The front of the eye is a D-shaped chamber in which the lens and iris (the coloured part) make up the straight wall, while the cornea (the surface of the eye) forms the curved wall. A gland behind the upper eyelid fills this chamber with a clear liquid (aqueous humour) that supplies the front of the eye with oxygen and nutrients and keeps it inflated. A steady supply of liquid is produced, and it drains out through a mesh of tiny holes behind the lower eyelid, called the trabecular meshwork.

In glaucoma, the liquid is produced normally, but the trabecular meshwork can't drain it due to clogging or some other reason. Liquid pressure builds up in the eye, pressing on the optic nerve (the nerve that links the eye to the brain). The nerve cells are then slowly strangled of blood, eventually dying. The outer nerves fail first, so vision loss tends to start at the edges, progressing to "tunnel vision" and blindness. Many people don't notice this at first, and there's usually no pain, so glaucoma can be quite advanced before it's detected. The US Glaucoma Foundation estimates that only 50% people with glaucoma are aware of the disease.

What is "normal" intraocular pressure?

As you would expect, "normal" varies from person to person. Normal eye pressures can fall within a range of 10 to 21 millimeters of mercury, with most people having normal eye pressure at 21 millimeters. However, eye pressure can fluctuate from hour to hour, day to day, and month to month.

Risk factors for glaucoma

  • elevated liquid pressure in the eye
  • being over 55 years of age
  • having a family history of glaucoma
  • being very nearsighted
  • having diabetes
  • being of African or Hispanic ethnicity
  • long-term use of corticosteroids (prednisone, cortisone)

original article by Frederick S. Mikelberg, MD
with revisions by the MediResource Clinical Team