Binocular Vision Disorders (BVD)
The number of patients who live with an undiagnosed Binocular Vision Disorder (strabismus), such as esotropia, exotropia, hypertropia, convergence insufficiency, et.al., is surprising. These are the persons you encounter who seem to bump into furniture, knock things over, have frequent headaches or difficulty reading, but what is it?
The easiest way to explain a Binocular Vision Disorder is to look at something with one eye closed, then switch to look at the same thing with the opposite eye closed. As you repeat this, whatever you are looking at appears to change position because each eye is seeing it from a slightly different angle. This means that each eye produces an independent image, yet we do not see two images when both eyes are open. The reason that we do not see double is that the brain takes the image from one eye and merges it with the other. The fusion of images produces depth perception and gives us the ability to move through space with ease. If a slight misalignment is present, the two images will not overlap and the brain will continuously try to fuse. Being unable to fuse results in vague eye discomfort that can lead to headaches and produces decreased depth perception; this is a Binocular Vision Disorder. The visual system is so precise that there are individuals with no eye health problems, who see clearly with each eye, yet incredibly see worse when using both eyes together because of BVD. Since they "see 20/20", they will be considered to have normal vision and tend to be missed by screenings.
BVD Causes
The majority of Binocular Vision Disorders are present at birth or caused by an injury to the face or head, but sometimes having different prescriptions between the two eyes can induce the problem. Some injuries are serious, like an automobile accident, while others are more common, like a fall or sports accident. Even a seemingly minor injury may be sufficient to produce symptoms such as an unusual posture or head position as in trochlear torticollis or Fourth Nerve Palsy.
The most common patient with a Binocular Vision Disorder is the average person seeking routine eye care who has various unexplained ailments affecting their vision, coordination, balance, or perception. Usually only a minimal amount of a prismatic lens correction is required for these patients to report physically feeling better once the eyestrain has been resolved.
While jumping on furniture with his brothers, a young teenager struck his head resulting in a concussion. He later developed chronic headaches and received care from a pediatrician, a neurologist, a physical therapist, and even biofeedback training, but the headaches continued. When we met months later, a thick ring binder containing MRIs, CT scans, and blood work test results was presented for review, but since the headaches had not resolved, it seemed unlikely that the answer would be found there. The history and symptoms were consistent with the eye on the side of the injury not aligning with the other and once a prismatic lens was provided, he immediately smiled; the eyestrain was relieved.
Prismatic Lenses
Prismatic lenses have been mentioned as a way to correct a Binocular Vision Disorder, but what are they? Prism is simply a way of manufacturing a lens so that it has uneven thickness on one end compared to the opposite end; this difference shifts the position of an image being seen towards the thinner end. Anyone who wears eyeglasses already knows that the thickness of their lenses can vary from the center to the edge, but it is an additional difference in thickness, between the right and left lens, that produces this effect. For the majority of patients, the difference is not visible, but the effect can be profound.
How does shifting an image resolve a Binocular Vision Disorder? Since each eye produces an image independent of the other, both images must be fused by the brain in order to see only one. In BVD, the brain is unable to align the images because one may be slighly higher, lower, to the left, to the right, or even rotated relative to the other. Without alignment, fusion cannot occur and the brain will begin a never-ending struggle. The constant eyestrain of trying to fuse produces discomfort, may lead to headaches, and giving up trying to use both eyes together. Once this happens, patients will often say "I don't like to read", "I don't like school", "I don't like sports", or "I'm clumsy" when it is not a voluntary choice, but a physical inability of the eyes. Prismatic lenses allow both images to realign, the brain no longer struggles to fuse, and the constant eyestrain and discomfort are relieved.
Retraining the Brain
Once prism is added, the brain will stop trying to constantly realign the images seen by each eye. It is common for people with a Binocular Vision Disorder to also turn their head, or even their entire body, slightly towards one side to help them compensate. While this appears to be poor posture, these patients are actually misaligning the body to realign the eyes. To them, “good” posture produces visual discomfort so they quickly return to what feels best. Once prismatic lenses are prescribed, these patients typically return to their poor posture out of habit, but since compensation is no longer needed, this may cause their eyes to shift in the opposite direction. Changes in posture must occur for prismatic lenses to have their maximum effect.
