Prescribing Prism

As soon as the cover/uncover test identifies an esophoria, the next step is to obtain an accurate cycloplegic refraction because many cases of esophoria are “accommodative,” due to uncorrected or undercorrected hyperopia. It is wise to perform the cover/uncover test for esophoria at both distance and near, for the deviation may be larger at one distance than the other.

After refracting the patient and ensuring that there is no uncorrected hyperopia, one may wish to try a small amount of plus sphere to decrease the accommodative demand, as some cases of esophoria are due to accommodative excess. Such intervention is often enough to treat the condition, but base-out prism may be necessary if refractive methods fail. In situations that require the use of prisms, base-out prism should be prescribed with only the minimum amount of power required to eliminate the symptoms.

Again, careful refraction of the patient can help the management of many cases of exophoria. With refractive correction in place, if any, cover tests should be performed, and accommodation should be evaluated by push-up measurement of accommodative amplitude, or, especially in children, by dynamic retinoscopy. Divergence excess (in contrast to convergence insufficiency) manifests as an increased angle of exophoria in the distance.

In exophoria, correcting both myopia and hyperopia can help improve symptoms, but additional cautions should be taken when correcting hyperopia, as full correction of hyperopia may worsen the symptoms. A several-minute test with hyperopic correction should be attempted to see if exophoric symptoms improve because of clearer imagery or worsen by relaxing accommodation. If they worsen, prescribe the largest correction possible to treat the hyperopia while avoiding exophoric symptoms. A good starting point is one-third of the spherical error. Just as plus lenses can be helpful for esophoria, decreased-power plus lenses or even minus lenses can improve exophoria.

Base-in prisms may also be helpful for the treatment of exophoria. As with esophoria, the least amount of prism that eliminates exophoric symptoms should be used. The cover test should provide an estimate of the power of the prism to be used. Additionally, treating a minor hyperphoria with vertical prisms can allow the patient to compensate for exophoria with no need for horizontal prisms (see below). If divergence excess is found to be the cause of the exophoria, prisms should be avoided. In this situation, base-in prism can cause esophoria at near, which patients do not tolerate well.

Both the measurement and treatment for hyperphoria are similar to those for the horizontal heterophorias. One important point regarding hyperphorias is that they often coexist with horizontal heterophorias, and the treatment of one may improve or eliminate the other condition. Therefore if either a horizontal heterophoria or a vertical heterophoria is found, it is important to investigate whether the other is present. Initial attempts at treatment should be focused on the primary phoria with the intention of treating both.

Often when patients with anisometropia receive a new pair of glasses, they will complain of double vision, particularly while reading. This double vision is due to the differential prismatic effects of the two lenses when the patient is looking off-center as when reading (as per Prentice’s Rule). In order to improve reading vision, vertical prism can be incorporated into the lower portion of one lens or the other to help compensate for the differential vertical prismatic effect and lessen or eliminate the double vision. These prisms are referred to as slab-off or reverse-slab prisms. The slab-off prism is placed on the more minus or less plus lens, more commonly on glass lenses, and in effect takes away base-down prism (adds base-up prism). The reverse-slab prism is placed on the more plus or less minus lens, most commonly on molded plastic lenses, and adds base-down prism.

Rather than a calculation of the amount of slab-off or reverse-slab prism to prescribe, trial-and-error measurements are preferred because one does not know how much the patient has already compensated to previous anisometropic glasses. Increasing the amount of prism handheld over the lower portion of one lens of the anisometropic correction until the patient can read comfortably is the most reliable way to determine the amount of slab-off or reverse-slab prism to prescribe. Up to 4^∆ to 6^∆ of slab-off or reverse-slab effect can be obtained when needed.

The position in which the prism is held can be critical when measuring a patient’s deviation. There are three intended positions for holding prisms: the Prentice position, the minimum deviation position, and the frontal plane position (Figure 6). Glass prisms are calibrated for the Prentice position and should be held in this manner when making measurements. Plastic prisms, including plastic prism bars, are generally calibrated for the minimum deviation position. As it is difficult to estimate accurately the minimum deviation position, the frontal plane position is a good approximation when using plastic prisms.

Stacking two prisms in the same direction, especially if one is of high power, can also lead to errors because of the same positioning issues mentioned above. If the two prisms are held in contact with each other, even if the first prism is held in the correct position, the second prism will not be in the correct position in relation to the light leaving the first prism. This will create a stronger prism effect than the sum of the two prisms, leading to a falsely low measurement. If the sum of the two prisms is prescribed, or surgery is based on the prism measurement, the patient will be left undercorrected. It is difficult to estimate accurately the correct position of the second prism in relation to the first, and it is therefore more accurate to simply stack the prisms together and use a conversion table to add their true effects.

One might assume that one way to avoid the difficulties of stacking two prisms is to split the prisms between the two eyes and add the powers together. Although this may work for low prism powers, it becomes increasingly inaccurate with increasing prism power. Splitting prisms is preferred over stacking two prisms together in the same direction before one eye, but again using a table of summed prism values is preferred to avoid errors.

There is a simple method to calculate oblique prism from combining a horizontal prism with a vertical prism that does not require trigonometric calculations and requires only a piece of paper, a ruler, a pen or pencil, and the protractor on a phoropter or trial frame.

First, sketch the vector addition of the two prism powers on polar coordinates to determine the approximate angle for the base direction of the oblique prism. Up is base-up, and right is base-in or base-out depending on which eye the prism is intended for.

Then measure off the number of prism diopters of the two component prisms on two adjacent edges of a piece of paper. (One can use any unit for this measurement, but the unit must be consistent. For example, if you have two prisms, one of 3^∆ and the second 2^∆, measure off 3 cm/inches/etc on one edge and 2 cm/inches/etc on the other edge). Connect the two measurement marks with a line, forming a triangle. Now measure the length of the connecting line you just drew, the hypotenuse of the triangle. This will give you the number of prism diopters for your oblique prism.

Then fold the paper along the hypotenuse, identify the acute angle of the triangle that you estimated using your polar coordinate plot, and measure this angle using the protractor on the phoropter or trial frame, giving you the base direction of the oblique prism.

For children with cranial nerve palsies, early treatment is important to prevent amblyopia. Prisms may be used for small, reasonably comitant deviations in order to maintain binocular function. A test with temporary plastic Fresnel prisms is recommended prior to grinding prisms into the child’s lenses. Treatment of a recognized underlying cause is essential, and if prism therapy fails, surgery should be considered.

For patients with no useful vision in the deviated eye, prisms can be extremely helpful in improving the appearance of the eyes and facilitating the patients’ communication and interaction with others by alleviating difficulties with eye contact. A prism held opposite the direction of a correcting prism can improve the apparent alignment of the eyes (ie, base-in for esotropia and base-out for exotropia). Expect approximately 1 mm of apparent eye shift for every 8^∆ of prism power.