Abstract
Purposes: To investigate the effectiveness of a dichoptic optokinetic nystagmus (dOKN) test to objectively quantify interocular suppression in intermittent exotropia (IXT) patients during the states of orthotropia and exodeviation.Methods: The OKN motion in subjects (15 controls and 59 IXT subjects) who viewed dichoptic oppositely moving gratings with different contrast ratios was monitored and recorded by an eye tracker. Interocular suppression in control subjects was induced using neutral density (ND) filters. The OKN direction ratios were fitted to examine the changes of interocular suppression in subjects under different viewing states. Two established interocular suppression tests (phase and motion) were conducted for a comparative study.Results: The dOKN test, which requires a minimal response from subjects, could accurately quantify the interocular suppression in both IXT and control subjects, which is in line with the established interocular suppression tests. Overall, although comparative, the strength of interocular suppression detected by the dOKN test (0.171 ± 0.088) was stronger than those of the phase (0.293 ± 0.081) and the motion tests (0.212 ± 0.068) in the control subjects with 1.5 ND filters. In IXT patients, when their eyes kept aligned, the dOKN test (0.58 ± 0.09) measured deeper visual suppression compared with the phase (0.73 ± 0.17) or the motion test (0.65 ± 0.14). Interestingly, strong interocular suppression (dOKN: 0.15 ± 0.12) was observed in IXT subjects during the periods of exodeviation, irrespective of their binocular visual function as measured by synoptophore.Conclusion: The dOKN test provides efficient and objective quantification of interocular suppression in IXT, and demonstrates how it fluctuates under different eye positions.
Highlights
Interocular suppression occurs during the dominance competition between two eyes and potentially contributes to the pathological processes of visual function impairment experienced by patients with amblyopia or strabismus (Maehara et al, 2011; Dorr et al, 2019; Webber et al, 2020)
The dichoptic optokinetic nystagmus (dOKN) test, which requires a minimal response from subjects, could accurately quantify the interocular suppression in both intermittent exotropia (IXT) and control subjects, which is in line with the established interocular suppression tests
Comparative, the strength of interocular suppression detected by the dOKN test (0.171 ± 0.088) was stronger than those of the phase (0.293 ± 0.081) and the motion tests (0.212 ± 0.068) in the control subjects with 1.5 neutral density (ND) filters
Summary
Interocular suppression occurs during the dominance competition between two eyes and potentially contributes to the pathological processes of visual function impairment experienced by patients with amblyopia or strabismus (Maehara et al, 2011; Dorr et al, 2019; Webber et al, 2020). IXT is the most common type of divergent strabismus, occurring in more than half of patients with exotropia (Mohney and Huffaker, 2003; Bergholz and Salchow, 2015), and it has been well recognized as variable and unstable (Hatt et al, 2008; Bergholz and Salchow, 2015; Economides et al, 2016). These intrinsic characteristics of IXT make it difficult for eye care professionals to determine the ideal treatment strategy and intervention timing for patients. An accurate objective quantification of visual suppression in patients with IXT is warranted
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