AC/A ratio

AC/A ratio. Basic to Advance

1. AC/A Ratio
Mahendra Singh
Assistant Professor and consultant
Optometrist.
CL Gupta Eye Institute. UP India

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 1/3
rd
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

10. Example
I.P.D =6cms
△1=4prism exo
△2=10prismexo
F1=3D
AC/A=6+(-10-(-4)_ )
3
=6+(-10+4)
3
=6+(-2)
=4

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

18. Examples
If 1=3 eso.
2=6 eso.
D1=1Dsph(concave)
AC/A=6-3
1
=3

19. The major Amblyoscope method

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.

27. Diopters of
accommodation
1
2
3
4
Prism diopter of convergence
(both eyes )
6(2metre angles3)
12(4metre angles3)
18(6 2metre angles3)
24(8 2metre angles3)

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
Correctionhave 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
Correctionand 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 nearmeasurements obtained after
prolonged monocular occlusion (typically1
hour).

46. When determined in that manner, the
presence of a highAC/A ratio before
surgery in an exotropic patient has been
reportedto 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 monocularocclusion.
Otherwise, many patients with a pseudo–
highAC/A ratio will be thought to have a
true high AC/A ratio.
Thepresence of a high AC/A ratio is
infrequent in patients withesotropia, but it
is highly predictive of a postoperative
esotropiaat 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

49. Accommodative esotropia with
normal 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")

54. The Exotropias

55. CHARACTERISTICS OF COMITANT
EXOTROPIA
AC/A ratio; normal or low
symptoms; photophobia, squinting
prognosis; usually good

56. INTERMITTENT EXOTROPIA OF THE
DIVERGENCE EXCESS TYPE
distance angle > near angle
deviation is intermittent
angle of deviation increases
with fixation distance
weak accommodative skills
high AC/A ratio?
near-far AC/A calculation -
gradient AC/A determination