Presbyopia

when discussing the mechanism of accomodation we saw that the increased refractivity of the eye was probably brought about by a change in the balance between the elasticity of the matrix of the lens and that of its capsule allowing it to assume a more globular shape. As age advances, however, several factors combine to diminish the accommodative power. The lens becomes harder and less easily moulded so that the elastic force of the capsule is no longer greater than the resistance of the lens substance. The lens tends therefore to set in an unaccommodated form. Moreover, the progrssive increase in the size of the lenstogether with similar changes in the ciliary body reduce the circumlental space so that the zonule becomes slackened and works at a disadvantage making the amplitude of accommodation less. Although there is avidence that some weakening of the ciliary muscle accours as age advances, especially in the later years of life, it is now clear from the work of Fisher that presbyopia can be explained solely by the physical changes in the kens itself.

    As a result it becomes more and more difficult to see near objects distinclty; that is, the near point gradually recedes. This loss of accommadation is not to be considered as abnormal, and it proceeds gradually throughout the whole of life without any sudden alterations. At first no inconvenience is experienced, but eventually a time comes when the near point has receded beyond the distance at which his arms allow him to hold the printed page, and then, being unable to see clearly, he becomes seriously inconvenienced. Such a condition a called presbyopia.

The variation of accommodation with age

The variation of the power of accommodation with age can be gathered from figure 10.1 which was compiled by Fisher from various sources, The diagram is a representation of the average of the result, on many subjects.

   It is seen that in the early years of life the amplitude of accommodation is about 14 D, and that the near point is situated at 7 cm distance. Thereafter is gradually and uninterruptelly rocedes; at the age of thirty-siz it has reached 14 cm, when the amplitude of accommodation has become halved and is now 7 D instead of the original 14 D. At the age of forty-five has reached 25 cm, and the amlitude of accommodation is only 4 D; at the age of sixty only about 1 D of accommodation remains.

    In the majority of cases near work is done at an average distance of 28 to 30 cm away from the eyes, and therefore in the emmetrope the actual limit of clear vision is reached at forty-five years when an amplitude of 3.5 to 4 diopters of accommodation remains. This, however, would entail working at the near point continuously and thus exercising the whole of the accommodation to obtain useful vision, a condition of strain that can rarely be tolerated over any length of time. Comfort demands that about one third of the accommodation be kept in reserve, so that when this limit has been reached and the near points is at a distance of 22 cm, presbyopia may be said to have set in. In the emmetrope this occurs between 40 and 45 years of age. Thereafter the accommodation must be supplemented by a convex lens if near work is to be done without strain. 

   A hypermetrop starts lift with this near point considerably further than that of an emmertope so that the hypermetope with an error of +3 D will require to exercise 7 D of accommodation in give himself an amplitude 4 D; he may therefore show presbyopic symptoms at about twnety-five years of age. In a myope, on the other hand, the opposite hold, and if he has an error of -4 D, presbyopia will never occur.

   Presbyopia is thus a relative term, depending not only on the age but also on the refraction. It also varies with the individual and with is habits. A person who has the habit of reading with his book on this knees complains of discomfort later than one who is used to reading more closely; and the carpenter or the book-keeper or the musician will be comfortable at his work at 30 or 35 cm or over, while the seamstress or the compositor or the engraverof the same age and with the same refractive state will have been forced to use spectacless in orer to see at his working distance of 20 cm. Or again, the ciliary muscle may fail in states on debility or disease and the physiological accommodation be at fault. There is thus no fixed presbyopric point, and there can there can be no rational rule-of-thumb treatment.

Symptoms

The failure of accommodation becomes evident gradually, and as a rule becomes apparent first in reading. Small print becomes indistinct, and in order to get within the limits of his receding near point, the patient tends to hold his head back and his book well forward until a distance a reached when clear vision in any circumstances is difficult. Trouble is experienced at first in the evening when the light is dim and the pupils are dilated, permitting large diffusion circles; at this time, too, after the work of the day, fatigue comes on easily. The presbyope consequently likes to read by brilliant illumination, and he tries if possible to get the light between the book and his eye or to read in the sunlight so that his pupils may be forced to contract down and diminish the aperture. For this reason, in more advanced years when the pupils becomes smaller in senility, an old person with no accommodation may see near objects with a fair degree of detail.

