orientation, the image is perceived to fall on the temporal retina and is projected to the opposite field, thus resulting in crossed

diplopia. (Modified with permission from Wright KW, Spiegel PH. Pediatric Ophthalmology and Strabismus. St Louis: Mosby; 1999:219.)

Testing for ARC

Testing for ARC is performed to determine how patients use their eyes in normal life and to seek out any vestiges of normal correspondence. As discussed earlier, ARC is a sensory adaptation to abnormal ocular alignment. Because the depth of the sensory rearrangement can vary widely, an individual can test positive for both NRC and ARC. Tests that closely simulate everyday use of the eyes are more likely to give evidence of ARC. The more dissociative the test, the more likely the test will produce an NRC response, unless the ARC is deeply rooted. Some of the more common tests (discussed at length in Chapter 7), in order of most to least dissociating, are the afterimage test, the Worth 4-dot test, the red-glass test (dissociation increases with the density of the red filter), amblyoscope testing, and testing with Bagolini striated glasses. If an anomalous localization response occurs in the more dissociative tests, the depth of ARC is greater.

Note that ARC is a binocular phenomenon, tested for and documented in both eyes simultaneously. It is not necessarily related to eccentric fixation (see Chapter 4), which is a monocular phenomenon found on testing 1 eye alone. Because some tests for ARC depend on separate stimulation of each fovea, the presence of eccentric fixation can significantly affect the test results (see also Chapter 7).

Monofixation Syndrome

The term monofixation syndrome is used to describe a particular presentation of a sensory state in strabismus. The essential feature of this syndrome is the presence of peripheral fusion with the absence of bifoveal fusion due to a central scotoma. Microtropia is a term that was separately introduced to describe small-angle strabismus with a constellation of findings that largely overlap those of monofixation syndrome.

A patient with monofixation syndrome may have no manifest deviation but usually has a small (<8Δ) heterotropia; the heterotropia is most commonly an esotropia but is sometimes an exotropia or hypertropia. Stereoacuity is present but reduced. Amblyopia is a common finding.

Monofixation syndrome is a favorable outcome of infantile strabismus surgery and is present in a substantial minority of patients with intermittent exotropia. It can also be a primary condition that causes unilaterally decreased vision when no obvious strabismus is present. Monofixation syndrome can result from anisometropia or macular lesions as well. If associated amblyopia is clinically significant, occlusion therapy is indicated.

Diagnosis

To diagnose monofixation syndrome, the clinician must demonstrate the absence of bimacular fusion, by documenting a macular scotoma in the nonfixating eye under binocular conditions, and the presence of peripheral binocular vision (peripheral fusion).

Vectographic projections of Snellen letters can be used to document the facultative scotoma of monofixation syndrome. Snellen letters are viewed through polarized analyzers or goggles equipped with liquid-crystal shutters so that some letters are seen with only the right eye, some with only the left eye, and some with both eyes. Patients with monofixation syndrome omit letters that are imaged only in the nonfixating eye. There are a variety of other tests for central suppression that are more commonly used (see Chapter 7).

Testing stereoacuity is an important part of the monofixation syndrome evaluation. Any amount of gross stereopsis confirms the presence of peripheral fusion. Most patients with monofixation syndrome demonstrate stereopsis of 200–3000 seconds of arc. However, because some patients with

this syndrome have no demonstrable stereopsis, other tests for peripheral fusion, such as the Worth 4- dot test and testing with Bagolini glasses, must be used in conjunction with stereoacuity measurement. Fine stereopsis (better than 67 seconds of arc) is present only in patients with bifoveal fixation.

Kushner BJ. The occurrence of monofixational exotropia after exotropia surgery. Am J Ophthalmol. 2009;147(6):1082–1085. Lang J. Microtropia. Arch Ophthalmol. 1969;81(6):758–762.

Parks MM. The monofixation syndrome. Trans Am Ophthalmol Soc. 1969;67:609–657.