Pupil Dilation in a 3 year old

I have a three year old that has one (left) eye that stays dilated almost always. What causes this? She is a healthy child on no medications. We just noticed this a couple months ago, thats when I got out pictures from the past and I had missed this earlier. It was visible in pictures as far back as 6months.

[John C Hagan III, MD, FACS, FAAO]

The most common cause would be "Addie's pupil" but it is very important to take her in to see an ophthalmologist Eye MD (not a optometrist).  When you do take several old pictures that demonstrate the differfence. Also do not let the technicians put any drops in the childs eye until you see the Eye MD and the Eye MD him/herself checks and measures the pupil. There are other causes that need to be excluded.

JCH MD