The AC/A ratio, age and presbyopia

Abstract

Previous reports concerning the effect of age on the AC/A ratio have been equivocal. Therefore, the present study investigated both the stimulus (AC/As) and response (AC/Ar) ratios using a subjective haploscope-optometer in a relatively large sample of subjects (n = 42) over a wide range of ages (22-65 years). The AC/As showed a small but significant decrease with age (approximately 0.04 delta/D/year). When the older subjects (> 45 years of age) were excluded, however, there was no systematic age effect. The AC/Ar exhibited a small but significant increase with age (approximately 0.08 delta/D/year) for subjects under 45 years of age. However, when the older pre-presbyopes and younger presbyopes (35-44 years of age) were excluded, there was no systematic age effect. In subjects 45 years of age and older, the AC/Ar could not be reliably assessed. This was attributed to physiological and instrumentation noise as a result of the minimal change in accommodative response. In the mid-aged subjects (35-44 years of age), the apparent increase in AC/Ar with age was speculated to be due to neural adaptation of the crosslink gain from the accommodative to the vergence system and/or slight intrusion into the upper non-linear response region of accommodation with the measurements. The finding of AC/Ar constancy with age when mid-aged pre-presbyopes and early presbyopes were excluded supports the non-linearity hypothesis, and thus there appears to be no real change in AC/Ar with increased age. The results support the Hess-Gullstrand theory of presbyopia.

Changes in fusional vergence limit and its hysteresis after viewing stereoscopic TV

“…AC/A ratio. Stimulus (clinical) AC/A ratio [8] was determined by the gradient method using a prism bar and a concave (minus) lens. The subjects' refractive errors were corrected by glasses.…”

The effect of incipient presbyopia on the correspondence between accommodation and vergence

“…One metre-angle is the convergence required to fixate an object at 1 m; an angle y in metre-angles can be converted to prism dioptres using the equation z=y.PD, where PD is the inter-pupillary distance in centimetres and z is the angle in prism dioptres. Other studies, however, have shown the AC/A ratio to remain stable or even decrease with age [1,7,23,34]. The majority of these studies were cross-sectional in nature and the age range of their subjects was relatively wide.…”

Change in the accommodative convergence per unit of accommodation ratio after bilateral laser in situ keratomileusis for myopia in orthotropic patients

“…Studies show that the AC/A ratio can alter after surgical correction of strabismus and with increasing age. [1][2][3] Laser in situ keratomileusis (LASIK) induces a state of emmetropia in a patient with previous myopia, suddenly increasing the amount of accommodation required for near vision. Moreover, in the absence of the prismatic effect of concave glasses, the convergence required for near fusion is also altered in myopia.…”