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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 pre-

scriptions, age-related physiological changes in the

positions of the eyes, blinking, high phorlas, etc. Ver-

gence adaptation Is useful in explaining previous dis-

crepancies between alternate and unilateral cover test,

pre- and postorthoptic ACA ratios, stimulus and re-

sponse 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 con-

vergence: tonic, accommodative, voluntary (which

he assumed was based upon a knowledge of near-

ness), and fusional convergence. Though not spe-

cifically included in his model, Maddox made ref-

erence to a fifth component based upon the knowl-

edge 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. Ac-

cording to Maddox, the remaining vergence dispar-

ity induces a fusional vergence movement to elim-

inate diplopia.

Though Maddox did not include a sixth type of

vergence in his model, he described a phenomenon

currently known as vergence adaptation. He re-

ported 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 reestab-

lish his initial phoria of 1/2k eso. A similar response

occurred with plus lenses, in which tonic conver-

gence 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 meas-

urement 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 conver-

gence 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~