The lumbar puncture is kind of invasive. It has some risks (albeit minor). It seems responsible of your doctor to go with the evoked potentials first. I am guessing here of course, if your evoked potential come back indicating MS then that seems like it would be enough paired with your MRI results. If your evoked potentials come back normal, then your doctor may request the lumbar puncture, or he may go with the more conservative "wait and watch" approach. I am glad to hear that the evoked potentials test has been requested.
Just for your information, evoked potentials have several sub-types. There is visual, auditory and lower extremities. There may be more than that but those are the ones I am familiar with. Each one is meant to time the response of stimulation along the central nervous system. If you are indeed having visual problems due to optic nerve damage, that will show on the test.
Gretchen co-moderator MS board diagnosed with MS July 2006