The 4Δ Base-Out Prism Test
The 4Δ base-out prism test can identify a small facultative scotoma in a patient with monofixation syndrome and no manifest deviation (see Chapter 5). A 4Δ base-out prism is placed before 1 eye during binocular viewing, while motor responses are observed (Fig 7-11); the test is then repeated with the prism over the other eye. Patients with bifixation usually show a version movement toward the apex of the prism, followed by a fusional convergence movement in which the eye with the prism maintains fixation while the fellow eye moves nasally to restore fusion. A similar response occurs regardless of which eye has the prism. In patients with monofixation, typically no movement is seen when the prism is placed before the nonfixating eye. When the prism is placed before the fixating eye, a refixation version movement occurs, but without any subsequent fusional convergence.
Figure 7-11 The 4Δ base-out prism test. A, When a prism is placed over the left eye, dextroversion occurs during refixation of that eye, indicating absence of foveal suppression in the left eye. If a suppression scotoma is present in the left eye, neither eye will move when the prism is placed before the left eye. B, Slow fusional adduction movement of the right eye is then observed, indicating absence of foveal suppression in the right eye. C, In a second patient, the absence of any such adduction movement (the right eye stays abducted after the dextroversion) suggests foveal suppression in the right eye. The patient does not experience diplopia. D, Weak fusion can also cause absence or delay of adduction movement, but in this case the patient experiences diplopia.
(Modified with permission from von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and Management of Strabismus. 6th ed. St Louis: Mosby; 2002:220.)
The 4Δ base-out prism test is the least reliable method of documenting a central suppression scotoma. Patients with bifixation may recognize diplopia when the prism is placed before an eye but make no convergence movement to correct for it. Patients with monofixation may switch fixation each time the prism is placed and show no version movement, regardless of which eye is tested.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.