Monofixation with eso-, exo-, or hypertropia: is there a difference?

Monofixation syndrome is considered the next best thing to bifoveal fixation because it allows excellent binocular vision and long-term stability of alignment in patients with small angle strabismus. The purpose of this study was to determine if there exists an anatomic or physiologic reason for the visual system to prefer small angle esotropia over exotropia or hypertropia. Of 259 cases of strabismus ≤10(Δ), esotropia was the most common and the most stable form of microtropia. Stability was statistically associated with sensory fusion, motor fusion, and stereopsis. Micro-ET cases outperformed the exotropic and hypertropic cases in all sensory categories, but particularly in the case of motor fusion. Instability was associated with the presence of an A- or V-pattern, oblique dysfunction, a history of infantile esotropia, dense amblyopia, and a significant change in refractive error. Micro-XT and HT were more likely to display these signs. In order to ensure long-term stability through fusion and stereopsis, the visual cortex must link the fovea of the dominant eye with the nasal retina in esotropic eyes, or the temporal retina in exotropic eyes. However, temporal retinal input is at a competitive anatomic and physiologic disadvantage. Compared to the connections necessary for binocular vision in micro-ET, those required for XT or HT may be more difficult to form and easily disrupted once established.