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The crossed prism test can be used for both heterophorias and heterotropias and in all the cardinal positions, but I shall discuss only its use in the primary position, for near vision, 13 inches (33 cm.), and for distant vision, 20 feet (610 cm.).
Of course, the muscles should be studied in all the cardinal positions for the proper diagnosis of any imbalance. The interpupillary distance for both near (13 inches) and distant (20 feet) vision is determined and the trial frame adjusted accordingly. The refraction for all patients under 40 years of age is done with a cycloplegic unless contraindicated.
With the proper correction in place to render the eyes emmetropic or as nearly so as possible, while the patient is still under the cycloplegic the test is performed for distant vision. It is repeated after mydriasis with the prescription in place, both for near and for distant vision.
BACON L. PRACTICAL APPLICATION OF CROSSED PRISMS. Arch Ophthalmol. 1943;29(5):772–774. doi:10.1001/archopht.1943.00880170092008
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