CHAPTER 32 Refractive presbyopia management
Introduction
Fundamental principles and anatomical considerations
The change in optical power of the eye in order to produce a sharp retinal image of objects at different distances is called accommodation. The accommodative apparatus of the eye consists of the ciliary muscle, the zonular fibers which form the suspension apparatus of the crystalline lens, and the crystalline lens itself. During accommodation (far–near focusing) the tension of the zonular fibers is reduced by the contraction of the ciliary muscle ring and under the influence of the elastic lens capsule the shape of the lens changes to its mechanical resting state12. This increases the surface curvature and the anteroposterior thickness of the lens and reduces its equatorial diameter. These changes contribute to an increase in optical power of the eye.
Clinical features and pathophysiology of presbyopia
The decrease of accommodative amplitude begins at an age of 10–12 years and ends with a nearly complete loss of accommodative ability by the age of 50–55. Presbyopia has been attributed to a loss of elasticity of the ciliary muscle and the choroid as well as geometric changes and hardening of the lens, especially the lens nucleus, with advancing age. Although the underlying causes are still a matter of scientific discussion, it is basically accepted that the hardening of the lens substance represents a substantial limiting factor for the accommodation in the presbyopic eye4. Experimental results suggest that other important structures such as ciliary muscle and lens capsule still remain at least partially functional even in the presbyopic eye.
Operation techniques
Pseudoaccommodative procedures
Monovision
The term monovision refers to correcting one eye (usually the dominant) to emmetropia and the other to a myopia between −1.0 and −2.0 D. which can be achieved with a variety of refractive surgical procedures (e.g. excimer laser surgery)2. In hyperopes, a steepening of the cornea with over-correction of one eye can be performed by laser thermokeratoplasty (LTK) or conductive keratoplasty (CK). All surgical techniques which can generate a unilateral myopia can be used in emmetropes.
