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A posterior vitreous detachment (PVD) is a condition of the eye in which the vitreous membrane separates from the retina. It refers to the separation of the posterior hyaloid membrane from the retina anywhere posterior to the vitreous base (a 3 - 4 mm wide attachment to the ora serrata.) Broadly speaking, the condition is common for older adults and over 75% of those over the age of 65 develop it. Although less common among people in their 40s or 50s, the condition is not rare for those individuals. Some research has found that the condition is more common among women.〔(【引用サイトリンク】title=Posterior vitreous detachment )〕 ==Causes and risk factors== The vitreous (Latin="glassy") humor is a gel which fills the eye behind the lens. Between it and the retina is the vitreous membrane. Over time the vitreous humor changes, shrinking and developing pockets of liquefaction, similar to the way a gelatin dessert shrinks, or detaches, from the edge of a pan over time. At some stage the vitreous membrane may peel away from the retina. This is usually a sudden event, but may also occur slowly over months. Age and refractive error play a role in determining the onset of PVD in a healthy person. PVD is rare in emmetropic people under the age of 40 years, and increases with age to 86% in the 90s. Several studies have found a broad range of incidence of PVD, from 20% of autopsy cases to 57% in a more elderly population of patients (average age was 83.4 years). People with myopia (nearsightedness) greater than 6 diopters are at higher risk of PVD at all ages. Posterior vitreous detachment does not directly threaten vision. Even so, it is of increasing interest because the interaction between the vitreous body and the retina might play a decisive role in the development of major pathologic vitreoretinal conditions, such as epiretinal membrane. PVD may also occur in cases of cataract surgery, within weeks or months of the surgery. The vitreous membrane is more firmly attached to the retina anteriorly, at a structure called the vitreous base. The membrane does not normally detach from the vitreous base, although it can be detached with extreme trauma. However, the vitreous base may have an irregular posterior edge. When the edge is irregular, the forces of the vitreous membrane peeling off the retina can become concentrated at small posterior extensions of the vitreous base. Similarly, in some people with retinal lesions such as lattice retinal degeneration or chorio-retinal scars, the vitreous membrane may be abnormally adherent to the retina. If enough traction occurs the retina may tear at these points. If there are only small point tears, these can allow glial cells to enter the vitreous humor and proliferate to create a thin epiretinal membrane that distorts vision. In more severe cases, vitreous fluid may seep under the tear, separating the retina from the back of the eye, creating a retinal detachment. Trauma can be any form from a blunt force trauma to the face such as a boxer's punch or even in some cases has been known to be from extremely vigorous blowing of the nose. 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「Posterior vitreous detachment」の詳細全文を読む スポンサード リンク
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