Prism Adaptation Study (PAS), The

• To determine whether the preoperative use of prisms in eyeglasses can improve the outcome of surgery for acquired esotropia, a type of strabismus.
• To determine whether patients who respond to prism adaptation by developing a new stable angle of -deviation have a better surgical result than do patients who do not respond to prism adaptation.
• To determine whether patients who respond to prism adaptation are more accurately corrected by operating for the prism-adapted angle or the original angle of deviation.
• To determine the usefulness of certain input variables (e.g., age at the time of surgery, size of the deviation, visual acuity, binocular function, refractive error) in predicting which patients are more likely to benefit from prism adaptation.

Acquired esotropia (crossed eyes that develop after a child reaches the age of 6 months) accounts for 25 percent of all patients with misaligned eyes. Surgery to correct esotropia is done primarily to attain functional use of the two eyes together. The cosmetic aspect of the surgery is secondary. In 40 to 50 percent of cases, more than one operation is needed to accomplish the primary goal, and in some cases even three and four operations are needed.

Preliminary studies from two eye care centers reported that the use of prisms on eyeglasses for about a month before surgery led to good results after a single operation in more than 90 percent of patients. These uncontrolled preliminary studies pointed to the need for a multicenter, randomized, controlled clinical trial designed to prove or disprove scientifically the beneficial effect of prisms.

The Prism Adaptation Study was a double randomization trial involving 286 patients. Three-fifths of the patients were randomly selected for prism adaptation before surgery. Of the patients who responded to the prisms, one-half were randomly selected to have surgery based on the amount of prism required to stabilize the deviation, and the other half had surgery based on the amount of esotropia originally measured. Patients who did not respond to the prisms also had surgery based on the amount of esotropia measured, as did the two-fifths of the patients who did not undergo prism adaptation.

Patients were examined postoperatively at 1 week, 1 month, 3 months, 6 months, and 1 year. An independent examiner, masked to the treatment assignment, evaluated the patient at the 6-month followup. The results were analyzed to determine whether the outcome was better in patients who underwent prism adaptation or in those who underwent conventional treatment. Because the examiner did not know what type of treatment a patient had received, he or she would have no bias in evaluating the results.

An eligible male or female must have been age 3 years or older (adults were included) and must have had esotropia that occurred at age 6 months or older, with no history of previous eye muscle surgery.

No longer recruiting. Comments: Recruitment began in March 1984 and ended in May 1989.

Completed, with results published. Comments: Completed.

Prism adaptation prior to surgery for acquired esotropia (crossed eyes) better aligns eyes, thus decreasing the possible need for additional surgery. Approximately 83 percent of patients who underwent prism adaptation prior to surgery achieved successful eye alignment following surgery. This compares with approximately 72 percent for patients who did not have prism adaptation prior to surgery. This difference was statistically significant.

The medial rectus muscles, which serve to keep the eyes aligned, work on a length-tension basis, and the closer to the front of the eye, the more the muscles will pull the eye inward. During the surgery, the muscles are detached from the eye and moved back a certain distance based on the amount of misalignment. This lessens the amount of tension to move the eyes inward. Prism adaptation helps determine the proper distance to move the muscles.

During prism adaptation, Fresnel lenses (thin plastic sheets with tiny ridges) are placed on the patient’s glasses. These ridged sheets are prisms that bend light rays, straightening the image the brain receives. This makes it possible for both eyes to see the same thing at the same time as soon as the prisms are in place. These prisms do not affect the wearer’s vision except possibly for a slight amount of blurring.

After patients have worn the prisms for a week or two, their eyes are re-examined to determine whether they are working together. If not, the power of the prisms may be changed. Several adjustments of the prisms sometimes are necessary to achieve just the right strength for a single image with the eyes.

By knowing how much prism it takes to allow the eyes to work together, ophthalmologists may be able to more accurately judge how much surgical adjustment of the eye muscles will be necessary.

The prism adaptation process appears to result in a higher percentage of patients with well-aligned eyes after surgery and a lower rate of re-operation.