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AC/A
1.
2. Measurement of the convergence induced by
accommodation, per diopter of
accommodation.
To determine the change in accommodative
convergence that occurs when the patient
accommodates or relaxes accommodates a
given amount.
Unit = ∆∕D
Normal range = ( 4 – 6) ∆/D
3. Abnormalities of the AC/A ratio are very
important causes of strabismus.
A high AC/A ratio may cause excessive
convergence & produce a convergent
squint during accommodation on a near
object.
A low AC/A ratio may cause a divergent
squint when the patient looks at a near
object.
4. 1. Stimulus AC/A Ratio
Ratio of accommodative convergence
to stimulus to accommodation.
2. Response AC/A Ratio
Ratio of accommodative convergence
to response to accommodation.
Usually stimulus AC/A > Response AC/A
6. Simple method, consists of comparing the
measurement of the latent deviation of eyes
Using prisms & alternate cover test at a point of
distance fixation ( 6m ) with refractive
correction.
At a point of near fixation ( 33cm ) with
refractive correction.
IPD should be measured.
+ve sign for esodeviation, -ve sign for
exodeviation.
7. AC/A ratio is calculated from this following
formula:-
AC/A = IPD + (∆n - ∆d/D)
8. The change in the stimulus to
accommodation is produced by means of
ophthalmic lenses not by a change in
viewing distance.
Concave lenses placed before the eyes
increase the requirement for accommodation
& convex lenses relax accommodation.
-1D lenses produce an equivalent of 1D of
accommodation, where as +1D lenses relax
accommodation by 1D.
9. Original deviation is found at near while the
patient wears optical correction & then with
additional lens.
Ac/A ratio is calculated from this following
formula:-
AC/A = ∆L - ∆O/D
10. Used by Ogle & co-workers.
Indirect method, more reliable due to test
under binocular condition.
Consists of 2 sets of data
First changes in fixation disparity induced by force
convergence using prism
Second change in fixation disparity induced by altering
the accommodative stimulus with lens
11. From these 2 sets of data they determined
stimuli for convergence & accommodation
that gave same fixation disparity.
Because of its complexity, this test is not
performed in routine clinical practice.
12. Convergence (in prism diopter) is plotted
along the x- axis.
Stimulus to accommodation is indicated
along the y-axis.
Slope of graph represents AC/A ratio.
13.
14. Diagnosis of convergence excess type of
esodeviation.
Divergence excess exodeviation
Divergence insufficiency
Convergence insufficiency
15. Apply in those patient who have chance of
obtaining some fusion when eyes are straight
or within 10PD in distance with optical
correction.
Try to keep eyes straight for distance upto
age of 8 years.
16. If eso deviation is less then 10PD in distance-
has chance of developing fusion with full
optical correction.
If eso deviation is greater than 10PD in
distance has no chance of developing fusion
unless the deviation is reduced to under
10PD.
17. Prescribing Prism
Prescribing Visual Training
Alteration of Sph. Portion of Correction
if AC/A sufficiently large additional plus for
convergence excess
18. CONVERGENCE EXCESS:
n>d
high AC/A ratio
Treatment
Near add (overcorrection of plus power
or under correction of minus power
most effective) e.g.
Base-out prism (alternative method)
Base-in training(not so effective)
19. CONVERGENCE INSUFFICIENCY
n> d
Low AC/A ratio
Treatment
Base-out training (most effective)
“Pencil push-up” training
20. DIVERGENCE EXCESS
n< d
high AC/A ratio
Treatment
overcorrection of minus power or
undercorrection of plus power
in mild case base-in prism for full time
wear