Monofixation syndrome is a concept defined by Marshall M. Parks in 1969. It consists of the following combination of features.
This syndrome is understood as a sensory adaptation to maintain peripheral fusion while avoiding diplopia in the presence of a central scotoma. Patients appear normal and are usually asymptomatic. It is often discovered due to a lack of fine stereopsis or mild to moderate amblyopia.
Strictly speaking, it is not a disease but a sensory adaptation state to avoid diplopia in the presence of a central scotoma. Many experts consider it a favorable outcome after strabismus surgery. Treatment is usually unnecessary, but if amblyopia is present, it should be treated.
Patients with monofixation syndrome are usually asymptomatic. Their appearance is also normal, and it is most often discovered incidentally through the following triggers.
There is no complaint of diplopia. This is because suppression due to central scotoma prevents double vision.
The following combination of findings is characteristic.
Evaluation of fixation patterns is also important. In patients with strabismus, the duration that the non-dominant eye maintains fixation is assessed under binocular viewing. The assessment is graded as no fixation, momentary fixation, or fixation for several seconds1). The induced tropia test is useful in microtropia or non-strabismic cases, where a 10 to 20 PD base-down prism is placed over each eye alternately to observe fixation behavior1).
Monocular fixation syndrome results from various conditions that cause functional or organic central scotomas.
Postoperative Strabismus
After surgery for infantile esotropia is the most common occurrence. It tends to develop in cases operated on before 2 years of age.
Favorable long-term prognosis: Patients who acquire this syndrome after surgery have a twofold higher likelihood of maintaining long-term ocular alignment.
It can also occur after exotropia surgery, but is relatively rare.
Anisometropia
Refractive asymmetry between the eyes can cause a monocular suppression scotoma.
Often accompanied by amblyopia.
Macular lesion
Unilateral macular lesions cause absolute scotomas.
Eye position is maintained by relying on peripheral fusion.
Primary
Congenital inability to fuse similar macular images.
Classified when there is no clear underlying disease.
Additionally, dense bilateral cataracts can cause long-term blockage of binocular foveal fusion. This has been reported in both children and adults.
When intermittent exotropia develops in infancy, it can lead to monofixation syndrome based on anomalous retinal correspondence due to eccentric fixation. In such cases, mild amblyopia is observed in about 5% of patients.
This is not a bad thing. Rather, many experts consider it a good postoperative outcome. It has been reported that patients who acquire this syndrome have improved ocular alignment stability through peripheral fusion, and the likelihood of long-term ocular alignment maintenance is doubled.
If a phoria or tropia of 8 prism diopters or less is observed on the cover-uncover test, this syndrome should be suspected. Definitive diagnosis requires demonstrating the presence of a central scotoma, reduced stereopsis, and the presence of binocular single vision in the periphery.
The main diagnostic tests are shown below.
| Test method | Evaluation item | Characteristics |
|---|---|---|
| Prism cover test | Deviation angle | Confirm 8 PD or less |
| Stereopsis test | Stereopsis | Less than 67 seconds of arc |
| 4PD base-out test | Central scotoma | Judged by ocular motor response |
| Worth 4-dot test | Fusion/scotoma | Response differs by distance |
| Bagolini lens | Central scotoma | Central break in light streak |
Confirm a phoria or tropia of 8 PD or less. Note that under binocular viewing conditions, peripheral fusion contributes to controlling deviation, so the simultaneous prism cover test or cover-uncover test may show a smaller angle of deviation than the alternate prism cover test.
Confirm reduced stereopsis (less than 67 arcseconds, typically 200–3000 arcseconds). In some cases, stereopsis cannot be detected at all despite the presence of peripheral fusion; in such cases, separately confirm peripheral binocular single vision using the following tests.
This is one of the most important tests for evaluating the presence of a central scotoma. With both eyes fixating on a distant target, a 4 prism diopter (PD) base-out prism is placed in front of one eye, and the binocular eye movements are observed.
False positives (when a patient with binocular foveal fixation does not make a convergence effort) and false negatives (when the fixating eye alternates) can occur, so caution is needed in interpretation.
False positives occur when a patient with binocular foveal fixation recognizes diplopia but does not make a convergence effort to correct it. False negatives occur when a patient with this syndrome alternates fixation each time a prism is placed, so no refixation movement is seen regardless of which eye is tested.
This test simultaneously evaluates the presence of peripheral fusion and the absence of a central scotoma. The patient wears red-green glasses and is tested at near and far distances.
The patient wears lenses with striae at 135 degrees for the right eye and 45 degrees for the left eye, and observes a point light source at a short distance. In an eye with a suppression scotoma, the center of the light line appears broken.
Evaluation of fixation abnormalities also aids diagnosis. Direct observation with a visuscope is the most common method, allowing accurate testing from age 3 onward. In infants, the corneal reflex method using a penlight is employed, checking whether the reflected light is centered on the pupil and assessing fixation maintenance by covering each eye alternately.
Monocular fixation syndrome is a sensory adaptation to avoid diplopia and plays a role in enhancing ocular alignment stability. Therefore, the main treatment strategies are as follows.
In patients with monocular fixation syndrome, when visual acuity in the dominant eye decreases and the fixating eye switches, they may newly experience diplopia (fixation switch diplopia) 2). Triggers include interocular differences after cataract surgery, introduction of monovision, and refractive surgery 2).
The essence of monocular fixation syndrome is a sensory adaptation to maintain part of binocular vision (peripheral fusion) while avoiding diplopia in the presence of a central scotoma.
Functional Scotoma
Suppression scotoma: Appears only under binocular viewing conditions. Under monocular conditions, foveal function is normal.
Causes include cortical suppression due to postoperative esotropia, anisometropic amblyopia, etc. Visual acuity may improve with amblyopia treatment, but the scotoma persists under binocular conditions.
Absolute scotoma
Scotoma due to macular lesions: Present under both monocular and binocular conditions. Based on organic retinal damage.
Caused by macular diseases such as macular degeneration and macular hole. The scotoma is irreversible, and dependence on peripheral fusion persists permanently.
Even if a central scotoma exists, the retina outside the scotoma (beyond 3–5 degrees) functions normally. Fusion is achieved through this peripheral retina, maintaining control of eye position. Peripheral fusion preserves a normal range of fusional convergence and divergence, so patients are less likely to experience deviation of eye position in daily life.
When intermittent exotropia develops in infancy, anomalous retinal correspondence based on eccentric fixation may become established, potentially progressing to monofixation syndrome. In such cases, the binocular visual field is reportedly 20–30 degrees even in orthophoria, narrower than the normal 40 degrees.
