Glaucoma is a group of related eye disorders that causes damage to the optic nerve which is responsible for carrying information from the eye to the brain. In the early stage of glaucoma it usually has no symptoms, which makes it so dangerous i.e. by the time problem is noticed with the sight, the disease has progressed to the point that irreversible vision loss has already occurred and additional loss may be difficult to stop.
In most of the cases, glaucoma is related with higher-than-normal pressure inside the eye a condition called ocular hypertension. But it mayalso happen when intraocular pressure (IOP) is normal. If not treated or not controlled, glaucoma first causes peripheral vision loss and eventually can lead to blindness.

Open-Angle Glaucoma (OAG)
Narrow Angle Glaucoma (NAG)
Glaucoma is generally known as the "silent thief of sight," since most of the times no pain occurs and no symptoms are produced until noticeable vision loss occurs.
Due to this reason, glaucoma generally grows undetected until the optic nerve already has been irreversibly damaged, with varying degrees of permanent vision loss. With acute angle-closure glaucoma, symptoms that occur unexpectedly may include blurry vision, halos around lights, intense eye pain, nausea and vomiting.

Diagnosis, Screening and Tests for Glaucoma
During routine eye exams, a tonometer is used to measure intraocular pressure, or IOP. Eye is numbed with eye drops, and a small probe gently rests against the eye's surface. Other tonometers send a puff of air onto the eye's surface.
An unusually high IOP reading shows a problem with the amount of fluid (aqueous humor) in the eye i.e. either the eye is producing too much fluid, or it is not draining properly.
Normally, IOP should be below 21 mmHg (millimeters of mercury) based on how much force is exerted within a certain defined area.
If the IOP is higher than 30 mmHg, the risk of vision loss from glaucoma is 40 times greater than someone with intraocular pressure of 15 mmHg or lower.
Other methods of monitoring the glaucoma involves the use of sophisticated imaging technology which are mentioned as follows:-
a. scanning laser polarimetry (SLP),
b. optical coherence tomography (OCT) and
c. confocal scanning laser ophthalmoscopy