Presbyopia

PRESBYOPIA, or old age hyperopia, represents a weakening of accommodation that physiologically occurs in old age. The volume of accommodation in general is in close dependence on age. The eye can work at close distances without tiring only in the case that it expends no more than ¹/₂ or ²/₃ of the available volume of accommodation. Most activities are performed at a distance of about 33 cm, which corresponds to 3.0 D of accommodation for an emmetropic eye. Such tension of accommodation can persist for a long time with an accommodation volume of at least 6.0 D, which corresponds to the age of 40 years. This age with emmetropic refraction is the beginning of presbyopic phenomena. P. is not caused by any disorder of the ciliary muscle or its innervation, but is caused by the hardening of the lens. Due to this, the lens loses its elastic properties, and therefore in response to the contraction of the ciliary muscle, the surfaces of the lens no longer change their curvature as occurs in young age. In P., the point of nearest clear vision moves away from the eyes. Therefore, presbyopes complain of poor vision at close distances. For the correction of P., converging glasses are used. These glasses replace the weakened accommodation of the patient and give him the ability, despite the more or less sharp decrease in the volume of accommodation, to still work at the usual distance. Since the volume of accommodation changes with age, it is natural that the strength of the prescribed corrective glass should also change. In practice, when fitting glasses for presbyopes with emmetropic refraction, a dual approach is possible. One can determine for a given patient the position of the nearest point of clear vision for each eye and, based on this and using the formula proposed by Donders (Donders), find the strength of the necessary glass, or one can use a special empirical table without any preliminary measurements, which indicates the strength of the glass depending on age. But since accommodation does not weaken in all people at the same age with the same force, this table gives only a guiding thread, and in fact, in each individual case, it is necessary to verify how well a given glass actually works for close work. Age

D 45-50 years......................1.0-1.5 50-55 years.....................1.5-2.0 55-60 years.....................2.0-2.5 60-65 years.....................2.5-3.0 In addition to age, when fitting glasses for presbyopes, a number of other factors must also be taken into account, namely: refraction, type of occupation, and visual acuity. 1. Refraction. The above calculations for the strength of the corrective glass in connection with age are only suitable for eyes with emmetropic refraction; if there is hypermetropia and the patient already wears converging glasses for distance, then the strength of the glass necessary by age for the correction of P. should be added to the distance glass. Conversely, with myopic refraction, the strength of the distance glass is subtracted from the presbyopic glass. The influence of refraction on P. is also manifested in the fact that with hypermetropia P. occurs at an earlier, and with myopia at a later age than with emmetropia. 2. The type of occupation has significance in the correction of P. insofar as the working distance varies for different professions. For example, in sewing, work is held farther from the eyes than in reading and writing. Engravers and watchmakers, on the contrary, work at closer distances. The above table takes into account a working distance of 33 cm. When working at closer distances, stronger glasses are prescribed, and when working at greater distances, weaker glasses than correspond to the age. 3. Visual acuity must be taken into account in cases where it is reduced due to some diseases. Any significant reduction in visual acuity leads to the need to bring the working object closer to the eyes. In connection with the need to bring the object very close in such cases, it is necessary to prescribe stronger glasses than correspond to the age. When the reduction in visual acuity reaches very high degrees, presbyopic correction must be combined with special magnifying systems and telescopic glasses must be prescribed (see Glasses). In cases where P. is accompanied by an anomaly of refraction, the patient needs correction for both distance and near. To avoid prescribing two pairs of glasses, bifocal glasses can be used for this purpose (see Glasses), which give the patient the ability to see clearly both at a distance and up close at the same time.