Dubbed the "silent thief of sight," glaucoma - a major cause of blindness among the middle-aged and elderly - can be prevented, diagnosed early and treated, but not cured. World Glaucoma Day will be marked here and abroad on Thursday to increase awareness of it. There are 4,000 Israelis who have been blinded by the disease. According to the Israel Glaucoma Society and the American Academy of Ophthalmology, glaucoma has long been considered a disease caused by elevated intraocular pressure (of fluid inside the eye). Yet, over the past two decades, an increasing number of non-pressure-dependent risk factors have been identified, suggesting that glaucoma can be defined broadly as the final common pathway of a number of different eye disorders. It may also be included in a larger group of neurodegenerative disorders - such as age-related macular degeneration and Alzheimer's - that share aspects of nerve cell death, oxidative damage and low-grade inflammation. Many patients with glaucoma have other risk factors, including reduced blood flow in their limbs (ischemia), and are more prone to sleep apnea (in which they stop breathing for seconds many times during the night). Glaucoma is diagnosed in as many as 10 percent of the population over 65, but the risk begins decades earlier and especially in those who have a family history of the eye disease, diabetes and perhaps shortsightedness (myopia), according to Dr. Dan Gaton, chairman of the Israel Glaucoma Society. "It is important to know that glaucoma does not show symptoms at the beginning. Therefore patients are not aware of the damage and have their eyes checked by an ophthalmologist only in advanced stages of the disease. There is no cure, but we know how to control it and usually prevent its critical loss of optic nerve fibers if it is detected early enough." Gaton noted that the damage starts at the periphery of the visual field, with the central visual acuity unaffected at first. Progressively the visual field becomes restricted. Most glaucoma patients are treated with special eye drops to lower the intraocular pressure; some may need pills, laser treatment or even surgery. Free glaucoma exams by ophthalmologists will be held Thursday between 10 a.m. and 4 p.m. at the Azrieli, Givatayim, Haifa and Rehovot malls. Such screenings were also held at the Knesset on Tuesday. Tel Aviv's Sourasky Medical Center has also invited the public over age 35 to a free test on Thursday, and the Rebecca Sieff Hospital in Safed will offer similar screenings the same day between 9 a.m. and 4 p.m.