AC/A ratio

AC/A ratio 

Ratio of the accommodative convergence AC (in prism dioptres) to the stimulus to accommodation A (in dioptres). The most common method of determining this ratio is by the gradient method (or gradient test) in which the phoria at near is measured after changing the accommodation with a spherical lens (usually +1.00 D or −1.00 D) placed in front of the two eyes. It is expressed as

AC/A = (α − α′)/F

where α is the phoria at near, and α′ is the phoria at the same distance but through a lens of power F. The deviation is measured in prism dioptres, with + for esodeviation and − for exodeviation. Example: if the initial phoria is 4 Χ exo and 8 Χ exo when a lens of +1.00 D is placed in front of the eyes, the AC/A ratio is equal to

[−4 − (−8)]/1 = 4 Χ/D

The average AC/A ratio is about 4 in young adults and tends to decline slightly with age. The gradient is not affected by proximal convergence, as the target distance and size are relatively constant. Syn. gradient.Another method of determining the AC/A ratio (often called the heterophoria method) compares the phoria measured at distance and at near. It is expressed as

AC/A = PD + ((N − D)/K)

where PD is the interpupillary distance in cm, N the deviation at near, D the deviation at distance and K the near fixation distance in dioptres. Example: A patient has a PD of 70 mm, a distance phoria of 4 Χ eso and a near phoria of 8 Χ exo at 33.3 cm from the eyes, the AC/A ratio is equal to

7.0 + ([−8 − (+4)]/3) = 3 Χ/D

See accommodative convergence; proximal convergence; prism dioptre.