My cat's eyes are perma-dilated ...

… but he can see, as evidenced by Ye Olde Wiggling String Test. We’re gonna take him in to the vet this week, but I was wondering if anyone had this happen to their animal, and if so, what the cause was? Poking around teh interwebz reveals retinal detachment (caused by high blood pressure) and kidney problems as possible causes.

He’s about 12 years old, a male Maine Coon - which comes with a risk of heart problems, I know. So we’re going to get him checked out by our favorite competent vet. But, seriously: no reaction to a flashlight in the eye. No contraction of the iris, and no squinting or turning his head as if the light hurt (he just held still and purred).
Thoughts?

Vet here. Primary rule outs are infectious disease (Toxoplasmosis, Bartonella, FIP, FeLV, FIV), retinal disease (including glaucoma), and intracranial disease. I would start with a really good ophthalmic exam and go from there.