how hypersensitivity to thimerosal might develop and play a role in clinical situation. Eight adult pigmented rabbits were used in the study. Toxicity threshold of the cornea to thimerosal was measured using various concentration of thimerosal drops and by soaking soft contact lenses in thimerosal of different concentrations. The main result is that the toxicity threshold of the cornea to thimerosal is ten times higher when applied as drops than when applied in a soft contact lens. In addition, local hypersensitivity to thimerosal did not develop after eight weekly of daily application of thimerosal drops to the eyes. These findings support the initial hypothesis that the threshold level for toxicity using thimerosal-soaked contact lenses would be lower than that of thimerosal drops. This probably occurs because the contact lens keeps the thimerosal in contact with the eye longer than following its topical instillation.