a. Calculate the AC/A ratio (accommodative convergence / 
    accommodation ratio) using the gradient method and the 
    following values (taken from an emmetropic patient):
 
    Alternating cover test 

for distance (6 metre)       = 10 dioptre esophoria
for distance with -3.00DS = 25 dioptre esophoria

b. A 6 year-old boy with 25 prism dioptres esotropia has a 
    + 4.00 DS cycloplegic refraction. He has a AC/A ratio of 6.
    What is the result of prescribing hypermetropic glasses?

 

Answer to a
AC/A = 5
Using the gradient method, AC/A ratio is given as
(Deviation without lens - Deviation with lens)/ Lens in dioptres
= (25 - 10) / 3
= 5
 

Answer to b
Significant reduction of esotropia.
The patient has +4.00DS hypermetropia and a AC/A
ratio of 6 (ie. 6 dioptre prism of esotropia with one
dioptre of accommodation). Therefore, prescribing the 
+4.00DS will reduce the accommodative convergence
by 4 X 6 = 24 prism dioptre. 
The residual esotropia = 25 -24 = 1 prism dioptre esotropia 
which is insignificant.

 

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