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C Bellusci, C Schiavi, R Bolzani, MG Benassi, EC Campos; Recognition Time as a Measure of Switch of Fixation in Small-angle and in Large-angle Exotropia . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4682.
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Purpose: A major but still unanswered question concerns the reasons for the stability of a post-operative small-angle deviation in strabismus patients. Why are relapses so rare in the presence of non-sophisticated central information, due to the absence of normal binocular vision? The possible role of a quick alternation of fixation for the maintenance of a small-angle deviation could be hypothesised. The aim of the study is to measure the time necessary for achieving a fixation switch in small-angle versus large-angle strabismus. Method: Twenty-two subjects participated in this study (8-30 years of age). Sixteen exotropic patients were divided in four groups: small-angle with free alternation (n=4), large-angle with free alternation (n=5), non-alternating small-angle (n=4), non-alternating large-angle (n= 3). Six healthy volunteers served as controls. Visual acuity of the examined eye was first measured with Snellen Es generated on a computer screen. Recognition time (RT) of the threshold stimulus was then measured in both eye sequentially (RT 1). Subjects pressed on a mouse when the stimuli were recognized. The Es disappeared and recognition time was recorded by the program. A second measure of recognition time (RT 2) was taken by moving a cover from the eye to be examined to the other while presenting the threshold visual stimulus. We calculated the difference between recognition time 1 and 2 as a measure of the time needed for the switch of fixation. Results: Multivariate Analysis of Variance for repeated measures indicated that the time of switch of fixation was statistically different in free alternating patients compared with non-alternating subjects (F4,17=24.206, p<0,001). Moreover, significant interactions were found between free alternating groups and time of switch of fixation (F4,17=37.8, p<0.001). In particular large angle free alternating patients showed larger differences between recognition time 1 and 2 than small-angle free alternators. In non-alternating patients, the difference between dominant and non dominant eye was statistically significant (F1,17=13.5, p=0.002). Conclusion: Small-angle free alternating patients showed a time of switch of fixation significantly reduced as compared to patients with large-angle strabismus. It appears therefore that alternation of fixation is achieved more readily in small-angle strabismus. It could be hypothesised that a rapid switch of fixation could be one of the elements involved in the stability of a post-operative small-angle deviation, in the absence of normal binocularity.
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