The Relationship between AC/A Ratio and Age
Abstract:
To obtain an understanding of the relationship between accommodation and convergence, one must keep in mind the elements involved in the process. These elements are: (1) The change in stimulus to accommodation, (2) The peripheral and central nervous system mechanisms that elicit and transmit the impulses and provide the motor impulses to the inner and outer muscles of the eyes, and (3) The effect or organs that provide the responses (the change in refraction of the eye and the change in position of the globe). These factors must be briefly analyzed. So far in this discussion of AC/A ratio determination, the degree of convergence achieved has been related to the stimulus to accommodation (the dioptric power of the lenses used or the change in viewing distance). This relationship has been termed the stimulus AC/A ratio by Alpern and coworkers. In laboratory studies one can arrange a haploscopic device so that the stimulus to accommodation, the response to the stimulus (the change in refraction of the eyes), and the change in position of the eyes can be determined simultaneously. With such an arrangement, one can relate the change in convergence to the stimulus to accommodation as well as to the accommodative response. The AC/A ratio related to the accommodative response has been termed the response AC/A ratio. This ratio differs from but parallels the stimulus AC/A ratio reported by Alpern and coworkers and by Ripps and coworkers. Alpern and coworkers stated that the response AC/A ratio could be predicted with reasonable accuracy by multiplying the stimulus AC/A ratio by a factor of 1.08. In other words, the response AC/A ratio exceeds the stimulus AC/A ratio by about 8%. Presumably, this applies only to non presbyopic adults. From the clinical standpoint, to determine the response AC/A ratio is impractical and unnecessary. The clinician must be concerned with the stimulus AC/A ratio. Various investigators have shown that the convergence response is generally linear and the stimulus is in the range within which the observers can respond. However, it is evident that a given stimulus to accommodation need not always elicit the required amount of change in refraction of the eye (the accommodative response), for example, in presbyopic patients. Although the impulses to accommodation sent out by the central nervous system may be adequate, or even excessive, and may result in adequate contraction of the ciliary muscle, the accommodative response will not be linear with the stimulus because of hardening of the crystalline lens.
