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OPTOMETRY AND VISION SCIENCE

VoL. 69, No. 4, pp. 300-307

symposium paper

Clinical Implications of Vergence Adaptation

JEFFREY COOPER*

State College of Optometry, State University of New York, New York, New York

ABSTRACT

Placement of a prism In front of an eye results In a change in the tonic position of the eyes, a shift in the forced fixation disparity curve, and a shift In fuslonal amplitudes. These changes remain In effect as long as motor fusion Is maintained. Elimination of fusion by occlusion or by removal of the prism results In a slow movement of the eyes back to the preprism position. This phenomenon, known as prism adaptation or slow fusional vergence, has Important clinical implications in maintaining binocular vision with anisometropic prescriptions, age-related physiological changes in the positions of the eyes, blinking, high phorlas, etc. Vergence adaptation Is useful in explaining previous discrepancies between alternate and unilateral cover test, pre- and postorthoptic ACA ratios, stimulus and response ACA ratios, changes in phorlas after orthoptics and the observation of patIents “eating up prism.” Vergence adaptation anomalies have been implicated in causing asthenopia. Adaptation has been shown ta change after orthoptic therapy. This paper reviews the clinical findings associated with vergence adaptation.

Key Words: vergence adaptation, fusion, phoria, stra bismus, asthenopla, orthoptics, vision training, fixatior disparity, exotropia

REVIEW OF VERGENCE ADAPTATION

Maddox’ in 1893 described four elements of convergence: tonic, accommodative, voluntary (which he assumed was based upon a knowledge of nearness), and fusional convergence. Though not specifically included in his model, Maddox made reference to a fifth component based upon the knowledge of nearness. Hofstetter’ described this type of convergence, which results from the sensation of nearness, as proximal convergence. Maddox’ as-

Paper presented at the Annual Meeting of the America:

Academy of Optometry, as part of the Symposium on Acconi modative-.Vergence Adaptation, Nashville, Tennessee, Decem bet, 1990.

Received September 25, 1991.

* Optometrist, M.S., Member of Faculty, F.A.A.O.

sumed that these components were additive. Tonic vergence or the distance phoria, which is a result Df persistent activity of the vergence system, brings ;he eyes in from the position of rest. Tonic vergence is supplemented by both accommodative vergence and proximal convergence to provide coarse tuning at any fixation distance other than infinity. According to Maddox, the remaining vergence disparity induces a fusional vergence movement to eliminate diplopia.

Though Maddox did not include a sixth type of vergence in his model, he described a phenomenon currently known as vergence adaptation. He reported that if he wore an 11 pd base-out prism for 10 min upon removal his tonic convergence increased by 5 pd. In other words, his phoria changed from ½A eso to 5.5 pd eso. Upon removal of the prism, Maddox noted that it took a few minutes for him to reestablish his initial phoria of 1/2k eso. A similar response occurred with plus lenses, in which tonic convergence decreased. Maddox reported that these changes were adaptive and served to relieve stress on the fusional vergence system.

Alpern3 reported that measurements on standard clinical fusional amplitude tests before the measurement of phorias resulted in a change in tonic vergence, i.e., distance phoria. The shift in the lateral phoria was always in the direction of the prism duction. Base-out prism had a greater effect on altering the phoria as compared to base-in prism when both ductions were performed, resulting in a greater propensity for esophoria. A similar finding was reported by Ellerbrock,4 who demonstrated a shift in the vertical phoria after wearing a vertical prism which persisted 30 min after its removal. These findings have led to the common clinical dictum that phorias should be measured before vergence amplitudes and divergence amplitudes should be measured and/or trained before convergence amplitudes.

Ogle and Prangen5 measured vertical forced duc

tion fixation disparity curves and found them to b

a straight line. Then they had their subjects wear

6A vertical prism for approximately 2 h. They foun

that the vertical fixation disparity curve measur

through the adapting prism was almost identical

300 OPTOMETRY & VISION ScI~NJ~