Fixation disparity exists when there is a small misalignment of the eyes when viewing with binocular vision. The misalignment may be vertical, horizontal or both. The misalignment (a few minutes of arc) is much smaller than that of strabismus, which prevents binocular vision, although it may reduce a patient's level of stereopsis.
In layman's words, Henry sees the world as a badly tuned TV, especially when it's up close, like when he reads. The images are ever so slightly blurred, because the two images from both eyes are not completely duplicated, therefore just a tiny bit away from each other. He complained about "double vision" two years ago, caused me quite a bit of worry, as I blogged here. At the time my eye doctor couldn't find anything wrong. Either it was the doctor, or Henry being older and more articulate in explaining his problem now, it took two years and two doctors to finally diagnose it.
It's anyone's guess if Henry's fixation disparity is hereditary from my lazy eye. It's a much milder misalignment than strabismus and amblyopia (lazy eye). Only girls in my family inherited the eye problems, with my mom's and mine most serious. But then it's entirely possible the boys are never diagnosed since they can all see very well in exams.
Now Henry has these cool Nike flexible prism glasses that he uses mostly for reading and playing his PSP. Rather cute, isn't he?