The AC/A ratio, age and presbyopia

Previous reports concerning the effect of age on the AC/A ratio have been equivocal. Therefore, the present study investigated both the stimulus (AC/A(s)) and response (AC/A(r)) 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/A(s) showed a small but significant decrease with age (~ 0.04Δ/D/year). When the older subjects (> 45 years of age) were excluded, however, there was no systematic age effect. The AC/A(r) exhibited a small but significant increase with age ( ~ 0.08Δ/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/A(r) 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/A(r) 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/A(r) 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/A(r) with increased age. The results support the Hess-Gullstrand theory of presbyopia.