What Causes Differently Dilated Eyes In A Child?

My two year old has differently dilated eyes. They dilate normally when looking in light or in a bright room. And they aren't always dilated differently. Sometimes they are both the same. We saw an eye doctor who said it's probably something she was born with and is no reason for concern. Im almost positive she wasn't born with it. He ordered an MRI where she will be sedated for 2 hours to look at her head and spine just to be cautious. I'd rather not do the MRI obviously, unless it's absolutely necessary. She has no symptoms with her eyes...no dizziness, headaches, etc. Should I be concerned? Follow through with MRI? Is this normal in kids? Thank you!!