Pilocarpine with reference to accommodative esotropia

The use of miotic drops in the treatment of accommodative strabismus is widely accepted, when there is present a higher than normal accommodative convergence/accommodation (AC/A) ratio. According to their pharmacological action, the miotics in current use fall into two groups-the anticholinesterases and the parasympathomimetics. Pilocarpine and Carbachol belong to the latter group, and while the value of Pilocarpine in accommodative esotropia is accepted, the value or otherwise of Carbachol appears to be unknown. This paper describes an experiment in which the cyclotonic effects of Pilocarpine and Carbachol are compared. Accommodation is simultaneously associated with convergence in order to maintain binocular single vision. The AC/A ratio expresses the degree of accommodative convergence, in prism dioptres, which occurs for a given amount of accommodation, measured in dioptres. The stimulus for accommodation has its origin in the central nervous system and is effected through the parasympathetic outflow to the ciliary muscle. In the presence of an abnormal AC/A ratio, excess convergence takes place for near, and if this escapes fusional control, it will result in esotropia. The mechanism by which miotics decrease the AC/A ratio lies not in miosis but in the peripheral stimulation of the ciliary muscle, thus lessening the amount of central innervational stimulus required and so reducing the accommodative response and convergence. Although miotic therapy for strabismus was described a century ago, it was not widely used until the work of Abraham (I949) was published. In the anti-cholinesterase group of drugs, Echothiophate (Phospholine) iodide and Isofluorophate (DFP) were favoured for their prolonged effects. Doubts were raised concerning the safety of these drugs when sideeffects, including iris cysts, lens opacities, irreversible miosis, angle-closure glaucoma, retinal detachment, and scoline apnoea in general anaesthesia, were reported (Pantuck, I966; Goldstein, I968; Roy, Hanna, Brown, and Clifton, 1970). Pilocarpine and Carbachol act directly upon the parasympathetic effector cell, but Carbachol also resists hydrolysis by acetylcholinesterase and might be thought to provide a longer acting and useful alternative to Pilocarpine in the treatment of accommodative esotropia.