How To Determine The AC/A Ratio Of A Patient

AC/A ratio is the accommodative-convergence that follows one diopter of accommodation. The normal AC/A ratio is 4/1 which means; for every +1.00 of accommodation, 4prisms of accommodative-convergence is being exerted. However, the normal value of AC/A ratio also ranges from 4/1 to 6/1.

The function of accommodation is to obtain a clearer focus while that of convergence is to obtain a single binocular vision. Accommodation is able to stimulate a definite amount of convergence (Accommodative convergence). If the influence of accommodation is excessive, it may produce a manifest deviation (esotropia) whereas if the influence is low (low AC/A ratio) and fusional convergence is inadequate, the individual may have exotropia.

When AC/A ratio is high (above 6/1), hyperopic patients should be given full correction while myopic patients should be under-corrected. If AC/A ratio is low (below 4/1), under-correct hyperopic patients and give myopic patients their full correction. This (low AC/A) presents problem if PFC is inadequate.

METHODS OF DETERMINATION

Calculation Method

AC/A ratio = 15 - Physiological exophoria/2.50

Where 2.50D = Accommodation at 40cm (100/40 = 2.50D).

15prisms = convergence at 40cm.

15prisms is gotten by multiplying the patients IPD (6.4cm) with the dioptric equivalent of distance from point of fixation to the center of rotation (i.e 40cm + 2.7cm = 42.7cm).

100/42.7 x 6.4 = 14.99prisms which is approximately 15prisms.

Physiological exophoria = induced phoria at near (#13B) minus induced phoria at far (#8). If the phorias are both in exodeviation, use the above formula but if the phorias are both in esodeviation, assign a plus sign instead of minus. If they are of different deviations, assign plus sign to the esodeviation and minus to the exodeviation.

 

Gradient Method

AC/A ratio = P – P’/F.

Where P = phoria at near.

P’ = Phoria at near through +1.00D lens power (F).

F = +1.00D lens power.

NOTE: esodeviation carries plus sign while exodeviation carries minus sign.

Heterophoria Method

AC/A ratio = PD + (N-D)/K.

Where PD = Interpupillary distance in centimeter.

N = Deviation at near.

D = Deviation at distance.

K = Near fixation distance in diopters.

NOTE: Eso carries plus sign while exo carries minus sign.

Relative Accommodation Method

AC/A ratio = Total accommodative range/Phoria range.

Where total accommodative range = PRA – NRA.

Phoria range = phoria with PRA lens in place minus phoria with NRA lens in place.

Therefore AC/A ratio = PRA – NRA/PRA deviation – NRA deviation.

Exercise1: A patient the following findings;

Phoria through subjective lenses at 6m (#8) = 1.00prism exophoria.

Phoria at 40cm through subjective lenses (#13) = 5prisms exophoria.

Phoria at 40cm through subjective lenses combined with +1.00D = 8prisms exophoria.

What is the calculated AC/A ratio?

What is the gradient AC/A ratio?

Calc. AC/A ratio = 15 – (5-1)/2.50 = 4.4/1.

Gradient AC/A ratio = 8 – 5/1 = 3/1.

Exercise2: Suppose a patient is orthophoric at 40cm with 10prisms exophoria at 6m, what is the calculated AC/A ratio and what will be the effect of plus and minus lens on that individual.

AC/A ratio = 15 – 10/2.50 = 2/1.

Plus lens will increase the exophoria and AC/A ratio while minus lens will decrease the exophoria and AC/A ratio.

Exercise3: A patient has the following findings:

Phoria at 6m through subjective lens (#8) = 2prisms esophoria.

Phoria at 40cm through subjective lens (#13) = 7prisms esophoria.

Phoria at 40cm through subjective lens combined with +1.00D = 1.00prism esophoria.

What is the calculated and gradient AC/A ratio?

Calc. AC/A ratio = 15 + (7 - 2)/2.50 = 8/1.

Gradient AC/A ratio = 7 – 1/1 = 6/1.

Exercise4: If the initial phoria is 4prism exophoria and 8prism exophoria when a lens of +1.00D is placed in front of the eye. What is the gradient AC/A ratio?

AC/A ratio = -4 + 8/1 = 4/1.

Exercise5: A patient has an IPD of 70mm, a distance phoria of 4prisms esophoria and a near phoria of 8prisms esophoria at 33.3cm from the eyes.

AC/A = 7 + (-8 – (+4))/3 = 3/1.