VT and Prism
Prisms play a valuable role in the success Vision Therapy. Since prisms are considered to be a medical device, their use should always be monitored and guided by a Developmental Optometrist, or other qualified eye doctor.
In our office, we use prisms in a variety of ways to enhance our Vision Therapy patient’s experience. Monocular prism jumps, which are accomplished by occluding one eye, and placing a prism in front of the open eye are important in establishing a just noticeable difference (JND’s) in a patient’s spatial awareness. The prism bends light toward the base, causing a change in the place on the retina where the light is falling. This in turn causes an apparent spatial shift or change in position of the viewed object towards the apex, causing a corresponding movement of the uncovered eye to re-establish fixation and center the image back on the fovea. The prism should be removed and replaced in front of the open eye. Depending on the needs and visual condition of the patient, the Vision Therapist may need to vary the starting point in terms of prism power. In our office, we generally begin with a 15 diopter prism and execute a shift in the four cardinal directions (BU, BD, BL, BR). The initial awareness we are hoping our patients gain is a change in space. That is to say the patient should have some awareness of the prism “moving the world” in the direction of the prism apex, as the light is bent towards the base of the prism. This awareness of the change in space can be enhanced with simple localization activities such as pointer in straw and pegboard. The power of the prism should decrease as the procedure progresses, and the patient’s awareness of the change created by the prism improves. The overall goal of monocular prism jumps is to enhance awareness of the just noticeable difference (JND’s) as well as feeling tone. Essentially, is the patient able to employ the discrimination skills necessary, both with feeling tone and spatial shift, to detect that a change has occurred. One unique situation that the Vision Therapist should be aware of while doing this activity is the patient who eccentrically fixates. These patients may have an extra amount of difficulty perceiving shifts in visual space with their amblyopic eye the further the eccentric point is away from the fovea, the less able to attend to detail. This is also true for the strabismic patient if the image has moved to a suppression zone or less sensitive area of the retina before it can be re-aligned on the fovea.
Binocular prism activities can also be very powerful to expand a patient’s accommodative vergence flexibility by increasing the vergence demand. For example:
- A patient who is wearing binocular base out prisms must stimulate vergence (convergence) to keep the target single, while actively relaxing accommodation to keep accommodation at the plane of regard, and thereby achieving a clear and single target.
- A patient who is wearing binocular base in prisms must relax vergence (divergence) to keep the target single, while actively stimulating accommodation to keep accommodation at the plane of regard, and thereby achieving a clear and single target.
Similar localization and spatial estimation activities as mentioned above would be beneficial including pointer and straw and peg rotator.
Yoked prisms are used to shift the patient’s visual spatial orientation into a state of disequilibrium. When a patient’s visual state is altered by yoked prisms, the patient must re-calibrate and re-orient their spatial awareness so that the input they are receiving from their other senses matches the new visual input. In our office, we will have out patients partake in such activities as walking on a walking rail, playing catch, or even just walking the halls, while wearing yoked prisms. As a Vision Therapist, we are monitoring the amount of time and effort that goes into the re-orientation process on the part of the patient. We will also note differences in adjustment from base right versus base left, base up versus base down, as well as how long it takes the patient to recover when all prisms are removed. These activities can be loaded by increasing the amount of prism, increasing the frequency of prism rotation, as well as increasing the difficulty of spatial judgments in the tasks we are choosing.
We also use dissociating prisms, also commonly referred to as bi-ocular activities, mostly to improve simultaneous awareness in cases of amblyopia. For this process, the patient is encouraged to maintain two images of the target which can be anything from a pencil, to a Hart Chart, or something larger. While holding a pencil in front of their face, the patient is asked to look back and forth from one image to the other. We also conduct this activity with a Hart Chart, where the patient is asked to read one letter on the chart image in their right eye, and the next letter in the sequence in the chart image in their left eye. The most powerful dissociating prism activity that I have ever witnessed with a patient is Squinchel (aka RK Diplopia), by Dr. Harry Wachs and/or Dr. Robert Kraskin, respectively. With this activity, the patient is wearing dissociated prisms and he or she is asked to either to on a table top or even rub the table top with the pad of their index finger. While this is going to, the Vision Therapist will ask “what finger is the sound coming from?” Inevitably the response will equal whichever eye is dominant. When the patient understands that the two images they are really are the same finger, they are asked to switch the noise or proprioceptive feedback back and forth from image to image; and finally the patient is asked to have the sound come from both fingers at the same time, thus performing and demonstrating simultaneous awareness.
Prisms are a valuable tool in any VT room and proper use can help patients in many ways. It bears repeating that prisms are considered medical devices and their use should be dictated and managed by a Developmental Optometrist, or other licensed eye care physician.