If the misalignment is small, realignment can be immediate, but larger amounts may require months, and gradual increases, for the full effect. A patient in their 50's, who had one eye inward and upward relative to the other since childhood, was prescribed prismatic lenses. Vision gradually improved in the affected eye from 20/30 the first year, to 20/25 the second year, and eventually to 20/20 by the third year. At the same time that their vision was improving, the eye that had been misaligned began to realign with the other. A small amount of prism was prescribed the first year, increased the second year (which had to be reduced slightly after a few weeks), and the full amount was prescribed by the third year. This is an important concept to understand; some patients with a Binocular Vision Disorder improve slowly, and may even reject a change, due to the brain being overwhelmed. Improvement is possible right away, but full correction may take time to allow the brain to learn how to use the eyes in ways that it has not done before.
As another example, a nursing student in her 20's was experiencing eyestrain and headaches due to the reading demands of her educational program. Her current eyeglass presription was correct, all test results were normal, and nothing seemed to relieve the eyestrain until prismatic lenses were prescribed. That Summer, she returned because the eyestrain and headaches were back, but seemed to improve by removing the eyeglasses. Her eyeglasses were replaced, to remove the prismatic lens correction, and the eyestrain resolved once again. It was possible that the eyes' muscles were now working properly and prismatic lenses were no longer needed. In the Fall, the nursing student returned because her vision problems were back. We had kept the original prismatic lenses from the Spring, replaced them in her eyeglasses, and she immediately felt better. In her case, the brain could not adapt to the reading demands during the semester and would begin to misalign, but would return to normal while on Summer break. Our solution was two pairs of eyeglasses, one with prismatic lenses and another without, so that she could use whichever pair met her needs depending upon the reading demands being made.
Relieving Headaches
There are many reasons why a person experiences headaches including glare from bright lights, dehydration, stress, strong smells, loud sounds, or foods aged to enhance flavor like pepperoni, sauerkraut, pickles, wine, or cheese. Glare can be reduced by wearing polarized sunglasses and having an anti-reflective coating on clear lenses, but the other causes listed do not have a visual component. Prismatic lenses only affect headaches caused by the constant eyestrain of trying to align the eyes. How do you know if your vision may be causing a headache? Vision headaches tend to be chronic and appear, or become worse, as the day progresses whereas headaches caused by a stimulus from outside the body, foods for example, only occur after consuming that particular food.
Some patients, like the nursing student, may have their eyes begin to perform normally and once that happens, the prismatic lens must be removed to avoid causing headaches (less than 1 out of 100 prism lens patients). Far more common is for these patients to have their prismatic lens prescription made incorrectly. Patients have brought eyeglasses made elsewhere for us to check and we find the wrong amount of prism added or even completely absent. Without the exact prism correction, relief is not possible.
Patient Comments
If you have good depth perception (can you see the bird?), then eye-hand coordination tasks are not a problem. For patients who have a Binocular Vision Disorder and poor depth perception, it is important to listen to their reactions to understand how they see.
"Flat, like a picture."
A patient wearing prismatic lenses for the first time simply said, "Whoa" as he sat in the examination chair and looked across the room. According to his description, he realized that items on a countertop in front of him were at different distances whereas before, everything had looked, "Flat, like a picture."
"It's right there!"
A young woman, with a previously undiagnosed Binolcular Vision Disorder, was prescribed prismatic lenses. She leaned back in the examination chair because the testing equipment suddenly seemed nearer. "It's right there!", she said due to being able to appreciate depth perception for the first time.
"The railing is curved?"
Another BVD patient stared at a handrail after putting on her new prismatic eye glasses and asked, "The railing is curved?" Apparently a nearby hand railing had appeared to have a flat surface, but now she could appreciate the subtle difference in depth of its curved edge.
