2. AC/A Ratio
Definition:
The AC/A Ratio is the ratio of the amount
of accommodative –convergence measured
in prism diopters to the number of diopter
of accommodation which causes this
convergence.
3. Different methods of measuring
Surgical method
The Heterophoria method
The gradient method
The major amblyoscope method
The graphic method
The fixation disparity method
4. Normal range of AC/A Ratio
The Normal range of AC/A Ratio is 3:1 to
5:1
It is expressed as 3 to 5
As the convergence measured is related to
one diopter of accommodation
5. The assessment of the AC/A
Ratio
The heterophoria method:
This method compare the measurements of the
latent deviation of the eyes, using the prism
&alternate cover method at point distant fixation
(6 meters)
The point of near fixation at 1/3rd meter with care
to ensure the steady accommodation at both
distance of fixation by the use of target which
contains detail like a snellen chart
6. The use of an appropriate spectacle
corrections when there is any significant
refractive error
It may be concluded that when the
measurement for distance and near is equal
the Normal range of AC/A Ratio is normal
When it is greater for distance than near the
AC/A Ratio is low
7. When it is greater for near than the distance
the AC/A Ratio is high
It must be conceded that some degree of
difference (possibly as much as 10 D)is
within normal limits
8. Formula for Heterophoria method
AC/A =I.P.D+△2-△1
F1
Where
AC=accommodative convergence in prism
diopters (△)
A=Accommodation in diopters (D)
9. I.P.D.=interpupillary distance in prism
diopters
△1=latent deviation for distance (6 M)
△2= latent deviation for NEAR(1/3M)
F1=Distance of near fixation in diopters
F2= Distance of near fixation in meters
11. The gradient method
The AC/A Ratio is measured by an estimation of
the difference between the deviation for the eyes
for a given distance
The Maddox rod is use in front of one eye and
correcting prism in front of the other eye
It is done before placing convex or concave
spherical lenses in front of the eye
12. The gradient method
The lenses are place so that there is change in
their accommodation and therefore in their
convergence
The convex lenses by decreasing the amount of
accommodation necessary for the given distance
decreases the amount of convergence
The concave lenses by increasing the amount of
accommodation necessary for the given distance
increase the amount of convergence
13. The gradient method
It is important to achieve this deviation of the eye
by ensuring that patient exerts the the full amount
of accommodation for the particular distance
This is achieved by the use of an object which
contain much fine detail in conjunction with the
alternate prism and cover test in preference to use
simply of a fixation light as in the maddox rod
test
14. The gradient method
It is of course necessary in measuring the
difference of the deviation to regard it as the
subtraction of the first deviation from the second
deviation
It is important to note down the sign properly
before calculation
Plus measurement when it is esodeviation
Minus measurement when it is exodeviation
15. The gradient method
In arriving for the final figure of the ratio it
is necessary to divide the difference in the
deviation by the power of the lenses used
in order to reduce it to single unit of
accommodation for ease of comparison
16. AC/A Ratio by gradient method
As a general rule the values of AC/A Ratio by the
gradient method is slightly lower than those
obtained by the heterophoria method
This is because the fixed distance used in the
gradient method (unlike the heterophoria method
which adopts two different distances of fixation)
This is due to the factor of proximal convergence
and this method has advantage that it induces
convergence mainly due to the patients subjective
accommodative effort
17. Formula of gradient method
AC/A=2-1
D
WHEREAC=accommodative convergence in prism
diopters ()
A=accommodation in diopters(D)
1=original deviation in prism diopters
2=deviation in prism diopters with the spherical
lenses
D1=power of lenses used in diopters
20. The major amblyoscope methods
There are two ways of assessing the AC/A Ratio
with the major amblyoscope
The instrument is adjusted to the patients
interpupillary distance in the usual manner
The correcting spectacle arte worn
Targets are used which ensure foveal fixation
The subjective angle is determined ,and the
reading taken from the prism
21. Diopter scale .minus lenses usually –3.00
D.S., are inserted in the lens holder of the
instument and the measurement is repeated
This method is comparable to yhe gradient
method when using the snellen’s test types
It has the advantage that small deviation can
be measured more accurately than when
done by prism and cover tet
22. Formula for the amblyoscope
method
AC/A = 2- 1
D
WHERE 1is the subjective angle
measured with patient’s own spectacles.
