Glaucoma
Glaucoma is the "silent thief" of vision. It is true that you may not be aware that anything is wrong until it is too late.
Vision lost from glaucoma cannot be recovered but only maintained at that level. This is why regular checking for glaucoma is important.
There are no warning symptoms that you have it and if you develop vision problems it may already be advanced glaucoma. So have a check up beginning at 40 years, particularly if you have a first degree relative who has glaucoma.
There are various tests you may need done including a eye pressure test (IOP), a visual field test (VFT), a pachymetry and an OCT. All of the above are painless and can save your vision.
Treatment of glaucoma usually involve the use of eye drops in most people. Additional treatment may involve laser and surgery if needed.