Ocular motor atavisms: the evolutionary basis of infantile strabismus and nystagmus

Infantile esotropia inaugurates a series of preternatural eye movements that include dissociated vertical divergence (DVD), primary oblique muscle overaction, and latent nystagmus. Congenital binocular visual loss leads to infantile nystagmus, a seemingly unrelated condition. These disorders seem to defy clinical explanation or nosologic classification. So where do these conditions come from and why are their neurologic pathways so elusive?

Analysis of these pediatric ocular motor disorders from the perspective of evolutionary biology and its applicability to humans.

All of these pediatric ocular motor disorders are evolutionary reversions or atavisms, characterized by the resurgence of ancestral visual reflexes under conditions that perturb early maturation of cortical binocular vision and visual motion processing. DVD and primary oblique muscle overaction originate from a primitive dorsal light reflex that is expressed when cortical binocular vision fails to develop. Latent nystagmus derives from a primitive subcortical visual motion bias that is ubiquitous in lateral-eyed afoveate animals. These atavistic ocular motor incursions originate within our ancient subcortical visuo-vestibular pathways and become incorporated into the human visual cortex when early infantile visual development is disrupted. Infantile nystagmus arises from an optokinetic "tug-of-war" between our older subcortical full-field optokinetic system and our newer cortical pursuit system.

The infantile forms of pediatric strabismus and nystagmus can be understood only through the prism of evolutionary biology. The evolutionary basis of these disorders confers unity on a set of clinical entities that have heretofore been viewed as being disparate and unrelated.