The term monofixation syndrome is used to describe a particular presentation of a sensory state in strabismus. The essential feature of this syndrome is the presence of peripheral fusion with the absence of bifoveal fusion due to a central scotoma. The term microtropia was introduced separately to describe small-angle strabismus with a constellation of findings that largely overlap those of monofixation syndrome.
A patient with monofixation syndrome may have no manifest deviation but usually has a small (≤8 prism diopters [Δ]) heterotropia; the heterotropia is most commonly an esotropia but is sometimes an exotropia or hypertropia. Stereoacuity is present but reduced. Amblyopia is a common finding.
Monofixation syndrome is a favorable outcome of infantile strabismus surgery and is present in a substantial minority of patients with intermittent exotropia. It can also be a primary condition that causes unilaterally decreased vision when no obvious strabismus is present. Monofixation syndrome can result from anisometropia or macular lesions as well.
To diagnose monofixation syndrome, the clinician must demonstrate the absence of bimacular fusion by documenting a macular scotoma in the nonfixating eye under binocular conditions and the presence of peripheral binocular vision (peripheral fusion).
Vectographic projections of Snellen letters can be used to document the facultative scotoma of monofixation syndrome. Snellen letters are viewed through polarized analyzers or goggles equipped with liquid-crystal shutters so that some letters are seen with only the right eye, some with only the left eye, and some with both eyes. Patients with monofixation syndrome omit letters that are imaged only in the nonfixating eye. Various other tests for central suppression are more commonly used (see Chapter 7).
Testing stereoacuity is an important part of the monofixation syndrome evaluation. Any amount of gross stereopsis confirms the presence of peripheral fusion. Most patients with monofixation syndrome demonstrate stereopsis of 200–3000 seconds of arc. However, because some patients with this syndrome have no demonstrable stereopsis, other tests for peripheral fusion, such as the Worth 4-dot test and testing with Bagolini lenses, must be used in conjunction with stereoacuity measurement. Fine stereopsis (better than 67 seconds of arc) is present only in patients with bifoveal fixation.
If associated amblyopia is clinically significant, occlusion therapy is indicated. Monofixation may decompensate to a larger heterotropia in adulthood, resulting in diplopia. Strabismus surgery may be required to restore fusion.
Ing MR, Roberts KM, Lin A, Chen JJ. The stability of the monofixation syndrome. Am J Ophthalmol. 2014;157(1):248–253.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.