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.
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.
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.
Monofixation syndrome is generally an asymptomatic condition that is diagnosed by a physical examination. Patients should be asked whether they have a history of eye muscle surgery, amblyopia, or retinal disease, as well as a family history of strabismus.
Adults with subnormal stereovision, mild anisometropia, and subtle asymmetry in best-corrected vision may have undiagnosed monofixation syndrome.
Patients may have an esotropia or exotropia measuring 8 prism diopters or fewer with a simultaneous prism cover test. Monofixation syndrome can also be observed in patients with vertical strabismus with a deviation measuring 6 prism diopters or fewer.[7] Stereoacuity as measured with polarized glasses will show reduced stereopsis, ranging from 3000 arc seconds to 67 arc seconds. There are many specialized tests to identify the presence of peripheral fusion with a central suppression scotoma, including vectographic projections of Snellen letters, Worth 4-dot test, Bagolini lenses, and 4-Prism diopter base-out test.[2]
Among patients with anisometropia leading to monofixation syndrome, refractive errors should be corrected with spectacles. In patients with amblyopia leading to monofixation syndrome, treatment includes correcting any refractive error and promoting use of the amblyopic eye. There is no treatment for stable monofixation syndrome.
Monofixation syndrome may deteriorate, leading to a larger tropia, decreased stereovision, and/or diplopia.[8] Decompensated subjects have demonstrated decreased horizontal fusional amplitudes as compared to patients with stable monofixation syndrome, warranting the possible benefit of orthotopic exercises.[9] In symptomatic patients, treatment also includes spectacle change, prisms, and strabismus surgery. The majority of patients are successfully returned to a monofixational state after treatment.
The small angle heterotropia associated with monofixation syndrome is functionally and cosmetically acceptable, rarely requiring surgery. In those with monofixation syndrome following strabismus surgery, there is increased stability of alignment, thereby decreasing the need for further surgery.
In decompensated monofixation syndrome, surgical treatment can often reduce the tropia and restore the patient to having monofixation syndrome.
Patients with stable monofixation syndrome should have routine follow-up care to monitor for deterioration. If patients are treated for amblyopia, then the frequency of visits is determined by the degree of amblyopia and intensity of treatment.
Monofixation syndrome has been shown to remain stable over decades. A small percentage of patients may have decompensation leading to decreased stereopsis and even diplopia. Among patients with secondary monofixation syndrome, fewer than 10% had any decompensation over 14 years of follow up.[10] Strabismus surgery can be utilized to improve alignment and stereopsis.