Four words in a patient’s chart can strike doubt in the mind of even the most accomplished optometrist: “Complains of double vision.”
What caused the diplopia? Is it sudden-onset or longstanding? Have they ever had prism in their glasses before? What about prior strabismus surgery? Could it just be undercorrected astigmatism or a corneal condition?
A comprehensive description of the diagnosis and management of all medical and ocular conditions that cause diplopia could fill several textbooks. This article will narrow that exhaustive list down to non-urgent, binocular vision causes of diplopia that are best managed with prism.
Prescribing prism is more of an art than a science, and can be very subjective. That subjectivity is part of the reason so many optometrists hesitate to tackle it.
Discover the patient’s main concern.
Constant, comitant diplopia due to strabismus or muscle paresis is the simplest case to manage.
The prism amount will most likely be stable, with no adaptation, and immediate patient relief. However, many binocular vision cases present with vague symptoms.
A young lawyer may only notice diplopia after several hours of intense computer work. An elderly patient may have had a small amount of occasional diplopia for several years, but now comments that it is happening more often and to a greater degree. An elementary school student may not complain at all, but consistently closes one eye to read comfortably. All of these patients may benefit from a prism prescription, and they may have gone years without proper binocular evaluation.
Binocular vision testing should be performed during all comprehensive eye exams.
This does not have to be a long process; a quick NRA/PRA after refraction can reveal an accommodative issue that may be subtly compromising binocular function. Technicians can be trained to perform cover test as part of the pre-testing process, and any questionable findings can be re-checked by the optometrist in the exam room.
Obtain objective and subjective measurements.
There are several articles that describe methods to measure and apply prism.
Ultimately, the choice of technique is up to each individual clinician, and several measurements may be prudent to determine the “best” value (1).
For example, a patient with a small vertical phoria may appear orthophoric to the clinician during a cover test, but when prompted, may note the target appearing to move up and down as each eye is uncovered.
Loose prisms may be used to measure this subjective motion, and the result compared to a Von Graefe phoria.
In the case of a vertical heterophoria, even a prescription with 0.5 prism diopter base up or down may be clinically significant (2).
The important factor is not which tests are performed, nor how many tests are performed, but rather, the consistency and validity of the results.
If four tests are performed, and four wildly different values are obtained, the patient will likely not benefit from a prism prescription. However, if the results of two tests make sense clinically when compared to patient complaints and exam findings, they may be enough to diagnose and manage a binocular condition.
Test the prescription in a trial frame.
Phoropter values can be deceptive when dealing with binocular vision complaints.
Patients with fragile binocular systems may be symptomatic while looking through the oculars, but function normally in glasses or contact lenses.
To avoid prescribing too much prism, always test the final prescription with prism in a trial frame.
Jannelli clips may be useful to simply check prism amounts over the top of a patient’s own frame if their base prescription is unchanged.
If the prism is correcting for a lateral phoria, allow the patient to wear the trial frame for ten minutes, then re-measure the alignment.
If their phoria has returned to the original measurement, they are not a good candidate for prism because they will adapt to (or “eat”) the amount (3).
Allowing the patient to adjust to the new correction will also give them adequate time to determine if the prism is comfortable for all distances.
Prescribe and educate the patient.
This step sounds deceptively simple, but there are several things to consider.
- Firstly, choosing to prescribe ground-in prism or stick-on Fresnel prism. Often, patients are able to choose which of these options is preferable.
- Secondly, should the prism be split or ground into only one lens? In most cases, patients with moderate or large phorias are more successful with equally split prism, including vertical. If a patient has a tropia, utilize a trial frame to determine if the patient subjectively prefers the prism to be placed only in front of the strabismic eye.
- Thirdly, don’t forget your optics lessons: lens decentration, frame size, optical center, and slab-off prism in a bifocal should all be considered when prescribing.
The patient should be made aware that their new prescription includes prism.
Take a moment to explain what prism is, why they need it, how long they can expect to adapt to their new prescription, and what symptoms to watch it out for if the prescription is not adequate. Trained technicians or opticians could handle this step if exam time is an issue.
Less is more.
Also known as, “If it ain’t broke, don’t fix it.”
If a patient is not symptomatic, they do not need prism. If they have prism in their current prescription and are happy with it, there is no need to change it.
If they can compensate by tilting or turning their head and they are content doing so, do not try to force them into a prism prescription.
Prescribe the least amount of prism that resolves their symptoms, and always keep patient satisfaction as the top priority.
As with most things in life, prescribing prism just takes practice. Practice those clinical techniques so adding binocular vision testing does not interrupt the flow of an exam. Practice describing convergence insufficiency, decompensated phorias, accommodative dysfunction, strabismus, and other conditions so that patients can understand their diagnosis.
Practice prescribing prism whenever it is necessary, and soon, you will be prescribing prism with confidence.
- Gray, Lyle S. “The prescribing of prisms in clinical practice.” Graefes Arch Clin Exp Ophthalmol, 2008 May; 246(5); 627-629. Published online 2008 Apr 1. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2292476/
- Weigel, O.D., Eric. “Vertical Prism: A Small Amount Goes a Long Way.” Journal of Behavioral Optometry, 2012. Volume 23, #5-6, pp. 129-133. http://www.oepf.org/sites/default/files/23_FINAL_WEIGEL.pdf
- Rosenfield, M. “Tonic vergence and vergence adaptation.” Optom Vis Sci, 1997 May; 74(5); 303-328. http://www.ncbi.nlm.nih.gov/pubmed/9219290