Use of ophthalmic topical anaesthetics

Pretreatment of the eyes of rabbits with a topical anaesthetic can be viewed as a refinement of the test for eye irritation. It reduces pain at the time of test-material administration, decreases animal distress and permits easier application of the test agent to the eye. In some cases, however, use of an anaesthetic either alone or in combination with the test substance may alter ocular responses or provide little benefit. Although anaesthetic pretreatment may result in decreased pain at the time of test-compound administration, it does not affect possible pain after the effects of the anaesthetic have dissipated. Some anaesthetics are themselves irritating to eyes. In addition, anaesthetics reduce blinking and tearing, thereby maintaining the test-material concentration at the surface of the eye longer. Corneal permeability may also be increased with pretreatment use of an anaesthetic, and may bring the test agent into contact with more structures of the eye. Some anaesthetics delay healing after ocular injury. All of these varied effects may result in increased irritation to the eye. Overall, pretreatment with anaesthetics has usually resulted in a tendency for slightly higher irritation scores; eye irritancy classification is usually unaffected.