How to prescribe prism for your patients?
How to Prescribe Prism for Your Patients
Prism is a type of lens correction that can help patients with eye misalignment, double vision, or binocular vision problems. Prism lenses bend light rays to shift the image of an object to a different location on the retina, allowing the eyes to work together more comfortably and efficiently.
However, prescribing prism can be tricky, as there are many factors to consider, such as the type, amount, direction, and stability of the eye deviation; the patient’s symptoms, preferences, and expectations; and the potential side effects, benefits, and limitations of prism therapy. In this article, we will share some tips and tricks on how to prescribe prism for your patients, based on the opinions and experiences of some optometrists.
What is Binocular Vision Dysfunction/BVD disorder
Tip #1: Use a systematic approach to measure and prescribe prism
One of the most important steps in prescribing prism is to measure the amount and direction of the eye deviation accurately and consistently. There are various methods and instruments that can be used for this purpose, such as cover tests, Maddox rod tests, prism bar tests, phoropter tests, or automated devices. However, no matter which method you use, you should follow a systematic approach that includes these steps1:
Perform a comprehensive eye exam to rule out any ocular or systemic diseases that may cause or contribute to the eye deviation.
Determine the type of eye deviation (horizontal, vertical, torsional, or mixed) and whether it is comitant (constant in all directions of gaze) or incomitant (variable in different directions of gaze).
Measure the magnitude of the eye deviation in primary gaze (straight ahead) and in different positions of gaze (up, down, left, right) using your preferred method.
Prescribe the minimum amount of prism that eliminates or reduces the patient’s symptoms (such as diplopia, eyestrain, headache, etc.) without causing any adverse effects (such as image distortion, spatial disorientation, prism adaptation, etc.).
Verify the prism prescription by having the patient wear trial lenses or a trial frame with the prescribed prism for at least 15 minutes and observe their response.
Educate the patient about the purpose, benefits, limitations, and potential side effects of prism therapy and set realistic expectations.
Follow up with the patient regularly to monitor their progress and adjust the prism prescription if needed.
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Tip #2: Use different methods for different types of eye deviations
Depending on the type and cause of the eye deviation, you may need to use different methods to measure and prescribe prism. Here are some examples
For horizontal deviations (esophoria or exophoria), you can use either a horizontal prism bar or a phoropter with Risley prisms to measure the deviation in primary gaze. You can also use a Maddox rod test to measure the deviation in different positions of gaze. You should prescribe prism with the base in the opposite direction of the deviation (base-in for esophoria and base-out for exophoria) and split it equally between both eyes if possible.
For vertical deviations (hyperphoria or hypophoria), you can use either a vertical prism bar or a phoropter with Risley prisms to measure the deviation in primary gaze. You can also use a Maddox rod test to measure the deviation in different positions of gaze. You should prescribe prism with the base in the opposite direction of the deviation (base-down for hyperphoria and base-up for hypophoria) and place it in front of the deviated eye only.
For torsional deviations (cyclotropia), you can use either a double Maddox rod test or a Bagolini striated lens test to measure the angle of rotation of each eye. You should prescribe prism with the base oriented at an oblique angle that compensates for the rotation of each eye.
For mixed deviations (combination of horizontal, vertical, and/or torsional components), you can use either a modified Thorington test or a Mallett unit test to measure each component separately. You should prescribe prism with the base oriented at an oblique angle that compensates for each component.
Tip #3: Use different strategies for different scenarios
Depending on the scenario and goal of prism therapy, you may need to use different strategies to prescribe prism. Here are some examples3:
For acute onset diplopia due to cranial nerve palsy or stroke, you may need to prescribe temporary prism (such as Fresnel press-on prisms) that can be easily adjusted as the deviation changes over time. You may also need to prescribe more than one pair of glasses with different amounts of prism for different distances or activities.
For chronic diplopia due to strabismus or decompensated phoria, you may need to prescribe permanent prism (such as ground-in prisms) that can provide stable binocular vision and prevent further deterioration. You may also need to prescribe vision therapy or surgery as adjunctive or alternative treatments.
For convergence insufficiency or excess, you may need to prescribe prism that can either stimulate or relax the convergence system and improve the patient’s near vision and comfort. You may also need to prescribe vision therapy or orthoptic exercises to enhance the patient’s binocular skills and reduce their dependence on prism.
For hemianopia or visual field loss, you may need to prescribe prism that can expand the patient’s field of view and improve their mobility and safety. You may also need to prescribe low vision aids or rehabilitation services to help the patient cope with their visual impairment.
Prism is a powerful tool that can help many patients with eye misalignment, double vision, or binocular vision problems. However, prescribing prism requires skill, knowledge, and experience, as there are many factors to consider and challenges to overcome. By following a systematic approach, using different methods for different types of eye deviations, and using different strategies for different scenarios, you can prescribe prism effectively and efficiently for your patients.