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
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.
Fusional convergence eliminates bitemporal retinal image disparity and controls an exophoria.
Fusional divergence eliminates binasal retinal image disparity and controls an esophoria.
Vertical fusional vergence controls a hyperphoria or hypophoria.
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:
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.
Change in vision: An improvement in vision may facilitate the fusional vergence mechanism and change a symptomatic intermittent deviation to an asymptomatic heterophoria.
State of awareness: Fatigue, illness, or drug or alcohol ingestion may decrease the fusional vergence mechanism, converting a heterophoria to a heterotropia.
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.
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.