Hello and thanks for your question. As an eye expert, I hope to shed a little more light on your son's condition with my answer.
A difference in the size of the two pupils is called anisocoria. For someone in good health, and typically most children are in good health, the prognosis is usually good. However, there are things to be on the lookout for. If the difference in the size of the pupils is more than 1.5 to 2.0 mm, or if it is a sudden occurrence associated with headaches, eye pain, double vision, or a droopy upper eyelid, then these can be warning signs of significant neurologic problems and should be addressed immediately. If there is one or more of the signs or symptoms associated with this change (difference in pupil size), then it would be prudent to see your ophthalmologist as soon as you are able to. If, however, there are none of these signs or symptoms and the difference in size between the two pupils is 1.5mm or less, then what you are likely observing is called a physiologic anisocoria. Physiologic means it is an abnormality that is not associated with any disease. This is a common occurrence and many people have a slight difference in the size of their pupils. It can even vary from one eye or the other and can even change in the amount that they look different from hour to hour and day to day. For many people, the sudden realization that there is a difference in the size of pupils is very alarming, but then it often times only amounts to a 1.5mm or less difference.
Additionally, sometimes trauma to the eye can cause what's called a traumatic mydriasis, which is damage to the muscle in the pupil that causes constriction of the pupil and it stays more dilated than it did before. The pupil can still change sizes, but does so with less amplitude than the opposite eye. Usually this happens from a direct blow to the eye, but I would not rule out general head trauma from doing this if the blow were in close proximity to the eye
If there really is a difference of more than 1.5mm or it is accompanied by any of the above symptoms I would suggest that you see your ophthalmologist as soon as possible. Even if this is not the case, I think this warrants an exam by an ophthalmologist in the near future.
Does this help address your concerns?
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My opinion is solely informative and does not constitute a formal medical opinion or recommendation