|Year : 2022 | Volume
| Issue : 1 | Page : 47
Harley E A. Bicas
Department of Ophthalmology, Faculty of Medicine of Ribeirao Preto, University of Sao Paulo, SP, Brazil
|Date of Submission||15-Sep-2022|
|Date of Decision||23-Sep-2022|
|Date of Acceptance||27-Sep-2022|
|Date of Web Publication||28-Oct-2022|
Harley E A. Bicas
Faculty of Medicine of Ribeirao Preto, University of Sao Paulo, Rua dos Tacapes, 430 - Quinta da Alvorada, Ribeirão Preto, SP
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Bicas HE. Accommodative tolerance. Pan Am J Ophthalmol 2022;4:47
Ophthalmology is a discipline not merely concerned to the resolution of quantitative visual problems (visual acuity and visual fields), but, on the contrary, great number of patients of an ophthalmological office complain of the bad quality with which the visual tasks are related. “Good” vision in each eye, but with discomfort (asthenopia) may be caused by the lack of binocular fusion, be it due to the images of different sizes (aniseikonia, either by inadequate corrections of anisometropias or by different components — axial and refractive — of isometropias), be it due to diplopia and confusion (in heterophorias and convergence insufficiency). Notwithstanding, the most prevalent cause of such asthenopia discomforts is, essentially, monocular (although it may also be binocular), the intolerance to the use of accommodation.
It is remarkable that in the history of ophthalmology, the accommodation is one of the newest ocular properties, only explained at relatively short time by Helmholtz, but with its theory till recently disputed,, it allowed the differentiation between the concepts of hyperopia and presbyopia and was quantified at the different ages by Duane. As a matter of fact, the accommodation is an ocular function used in a practically continued mode. By definition, it should be in rest only if an eye is required to “see” objects at infinite distances while keeping the condition of being emmetropic or having hyperopia totally corrected by optical means. Hence, the natural, synchronic, association between accommodation and convergence for near fixations (a synkinetic relationship) led to propositions of formulations for an “objective” calculation of visual comfort., The very laborious methods demanded the knowledge of the oculomotor balance at different positions of gaze, that of the respective binocular fusional capabilities and of the amplitude of accommodation. It is obvious that the evaluations of possible deviations of the visual axes at given distances (heterophorias) and the respective fusional capabilities of compensation are basic propaedeutic elements of the ophthalmological examination. However, in daily practice, such disturbances are of fewer clinical occurrences than those of accommodation and should be treated independently of it by several reasons, among which: (1) The relationship between accommodation and convergence is variable in different people; (2) it is not reciprocal (accommodation evokes convergence — read as “ophthalmic lenses change convergence” — while convergence almost does not change accommodation — read as “prisms practically do not influence accommodation”; and (3) although be assumed that the relationship between the stimulus to the accommodation, A (not necessarily its response) which causes a convergential effect (AC, the accommodative convergence) remains relatively constant during a subject's life (the AC/A relationship), the convergence is, at the different ages practically unchangeable, while accommodation progressively decreases.
At last, the accommodation performance may be even quantitatively normal for a specific age but — due to the continuity of the exercitation (to compensate for total or residual hyperopia) and/or by the relatively excessive and prolonged efforts to adjust images on the retina of objects at near distances — determine signs (esodeviations, by the accommodative convergence) and symptoms (headache, ocular pain, conjunctival hyperemia, lachrymation, blurring of images, “aversion” to read and write, eyes rubbing, hordeola, etc. Therefore, as depending on the extension of time by which accommodation is exerted, the knowledge of its total capability (the measurement of its amplitude) is not so relevant than the amount by which its prolonged efforts (though a small part of such total capability) may be tolerated. Actually, for example, although a weightlifter may succeed to lift 1100 lb. by a very short time (a proof of maximal strain, as it is the measurement of the near point of accommodation, with which the amplitude of accommodation is calculated), he does not support to carry about a much smaller weigh all day long. Therefrom emerge the concept of accommodative tolerance.
