Prentice’s Rule states: The power of the prism is equal to the power of the lens in diopters times the amount of decentration in millimeters divided by 10. dec = decentration or distance in mm away from the optical center of the lens.

Piaget coined the term “precausal thinking” to describe the way in which preoperational children use their own existing ideas or views, like in egocentrism, to explain cause-and-effect relationships.

The formula used to calculate the amount of prism is called Prentice’s Rule. The formula for Prentice’s Rule is: Prism (diopters) = Power (diopters) X Decentration (centimeters). Prentice’s Rule can be used to create prism in a lens.

Determine the prism direction by checking the relationship of the bridge to the OC. If the OC is ground nasally on the 180 degree line, and if you are verifying the OD lens, then the direction of the prism is base in (BI).

The formula: Prism needed = 2/3(phoria) – 1/3(compensating fusional vergence). So, if a patient has 6∆ exophoria and base-out (BO) to blur is 6∆, the prism needed would be 2/3(6) – 1/3(6), or 4 – 2. You would prescribe 2∆ base-in (BI), since deviation is exophoria. ~Percival Criterion.

Induced Prism If the lens power is sufficient, to induce the prescribed prism, the lens can simply be cut off-center to achieve the required results. This is known as prism by decentration. If the power is insufficient, however, the prism must be cut into the surface of the lens.

Normally, when prism is ordered horizontally, both lenses are base in or base out. However, when prism is ordered vertically, one lens is base up, and the other, base down.

What is slab-off? It is a special lens grinding technique used to neutralize the unwanted prism effect when looking down through the reading area (usually a bifocal) of widely differing lens powers. Slab off insures that the near image will not be displaced.

Esophoria or esotropia: An inward (toward the nose) turned eye or inward “seeing” eye. Base-out prism is used to correct for this and can be all in one eye or split between the two eyes. If the prism is split between both eyes, the orientation for the prism will still be base-out for both eyes.

Eye care practitioners who are prescribing prism, and do not want it to be split, need to annotate the prescription accordingly. Prism is split equally, or unequally, depending upon the equality or otherwise of the ametropia between the two eyes.

Once properly diagnosed, exophoria can be treated and corrected. It usually takes several months of regular treatment or exercises to correct exophoria. Most treatments are done at home, so it’s important that you do your exercises regularly as prescribed by your doctor.

Exophoria is a condition in which your eyes drift outward out of your control. It usually appears for a short time while you’re doing certain types of tasks. It’s not a serious condition and can be corrected with the right treatment.

The service-connected post-operative congenital alternating strabismus (also called exotropia) is currently rated as 30 percent disabling under 38 C.F.R. § 4.84, Diagnostic Code 6090 for diplopia.

By about 4 months of age, the eyes should be aligned and able to focus. If you notice misalignment after this point, have it checked out by an eye doctor. Experts note that untreated exotropia tends to get worse over time and will rarely spontaneously improve.

Generally, exotropia progresses in frequency and duration. As the disorder progresses, the eyes start to turn out when looking at close objects as well as those in the distance. If left untreated, the eye may turn out continually, causing a loss of binocular vision or stereopsis.

How is exotropia treated? Non-surgical treatment may include glasses and in some instances, patching therapy may be recommended. If the eyes are misaligned more often than they are straight, surgery on the eye muscles may be recommended in order to realign the eyes.

Although the determinants of increased near disparity in intermittent exotropia have yet to be defined, this form of strabismus appears to be a “soft” sign of neurological disease in children should prompt a search for other signs of neurological disease.

Families are usually concordant for either esotropia or exotropia, but families with both forms have been reported. This finding may reflect the presence of 2 relatively common genes or 1 gene with variable expressivity.

Vision Therapy — strabismus treatment without surgery; with or without corrective lenses — is the most effective and non-invasive treatment for Strabismus. In a Vision Therapy program, eye exercises, lenses, and/or other therapy activities are used to treat the brain and nervous system which control the eye muscles.