A Model Explaining Infantile Esotropia: Dissociated Strabismus as a Result of Sensory Interhemispheric Dissociation and Crossed Dominance in the Brainstem

Abstract

Over 150 million years, modern reptiles, birds and mammals evolved. Predatory birds and mammals have eyes on the front of their heads. In these animals, binocular correspondence, voluntary eye movements, eye-hand coordination, partial decussation at the optic chiasm and cortical interhemispheric pathways could develop at the cost of a smaller visual field. The subsequent enlargement of the cerebral neocortex and the hemispheric pathways are of central importance in binocular vision, sensory fusion and infantile strabismus. To investigate visual interhemispheric fibers, tractography was used in subjects with infantile esotropia (IE), callosal agenesis and control subjects with normal binocularity. In human callosal agenesis, normal binocularity could be explained by a different interhemispheric connection. In subjects with infantile esotropia, the analyses starting from the primary visual area on one side appeared different from the analysis from the other side. The distribution areas are asymmetrical between sides. Binocularity and alignment not only rely on correspondence between crossed and uncrossed hemi-representations but, in addition the corpus callosum, the most important interhemispheric pathway in placental mammals, seems important in the development of human binocularity. When correspondence is possible between the hemi-representations of the crossed and the uncrossed pathway on both sides of the cortex, the uncrossed pathway finds its way via the corpus callosum toward the contralateral hemi-representation of the same eye. A vertical midline must come out of this process. Meanwhile, however, the other uncrossed pathway strives to find the vertical midline of the other eye. Likely, the corpus callosum is important during the tuning of the pathways involved in sensory fusion. The anterior commissure might be an alternative in callosal agenesis. A failure in the offset will result in strabismus and crossed dominance after V1. Consequently, the normal binocular development of the brainstem (i.e., the superior colliculus), will not take place and latent nystagmus, as well as dissociated divergence, may ensue.