accommodative convergence

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ac·com·mo·da·tive con·ver·gence

the meter angle of convergence expressed in diopters; equal to the product of the meter angles of convergence multiplied by the interpupillary distance measured in centimeters.
Farlex Partner Medical Dictionary © Farlex 2012

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.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann
References in periodicals archive ?
Myopia has long been associated with inaccurate and insufficient accommodative behaviour at near and increased accommodative convergence in comparison to emmetropes (3-7)--detecting these conditions in both the at-risk emmetrope and myopic child can reveal the picture of myopia progression risk.
It is generally believed that in IXT patients, tonic convergence and accommodation through accommodative convergence contribute to maintain ocular alignment at near fixation [5].
Refractive accommodative esotropia (RAE) includes accommodative convergence, uncorrected hyperopia and inadequate fusional divergence.1-3
Of the subjects, 50 percent exhibited reduced values for relative accommodation (positive relative accommodation [PRA] and negative relative accommodation [NRA]) and 50 percent had an abnormal accommodative convergence to accommodation ratio.
We conclude that, in the context of the present study, accommodative convergence had a minimal impact on our results.
Around two thirds of the convergence total is said to be accommodative convergence, with fusionai convergence tidying up the final bits of the vergence response.
Retinal image blur induces accommodation to focus the image; then, this accommodative effort elicits accommodative convergence when fixating at distance or near.
As the amount of accommodative convergence varies with accommodation, a difference in the size of the deviation with and without accommodation can give useful clues to its nature.
Accommodative responses, including both monocular and binocular accommodative amplitude, accommodative convergence to accommodation ratio, and negative and positive relative accommodation, were reduced in the TBI population compared with normative values.
(19) As always when prescribing prism, care must be taken to balance the refractive and prismatic prescriptions with respect to the accommodative convergence: accommodation ratio.
Myopes tend to show more enhanced accommodative convergence (esophoria and elevated AC/ A ratios), along with insufficient accommodative responses (accommodative lag) compared to their emmetropic counterparts.
d Decreased accommodative convergence when using the reading spectacles