2is the subjective angle measured with the
adddition of the –3.00D.S.
Dis the strength in dioters of the concave
spherical lens used
23. Example of the major
amblyoscope method
E.g. if 2=19 eso
1=7 eso
D= -3.00Dsph
AC/A =+19-(+7)
3
=4
24. The Graphic Method
The instrument is adjusted to suit the I.P.D. of
the patient
The object is to determine the patient’s subjective
angle but instead of using the usual S.F.P. slides
and recording the deviation on the degree scale , a
reduced test-type slide is placed before one eye
and a black vertical line before the other,such test
type ensures accurate fixation
25. Correct accommodation is achieved by the patient
being told to read the test-type from largest to the
smallest row of letters
Then by moving the slide carrier containing the
vertical line to cause the line to bisect the
smallest row of letters which he can see clearly
The angle at which he achieves is recorded on the
prism diopter scale and is noted as the patients
subjective angle
26. The measurement is repeated with the
introduction of –1.0.-2.0,-3.0,-4.0 dioptre
spheres successively before the patient’s
correcting spectacles
The measurement represent the
accommodative –convergence.
28. The fixation disparity method
In the fixation disparity method the AC/A Ratio is
measured by an assessment of the The fixation
disparity which is found in the heterophoria
,when targets mainly identical features but also
certain dissimilar features are presented to the
eyes by means of haploscopioc device
Fusion occurs readily and is mainted for the
identical features of the targets but a displacement
occurs for the dissimilar features according to the
direction of the hetrophoria
29. The fixation disparity method
The displacement is measured in two
different circumstances
First-it is measured in various positions of
positive convergence and negative
convergence ,as induced by the
superimposition of varying prisms (base-
out and base-in respectively )
30. The fixation disparity method
The production of positive convergence by the
base out prism causes an exo-disparity and the
production of negative convergence by the use of
base-in prism causes an eso-disparity
These results are recorded graphically with the
strength of the prisms along the abscissa (base-
out to the right and base-in to the left) and the
degree of disparity along the ordinate (eso
disparity upwards,exo-disparity downwards )
These findings are termed as disparity –prism
data
31. The fixation disparity method
Secondly it is measured during the various
changes in the stimulus to positive and negative
accommodation as induced by the
superimposition of varying spherical lenses
(concave or convex)
This are place in front of one or both eyes during
the maintenance of binocular fixation on a target
at constant distance usually 40 cms
32. The fixation disparity method
The stimulus to the positive
accommodation by the use of concave
spherical lenses causes an eso-disparity
The stimulus to the negative
accommodation by the use of convex
spherical lenses causes an exo-disparity
This is recorded on the graph
33. The significance of the AC/A
Ratio
An appreciation of the AC/A Ratio gives an
indication that there are two types of
accommodative esotropia
There is accommodative esotropia in which the
determining fact is the uncorrected hypermetropia
with its excessive accommodative requirement
leading to an excessive convergence the AC/A
Ratio is normal
34. The significance of the AC/A
Ratio
Secondly, there is an accommodative esotropia in
which the determining factor is the excessively
high response of the convergence mechanism to
any accommodative effort because the AC/A
Ratio is high
This distinction between these two different
forms of accommodative esotopia is importyant
in determining the method of treatment of such
cases
35. The significance of the AC/A
Ratio
When the AC/A Ratio is relatively normal –
correction of refractive error alone may be
sufficient to correct the ocular deviation but not
when the AC/A Ratio is high
When AC/A Ratio is high –occlusion(although
the value in eliminating suppression or
amblyopia) causes an esotropia to increase in
amount o that it ceases to have any latent
component
This is because an over convergence inevitably
follows the loss of the fusional control which
occurs during occlusion
36. The influence of method of
treatment on the AC/A Ratio
Correcting spherical lenses
Miotic therapy
Surgical method
37. Correcting spherical lenses
This alters the AC/A Ratio
For example a case in which there is
orthophoria on distant fixation and
esotropia on near fixation without any
spectacle correction may show orthophoria
on both distant and near fixation after the
use of spectacle correction
38. Miotic therapy
Miotics such as pilocarpine, DFP, causes the
change in the AC/A Ratio
This is because the lesser innervations which is
require to produce a diopter of accommodation is
associated with the decrement in the innervations
of the ciliary muscles and a proportionate
decrement in the innervations to the medial recti
so that there is decrease in the AC/A Ratio
39. Surgical method
There is evidence that a decrease in the
effectiveness of the medial rectus muscle or
muscles by a recession in accommodative
esotropia results in a reduction of
abnormally high Surgical method
40. Medical care of Esotropia, with High
AC/A Ratio
Treatment of nonrefractive accommodative
esotropia consists of full correction for the
distance refractive error and bifocals for near
vision to suspend the accommodative drive and to
lessen accommodative convergence.
