Spectacle prescription change encountered with an optimization of visual acuity

The author has recently documented the improvement in logMAR visual acuity (VA) encountered in a clinical population when the habitual distance refraction was adjusted to an optimal level (Pointer, 2008). Subsequent to publication several persons have enquired what magnitude of (clinical) prescription change at the spectacle plane might be anticipated to produce this VA refinement – recognising that of the 6-in-10 persons who manifested an acuity improvement this was in 43% of cases within a single logMAR chart line (£5 letters), but in a few cases was by as much as four lines (Pointer, 2008: Figure 1, page 461). Reference to the original data set (n = 1288 normallysighted individuals) indicated that a relevant refractive history was available for 78% of cases [n = 1002; mean age 47.2 years (standard deviation, S.D. 22.9), range 6.7–89.3 years; 49% males; 47.5% habitual spectacle wearers]. Both the habitual (previous) and the optimal (current) monocular (right and left) distance refractions of these individuals had been determined under consistent test circumstances by a single investigator (the author): mean inter-test period was 1.92 years (S.D. 1.11). The composition of this sub-group was not significantly different to that of the total data set in terms of gender, age and refractive distribution, habitual spectacle use and inter-test period (p > 0.1: t-test or Chi-square test, as appropriate). The logMAR acuity values determined at 6 m under typical consulting room conditions for the patients comprising this clinical population cannot be regarded as definitive, being subject possibly to temporal and other variations. In addition, for the habitual vs optimal prescription comparison planned here, reference to monocular results will be necessary: the question thus arises as to which of an individual s pair of eyes should provide the data for analysis. It was decided to mirror the approach adopted in similar circumstances by Heravian et al. (1990): four successive correlation analyses were undertaken, plotting binocular logMAR acuities vs monocular values for right, left, bettersighted or worse-sighted eyes (as recorded at the most recent examination). Closely-coincident distributions were obtained but the highest correlation (r = 0.79: p < 0.001) was recorded between binocular and bettersighted VA values: this is an outcome in accord with previous observations (Coren, 1987) which had indicated that, for distance viewing in particular, binocular VA is determined primarily by the monocular VA of the better-sighted eye (but note that inter-eye differences in refraction are also likely to be clinically insignificant: Adler-Grinberg, 1986). Consequently the refractive and associated data of each patient s better-sighted eye were utilised in this report: incidentally, this choice corresponded to the right eye in 62% of cases, with no gender difference (p = 0.4: Chi-square test). Comparative statistical analysis of the refractive material was achieved by the application of power vector manipulation to the standard clinical prescription notation (theory, nomenclature and methodology after Thibos et al., 1997; Miller, 2009). Table 1 sets out the mean habitual and optimal monocular prescription values, grouped on the basis of whether there was no (0) acuity change recorded with an optimized prescription or up to 1, 2, 3 or maximally 4 lines of logMAR chart improvement. In similar proportions to results reported previously for the original full data set (Pointer, 2008), a total of nearly 86% of patients showed nil or up to 1 line of acuity improvement in balanced numbers: likewise, those showing a greater acuity improvement were much less numerous. Consequently, while comparisons here must be regarded as somewhat speculative, it is evident from Table 1 that for a 1 or 2 chart line acuity improvement there is no statistically significant change in the prescription power vectors M, J0 or J45. However, a 3 line improvement is likely the result of a negative vertical Jackson cross cylinder adjustment: vector J0 is statistically significantly different (p = 0.001: Wilcoxon matched pairs test) when habitual and optimal group prescriptions are compared. For a 4 line acuity change (the maximum difference recorded) a clinical adjustment to the spherical equivalent prescription (M: p = 0.001) in tandem with a change to the vertical cylinder (J0: p = 0.04) is inferred. In no instance is there a statistically significant change recorded in the oblique cylinder vector (J45: p > 0.08). Table 2 corroborates these conclusions on the basis of a comparison of power vectors for the no acuity difference (i.e., optimal = habitual) control data vs 1, 2, 3 or 4 line changes. Statistically significant differences between optimal and habitual values of the M and J0 power vectors are only evident where a 3 or 4 line acuity improvement is recorded: but again, no (statistically) significant comparative variation in the Ophthal. Physiol. Opt. 2010 30: 405–407