Hand dominance, eye laterality and refraction

Sir, Hand dominance has been known of since antiquity. There are 151 verses in the Old Testament and 49 in the New Testament containing references to right and left hands (Fabbro 1994). About 90% of human beings are right-handed and hand dominance appears to be genetically determined (Fabbro 1994). Little is known about the role hand dominance may play in refractive differences between the right and left eyes. To this end, we analysed the refractive status of eyes without ocular pathology. We carried out a retrospective study of the objective cycloplegic (two applications of tropicamide 1% in adults in addition to cyclopentolate 1% in children) refraction of consecutive subjects attending a private clinic. The subjects' ages ranged from 3 to 49 years. Subjects with ocular or systemic diseases were excluded. Hand dominance was ascertained by telephone calls. Statistical analyses were carried out using the paired t-test and anova. Data for a total of 1336 consecutive subjects were analysed. These comprised 623 (46.6%) male and 713 (53.4%) female subjects, with a mean age of 22.6 years (standard deviation = 12.0). Hand dominance was right-sided in 1242 (93.0%) and left-sided in 94 (7.0%) subjects. Mean astigmatism (expressed in positive cylinders) was 0.51 dioptre (SD = 0.93) in the right eye and 0.49 D (SD = 0.92) in the left eye. The mean astigmatic axis was 100.4 degrees (SD = 30.2) in the right eye and 93.1 degrees (SD = 31.1) in the left eye (t = 4.666; p = 0.000). The mean absolute spherical power (excluding positive astigmatism) was − 1.49 D (SD = 2.68) in the right eye and− 1.36 D in the left eye (SD = 2.58) (t = 4.94; p = 0.000). The mean spherical equivalence was significantly less in the right eye, at − 1.23 D (SD = 2.64), than in the left eye, − 1.11 D (SD = 2.54) (t = 4.86; p = 0.000). Analysis by anova did not confirm any role of hand dominance in the difference between the mean spherical equivalents of right and left eyes (F = 1.233; p = 0.267) (Table 1). Results in the present study demonstrated that the right eyes were, on average, slightly more myopic than the left. No correlation of this difference with hand dominance was established, probably because of the small sample size. The present study is disadvantaged by being small and retrospective. A larger sample size with multivariate analysis may further clarify the role of hand dominance. What is needed is a large, prospective study, which takes into account multiple variables such as ocular dominance, ocular rubbing, facial asymmetry, sleeping position and hand dominance. Ocular dominance studies have demonstrated around 80% right eye dominance, 10% left eye dominance and 10% no dominance (Fink 1938; Walls 1951). Fink (1938) found 25% of subjects to have crossed dominance (right hand preference was associated with right eye preference in 75% of cases). Accommodation has been implicated in the pathogenesis of mild myopia (Philips 1990). As the dominant eye is used for near fixation, it is conceivable that fixating with the dominant eye exerts an accommodative overload on that eye compared to the non-fixating contralateral eye, resulting in excess myopia in the dominant eye. Ocular rubbing by the dominant hand alters the corneal curvature as in keratoconus (Koenig & Smith 1993) and possibly over-stretches the sclera by causing a transient intraocular pressure rise during rubbing. Sleeping on one side, thereby squeezing the eyeball against a pillow, is a potential source of eyeball compression. Using a three-dimensional evaluation, Ferrario et al. (1995) found that the right side of the face can be significantly larger than the left side. It is possible that asymmetric eyeballs accounting for mild myopic differences between right and left eyes accompany asymmetric faces.