Maltese Mix Who Squints in Bright Light

What she describes sounds like iris atrophy, a gradual thinning of the muscle that closes the pupil. It's a common aging change in middle-aged and older dogs, especially small and toy breeds, so it's most likely if he's no longer young. The pupil stays larger and can't shut out bright light, so dogs squint or close their eyes outdoors and under bright lights. It doesn't affect vision.

It's slowly progressive and can't be reversed, but it doesn't need treatment, so there's no ongoing medicine cost. Other problems can also cause a large pupil, including glaucoma, inflammation and nerve problems, so it's worth a proper eye exam to confirm. Go sooner if his pupils suddenly look different in size, or the eye turns red, cloudy or painful.

To keep him comfortable, walk early or late in the day, give him shade and a dim place to rest, and try tinted dog goggles if he'll accept them.

See iris atrophy and light sensitivity.

This is general information, not a diagnosis. Please follow your own vet's advice.