As a vision therapy and rehabilitation resident, I have the opportunity to work with primary care providers within New York and examine patients on a referral basis. Most referrals are due to an observed strabismus during an ocular wellness exam. The big question that providers want to be answered is: What is this patient’s prognosis for vision therapy? Patients are eager to learn about their treatment options, hence the integrative process of a tertiary care exam begins.
During my residency, I’ve learned that there are 6 major elements of testing which provide a lot of information when evaluating a patient with strabismus. Granted in a tertiary care setting, time is on your side and the diversity of equipment for evaluation is a luxury. Yet, there are multiple ways to perform a test and oftentimes the very equipment you use every day can be used to modify a test. So, let’s find out how to evaluate a strabismic patient in a primary care setting. This may help you gain a better understanding of your patient’s binocularity and make the appropriate referral.
The 6 major elements assessed in a strabismus evaluation are visual acuity, ocular alignment ocular motility, fusional status, fixation status, and retinal correspondence.
Let’s begin with visual acuity.
This can be performed with a basic Snellen chart. If the acuities are asymmetric, further investigation is warranted. Crowding vs. single letter acuity is a quick way to determine if amblyopia could be a factor. However, since amblyopia is a diagnosis of exclusion, don’t forget to drop the pinhole in your occluder to rule out a simple refractive cause for the reduced vision.
Next, ocular alignment can be assessed with a cover test.
Distance vs. near cover test tells you about the change in deviation and relate it back to the patient’s chief complaint. You would expect the deviation to be larger or more frequent at the distance where the patient is most symptomatic. Don’t forget to ask your patient about their work setting to figure out where exactly their symptoms are heightened. When looking at ocular alignment you want to report laterality, direction, frequency, and magnitude.
Let’s now move along to ocular motility.
Here we are looking for over or under actions of extraocular muscles and assessing for comitant deviation. The basic equipment you can use to accomplish this is a penlight and a red lens (most trial lens sets are equipped with a red lens). Subjectively you can ask a patient to tell you if the separation between the two lights (if it exists) increases or decreases as you move into the nine cardinal gazes. If your patient is suppressing (they only see one color of light which is not a combined luster), perform an objective test such as an alternate cover-test in the cardinal gazes and note if the magnitude of the deviation changes. Always look for anomalous head posture! This tells you about how the patient adapts to their deviation.
Fusional status can be measured with the same red lens and penlight from #3 and a prism bar from #2.
If your patient is an esotrope you need to determine the centration point. This means, figure out the location in space where the visual axes intersect and the patient can fuse. Fusion is appreciation of luster of a red and white light. Motor range of fusion can be assessed by adding some prism with your prism bar and seeing where they lose appreciation of luster. A patient with adequate sensory and motor fusion may be a good candidate for vision therapy! In an exotrope, the centration point is at infinity so you may need some prism initially to get that to move out into space. Start close to the motor angle measured on cover test to determine if they can see luster. Then just like the esotrope find the motor fusional range by adjusting the prism amount.
It’s now time to evaluate your patient’s fixation status.
If visual acuity was 20/20 in the right and left eye you can assume that the patient has central fixation, and boom you’re done! Another method which gives you a lot of information is the random dot stereogram. In order for a patient to achieve RDS they need to be bi-foveal so, if they can perform this test you can assume they have central fixation. If you suspect eccentric fixation, you can brush off the cobwebs from your direct ophthalmoscope and use the bull’s eye target to look at their fixation. Remember this test is administered monocularly!
Retinal correspondence can be measured with a red lens and a cover test.
In the presence of sensory fusion i.e. when a patient appreciates luster, if a tropia is elicited on a unilateral cover test then the patient has anomalous correspondence. So find the point of sensory fusion and perform a unilateral covertest. If there is no movement of the suspected eye, the patient likely has normal retinal correspondence.
The equipment we used to evaluate our strabismic patient in a primary care setting was: Occluder with pinhole, red lens, prism bar and direct ophthalmoscope. That’s it! Now, something important to keep in mind is that this is not a comprehensive analysis and often times there are additional tests that need to be performed. However, as a practitioner, these tests will help you better understand the binocular status of your patient.