Topical Anaesthesia

In the examination of the anterior segment of the eye an anaesthetic agent can be very helpful. Topical anaesthesia of a painful cornea/eye will let the ophthalmologist examine the cornea, and conjunctiva as a blepharospasm decreases. Without anaesthesia measuring of the IOP is not possible, and anaesthesia should be applied before any vital stains, as these cause stinging and pain (reported in humans). Before corneal scrapings are undertaken an anaesthetic agent should be topically applied (Murphy 1988). The most popular anaesthetic agent in veterinary ophthalmology is currently 0.5% proparacaine (Ward 1999). In Denmark 0.5% proparacaine is not commercially available and 4% oxybuprokain can be used instead. In humans, onset of anaesthesia is approximately 13 seconds after application, in canines the onset is probably 15seconds to 2mins after application; in dogs the anaesthesia of the cornea lasts 10-20 minutes (Ward 1999). Local anaesthetics' effect on bacteria have been investigated, concluding that 0.5% proparacaine didn't inhibit bacterial growth, thus 0.5% proparacaine without a preservative in single-dose vials would be optimal (Kleinfield and Ellis 1966); other anaesthetics were proven to inhibit bacterial growth. Benoxinate Hydrochloride 0.4% is available without preservatives (Minims®, Chauvin Pharmaceuticals Ltd, Essex, England) in Denmark. Benxinate 0.4% has been shown to be lethal to Staphylococci but does not affect growth of Pseudomonas (Kleinfield and Ellis 1966). The nerves of the canine cornea have been examined histologically by Spreull (1966) : "Unmyelated nerve fibers pass from the periphery of the cornea towards the centre of the stroma. They are arranged in two planes, the more superficial of which forms a plexus which terminate in the epithelial layer where they have both tactile and pain terminals. The deeper fibers which are distributed throughout the substance of the stroma have only tactile terminals".

This page was authored by T. F. Evans October 2000.