Fusional amplitude is a handy concept to understand if a patient has ocular misalignment or double vision.
There are two kinds of double vision or diplopia. Monocular diplopia means one eye causes double vision all by itself.
This is due to some eye condition. It could be due to cataract or dry eyes or inadequate glasses, etc. But it is not neurologic. This is not what we are talking about here.
Neurologic double vision is always binocular diplopia. This means both eyes see just fine independently, but when you try to put them together they don't line up together and you see double.
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| (If you know how to cross your eyes, you can simulate this type of double vision.) |
A neurologic pattern of double vision (binocular diplopia) could be due to the brain or the nerves or the muscles, but something is not working properly to team up the two eyes.
However, sometimes our awesome brain steps in to correct the problem.
If the misalignment is just off a little bit, then sometimes the brain can take those two separate images and bring them together. This is a form of compensation for a deficit.
When the brain does this it is called "fusion".
The brain takes those two images and fuses them back together.
That is a good thing, of course, but it can lead to some confusion. If a patient has had a chronic misalignment of their eyes, the brain may be masking it. Then as the brain ages, and especially when fatigued, the symptoms start to show up. This creates a curious situation where a patient has a longstanding issue, but just recent development of symptoms. It is possible that the patient always had misalignment their whole life and no one ever noticed it until late in life. So, in that case, they didn't develop any new concerning eye movement problem, they just developed a loss of fusional amplitude.
It is common to lose some fusional amplitude as we get older. This is further exacerbated if we feel sick, tired or stressed.
Patients who have longstanding eye alignment issues usually have really high fusional amplitude. If it has always been like that, then the brain has had a lot of practice at fusing. This is a good clue if we are trying to figure out if a misalignment is new or old. If the patient has a really high fusional amplitude they have probably had the eye alignment issues for a long time (but maybe just aren't compensating as well as they used to.)
Fusional amplitude is measured with prisms. We can place stronger and stronger prisms in front of the eye and check if it causes double vision.
The strongest prism that does not create double vision is the fusional amplitude.
This can be measured both vertically and horizontally. Since the eyes are aligned side-by-side most people have really high horizontal fusional amplitude. I think the vertical fusional amplitude is more interesting. I can only fuse 2 or 3 prism units. So, I'm pretty sure that if my eyes went out of alignment it would bother me a lot. Most people can fuse at least 3. Some patients with chronic issues of eye alignment can fuse a whopping 10 prism units. This is helpful to measure, because it can help us understand when something is new vs. old. Usually an old problem is less serious.
In summary, fusional amplitude is the brain's ability to overcome double vision and create a single image. It is a helpful adaptation, but it makes diagnosis more tricky. The older a defect is, the more practice the brain should have at making this adaptation. But, as we age the brain sometimes can't keep it up anymore and patients can start to have seemingly new symptoms. So, new symptoms don't always mean the problem is new.