eMedicine Specialties > Ophthalmology > Extraocular Muscles

Monofixation Syndrome

Author: Balaji K Gupta, MD, Clinical Assistant Professor, Department of Ophthalmology and Visual Sciences, University of Chicago
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Updated: Sep 25, 2007

Introduction

Background

Monofixation syndrome is essentially a form of subnormal binocular vision. It may also be considered a form of binocular single vision without bifixation due to an absolute scotoma of less than 3 degrees in one eye during binocular vision.  It can be a primary disorder of binocular vision, or it can be a secondary sensory status from a variety of primary causes.

Monofixation syndrome is a sensory adaptation that is characterized by small angle strabismus, unilateral facultative central suppression scotoma, and peripheral fusion.

Pathophysiology

The main defect is a central suppression scotoma.

The central retina has small receptive fields and is 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.

Some patients seem to have an inherited inability to bifixate.

Frequency

United States

The prevalence of monofixation syndrome in the general population is 1%.

Age

This condition is recognized mainly in children but can occur at all ages.

Clinical

History

• Because monofixation syndrome is an asymptomatic condition, the diagnosis is often based on the physical examination.
  • Patients should be asked whether they have a history of muscle surgery, amblyopia, or retinal disease, as well as a family history of strabismus.
  • Adults with subnormal stereovision, mild anisometropia, and subtle asymmetry in best-corrected vision may have undiagnosed primary monofixation syndrome.

Physical

• Prism cover tests
  • 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 degrees 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 degrees 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.
• Binocular visual fields

Causes

• Corrected strabismus
  • 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.
  • 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.
• 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.
• 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.
  • 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.

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