Glaucoma is an increase in pressure within the eyeball. The problem results from blockage of the gland that drains intraocular fluids (vitreous humor and aqueous humor) and overproduction of the glands that secrete the fluids. The condition is painful and symptoms include a bulging reddened eye with prominent blood vessels. A tonometer is a special instrument capable of measuring intra-ocular pressure. Left untreated, glaucoma can cause blindness. Glaucoma is treated with drugs and or surgery.