Up to now, there are no rigorously established measurements for the accommodative tolerance, in different age groups. However, they may be estimated by empirical knowledge of approximate medium values for each age group and easily memorized [Table 1]. Variations of small magnitudes around them (e.g. ±1.5D or 2.0D for the 5-year-old group, but decreasing for older age groups) might be aggregated.
|Table 1: Approximative medium values of the accommodative tolerance (TA) for different age groups and respective estimated variations|
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As a matter of fact, one knows that children with “high” hyperopia do not compensate them (though they might have, theoretically, higher accommodative capabilities to do so) and develop bilateral amblyopia by the worse quality of retinal images of such no conveniently corrected refractive unbalances. On the other side, it is well accepted that up to the 5 years of age hyperopia of +4D be “safe,” well tolerated (continuously compensated, with no clinical problems). Therefore, for the 5-year-old children, that value (+4D), as a rule, should be the maximum dioptric value of no corrected (residual) hyperopia to be left, that is, the value of the accommodative tolerance for such an age group. For the 15-year-old group, it is exceptional that hyperopia over +3D (accommodation for “near distances” is about 6D, or more, depending on the distance to the fixation point) be well tolerated. For 25-year-old people, the values of (sub-corrected or no corrected) well-tolerated hyperopia are reduced to about 2D (remember that 5D of accommodation is needed for the conventional near work distance); for the 35 years age group, the accommodative tolerance drops to, approximately, 1D; and at people 45 years old, it becomes null. A hyperope of +0.5D (who requires 3.5D — or more — for near distances) though might have an accommodation of up 6D does not, commonly, support exert it durably for his/her daily tasks. Therefore, one loses 1D of accommodative tolerance each 10 years, or about 0.1D/year, starting from the age of 5 years. This lead to the empirical equation of the accommodative tolerance (TA) as function of an age (y) as:
TA = 4,5 − 0,1 y
How, then, is measured the accommodative tolerance of a patient, to know if it is, or not, adequate for his/her age? By now it is not measured. Nor, perhaps, such measurement shall one day be necessary, since the subject, or those who answer for the subject ”know” that value. Therefore, simply ask for such a value, something as “What is the consultation reason?” Follow some examples about how to interpret the answer.
Suppose three children, all of them with a +4D hyperopia in each eye and normal complementary examinations. Theoretically, the expected normal measurement for their accommodative tolerances should be 4.1D, practically identical to the value of the hyperopia. The “A” child's parents relate that she started presenting a sporadic, intermittent, convergent strabismus some months ago, which became constant the last week. Till an opposite proof, we face a simple accommodative esotropia, to which the classical recommendation is the neutralization of the cause, a total correction of the hyperopia (+4D). The “B” child's parents refer that they come to the ophthalmological examination only to know if everything is normal, there are no complaints, or manifestations which worry them. They are answering “Our child has accommodative tolerance for his 4D (minimum) of continued accommodative efforts, maybe more.” Hence, there is no validity to correct what is already naturally corrected, without signs and symptoms. Prescription zero. The “C” child who presents a whole collection of very typical accommodative asthenopia signs draws forth more alternatives of optical prescriptions. Some would prefer to start by low prescriptions, as +1D. Now, a child who does not tolerate continuously accommodate 4 D for distant fixation and much more for near fixations (7 D or above), very much probably will follow with practically the same manifestations imputable to the use of accommodation, if it is kept in almost identical values (respectively, 3 and 6D). In an eventual return, the one decides to increase the correction to +1.5D with, probably, the same failure, for the mentioned reasons. It should be better to start with a “high” prescription, as +3.5D, which would neutralize, practically, almost all accommodative effort to distant fixations and would greatly reduce it for near visual work. Anyway, such a prescription very much probably would “elucidate” the question, be it by making the signs and symptoms disappear, or by informing — if they were kept — that they are not due to the accommodative efforts (residual hyperopia of +0.5D in a child is too low as a possible cause). Obviously, would come the question: “But the prescription could not be +3D?” to which the right answer is “I do not know. Let us ask the subject, or his/her family next time”.(It is expected that contented clients come back.). So, when the subject comes back, this value (+3D) is prescribed. Then, two things may happen: The return of symptoms and signs, though attenuated (conclusion: “We have hit the bulls-eye with the first prescription, so that +3.5D is the accommodative tolerance of such a child”) or the child remains healed (and the family happy, because “The Doctor is decreasing the eyeglasses power”). Now, what about +2.5D? We do not know. Let us check next time, always with two alternatives: That by which the “corrected” value of the accommodative tolerance for the child is found (then, +3D), or that with which the child follows without signs and symptoms and the family is grateful because the optical prescription is still lower (with the possibility it might be still reduced…).
