In Darian Lusk’s recent CBS News article, he discusses the medical reason Bono always wears sunglasses. In the article, Bono says he’s had glaucoma for over 20 years.
Glaucoma is usually symptomless, gradually stealing away peripheral vision without pain or light sensitivity. That’s one reason it is a scary disease. Bono likely suffers from a less common form of glaucoma like “narrow angle glaucoma” which can cause eye pain when exposed to changing light levels – for instance, going from a dark room to a brightly lit area.
In the vast majority of cases a thorough eye examination is the only way to detect glaucoma in its early stages.