Vergence Response

Tonic

Proximal

Accommodative

Fusional

AC/A Ratio

Accommodation response produces a convergence response

Accommodative convergenceAC/A expressed in ∆/D

CA/C Ratio

Less commonly measured in practice

Convergence response produces an accommodation response

Convergence accommodationCA/C expressed in D/∆

Near Response (Near Triad)

Accommodation

Vergence

Pupil

Accommodation

Blur driven responseOther optical cues such as chromatic

aberration contributeNon-optical cues such as size, rate of

change, time to contactVergence

Vergence

Disparity driven responseEye movements usually complex

with version and vergence interactions

Non-optical cues contributeAccommodation

Pupil Response

Miosis observed for near viewingNo drive from accommodationNo drive from vergenceNon-optical cues such as perceived

distance are main drivers

Accommodation & Vergence

AC/A

Response AC/A: amount of accommodative convergence produced by an actual change of 1D of accommodation

Stimulus AC/A: amount of accommodative convergence produced by a change of 1D in the stimulus to accommodation

Accommodation Response

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6 7

Unit ratioResponse

AC/A Distribution

Measurement of AC/A

Difficult to measure response AC/A

Stimulus AC/A used in practiceNot appropriate to measure in

older age groups Useful in younger patients with

BV problems

Heterophoria Method

Comparison of phoria at D & NAC/A = PD +((∆N - ∆D)/D)

Example

PD 60mm; Distance 3SOP; Near 12SOP

AC/A = 6 + ((12 - 3)/3)AC/A = 6 + (9/3)AC/C = 9:1

Very High AC/A producing anomalous phoria at near

Gradient Test

-15

-10

-5

0

5

10

15

-4 -2 0 2 4

Stimulus to Accommodation (D)

Induced Phoria (∆)

AC/A

Other Methods

Synoptophore Method: Similar to gradient but using synoptophore

Fixation Disparity Method: Indirect method and not appropriate clinically

Significance of AC/A

Has effect on near phoriaLow AC/A leads to large xopHigh AC/A leads to large sopMay be associated with

convergent strabismusHyperopia and high AC/A

associated with convergent squint

Accommodative Strabismus

Innate or Acquired?

No relationship between Rx and AC/A

AC/A similar in mon and binocularStudies of twins show similar AC/AAC/A unchanged by vision training

Effect of Drugs

Miotics

Cycloplegics

Effect of local drugs

Effect of Age

No value in presbyopiaRemains constant up to presbyopia

Summary

A useful measure in practiceVery easy to measureConsult Primary Care text for clinical

variations in techniqueHelps understand BV problemHelps management of BV problem