Michelle McFarland, age 14, of Portland, Ore., for her question:

WHAT IS GLAUCOMA?

Glaucoma is an eye disease that may cause blindness. It occurs when the eye's aqueous fluid cannot drain into blood vessels outside the eye. Pressure that builds up may damage the optic nerve.

The most common of four main types of glaucoma is chronic simple glaucoma. It usually affects people over 40 years of age. This type of glaucoma is usually inherited and frequently escapes notice as it slowly and permanently destroys the optic nerve.

Side vision is affected first and central vision later. Most victims retain their sight for many years when treated with drugs and, if necessary, surgery.

Congenital glaucoma may appear at birth or soon afterword. It is also called "ox eye" because the eyes become greatly enlarged. Blindness may result unless an operation is performed.

Acute congestive glaucoma may occur suddenly at any time. Its symptoms include seeing rainbowlike rings around lights, redness of the eye and severe pain.