Vergence and accommodation responses in the control of intermittent exotropia

Abstract

Purpose Individuals with different types of intermittent exotropia (IXT) may use neurally coupled accommodation and vergence responses differently from those without exotropia to achieve eye alignment. This study examined the relationship between simultaneously recorded accommodation and vergence responses in children and young adults with a range of types of IXT while aligned and deviated. Methods Responses of 29 participants with IXT (4–31 years) and 24 age‐matched controls were recorded using simultaneous eye‐tracking and eccentric photorefraction while they watched a movie in binocular or monocular viewing at varying viewing distances. Gradient response AC/A ratios and fusional vergence ranges were also assessed. Eight participants had divergence or pseudo‐divergence excess type IXT, 5 had convergence insufficiency and 16 had basic IXT. Results Control and IXT participants accommodated similarly both in monocular and binocular‐aligned conditions to visual targets at 80 and 33 cm. When deviated in binocular viewing, most participants with IXT exhibited changes in accommodation <0.5D relative to alignment. Gradient response AC/A ratios were similar for control [0.56 MA/D (IQR: 0.51 MA/D)] and IXT participants [0.42 MA/D (0.54 MA/D); p = 0.60]. IXT participants showed larger vergence to accommodation ratios with changes from distance to near fixation [1.19 MA/D (1.45 MA/D)] than control participants [0.78 MA/D (0.60 MA/D); p = 0.02], especially among IXT participants with divergence or pseudo‐divergence excess. Participants with IXT exhibited typical fusional divergence ranges beyond their dissociated position [8.86 Δ (7.10 Δ)] and typical fusional convergence ranges from alignment [18 Δ (15.75 Δ)]. Conclusions This study suggests that control of IXT is typically neither driven by accommodative convergence alone nor associated with over‐accommodation secondary to fusional convergence efforts. These simultaneous measurements confirmed that proximal vergence contributed significantly to IXT control, particularly for divergence or pseudo‐divergence excess type IXT. For IXT participants in this study, achieving eye alignment did not conflict with having clear vision.

Continuous psychophysics shows millisecond-scale visual processing delays are faithfully preserved in movement dynamics

“…Moreover, instructions that seem simple for experienced psychophysical observers, such as “press the left arrow key when the stimulus appears to be moving left when it is in front of the screen, and the right arrow if it appears to be going the other way” might be confusing to young children and unintelligible to preverbal infants. Indeed, because infants naturally follow moving visual targets, continuous psychophysics should enable the behavioral assessment of visual function early in the developmental arc ( Mestre, Neupane, Manh, Tarczy-Hornoch, & Candy, 2023 ). Finally, if one wishes to study effects that have large individual differences, then a technique is needed in which it is possible to obtain a satisfactory amount of data from a single observer in a short amount of time so that it is, in turn, possible to collect good data from a very large number of observers.…”

Myopic progression in school-aged children with moderate intermittent exotropia

“…In contrast, the moderate IXT children in Group B showed slow myopic progression than the normal control in Group C. The moderate IXT children had relatively better control of exodeviation, which indicated that sufficient accommodative response compared with accommodative demand in these moderate IXT children to control normal ocular alignment. The mechanism involved in the control of IXT is still of limited understanding ( 19 ), and coupled accommodation and vergence responses are needed for compensating the additional convergence demand when overcoming the exodeviation to achieve normal alignment in IXT patients ( 20 ). The control of IXT is typically driven by neither accommodative effort alone nor convergence effort alone.…”

Heterophoria, fusional vergence, and near point of convergence in Sudanese hospital-based population

“…Jampolsky 23 suggested that the shift from alignment to deviation in strabismus is associated with the relaxation of fusional convergence ranges. Researchers 24,25 have shown that there is a significant correlation between the width of the fusional range and the control of exodeviation. This suggests that the ability to maintain comfortable binocular single vision is related to the strength and flexibility of FV.…”