The Prism Adaptation Study (PAS)



National Eye Institute (NEI)

Brief Summary

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.

Detailed Description

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.

Overall Status


Start Date


Completion Date


Primary Completion Date



Phase 3

Study Type




Intervention Type


Intervention Name



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.



Minimum Age

3 Years

Maximum Age


Verification Date


Lastchanged Date


Firstreceived Date



Has Expanded Access


Condition Browse

Firstreceived Results Date




Efficacy of prism adaptation in the surgical management of acquired esotropia. Prism Adaptation Study Research Group. Arch Ophthalmol. 1990 Sep;108(9):1248-56.




Repka MX, Connett JE, Scott WE. The one-year surgical outcome after prism adaptation for the management of acquired esotropia. Ophthalmology. 1996 Jun;103(6):922-8.



Firstreceived Results Disposition Date


Study Design Info



Primary Purpose


Study First Submitted

September 23, 1999

Study First Submitted Qc

September 23, 1999

Study First Posted

September 24, 1999

Last Update Submitted

September 16, 2009

Last Update Submitted Qc

September 16, 2009

Last Update Posted

September 17, 2009 processed this data on August 24, 2018


Conditions usually refer to a disease, disorder, syndrome, illness, or injury. In, conditions include any health issue worth studying, such as lifespan, quality of life, health risks, etc.

Interventions refer to the drug, vaccine, procedure, device, or other potential treatment being studied. Interventions can also include less intrusive possibilities such as surveys, education, and interviews.
Study Phase

Most clinical trials are designated as phase 1, 2, 3, or 4, based on the type of questions that study is seeking to answer:

In Phase 1 (Phase I) clinical trials, researchers test a new drug or treatment in a small group of people (20-80) for the first time to evaluate its safety, determine a safe dosage range, and identify side effects.

In Phase 2 (Phase II) clinical trials, the study drug or treatment is given to a larger group of people (100-300) to see if it is effective and to further evaluate its safety.

In Phase 3 (Phase III) clinical trials, the study drug or treatment is given to large groups of people (1,000-3,000) to confirm its effectiveness, monitor side effects, compare it to commonly used treatments, and collect information that will allow the drug or treatment to be used safely.

In Phase 4 (Phase IV) clinical trials, post marketing studies delineate additional information including the drug's risks, benefits, and optimal use.

These phases are defined by the Food and Drug Administration in the Code of Federal Regulations.

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