In this study, we investigated the translatory movement during lateral gaze in patients with horizontal strabismus using the coordinates of the centroid of the eyeball in 3D reconstructed MR images. Our study showed different patterns of positional change of the eyeball during horizontal movement in the esotropia and exotropia groups. In patients with horizontal strabismus, the translatory movement is more marked when the eye turns in the direction of horizontal deviation. During abduction, the centroid moved more posterior in the exotropia group than in the esotropia and control groups. The centroid moved farther in the posterior direction in the esotropia group during adduction compared with that in the exotropia and control groups. In contrast, healthy individuals showed a similar distance of centroid movement during abduction and adduction as reported in our previous study.
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Eye movements consist of rotation and translation, which implies that the centroid of the eyeball cannot be the axis of rotation. Any eccentricity of the rotation axis would change the relative lever arm lengths for the rectus muscles. During abduction, the eccentricity of the rotation axis would shorten the lateral rectus lever arm while lengthening the medial rectus lever arm. In contrast, during adduction, the eccentricity of the rotation axis shortens the medial rectus lever arm and lengthens the lateral rectus lever arm.
3 This change would be large in the exotropia group during abduction and in the esotropia group during adduction, inducing an imbalance between the agonist and the antagonist of horizontal gaze. The cause of the change in the eccentricity of the rotation axis may be due to nervous or muscular factors, or both. Regardless, the change in eccentricity would be associated with the pathogenesis of comitant horizontal strabismus. Further study of eccentric rotation in strabismus patients may help to understand the ocular mechanics in strabismus.
The mechanism of the translatory movement is not understood fully. One of the candidate actuators for translation is the orbital layers of the rectus muscle, and the other is the smooth muscle present in the pulley suspensions.
8,9 A previous study reported a difference in the location of the pulley of the medial rectus muscle between esotropia and exotropia under general anesthesia.
10 The smooth muscles around the pulley receive rich innervation, suggesting complex excitatory and inhibitory control.
11 Considering the dynamic role of the pulley in ocular motility, it likely contributes to horizontal strabismus, at least partially, and induces a change in translatory movement.
In addition to this active force, structural factors should also be considered. Unfortunately, we were not able to obtain data regarding the duration of the disease in all participants. However, some reported having strabismus for more than 10 years. The long-standing eccentric deviation of the eyeball, by itself, causes structural changes in the extraocular muscles and soft tissues of the orbit.
12 The material properties of the extraocular muscles and other soft tissues affect the amount of translatory movement because the eye is suspended with the extraocular muscles in the orbit surrounded by soft tissues.
Recently, eye-tracking systems have become easier to integrate into research and clinical practice. However, most eye tracking techniques assume that the eye only has rotational movement, which induces intrinsic errors because real eye movement is composed of both rotation and translation.
13–15 Furthermore, patients with horizontal strabismus show asymmetric translation during horizontal eye movement. Therefore, errors could be exacerbated when using eye-tracking systems to evaluate eye movements in patients with strabismus. Although translation comprises a smaller portion than rotation, it should be addressed, especially for eye movement evaluation in patients with strabismus.
This study had several limitations. First, we only included patients who were able to undergo MRI, and only five patients were children less than 15 years of age. Thus, selection bias should be considered when interpreting our results. Second, we evaluated the eye movements in a fixed experimental setting. The subjects only fixated on the left target with their left eye and the right target with their right eye, with the other eye occluded. Hence, the rotation of the eye for adduction did not correspond to the actual target displacement, resulting in smaller adduction in the exotropia group and larger adduction in the esotropia group. The eye movements in this study were also limited to the conjugate version, and vergence, such as convergence and divergence, was not evaluated in this study. Third, the individual subject parameters, such as pupillary distance and head diameter, were not considered fully in this experimental setting. Although the real rotation angle was not exactly 40° owing to various individual properties, the three groups had no significant difference in rotation angles. Therefore, the error induced by the individual properties might not have had significant effects on the results. Finally, owing to the small sample size in each group, we were not able to analyze the associated factors other than the type of strabismus. Further studies with larger sample sizes are needed to identify the clinical factors associated with translatory movement. Despite these limitations, to our knowledge based on the PubMed (Medline) databases search for “strabismus” and “translation,” this study is the first to evaluating eye movements in patients with strabismus that focused on translation. This study was able to assess physiological eye movement using 3D reconstructed MRI, which was one of the strengths of this study.
In conclusion, translatory movement was more prominent when the eye was rotated in the direction of horizontal deviation in patients with horizontal strabismus. The evaluation of translatory movement may help to better understand the eye movements and biokinetics of strabismus.