Anterior chamber (chamber in the front portion of the eye) is filled by a fluid called aqueous humour, which bathes and nourishes the tissue of the eye. From one portion it enters the eye and from other portion it gets drained. It is a continuous dynamic process. If the drainage of this fluid is restricted for reasons yet unknown, pressure gradually builts up within the eye causing glaucoma.
Persons above 40 years are likely to be affected, however the disease may appear at any age. Persons with myopia, diabetes, systemic hypertension and family history of glaucoma have an increased risk.
Most of the time it is symptoms less. There is a gradual loss of peripheral vision which is not possible for a human being, to experience in initial stage. It usually diagnosed during a routine eye examination.
    A small group of patients has narrow angle glaucoma where they get symptoms like frequent attacks of colour halo rings around light, headache with blurring of vision. They on account of symptoms present early and get the treatment.
Apart from measuring  intraocular pressure  by applanation tonometer  other test like gonioscopy , optic nerve head evaluation , visual field charting by automated perimeter  are carried out to establish the diagnosis of glaucoma.
Loss of vision due to glaucoma is irreversible, therefore earlier the diagnosis, better the chances of arresting visual impairment.

For most people with glaucoma regular use of medicines will control the increased intraocular pressure. However in some drugs may stop working after a period of time. In this situation patient again may not appreciates the further loss of vision. The ophthalmologist can help by adding or changing medication or choosing another type of treatment like surgical method. These periodic eye examinations are very essential to ensure that the medications are working.