Monofixation Syndrome

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Article initiated by:

Pedro Mario González Madrigal, MD

All contributors:

Alison Jess, Pedro Mario González Madrigal, MD, S. Grace Prakalapakorn, MD, MPH, Donny W. Suh, MD, FAAP, MBA, FACS, Dr. Priyanka Kumar, MD, Angeline Nguyen, MD

Assigned editor:

Angeline Nguyen, MD

Review:

Assigned status Update Pending

 by Angeline Nguyen, MD on July 25, 2023.

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Contributing Editors:	add

Monofixation Syndrome

DiseasesDB

33536

ICD-10

H50.4

Disease Entity

Monofixation syndrome, defined by Marshall M. Parks in 1969, constitutes a combination of orthophoria or near-orthophoria (≤ 8 prism diopters [PD] heterophoria or uncompensated heterotropia), peripheral fusion, normal fusional vergence amplitudes, macular scotoma (measuring 3-5º) in the non-fixating eye, and the presence of gross stereopsis. This state is believed to be a sensory adaptation to prevent diplopia in conditions that cause a central scotoma while still allowing for peripheral fusion. The central scotoma may be facultative (due to suppression) or absolute (due to macular pathology). Patients with monofixation syndrome appear cosmetically normal and have no symptoms. Their condition is typically detected due to a lack of fine stereopsis and presence of mild-to-moderate amblyopia.

Etiology

Monofixation syndrome has been described to result from a various causes of a facultative or manifest central scotoma.

1. Surgically corrected strabismus: Monofixation syndrome most commonly occurs after strabismus surgery in patients younger than 2 years old with esotropia (less commonly occurring in those with exotropia). It has been reported that patients with congenital esotropia who develop monofixation syndrome after surgery are twice as likely to maintain long-term alignment as a result of this sensory adaptation; thus, monofixation is considered by many to be a favorable surgical result following surgery for infantile esotropia.
2. Anisometropia: Anisometropia may lead to a unilateral suppression scotoma and amblyopia.
3. Macular lesion: A unilateral macular lesion may lead to an absolute scotoma and thus patients rely on peripheral fusion to maintain ocular alignment.
4. Primary: Primary monofixation syndrome may occur due to a primary inability to fuse similar macular images.
5. Bilateral cataracts: Interruption of bifoveal fusion for an extended period of time due to dense cataracts has also been described as a cause of monofixation syndrome in children and adults.

Diagnosis

Physical examination

Clinical suspicion should arise in the presence of the following:

• Small angle strabismus
• Subnormal stereopsis
• Amblyopia
• Anisometropia
• History of strabismus surgery
• Retinal disease

Diagnostic procedures

The presence of monofixation syndrome may be suspected with cover-uncover testing demonstrating ≤ 8PD heterophoria or heterotropia. Confirming the diagnosis involves demonstrating the presence of a central scotoma, decreased stereopsis, and peripheral binocular single vision.

• Prism cover testing: should demonstrate ≤ 8 prism diopters of heterophoria or heterotropia. Of note, since under binocular conditions peripheral fusion can help exert some control over their deviation, patients may demonstrate orthophoria or a smaller heterotropia by simultaneous prism or cover-uncover testing as compared to prism and alternate cover testing.
• Stereoacuity testing: should demonstrate reduced stereopsis (<67 seconds of arc, typically 200-3000 seconds of arc). Some patients may not have any demonstrable stereopsis despite peripheral fusion, thus the tests below may be necessary to demonstrate peripheral binocular single vision.
• 4-Prism diopter base out testing: should demonstrate the presence of a central scotoma. To perform the test, a 4 PD base-out prism is placed before one eye during binocular viewing of letters at a distance. The examiner observes the motor response with the prism held before each eye respectively. In a patient with normal bifoveal fixation, the motor response involves an initial refixation movement of the eye behind the prism toward the apex of the prism followed by a fusional movement in which the fellow eye converges nasally to restore fusion. In a patient with monofixation syndrome, there is no movement noted when the prism is placed before the non-fixated (the affected eye). When the prism is placed before the fixating eye (the unaffected eye), an initial refixation movement towards the apex of the prism occurs, but this is not followed by the fusional convergence of the fellow eye. Unfortunately, false results may occur with this test, limiting its utility. For example, a false positive result may occur in patients with bifoveal fixation who recognize diplopia when the prism is placed before an eye but make no convergence effort to correct for it. A false negative result may occur in a patient with monofixation syndrome if they switch fixation each time the prism is placed before an eye and thus shows no refixation movement regardless of which eye is tested.
• Worth-4-dot testing: should demonstrate both the presence of peripheral fusion (with testing at near) and a central scotoma with absence of bifoveal fusion (with testing at distance). With the red-green glasses placed on the patient with the red lens placed in front of the right eye by convention, the test is performed at both near and distance. Because the standard Worth 4-dot dots at distance (3m or 10 ft) projects onto the central 1º of the retina, which is within the 1-4º scotoma common to monofixation syndrome, the patient will report only seeing 2 or 3 lights, depending on which eye is preferred. When tested at near, the Worth 4-dot flashlight projects onto the peripheral retina outside the central scotoma, and thus the patient reports a fusion response of seeing 4 lights.  
• Bagolini lens testing: should demonstrate presence of a central scotoma. When the lenses are placed with the striations at an angle of 135º and 45º in the patient’s view over the right and left eyes, respectively, the patient views a small point light source at a near distance. The eye with the suppression scotoma would perceive a line with a central gap.

Management

Monofixation syndrome is a sensory adaptation to avoid diplopia, which may serve to improve the patient's alignment stability (even when a small angle heterotropia is present). Thus, monofixation syndrome is a favorable surgical result after strabismus surgery and does not require further surgery or prismatic correction for sensory or motor reasons.

In cases wherein visual acuity is markedly abnormal due to amblyopia, treatment methods for amblyopia should should be applied. Of note, treatment of amblyopia (a sensory defect under monocular conditions) does not affect the scotoma (a sensory defect present under binocular conditions) in monofixation syndrome.  

In cases of alignment decompensation (occurring rarely, in <10% of cases in 14 years of follow-up), patients may benefit from additional corrective strabismus surgery.