Monofixation syndrome is a form of subnormal binocular vision without bifixation characterized by small-angle strabismus, unilateral absolute facultative central suppression scotoma of less than 3º, and peripheral fusion. While monofixation syndrome can be a primary disorder of binocular vision, it is more commonly a secondary sensory status due to various primary causes.
The main defect is a central suppression scotoma, which prevents bifixation.
The central retina has small receptive fields and is therefore more sensitive to image blur or image disparity than the peripheral retina. Conditions that cause a suppression scotoma in the central retina but allow for peripheral fusion cause monofixation syndrome. 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 pd of monofixation syndrome.[1]
Some patients have an inherited inability to bifixate.
Monofixation syndrome has been shown to remain stable over decades. Among patients who have been monitored for several years, sensory status has improved with higher-grade stereoacuity in a small percentage. A small percentage of patients may have decompensated motor loss over time.[2]
Patients should display less than 8 prism diopters of heterotropia with a simultaneous prism cover test.
Alternate prism cover testing may result in 2-3 times greater measurement than simultaneous prism cover testing due to fusional vergence mechanisms.
Up to one third of patients have no deviation with alternate cover.
Stereoacuity tests
Patients with monofixation syndrome have reduced stereopsis, ranging from 300 arc seconds to 67 arc seconds.
Stereopsis tends to be better at near than at distance.
Central suppression scotoma tests
Worth 4-dot test
At near (one-third meter), the standard flashlight tests peripheral fusion by subtending 6 º of the retina.
When performed at a distance of 6 meters, the patient with monofixation syndrome should only visualize from one eye because the target subtends only 1.25 º and slips into the suppression scotoma.
4-Prism diopter base-out test
While having the patient read letters at 20 feet, a 4-prism diopter base-out is placed over one eye.
This is repeated on the other eye.
The test is positive when the eye that is suspected of having a suppression scotoma does not move after being covered with the prism.
This test has a high number of false negatives, especially if no manifest deviation is present.
Bagolini lenses
Lenses are placed over the eyes, and the patient views a small handlight that is located one-third meter away in a normally lit room.
The eye with the suppression scotoma should have a line with a central interrupted portion.
Patients with corrected strabismus comprise the largest subgroup of patients with monofixation syndrome seen in clinical practice.
Patients who are surgically treated for esotropia more commonly develop monofixation syndrome than exotropia.
Of patients with congenital esotropia, 40-50% will develop monofixation syndrome if corrected to less than 8 prism diopters of deviation by age 2 years.[3]
Late correction dramatically decreases the chances of developing monofixation syndrome.
Patients with congenital esotropia who develop monofixation syndrome are almost twice as likely to maintain long-term alignment.[3]
While monofixation syndrome can be an acceptable outcome for patients with preexisting esotropia, many clinicians feel that it is an undesirable outcome for patients with intermittent exotropia. However, monofixation syndrome in postoperative patients with exotropia may have had underlying primary monofixation prior to surgery.[4]
Anisometropia
Blurred image on one macula due to uncorrected refractive error leads to unilateral suppression scotoma and mild-to-moderate amblyopia.
Almost 50% of such patients show no manifest deviation.
Adult patients who have no microtropia, uncorrected mild anisometropia (meridional or hyperopic anisometropia), subnormal stereovision, and a subtle difference in visual acuity likely have monofixation syndrome.
Primary monofixation syndrome
Most patients with primary monofixation syndrome are difficult to diagnose because of the lack of symptoms.
These patients have an inherent defect in central fusion, and most have mild amblyopia.
Prevalence of primary monofixation syndrome is 9% in families who have children with congenital esotropia.[5]
Monofixation syndrome may represent the forme fruste of congenital esotropia.
Macular lesion
Patients with organic macular lesions have a central scotoma.
They often maintain alignment because of peripheral fusion; they are the only group without a facultative scotoma.
A variety of devices have been developed to detect microtropia based on the Bruckner reflex. These methods typically have a higher sensitivity in young children, who lack subjective test cooperation.[6]
Monofixation syndrome is often the desired result of strabismus surgery.
If a manifest strabismus is present, it is small and cosmetically acceptable.
Monofixation syndrome allows for a higher likelihood of diplopia-free long-term alignment.
Monofixation syndrome with esotropia of less than 8 prism diopters is more stable than monofixation syndrome with exotropia of less than 8 prism diopters.
Associated amblyopia
Treatment is given for any associated amblyopia in accordance with standard ophthalmic principles.
Amblyopia in children should be treated with appropriate spectacle correction. A careful cycloplegic refraction should be performed to check for anisometropia.
Occlusive patching or optical penalization should be considered as the clinical situation warrants. The amount of occlusive patching is dependent upon many factors, including age, density of amblyopia, and underlying etiology. In cases of mild amblyopia secondary to anisometropia, part-time occlusion may be all that is needed.
Optical penalization is a method of amblyopia treatment. Optical penalization relies on optically blurring the image of the preferred eye. The most common method relies on using atropine 1% ophthalmic solution in the preferred eye.
Decompensated monofixation syndrome
Older patients with monofixation syndrome may develop double vision if their fusional status decreases. This is termed decompensated monofixation syndrome.
This is less likely in patients treated earlier in childhood, those with no manifest strabismus, and patients with monofixation due to anisometropia.
Decompensated subjects have demonstrated worse horizontal fusional amplitudes, greater torsional fusional amplitudes, and a higher likelihood of having small vertical strabismus than patients with stable monofixation syndrome.[7]
In symptomatic patients, treatment ranges from correcting refraction, orthoptic exercises, prisms, and strabismus surgery. The majority of patients can be successfully returned to a monofixation state.
Heterotropia is cosmetically acceptable, so patients rarely need surgery. Monofixation syndrome tends to stabilize the alignment, thereby decreasing the need for further surgery, with over 90% of patients having stability of alignment over the first 2 decades of follow up.[2]
Patients with stable monofixation syndrome should have routine follow-up care.
If patients have been treated for amblyopia, then the frequency of visits is determined by the intensity of their treatments.
Monofixation syndrome may deteriorate, and patients may develop larger tropia, decreased stereovision, or diplopia.[8] Risk factors for deterioration may include timing of initial treatment, exotropia, and strength of fusion. Surgical treatment can often reduce the tropia and restore secondary monofixation syndrome.
Balaji K Gupta, MD, Clinical Assistant Professor, Department of Ophthalmology and Visual Sciences, University of Chicago
Disclosure: Nothing to disclose.
Specialty Editors
Simon K Law, MD, PharmD, Clinical Professor of Health Sciences, Department of Ophthalmology, Jules Stein Eye Institute, University of California, Los Angeles, David Geffen School of Medicine
Disclosure: Nothing to disclose.
J James Rowsey, MD, Former Director of Corneal Services, St Luke's Cataract and Laser Institute
Disclosure: Nothing to disclose.
Chief Editor
Hampton Roy, Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences
Disclosure: Nothing to disclose.
Additional Contributors
Gerhard W Cibis, MD, Clinical Professor, Director of Pediatric Ophthalmology Service, Department of Ophthalmology, University of Kansas School of Medicine
