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Glaucoma is a group of eye diseases which result in damage to the optic nerve and vision loss. A major risk factor is increased pressure in the eye. The disorders can be roughly divided into two main categories: "open-angle" and "closed-angle" (or "angle closure") glaucoma. Open-angle chronic glaucoma is painless, tends to develop slowly over time and often has no symptoms until the disease has progressed significantly. Closed angle glaucoma is usually chronic and asymptomatic but can present all of a sudden as well. This involves sudden eye pain, blurred vision, mid-dilated pupil, redness, nausea and vomiting, resulting from a sudden spike in intraocular pressure from iridotrabecular contact. Glaucoma can permanently damage vision in the affected eye, first by decreasing peripheral vision (reducing the visual field), and then potentially leading to blindness if left untreated. The many subtypes of glaucoma can all be considered to be a type of optic neuropathy. The nerve damage involves loss of retinal ganglion cells in a characteristic pattern. Raised intraocular pressure (above ) is the most important and only modifiable risk factor for glaucoma. Some may have high eye pressure for years and never develop damage, a condition known as "ocular hypertension". Conversely, the term 'low tension' or 'normal tension' glaucoma is used for those with optic nerve damage and associated visual field loss, but normal or low intraocular pressure. Closed-angle glaucoma also involves damage to the optic nerve, however, is characterized by closure of part of the filtration angle as a result of iris apposition to the trabecular meshwork(iridotrabecular contact). If the condition is detected early enough, it is possible to arrest the development or slow the progression with medical and surgical means. Open angle chronic glaucoma is treated with either glaucoma medication to lower the pressure, or with various pressure-reducing glaucoma surgeries. Treatment of closed angle glaucoma involves medication to bring the intraocular pressure down and laser surgery to the iris to open the drainage angle and hence reduce the eye pressure. Glaucoma has been called the "silent thief of sight" because the loss of vision often occurs gradually over a long period, and symptoms only occur when the disease is quite advanced. Worldwide, glaucoma is the second-leading cause of blindness after cataracts. It is also the leading cause of blindness among African Americans.〔(【引用サイトリンク】publisher=National Eye Institute )〕 Although the term "glaucoma" has a history relating to disorders of the eye going back to ancient Greece, in English the word was not commonly used until after 1850, when the development of the ophthalmoscope permitted visualization of the optic nerve damage caused by glaucoma. == Types== Glaucoma is an umbrella term for eye conditions which damage the optic nerve, and which can lead to a loss of vision. The main cause of damage to the optic nerve is intraocular pressure (IOP), excessive fluid pressure within the eye, which can be due to various reasons including blockage of drainage ducts, and narrowing or closure of the angle between the iris and cornea. The primary division in categorizing different types of glaucoma is open-angle and closed angle (or angle-closure) glaucoma. The open angle refers to the angle where the iris meets the cornea being as wide and open as it should be, allowing the fluid from inside the eye to drain, thus relieving the internal pressure. Where this angle is narrowed or closed, pressure can build up, and eventually damage the optic nerve leading to loss of vision. Primary open-angle glaucoma (also, ''primary glaucoma'', ''chronic glaucoma'') refers to slow clogging of the drainage canals resulting in increased eye pressure which causes progressive optic nerve damage. This manifests as a gradual loss of the visual field, starting with a loss of peripheral vision, but eventually the entire vision will be lost if not treated.〔(【引用サイトリンク】title=Primary Open-Angle Glaucoma: Glaucoma: Merck Manual Professional )〕 This is the most common type of glaucoma, accounting for 90% of cases in the United States, but fewer in Asian countries. Onset is slow and painless, and loss of vision is gradual and irreversible. Narrow angle glaucoma (also ''closed angle glaucoma'') the iris bows forward, narrowing the angle that drains the eye, increasing pressure within the eye. If untreated, it can lead to the medical emergency of angle closure glaucoma. In angle closure glaucoma (also ''closed angle glaucoma'', ''primary angle closure glaucoma'', ''acute glaucoma'') the iris bows forward and causes physical contact between the iris and trabecular meshwork, which in turn blocks outflow of the aqueous humor from within the eye. This contact may gradually damage the draining function of the meshwork until it fails to keep pace with aqueous production, and the intraocular pressure rises. Onset of symptoms is sudden, and causes pain and other symptoms that are noticeable, and is treated as a medical emergency. Unlike open-angle glaucoma, angle-closure glaucoma is a result of the angle between the iris and cornea closing. Tends to occur in the far-sighted, who have smaller than normal anterior chambers, making the physical contact more likely. Normal tension glaucoma (also ''NTG'', ''low tension glaucoma'', ''normal pressure glaucoma'') is a condition where the optic nerve is damaged although intraocular pressure (IOP) is in normal range (12-22mm Hg). At higher risk are those with family history of NTG, those of Japanese ancestry, and those with history of systemic heart disease. The cause of NTG is unknown. Secondary glaucoma refers to any case in which another disease, trauma, drug or procedure causes increased eye pressure, resulting in optic nerve damage and vision loss, and may be mild or severe. It can be due to eye injury, inflammation, a tumor, or advanced cases of cataracts or diabetes. It can also be caused by certain drugs such as steroids. Treatment depends on whether it is open-angle or angle-closure glaucoma. In pseudoexfoliation glaucoma (also, ''PEX'', ''exfoliation glaucoma'') the pressure is due to the accumulation of microscopic granular protein fibers, which can block normal drainage of the aqueous humor. PEX is prevalent in Scandinavia, primarily in those over 70, and more in women. Pigmentary glaucoma (also, pigmentary dispersion syndrome) is caused by pigment cells sloughing off from the back of the iris and floating around in the aqueous humor. Over time, these pigment cells can accumulate in the anterior chamber in such a way that it can begin to clog the trabecular meshwork. A rare condition, it occurs mostly among Caucasians, mostly males in their mid 20s to 40s, most nearsighted. Primary juvenile glaucoma is a neonate or juvenile abnormality where ocular hypertension is evident at birth or shortly thereafter and is caused by abnormalities in the anterior chamber angle development that blocks outflow of the aqueous humor. Uveitic Glaucoma is due to uveitis, the swelling and inflammation of the uvea, the middle layer of the eye. The uvea provides most of the blood supply to the retina. Increased eye pressure in uveitis can result from the inflammation itself or from the steroids used to treat it. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Glaucoma」の詳細全文を読む スポンサード リンク
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