2020–2021 BCSC Basic and Clinical Science Course™
6 Pediatric Ophthalmology and Strabismus
Part I: Strabismus
Chapter 7: Diagnostic Evaluation of Strabismus and Torticollis
Assessment of Eye Movements
Fusional Vergence
Vergences are movements of the 2 eyes in opposite directions (see Chapter 4). Fusional vergences are motor responses that eliminate horizontal, vertical, and, to a limited degree, torsional image disparity.
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Fusional convergence eliminates bitemporal retinal image disparity and controls an exophoria.
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Fusional divergence eliminates binasal retinal image disparity and controls an esophoria.
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Vertical fusional vergence controls a hyperphoria or hypophoria.
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Torsional fusional vergence is cyclovergence that controls an incyclophoria or excyclophoria.
Fusional vergence can be measured using an amblyoscope, rotary prism, or bar prism; the prism power is gradually increased until diplopia occurs. Accommodation must be controlled during fusional vergence testing. Normal fusional vergence amplitudes are listed in Table 7-1. Fusional vergence can be altered by the following:
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Compensatory mechanisms: As a deviation evolves, a larger-than-normal fusional vergence develops. Large fusional vergences are common in compensated, long-standing vertical deviations and exodeviations.
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Change in vision: An improvement in vision may facilitate the fusional vergence mechanism and change a symptomatic intermittent deviation to an asymptomatic heterophoria.
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State of awareness: Fatigue, illness, or drug or alcohol ingestion may decrease the fusional vergence mechanism, converting a heterophoria to a heterotropia.
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Orthoptics: Orthoptic exercises may increase the magnitude of the fusional vergence mechanism (mainly fusional convergence). This treatment works best for near fusional convergence, particularly in convergence insufficiency.
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Optical stimulation of fusional vergence: In controlled accommodative esotropia, reducing the strength of the hyperopic or bifocal correction induces an esophoria that stimulates compensatory fusional divergence. In convergence insufficiency, base-out prism stimulates fusional convergence. Similarly, the power of prisms used to control diplopia may be decreased gradually to stimulate compensatory fusional vergence.
Table 7-1 Average Normal Fusional Vergence Amplitudes in Prism Diopters (Δ)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.