Etiology of Heterophoria and Heterotropia

In heterophoria there is a relative deviation of the visual axes held in check by the fusion mechanism, whereas in heterotropia there is a manifest deviation of the visual axes. The relative position of the visual axes is determined by the equilibrium or disequilibrium of forces that keep the eyes properly aligned and of forces that disrupt this alignment. Clearly, the fusion mechanism and its anomalies are involved in some manner in producing comitant heterotropias. To understand the etiology of neuromuscular anomalies of the eyes, therefore, one should also gain an insight into other factors that determine the relative position of the visual axes. First, there are anatomical factors, which consist of orientation, size, and shape of the orbits; size and shape of the globes; volume and viscosity of the retrobulbar tissue; functioning of the eye muscles as determined by their insertion, length, elasticity, and structure; and anatomical arrangement and condition of fasciae, ligaments, and pulleys of the orbit. Second, there are innervational factors, that is, all the nervous impulses that reach the eyes. These factors include the co-movements of extraocular muscles with intrinsic ocular muscles, psychooptical reflexes (fixation reflex, fusional impulses), influences of the static apparatus on extraocular muscles and their tonus (endolymph, vestibular system, reflexes from neck muscles), and influences of the several nuclear and supranuclear areas that govern ocular motility.