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Yeditepe Üniversitesi Hastanesi Göz Hastalıkları A.D., İstanbul, Yard. Doç. Dr. Accommodation is the dioptric change in the refractive power of the eye, so that the eye can change its point of focus from distance to near. Presbyopia is the receding of the eye's near point away from the eye, due to narrowing of the accommodative amplitude. Ciliary body, ciliary muscle, choroid, anterior and posterior zonular fibers, lens capsule and crystalline lens make up the accommodative anatomy of the eye. The role of the vitreus is both supported and opposed by different theories. The ability and mechanism of accommodation vary among vertebrates, as much as their environmental/visual circumstances and accommodative needs do.
The accommodative mechanism has been studied since fourteenth century. Currently widely accepted Helmholtz's “Crystalline Lens Elasticity Theory” states that during accommodative maneuver, ciliary muscle contracts anteriorly and towards the axis of the eye. Zonullar tension system works as a pulley for the entire system and transfers the tension force to the posterior zonules, so that the anterior zonules are relaxed for 360°. The crystalline lens becomes more spherical with its own elasticity. The anterior lenticular curvature increases, radius of curvature decreases and the vertex displaces anteriorly. Posterior lenticular curvature also increases slightly, but remains in its own place. Thus, the lens is thickened centrally. The anterior surface never becomes steeper than the posterior surface. Glasser et al have proved the Helmholtz's theory of accommodation by studying the accommodative mechanism in vivo in rhesus monkeys, by electrically stimulating the parasympathetic pathways under surgical anesthesia.
Although age related changes take place in all accommodative structures, lens and capsule based changes have the greatest role in the occurence of presbyopia. Keywords : Presbyopia, lens