AC/A ratio (Accommodative convergence/ Accommodation ratio)

Accommodative convergence/ Accommodation ratio also known as AC/A ratio, is a measurement of changes in accommodative convergence in prism diopters induced when the patient exert or relax 1 diopter of accommodation.

Changes in the accommodation are either evoked by placing plus lens which relaxes accommodation or by minus which activate accommodation or when a person fixates near the target.

The assessment of the AC/A ratio is significantly important. AC/A ratio plays a crucial role in reaching a final diagnosis and is an important factor to consider in determining the appropriate treatment approach. E.g., in a High AC/A ratio patients respond well to lens treatment whereas, in the case of a normal or low AC/A ratio, the appropriate treatment approach should be prism and/or vision therapy.

There are two methods used clinically to determine the AC/A ratio

• Calculated heterophoria method
• Gradient method

Calculated Heterophoria method

Calculated heterophoria method is based on the difference in deviation between distance phoria & near phoria and this method takes account of patient Interpupillary distance (IPD), which is an important factor if accommodative changes are induced by placing the target at near. As convergence require for a patient with a larger IPD is more compared to a patient with a small IPD, when the target is placed 40cm far away from the eyes. AC/A ratio is determined by using the formula:

• Where IPD is intra pupillary distance in centimeters.
• NFD is Near fixation distance in meters.
• Hn is Heterophoria at Near in prism diopters.
• Hd is Heterophoria at distance in prism diopters.
• Note: Esophoria is plus, Exophoria is minus.

For example:

Calculate AC/A ratio, Patient IPD is 70 mm, Distance deviation is 2 PD esophoria and Near deviation is 6 PD esophoria at 40 cm.

IPD + NFD (Hn – Hd)

7 + 0.4 [6 – 2]

7 + 0.4 (4)

AC/A ratio = 8.6: 1

Procedure of Calculated heterophoria method

• Make sure the patient is wearing their appropriate correction.
• Measure the patient binocular Interpupillary distance
• Ask the patient to look at a 6m faraway target and measure the patient’s distance phoria.
• Ask the patient to look at the near target (usually 40 cm) and measure the patient near phoria.
• Use the formula to calculate the AC/A ratio

Gradient method

The gradient AC/A measures the amount of convergence generated by a dioptre of accommodative effort. Changes in the accommodation are induced by placing plus lens which relaxes accommodation or by a minus which activates accommodation. AC/A ratio is determined by placing +1.00 DS or +2.00 DS in front of each eyes. AC/A ratio will be given by:

Note: Esophoria is plus, Exophoria is minus.

For example,

Baseline phoria is 2 exophoria, with additional 2D plus lenses 12 exophoria.

AC/A ratio = [12 – (2)] / 2 = 5

AC/A ratio = 5 : 1

Procedure of Gradient method

• Make sure the patient is wearing their appropriate correction.
• Measure horizontal baseline near phoria without any additional lenses.
• Add +1.00 DS or +2.00 DS lenses in front of both eyes and measure again horizontal near phoria with additional minus lenses. Calculate the AC/A ratio
• Calculate AC/A using formula mentioned above

Kamal Thakur

Lecturer (Nethradhama School of Optometry)

Moptom