- 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 1/3rd 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
- 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 angles3) 12(4metre angles3) 18(6 2metre angles3) 24(8 2metre angles3)
- 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
- 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