Binocular adaptation to near addition lenses in emmetropic adults

Near addition lenses are prescribed to pre-presbyopic individuals for treatment of binocular motor problems such as convergence excess and to control the progression of myopia. To date, no investigation has looked at the complete sequence of binocular motor responses during a period of near work with +2 D lenses. This investigation evaluated changes to accommodation and vergence responses when young adults sustained fixation at 33 cm with +2 D addition lenses. In addition, the effect of the accommodative vergence cross-link (AV/A) on the magnitude and the completeness of binocular adaptation to these lenses were evaluated. The results showed that +2 D lenses initiate an increase in exophoria and convergence driven accommodation. The degree of the initial induced phoria was dependant upon the magnitude of the AV/A ratio. Vergence adaptation occurred after 3 min of near fixation and reduced the exophoria and convergence driven accommodation. The magnitude of vergence adaptation was dependant upon the size of the induced phoria and hence the AV/A ratio. The completeness of adaptation was seen to vary inversely with induced exophoria and thus the AV/A ratio.