Monofixation syndrome, defined by Marshall M. Parks in 1969, is a form of subnormal binocular vision without bifoveal fixation, characterized by small-angle strabismus, unilateral absolute facultative central suppression scotoma of less than 3º, and peripheral fusion.   Conditions that cause a suppression scotoma in the central retina but allow for peripheral fusion cause monofixation syndrome.
Some causes associated to monofixation syndrome:
- Surgically corrected strabismus- the most commonly identified cause. Develops after surgery, mostly on patients younger than 2 years old, with a residual deviation less than 8 prism diopters. It has been reported that patients who had a diagnosis of congenital esotropia before surgery and develop monofixation syndrome are twice as likely to maintain long-term alignment.
- Anisometropia- anisometropia may lead to a unilateral suppression scotoma and amblyopia. Many of these patients develop monofixation syndrome as an adaptation mechanism.
- Macular lesion- a macular lesion will induce a central scotoma (caused by the structural pathology NOT suppression) and in an effort to maintain alignment, patients will develop peripheral fusion.
- Primary- very difficult to diagnose in the absence of symptoms, but most patients will have mild amblyopia.
Clinical suspicion should arise if:
- Subnormal stereopsis
- Asymmetry in best corrected visual acuity
- History of strabismus surgery
- Retinal disease
- Prism cover tests – less than 8 prism diopters of heterotropia in cover/uncover test.
- Stereoacuity tests – reduced stereopsis (typically less than 60 arc seconds). Usually better at near since the image reflected from a distant point will tend to fall into the suppression scotoma.
- Worth-4-dot– Near: 4 lights, Distance: 2-3 lights depending on the eye with the scotoma. This will prove peripheral fusion and the absence of bifoveal fixation.
- 4-Prism diopter base out test- with the patient reading letters at 6m, a base-out 4-prism diopter is placed over one eye (the one in which we are suspecting a suppression macular scotoma). If the eye does not move after being covered with the prism, this will mean that the patient is experiencing single binocular vision in spite of the shifted retinal image (without a corrective eye movement). Many false negatives make this test of limited utility.
- Bagolini lenses- lenses are placed over the eyes, and the patient views a small handlight at near distance. The eye with the suppression scotoma should have a line with a central interrupted portion.
Monofixation syndrome is an adaptation to avoid diplopia which also gives the patient alignment stability (even when a small angle heterotropia is present). Put simply, monofixation is an acceptalbe surgical result after strabismus surgery, and does not require further treatment.
In cases where the visual acuity is markedly below normal, amblyopia treatment (patching or penalization) might be considered.
Since monofixation syndrome is a “compensatory” mechanism, it can decompensate. It has been studied that even though this is not common (less than 10% of cases in 14 years of follow-up), these patients might benefit from additional corrective strabismus surgery.
- Parks, M M. "The Monofixation Syndrome." Trans Am Ophthalmol Soc 67 (1969): 609–657.
- Gupta, B. "The Monofixation Syndrome” Medscape (2014). Medscape. Web. 1 Jan. 2015. <www.medscape.com>.
- Ing,M.“The stability of the monofixation syndrome”. Am J Ophthalmol. Jan 2014;157 (1):248-253
- Noorden, G. "Binocular Vision and Ocular Motility: Theory and Management of Strabismus. " 4th ed. St. Louis: Mosby, 1990.