prism adaptation test
Also called PAT, prism adaptation, preoperative prism adaptation, eating up the prism
A test that asks whether the eyes re-deviate behind a prism that neutralizes a deviation: the prism is worn for minutes to days and the deviation is measured again through it1,3. Adapting to prism is a normal response, since within two to three minutes a healthy binocular system adjusts its muscle tone and the phoria returns to its old value1.
The test serves two purposes that run in opposite directions. In heterophoria, adaptation means a relieving prism will not help, and the patients who adapt poorly are the ones with symptoms1,2. In planning strabismus surgery, some surgeons operate on the larger adapted angle4.
More detail
In strabismus with anomalous correspondence the eyes can absorb the prism and the angle grows, sometimes back to the original deviation, a response described as eating up the prism1,4. For a phoria, the proposed prism is worn in a trial frame for five minutes during a real visual task; if the cover test or fixation disparity through the prism is larger than the prism predicts, adaptation is occurring3. For surgical planning the prism is worn for one to fourteen days, and the angle increased in more than half of esotropes even with normal correspondence1. About 80 percent of people adapt to a small vertical prism within 15 minutes2.
Sources (4)
- Pickwell's Binocular Vision Anomalies, ch. 6 and ch. 14
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 14
- Eperjesi and Rundström, Practical Binocular Vision Assessment, ch. 7
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 13 and ch. 26
Reviewed October 2026