An Ophthalmic Migraine. An Ophthalmic Migraine occurs when zig-zag lines or "heat waves" show up near the center of one's field of vision and a blind spot expands on to one side--of the field of vision--for several minutes, though, usually less than an hour. These are sometimes followed by a headache, sometimes it's not. Some people experience them once and some people experience more episodes throughout their life. No medical treatment is entirely necessary, though, if these occur on a regular basis, you should contact an Ophthalmologist, preferably your family Ophthalmologist.
I was at school doing my Geometry work and out of the blue, I had an eyegrain. It bothered me and I couldn't finish the rest of my work until it was over, though, the bell rang right after it finished.