Why do we give miotics like ECOTHIOPHATE in case of non refractive accomodative squint … and what do we mean by AC/A ratio ?
Hi Wake Awake,
The ratio accommodative-convergence (AC) over accommodation (A) (AC:A Ratio) delineates the relationship between the amount of convergence produced by a stimulus to accommodate and the amount of accommodation which produces that convergence.
Non-refractive accommodative squint has a high AC:A ratio. The accommodative effort results in an abnormally high convergence response. Because more accommodation occurs at near, the excess convergence is more, so the near esodeviation is larger than that for distance.
Echothiophate reduces near esodeviation. It has a parasympathomimetic effect on the iris sphincter and ciliary muscle, thus reducing the accommodative effort required to obtain a clear retinal image. As a result, the reflex convergence also reduces.
Thank you.
Parasympathomimetic effect (ecothiphate) on ciliary muscle causes - contraction - thus accomodation type effect, thus less accomodation needes to see a near object which inturn leads to less convergence