Glaucoma is something we think about when it comes to getting old, and indeed it affects the elderly more than younger people, although childhood glaucoma can occur. There are several kinds of glaucoma and all can and often do lead to impaired vision and even blindness. Most sufferers of glaucoma have a version called "open-angle glaucoma" that produces pressure inside the eye itself. Glaucoma is the second leading cause of blindness (second only to diabetes) and is often only diagnosed when fairly advanced as it takes some years for symptoms (vision loss) to become noticeable. This is why regular eye exams are important.
Glaucoma occurs when the fluids that enter and leave the eye area, in order to keep it healthy and functioning, slow down.
https://www.empowher.com/glaucoma/content/glaucoma-what-it-and-how-its-treated