Background
Monofixation syndrome is characterized by the presence of peripheral retinal fusion with a unilateral facultative central suppression scotoma. Individuals with monofixation syndrome have reduced binocularity due to this suppression scotoma. Specifically, monofixators tend to have small angle strabismus measuring 8 prism diopters or fewer without presence of diplopia. [1] Patients with monofixation syndrome cannot achieve fine stereopsis (binocular depth perception). Instead, they can only achieve gross stereopsis measuring at best 3000 to 67 seconds of arc, due their inability to have bifoveal fusion.
Pathophysiology
Conditions that cause a suppression scotoma in the central retina while allowing for peripheral fusion can lead to monofixation syndrome. Monofixation syndrome is commonly found among patients with infantile esotropia after correction, anisometropia (unequal refractive error between eyes), infantile cataracts, and macular disease. Amblyopia is commonly associated, which is a failure of normal neural development in the visual system leading to poor vision. [2] Monofixation syndrome may also be a primary defect causing unilateral decreased vision.
Monofixators have a foveal suppression scotoma measuring 3-5 degrees in the non-dominant eye during binocular viewing. The central retina has small receptive fields and is therefore more sensitive to image blur or image disparity than the peripheral retina. Studies in macaque monkeys have demonstrated that 2 adjacent neurons in the visual cortex could join receptive fields up to 5°, which correlates to the maximum deviation of 8 prism diopters of monofixation syndrome. [3]
Patients with corrected strabismus comprise the largest subgroup of patients with monofixation syndrome seen in clinical practice. Patients with monofixation syndrome in the setting of corrected esotropia are more likely to maintain long-term alignment. While monofixation syndrome is a preferred outcome for patients with treated for esotropia, it is generally considered an undesirable outcome for patients with intermittent exotropia. However, monofixation syndrome may be an underlying condition in some patients with exotropia, present even prior to surgical correction. [4]
Approximately one third of patients with monofixation syndrome do not have any manifest strabismus. Unequal and uncorrected refractive error can lead to a blurred image on one macula, leading to a unilateral suppression scotoma and mild-to-moderate amblyopia. Macular disease can also lead to unilateral image blur. Patients with macular disease have a constant, manifest scotoma rather than a facultative scotoma.
Most patients with primary monofixation syndrome are undiagnosed because of a lack of symptoms. These patients have an inherent defect in central fusion, with most having mild amblyopia.
Epidemiology
The prevalence of primary monofixation syndrome (without strabismus or other ocular pathology) in the general population is less than 1%. [5] Primary monofixation syndrome is more prevalent among families with infantile esotropia.
Roughly half of patients with infantile esotropia develop monofixation syndrome, if corrected to less than 8 prism diopters of deviation prior to 24 months old. [6] Monofixation syndrome is found to be 9 times more common after surgical correction for esotropia as compared to exotropia correction. [4] Monofixation syndrome is recognized mainly in children but may be present at all ages.
