Bilateral Occlusion Reduces Ocular Deviation in Exotropia

This study evaluated the eye position of individuals with intermittent exotropia during occlusion of one eye or both eyes. The authors found that the deviation decreased under binocular occlusion by almost 60% compared with covering only one eye at a time. They did not control for accommodation but provided an explanation of why their results could not be explained by accommodative / vergence responses. These findings support the notion that intermittent exotropia occurs as a result of active divergence innervation and is not due to the failure of convergence to hold the eyes straight.

This paper, which came from the Laboratory for Neuroscience at the University of California, San Francisco, is where Jonathan Horton MD, PhD, and his associates, have performed a number of studies on humans and animals to determine the sensory status of intermittent exotropia while deviating.^{1, 2} Like our earlier work,^3-5 they have demonstrated that, during deviation, there is an increase in the binocular field of vision (panoramic viewing); non-suppression of the foveas of each eye; harmonious retinal correspondence; and temporal suppression of the deviating eye. These findings are important because they need to be addressed for any treatment of intermittent exotropia to be successful.

References

1. Adams DL, Economides JR, Sincich LC, Horton JC. Cortical metabolic activity matches the pattern of visual suppression in strabismus. J Neurosci. 2013;33(9):3752-3759. https://www.jneurosci.org/content/33/9/3752.long
2. Economides JR, Adams DL, Horton JC. Perception via the deviated eye in strabismus. J Neurosci. 2012;32(30):10286-10295. https://www.jneurosci.org/content/32/30/10286.long
3. Cooper J, Feldman J. Panoramic viewing, visual acuity of the deviating eye, and anomalous retinal correspondence in the intermittent exotrope of the divergence excess type. Am J Optom Physiol Opt. 1979;56(7):422-429. https://journals.lww.com/optvissci/Abstract/1979/07000/Panoramic Viewing,_Visual_Acuity_of_the_Deviating.3.aspx
4. Cooper J, Record CD. Suppression and retinal correspondence in intermittent exotropia. Br J Ophthalmol. 1986;70(9):673-676. https://bjo.bmj.com/content/70/9/673.long
5. Cooper J, Feldman J, Pasner K. Intermittent exotropia: stimulus characteristics affect tests for retinal correspondence and suppression. Binocul Vis Strabismus Q. 2000;15(2):131-140. https://sa1s3.patientpop.com/assets/docs/58075.pdf

In this prospective study of 18 patients, the authors "compare the ocular deviation in patients with intermittent exotropia under conditions of monocular vs binocular occlusion." The authors found that "[t]he majority of patients (11/18) had a smaller deviation when both eyes were covered." Another significant quote is the following: "Our experiments have revealed in intermittent exotropia a major, unsuspected difference between abolishing fusion and abolishing fixation." They state, moreover, that "[o]cclusion of both eyes gives rise to a third sensory state." The authors conclude that "[t]his finding demonstrates that the deviation angle in patients with intermittent exotropia is actively mediated by visual feedback, which the fixating eye is capable of providing alone." The findings should prove very interesting for those interested in the mechanism underlying this disorder.

This small-scale study presents the interesting result that bilateral occlusion, on a short-term basis, decreases the angle of deviation in intermittent exotropia (IXT). This is unexpected, as conventional wisdom says that IXT going to its position of rest, as in unilateral occlusion, should increase the strabismic angle. This speaks to the probability that IXT is not as much as an eye muscle problem as it is an adaptation at the level of the brain, revealed in the neurology and exhibited by alignment.