Monofixation syndrome, defined by Marshall M. Parks in 1969, 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.   Conditions that cause a suppression scotoma in the central retina but allow for peripheral fusion cause monofixation syndrome.
Some causes have been associated to monofixation syndrome.
- Surgically corrected strabismus- the most commonly identified cause. After surgery, mostly on patients with a residual deviation less tan 8 prism diopters and younger than 2 years old. A patient 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 supression scotoma and amblyopia. Many of these patients develop monofixation syndrome as an adaptation mechanism.
- Macular lesion- a macular lesion will induce a scotoma (which won´t be because of suppresion) and in an effort to maintain alignment they will develop peripheral fusion.
- Primary- Very difficult to diagnose since there is a lack of symptoms. Most patients will have a mild amblyopia.
Typically asymptomatic. Clinical suspicion should arise if:
- Subnormal steropsis
- 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 is 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 bifixation.
- 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 a 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 will also give the patient alignment stability (even when a small angle heterotropia is mantained). Put in a simple way, monofixation is a good surgical result in an strabismus surgery so no treatment is required.
In cases were the visual acuity is markedly below normal, amblypia treatment (patching or penalization) might be considered.
Since monofixation syndrome is a “compensatory” mechanism, it might decompensate. It has been studied that even when this is not common (less than 10% of the cases in 14 years of follow-up), this patients might benefit from a 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.