AC/A Ratio

Question 1

What is accommodative convergence?

Answer 1

Component of convergence that occurs because of accommodation that can be calculated through the AC/A ratio

Question 2

What is the AC/A ratio?

Answer 2

The amount of accommodative convergence (in PD) that occurs per dioptre of accommodation which is typically a 4:1 (AC/A)

This means for every 1D of accommodation there’s 4PD of accommodative convergence BUT isn’t always 4:1 for everyone

Question 3

What does the AC/A ratio measure?

Answer 3

Measures the effect of accommodation on the angle of deviation and can be measured by changing accommodation level and assessing the effect on deviation

Question 4

How do you calculate the AC/A ratio?

Answer 4

Change the stimulus to accommodation and measure the amount of accommodative convergence induced by the change
The change in accommodation level also effects angle of deviation so if accommodation is relaxed in an Exo we can see how the measurements change when we INDUCE accommodation
BUT, in an Eso where accommodation is too much we can see how the measurements change when we relax accommodation

Question 5

When calculating AC/A what must we be aware of?

Answer 5

Tonic and proximal convergence

Question 6

What is the Gradient Method?

Answer 6

A method of calculating AC/A ratio - changes the amount of accommodation by using convex and concave lenses. It’s the most accurate method as it excludes both proximal and tonic convergence

Question 7

What is the Heterophoria Method?

Answer 7

A method of calculating AC/A - change the amount of accommodation by changing the distance of the fixation target

Question 8

How does the gradient method work with minus (concave) or plus (convex) lenses?

Answer 8

Minus (concave) at 6m (-3D)
Plus (convex) at 1/3m (+3D)

Question 9

How do you use the gradient method to work out a minus lens at 6m?

Answer 9

(Measurement with induced accommodation - Measurement without accommodation) / Change in accommodation

REMEMBER that no matter the sign of the trial frames Eso is always POSITIVE and Exo is always NEGATIVE

Question 10

How do you use the gradient method to work out a plus lens at 1/3m?

Answer 10

(Measurement on accommodation - measurement with relaxed accommodation) / Change in accommodation

REMEMBER that no matter the sign of the trial frames Eso is always POSITIVE and Exo is always NEGATIVE

Question 11

What’s the gradient method formula for convex and concave lenses?

Answer 11

(Measurement with accommodation - Measurement without accommodation) / Change in accommodation (the lens amount)

REMEMBER that no matter the sign of the trial frames Eso is always POSITIVE and Exo is always NEGATIVE

Question 12

What’s the method for the gradient methods?

Answer 12

• Use trial frames with lenses over either eye
• Perform with glasses if worn
• Use detailed accommodation target
• Allow time for accommodation to take place & ensure the patient can see the target clearly
• Maintain dissociation (occlusion) throughout the PCT

REMEMBER that no matter the sign of the trial frames Eso is always POSITIVE and Exo is always NEGATIVE

Question 13

PCT
6m: 45BI XT
6m c/ -3DS: 30BI XT

What’s the AC/A ratio using the gradient method?

Answer 13

(Measurement on accommodation - Measurement without accommodation) / Change in accommodation

(-30) - (-45) / 3 = -5
AC/A - 5:1

Used negative values because both are EXO deviations

Question 14

PCT
1/3m: 18BO ET
1/3m c/ +3DS: 3BO E

What’s the AC/A ratio using the gradient method?

Answer 14

(Measurement on accommodation - Measurement with relaxed accommodation) / Change in accommodation

(18) - (3) / 3
AC/A - 5:1

Used positive values because both are ESO’s

Question 15

PCT
1/3m: 3BO E
1/3m c/ +3DS : 6BI X

Answer 15

Need to be aware that one is an eso (+ sign) & one is exo (- sign) so:

(Measurement on accommodation - Measurement without accommodation) / Changes in accommodation

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

Question 16

How do you conduct the gradient method on the Synoptophore?

Answer 16

The patient isn’t accommodating on a synoptophore as it’s a distance test so can only do the 6m formula:

(Measurement on accommodation - Measurement without accommodation) / Change in accommodation

• Test with any refractive correction
• Correct IPD
• Use foveal simultaneous perception slides
• Measure objective angle in PD
• Put -3DS lenses in the lens holders of trial frames
• Ask if the picture is clear as if it’s blurred then decrease lens strength like to reduce to -2DS
• Re-measure objectively
• Use distance test 6m with the minus lens

Question 17

What’s the formula for the Heterophoria method?

Answer 17

AC/A = IPD + ((n - d) / D)

IPD = Interpupillary Distance
n = PCT results at 33cm
d = PCT results at 6m
D = Accommodation in D which is calculated by 1/focal distance

IPD is added to allow for fixation movement created by change in fixation distance

Question 18

What’s the method of the Heterophoria method?

Answer 18

• Refractive correction if worn
• PCT measured at near & distance
• Measure IPD
• Measure AC/A using the above

Question 19

What does the Heterophoria method not eliminate?

Answer 19

Doesn’t eliminate proximal convergence and there’s a change in fixation distance

Question 20

Why is AC/A important in Eso?

Answer 20

A normal AC/A ratio is 4:1 so for every 1 dioptre of accommodation we have 4PD of accommodative convergence

A high AC/A ratio may be 8:1 which would cause an Eso to get bigger, hypermetropic/convex/+ve will help control it as it relaxes the accommodation

E.g. Convergence Excess ET
1/3m: Near ET when looking at target
6m: Esophoria
= but need more convex at near to relax accommodation but distance so need bifocals

Question 21

Why is AC/A important in Exo?

Answer 21

If someone has an intermittent XT we want to know if it’s “real” or if due to a high AC/A ratio:
True would have near X and distance XT

A stimulated XT would be both a near XT and distance XT (so due a high AC/A ratio they could be controlling it at near)

Question 22

What’s the accommodation in dioptres for a 24, 32, 40, 48 & 56 yo?

Answer 22

24 - 11.3
32 - 8.9
40 - 6.5
48 - 4.1
46 - 1.7

(Goes down 2.4 per 8 years)

Question 23

What is near point of accommodation?

Answer 23

Closest point at which an object can be clearly seen

Question 24

What is far point of accommodation?

Answer 24

Furthest point at which an object can be seen clearly

Question 25

What is range of accommodation?

Answer 25

Distance between near and far point