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Michael C. Brodsky, MD; Katherine J. Fray, CO

Arch Ophthalmol. 2007;125(12):1703-1706.

Tonus refers to the effects of baseline innervation on musculature in the awake, alert state.¹ Since the normal anatomical resting position of the eyes is one of exodeviation, extraocular muscle tonus plays a vital physiologic role in establishing ocular alignment. Under normal conditions, binocular esotonus is superimposed on the baseline anatomical position of rest to maintain approximate ocular alignment, save for a minimal exophoria that is easily overcome by active convergence. When binocular visual input is preempted early in life, monocular fixation may give rise to a larger dissociated esotonus that gradually drives the 2 eyes into a "convergent" position, resulting in infantile esotropia.² 

Author Affiliations: Departments of Ophthalmology, Mayo Clinic and Mayo Foundation, Rochester, Minnesota (Dr Brodsky), and University of Arkansas for Medical Sciences, Little Rock (Ms Fray).

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Dissociated Horizontal Deviation After Surgery for Infantile Esotropia: Clinical Characteristics and Proposed Pathophysiologic Mechanisms
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Arch Ophthalmol. 2007;125(12):1683-1692.
ABSTRACT | FULL TEXT