Bifocal power should be +2.50 to +3.00 diopters,
and bifocals should be placed such that the upper
boarder of the bifocal segment bisects the pupil.
41. Miotics, which lower the AC/A ratio, are
successful in some patients.
In cases of amblyopia, early treatment
of patching the normal (unaffected) eye
is the mainstay of treatment.
42. Surgical care Esotropia,
with High AC/A Ratio
Surgery may be required if the esodeviation
becomes refractory to optical treatment. Surgery
often is needed when optical treatment is delayed.
Surgical treatment typically entails recession or
weakening of the inward-pulling medial rectus
muscle in each eye.
Surgery is performed for the nonaccommodative
component only. The operation is not intended to
discontinue use of glasses.
FOLLOW-UP Section 6 of 8
43. Esophoria With Myopic
Correction have high AC/A Ratio
When myopes are esophoric this may mean a
several things.
Two of which are they have a high ac/a or are
overminused
There is some suggestions that this may be one
type of myope that can lead to myopic
progression if overminused (there was a study)
With myopic esophores that were fit with bfl
contacts
44. Treatment for Esophoria With Myopic
Correction and high AC/A Ratio
1. Make sure they are not overminused
2. Can consider a PAL or BFL (add is clinically
figured out by finding the the add that makes
them ortho at near)or a SV near rx.
3. For those you mentioned they can just take off
their Rx to read.
4. Or rx the least amount of minus that gets them
20/happy...dont over minus them!
45. Diagnosis and treatment of exotropia
with high AC/A Ratio
Patients with exotropia often have a fusional
mechanism at near, which masks the true near
deviation.
Consequently, determination of the
accommodation convergence–accommodation
(AC/A) ratio in patients with exotropia must be
based on near measurements obtained after
prolonged monocular occlusion (typically 1
hour).
46. When determined in that manner, the
presence of a highAC/A ratio before
surgery in an exotropic patient has been
reported to be predictive of an esotropia at
near after surgery.
47. Importance of measurement of
AC/A Ratio
Near measurements used to calculate the
AC/Aratio in exotropic patients must be
made after prolonged monocular occlusion.
Otherwise, many patients with a pseudo–
highAC/A ratio will be thought to have a
true high AC/A ratio.
The presence of a high AC/A ratio is
infrequent in patients with esotropia, but it
is highly predictive of a postoperative
esotropia at near fixation.
48. Accommodative esotropia with
normal AC/A Ratio
Total esotropia eliminated
a. refractive - high hyperopia
bilateral not unilateral
normal range of AC/A ratio
50. Low AC/A Ratio
Convergence Insufficiency - (clinical
condition) the inability of the eyes to turn
inward and/or sustain an inward turn.
Symptoms include eye strain with reading
and using a computer, headaches, loss of
comprehension, difficulty concentrating,
blurred or double vision, and eye fatigue.
51. Low AC/A Ratio
Clinical signs include: near point of
convergence of greater than 4 inches
(10 cm), greater exophoria at near than
at distance, and low AC/A ratio.
Vision therapy is an effective
treatment option.
52. High AC/A Ratio
Convergence Excess - a clinical
condition in which the eyes have a
tendency to turn excessively inward
when viewing an object at a near
distance.
Symptoms may include visual fatigue
while reading or using a computer,
occasional blurred or double vision,
and inability to comprehend or
concentrate while reading.
53. High AC/A Ratio
Clinical signs include: greater
esophoria at near than distance,
high AC/A ratio, and a high lag of
accommodation. Can be improved
with vision therapy and/or glasses.
(See "Esophoria")