Purchase this article with an account.
Sonia Toor, Anna Horwood, Patricia Riddell; Disparity Is A Less Important Cue In Asymptomatic Near Exophoria. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6366.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Convergence is necessary to align the eyes for near fixation but large exophorias for near are atypical, with a low accommodative convergence to accommodation (AC/A) ratio being implicated in the etiology. Near exophoria can lead to asthenopia for close work, but may remain well compensated and asymptomatic. We studied asymptomatic near exophoria in comparison to orthophoria to assess vergence and accommodation responses to blur, disparity and proximity cues, to test the hypothesis that blur cues would drive less vergence.
17 participants with asymptomatic near exophorias (8.82±1.11PD) and 23 participants with orthophoria (0.78±0.47PD) were compared. None had exophorias >2PD for distance fixation. All were aged between 17-26 years, emmetropic, naïve to vision experiments and had equal VA, stereoacuity <60secs arc, and normal fusion ranges. Vergence and accommodation were assessed with a PlusoptiXSO4 photorefractor set in a haploscopic device. Target manipulations presented all combinations of blur/disparity and proximity cues at four distances between 33cm and 2m. Stimulus and response CA/C and AC/A ratios were calculated from the PlusoptiXSO4 data.
The exophoric accommodation responses were elevated and generally more appropriate for target demand at all distances and targets, with less lag at 33cm (p=0.003) and more lead at 2m (p=0.0003). There was higher accommodation gain to the proximity-only cue in exophoria (p=0.04). Exophoric blur-driven vergence responses (AC/A) were very similar to the controls, but vergence gain to disparity-only cues was lower (p=0.03). CA/C ratios were elevated (p=0.023) in the exophoric group, due to slightly more accommodation and significantly less convergence to the disparity-only target. Presenting proximal cues (p=0.03), or removing them in isolation (p=0.03) had a greater influence on exophoric vergence than in the controls.
Accommodation is more appropriate across all targets in near exophoria with lead at 2m, and CA/C ratios are elevated. Against the prediction, AC/A ratios and response weighting to blur are no different. Although disparity remains the main near cue, disparity cues appear less strongly weighted and proximal cues more so in near exophoria.
This PDF is available to Subscribers Only