For clarification, anyone know whether a lumbar MRI is relevant to search for lesions or is it scanning an area completely below the level of the CNS and therefore more used for detecting disc related problems? If you were looking for lesions that could explain incontinence (both kinds) or a leg giving way, what level(s) for an MRI would be relevant?
MS affects the brain and the spinal cord. Because the cord finishes fairly early into the lumbar spine, most neuros don’t bother with it, but it is possible to have lesions there - they would be high up the area though, in the cord.
There are multiple things involved in bowel and bladder control so it’s not as straightforward as looking somewhere specific. The actual nerves that control the muscles there are low in the spinal cord (see, e.g. http://emedicine.medscape.com/article/321172-overview#a0104), but a lesion anywhere on the cord can affect continence.
The legs giving way thing is even more complicated because it could be caused by the brain and/or the cord.
Sorry I can’t give you specific answers.
I should say though - having a lesion in the lumbar spinal cord would be unlikely to get someone a diagnosis of MS - MS most commonly attacks higher up.