Two Criteria For Determining Prisms

When prism is the only reliable and feasible treatment for a symptomatic heterophoric patient, you may apply these two methods.

Sheard’s Criterion

This states that for single clear binocular vision, the compensating fusional reserve should be at least twice the demand (phoria).

Mathematically, prism needed to satisfy sheard’s criterion =2(phoria) – compensating fusional reserve/3

Prism is only prescribed if the value gotten is in plus sign but if it is in minus sign, then prism is not needed.

Take note of the following;

For exophoria, base-out to blur finding (16A)

is used to compensate but the base direction is base-in (Apex towards base direction).

For esophoria, base-in to blur finding (17A) is used to compensate but the base direction is base-out.

Exercise1: find the prism needed to satisfy sheard’s criterion with the following findings; #16A = 12prisms, Phoria = 9prisms exophoria at near.

From the formula above, prism needed = 2(9) – 12/3  = 2prisms base-in.

Exercise2: #17A = 8prisms, Phoria = 3prisms esophoria at near.

Prism needed to satisfy sheard’s criterion = 2(3) – 8/3  = -2/3

No prism is needed since the value is in minus sign.

NOTE: in cases were lens modification is required to satisfy sheard’s criterion, the following should be carried out.

1. Divide the prism needed to satisfy sheard’s criterion with the AC/A ratio.
2. Add the value gotten to the spherical component of the prescription if the prism is in base-out direction.
3. If the prism is in base-in direction, subtract the value from the spherical component.

Exercise3: a patient is found to have 10prisms exophoria at 40cm, base-in finding of 18/28/12 and base-out finding of 10/16/8 with AC/A ratio of 3/1.

How much prism and in what direction is needed to satisfy sheard’s criterion?

If patient prescription is +1.50/-0.50 x 180, what is the lens alteration needed to satisfy sheard’s criterion.

Prism needed = 2(10) – 10/3 = 3.25prism base-in.

10/3 x 1/3 = +1.11.  Which is approximately = +1.00 or + 1.25

Subtract the above value from the sphere since it is in base-in direction.

+1.50 - 1.00 = +0.50 OR +1.50 – 1.25 = +0.25

The new prescription will now be;

+0.50/-0.50 x 180 OR +0.25/-0.50 x 180.

Exercise4: A patient is found to have 5prisms esophoria at 40cm, base-in finding of 5/16/10 and base-in finding of 13/24/10 with AC/A ratio of 4/1.

How much prism and in what direction is needed to satisfy sheard’s criterion.

What is the lens modification needed to satisfy sheard’s criterion if patients prescription is +1.75D?

Prism needed = 2(5) – 5/3 = 1.75prism base-out

5/3 x 1/4 = +0.45. Which is approximately = +0.50 OR +0.25.

Add the value to the sphere since it is in base-out direction.

i.e +1.75 + 0.50 = +2.25D OR +1.75 + 0.25 = +2.00D

Percival’s criterion

This states that the demand (phoria) should fall in the middle third of the total range of fusional reserve (PRC and NRC).

For example if PRC is 14prisms and NRC is 10prism and fixation were at 40cm, the fixation point would fall in the middle third as shown below;

14,13,12,11,10,9,8,7/6,5,4,3,2,1,0,1,2/3,4,5,6,7,8,9,10. The fixation point zero falls at the middle third, hence percival’s criterion is satisfied.

Prism needed to satisfy percival’s criterion = G – 2L/3

G = greater reserve (PRC or NRC).

L = lesser reserve (PRC or NRC).

Prism is prescribed if the value is in plus sign but if the value is in minus sign prism is not needed.

In cases where lens modification is needed to satisfy percival’s criterion, then carryout the following;

1. Divide the prism needed to satisfy percival’s criterion with the AC/A ratio.
2. Add the value to the sphere if the correcting prism is in base-out direction.
3. If the base direction is in base-in, subtract the value from the sphere.

Exercise1: A patient has 6prism exophoria, base-out finding of 6/10/8 and base-in finding of 21/26/22. Find the prism needed to satisfy percival’s criterion.

Prism needed = 21 – 2(6)/3 = 3prisms base-in.

Exercise2: A patient has 6prism exophoria with base-out finding of 18/24/10 and base-in finding of 21/16/10. Find the prism needed to satisfy percival’s criterion.

Prism needed = 18 – 2(12)/3 = -2.00. Prism is not needed since the value is in minus sign.

Exercise3: A patient is found to be 5prism esophoria at 40cm with base-out finding of 19/24/20 and base-in finding of 5/16/10. AC/A ratio is 4/1.

 How much prism and in what direction is needed to satisfy percival’s criterion?

What is the lens alteration needed to satisfy percival’s criterion if patients prescription is +1.75/-0.50 x 90.

Prism needed = 19 – 2(5)/3 = 3prisms base-out.

3 x 1/4 = +1.75D.

 Add the value to the sphere since the correcting prism is in base-out direction.

+1.75 + 0.75 = +2.50.

Final prescription will now be:

+2.50/-0.50 x 90