AC/A and CA/C ratios

What is AC/A ratio?

the change in acconodative convergence over the change in accommodation

How do we measure AC/A ratios in clinic?

gradient AC/A ratio

Why does the patient have to be disocciated inorder to get gradient AC/C ratio?

to prevent changes in disparity vergence once accommodative convergence has happened

Why does the target have to stay at the same distance throught out AC/A testing?

to keep proximal vergence constant

Why is it called the gradient AC/A ratio

because you are calculating the slope of their AC/A

If you have a large variation between the values what should you do when testing AC/A ratio?

try testing through +1.00 and +2.00 instead

How many data points do you need to calculate AC/A ratio?

3 at least but more is better

How do you determine calculated AC/A ratio?

compare the near and distance phoria:

change in vergence response

---------------------------------

change in accommodative stimulus

When measuring a calculated AC/A ratio what is taken into consideration?

both the accommodative vergence and the proximal vergence

How can you estimate the proximal vergence response?

calculated AC/A- gradient AC/A

What measurements do you need in order to get the calculated AC/A ratio?

PD and near and distance phoria

What is the true stimulus to accommodative convergence?

the accommodative response

If were to measure the response AC/A ratio how woud we do it?

we would figure out the accommodative response with dynamic retinoscopy and then measure the phoria

Can you measure an AC/A ratio for presbyopes?

NO they do not have any accommodation to measure

Do presbyopes have an AC/A ratio?

Yes you just cannot measure it

When measuring the AC/A response it is important to ask the patient what?

if the target is clear

-if the target is not clear the patient is most likely not converged properly and the ratio will not be accurate

What is the albern approximation?

estimation to calculate the response AC/A from the stimulus AC/A

response AC/A=stimulus AC/A (1.08)

Approximately how much bigger is the response AC/A than the stimulus AC/A?

about 10%

Why is determining an AC/A ratio important clinically?

-help determine an appropriate add for a patient with a phoria

-determine if patient is ok to read without their glasses or not

What is a formula to determine required add from the AC/A ratio?

required add= phoria/ (AC/A ratio)

What type of patient with a phoria would we prescribe an add for?

patient with a high AC/A ratio

What type of patient with a phoria would we prescribe prism for?

patient with a low AC/A ratio

-a patient where the add would have to be too high to make sense to prescribe to treat a phoria

For a patient with an eso deviation what type of add would you prescribe?

plus

Fow a patient with an exo deviation what type of add would you theoretically prescribe?

minus (but most likely over minus the patient)

Are low myopes with a higher or lower AC/A going to be more comfortable taking off their glasses to read?

low AC/A ratio

What target do we use for CA/C ratio?

Difference of Gaussian grating (DOG) target

Why do we use DOG grating as target for CA/A ratio?

it is a good stimulus to vergence and poor stimulus to blur driven accommodation

What is the formula for CA/C ratio?

change in AR/ change in VR

What is the expected result for CA/C ratio?

0.40D/MA (or 0.40D/6 prism diopters)

When do we use CA/C ratio?

a person with uncorrected refractive error appears to have a tropia

-once we correct their refractive error they no longer have the tropia