Consider a young man, 20 years old, hyperopia +2.0D in each eye. The expected accommodative tolerance for his age is 2.5D which, theoretically, should be enough for the continued compensation of the hyperopia, but not, necessarily, for greater and persistent use of accommodation. Let us ask the young man. And he answers that he feels headache and visual blurring after some hours of using the personal computer (at 40 cm, i.e. +2.5D, which corresponds to 4.5D of prolonged accommodative efforts), but no complaints when the accommodation for near work is scarcely required (that is, he keeps an accommodation of 2.0D, or eventually little more, but not more and persistently). He is, clearly saying: “Although my accommodative tolerance be sufficient to keep me without continued correction of my hyperopia (2D of accommodation) I do not tolerate the sustained use of 4.5D of accommodation. I need eyeglasses for close proximity”. If he had, also, complains for far visual attention, the prescription +1.5D for constant use (leaving an accommodation of only 0.5D for fixation to far objects and 3.0D for close work) should be convenient. However, by the young man report, perhaps, the prescription of +1.5D uniquely for the personal computer (at 25 cm) use be convenient, but the suggestion that he may use it also in a cine; or still a little more, eventually +2.0D (but then, only for near distances), and always based on the magnitude and subtleties of the informed complains.
The benefits of using the accommodative tolerance concept for optical prescriptions imply the knowledge of the ocular refraction measurement with the accommodation totally relaxed, a condition which, by the way, corresponds to the proper definition of the balance state of the optical ocular system. However, as the accommodation is evoked by the command of an autonomic nervous system, the guarantee of such a complete relaxation is only given by cycloplegia. At last, optical prescriptions (mainly those of hyperopia) necessarily require two knowledges: That of the “static” ocular refractometry (which is a kind of pleonastic construction, since there is no possible assurance of this measurement, if the accommodation is active, or “excessive”, or spastic); and that of the accommodative tolerance (related to the subject's age and complains). Fortunately, however, those two only information, saving time of (other) examinations, are sufficient for almost the totality of good optical prescriptions.
The Spanish and the Portuguese versions of the editorial can be viewed below:
| References|| |
Helmholtz H. Uber Die accommodation des Auges. Albrecht Graefes Arch Ophthal, 1855; 1(Abt 2):1-74.
Schachar RA. Cause and treatment of presbyopia with a method for increasing the amplitude of accommodation. Ann Ophthalmol 1992;24:445-7, 452.
Schachar RA. The mechanism of accommodation and presbyopia. Int Ophthalmol Clin 2006;46:39-61.
Donders FC. On the Anomalies of Accommodation and Refraction of the Eye. London: The New Sydenham Society; 1864.
Duane A. Normal values of the accommodation at all ages. Trans Ophthalmol Am Med Assoc1912:383-91.
Percival AS. The relation of convergence to accommodation and its practical bearing. Ophthalmol Rev 1892;11:313-28.
Sheard C. The zones of ocular comfort. Am J Optom 1930;7:9-25.
Ogle KN, Martens TG. On the accommodative convergence and the proximal convergence. AMA Arch Ophthalmol 1957;57:702-15.
Bicas HE, Nóbrega, JF. Resposta acomodativa à convergência induzida por prismas em pessoas normais. Bol Bras Ortop 1973;5:46-51.
Davson H. Muscular Mechanisms. The Eye. Vol. III. London: Academic Press; 1969.
Bicas HE. Tolerância Acomodativa. In: Bicas HE, Alves AA, Uras R, editors. Refratometria Ocular. Rio de Janeiro: Cultura Médica; 2005. p. 147-9.