The use of prisms can improve anomalous head positions by shifting perceived object location toward the null point. For a patient with a left head turn and a null point in right gaze, the prism before the right eye should be oriented base-in, and the prism before the left eye oriented base-out. This shifts the retinal images to the patient’s left and perceived object location to the right; objects in front of the patient are now imaged on the fovea when the patient is in right gaze, reducing the amount of head turn required to use the null point gaze position.
In patients with binocular fusion, bilateral base-out prisms can improve vision by inducing convergence, which damps nystagmus (amounts are determined by trial and error).
Prisms can be used as the sole treatment of nystagmus or as a trial to predict surgical success. With powers ranging up to 40 prism diopters, Press-On (Fresnel) prisms (3M, St Paul, MN), inexpensive plastic pieces that can be cut and then applied to glasses, can be used for both purposes. Ground-in prisms cause less distortion and are preferred for patients who require only small amounts of prism.
Other nonsurgical treatment options for nystagmus are discussed in Chapter 9 of BCSC Section 5, Neuro-Ophthalmology.
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.