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A. M. Horwood, P. M. Riddell; Disparity is Used to Control All Types of Intermittent Distance Exotropia. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1587.
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© ARVO (1962-2015); The Authors (2016-present)
Most classifications of intermittent distance exotropia (X(T)) suggest that near control is achieved by either fusional, accommodative or proximal convergence, with characteristic clinical patterns associated with each group. We investigated use of disparity, blur and looming according to clinical pattern to determine whether different patterns of cue use could be indentified.
19 children (age 4-11yrs) with X(T) controlled for near but not distant fixation, and 27 controls, were assessed with a remote haploscopic off-axis videorefractor to measure simultaneous accommodation and vergence to a detailed picture stimulus at 4 target distances (0.3, 0.5, 1 & 2m). Blur, disparity and looming cues were removed by using a DoG patch, remote occlusion, and scaled targets respectively. Participants were tested with all combinations of cues (8 conditions) to assess the relative contributions of each cue to the near response in relation to clinical characteristics.
Individuals could not be grouped on the basis of clinical changes in angle of deviation after 30 min occlusion and with +/- lenses and showed a wide spectrum of responses. Decompensation to exotropia occurred in 20% of test runs where disparity was available and 46% where it was not. When decompensated, response slopes to targets containing disparity flattened significantly (p<0.0001). The exotropes showed less accommodation lag for near than controls (p=0.026).Correlation between changes in vergence and accommodation on decompensation was weak (r2=0.17). Individuals diverging most with occlusion showed no difference in cue use from those with greater vergence changes with lenses ("high AC/A" type). Despite high clinical AC/A ratios, response AC/A ratios were slightly lower than controls (0.87 vs 0.93 MA/D; p=0.85), but response CA/C ratios were significantly higher (1.46 vs 0.87 D/MA; p=0.02).
We suggest that disparity (motor fusion) drives control in all types of X(T). The group with high clinical AC/A ratios had lower response AC/A ratios, but were found to have high CA/C ratios, thus the additional convergence being used for control was driving more accommodation than in the controls. Clinical findings can be better explained by this model than previous theoretical models.
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