    Complaint is usually made of visual failure rather than visual fatigue. Sooner or later, however, symptoms of eye-strain-appear. The ciliary muscle working near its limit becomes fatigued, and the accommodative effort, strained in excess of the convergence, gives rise to distress. Headaches may occur, and the eyes feel tired and ache and sometimes tend to assume a chronically suffused appearance.

treatment

The treatment of presbyopia is to privide the patient with convex lenses so that his accommodation is reinforced and his near point brought within a useful working distance. To do this adequately we must first know the working point of the individual, estimate his refraction and, in theory, determine the amplitude of his accommodation, and then supplement this by the appropriate strenght of lens allowing him a sufficient reserve accommodation.

   Thus , if the patient  is emmtetropic and whises to work at 25 cm, he will require  an amplitude of accommodation of 4 D. Let us suppose that his near point has receded to 50 cm; he has, in this case, 2 D of accommodation of his own. But he must, if he is to work comfortably, be able to keep one-third of this in reserve, so that he has an amplitude of 1.3 D i..e..f...of 2 D. The lens he requires theoretically is therefore one of 2.7 D. If he were ametropic, his refraction msut be determined, and his near point estimated whilst wearing the correcting lenses which rener him emmetropic.

    The techniques for determination of the accommodation and the other tests of near vision will be discussed later and should certainly be applied in appropriate circumstances. However, in practicea knowledge of the patient's static refraction and of the distance at which close work is carried out, are usually sufficient to allow the near vision wtih the addition of appropriate convex lenses and to prescribe on the basis of the result.

   The appropriate strength of these additions raises important questions. It is certainly true that presbyopic spectacles should never be prescribed mechanically by ordering an appropriate addition based on the age of the patient. Each patient should be tested individually and those lenses should be ordered in each case which the spectacles are intende. Nevertheless, there is not a great variation between individuals in the way that their accommodation declines. While it si wrong to give a dogmatic rule such as the addition of 1 dioptre for evert 5 years above 40, most refractionists formulate their own loose realtionship between the age and the presbyopic correction needed by the average patient. It is customary to start with an addition of +0.75 D to the distance correction in the first reading spectacles presribed for a presbyope showing the typical early symptoms of difficulty with mewsprint in the evenings or in poor illumanation, and it is a safe rule to be wary of increasing a reading addition by moe than this amount (0-75 D) for the presbyope whose near correction is no longer adequate.

   In all cases it is better to under-correcy than to overcorrect since, if the lenses tend to be too strong, difficulties will be experienced with the association of accommodation and convergence and the range of vision will  be inconveniently limited.  Short of a formal determination of the positive and negative realtive accommodation (p.94), agood practical hint is to make sure that with the reading correction it is intended to prescribed the patient is able to read the near-vision chart satisfactorily not only at his reading distance but also some 12 to 15 further away. This will guard againts over-correction.

    The average subject's accommodation declines so that in the late fifties an addition of about +2.50 D becomes necessary and thereafter little further change is required. In any case, a lens which brings the near-point closer than 28 cm is rarely tolerated (that is, a total power of 3.5 D), and if for any reason the demands of fine work require a higher correction, the convergence should be sided with prism as well as the accommodation with soheres. Unequal or very powerful addition for near work are often indicated in the presence of medical lesions causing poor visual aculty in one or both eyes. Thus patients with early cataract will often be enabled to read more comfortably with a +3.5 D or + 4 D addition. Even higher additions may be considered as visual aids.

    In the normal subject however, it cannot be too strongly emphasized that the usual cause of strain and discomfort following the prescription of spectacles for presbyopia is over-correction. If the lenses can be reduced in strength without causing a serious deterioration in visual acuity for work at the required range, they should be so reduced; but if this is impossible, the discomfort is usually relieved by adding to the lenses a prism with the base inwards or, alternatively, by decentring the lenses by a corresponding amount. Thus, while the sphere relieves the accommodation, the prism relieves the convergence.

     If near works with the reading spectacles still gives rise to trouble which cannot obviously be explained, the relative accommdation and convergence for the working distance must be further considered. It is to be remembered that the posivtive portion of the relative accommodation (that is, the amount in reserve) must be large as possible, certainly larger than the negative portion. Similarly, the positive portion of the relative convergence should also be large. If the relative accommodation is dificient, the spherical addition for the working spectacles should be altered; if the patient is working outside the "area of comfort" of his convergence orthoptic exercises should be prescribed or a prismatic correction should be ordered which brings his convergence within it.

    Presbyopia is one of those conditions wherein a monocle or a lorgnette mat be of real service: on the many little occasions in everyday life-when out shopping, loking at a ticket, or consulting a time-table-when takig out a pair of spectacles and putting them on for a moment become irksome, the more easily manipulated monocle may save much time and not a little annoyance. This, of course, apllies to short periods only; if persisted in, the enforced uniocularity of the monocle may introduce strains of another kind.