The patient with established glaucoma has already lost some part of the visual field, usually in the area. This area of lost vision is called "scotoma".
It should be noted however that "scotoma" caused by glaucoma is not easily perceivable. Although for reasons of conception we often depict this as being 'black', as if an obstacle is hidden in the area, it is in actual fact the area that is missing from our visual field simply ceases to exist for our brain. Thus, this loss of visual field does not easily "bother" us, in the same way that it does not bother us that we cannot see behind us.
But as the damage becomes more extensive, active people starting having problems realizing that they do not see things that they know for sure are in front of them (eg sections in the page of a book while reading).
This is truly a significant disability with serious consequences for the patient, not only because it reduces many of their daily activities, but also because they may be faced with risks that should have been perceived, but were not.
Although possible, it is difficult in practice to self-examine ourselves for glaucoma and the surest way is to visit an ophthalmologist. He will take all the predisposing factors from the patient's history into account, he will measure the intraocular pressure and with modern imaging will study the morphology of the optic nerve and be able to promptly identify the slightest damage.