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Posterior Ischemic Optic Neuropathy (PION) is a medical condition characterized by damage to the retrobulbar portion of the optic nerve due to ischemia, a restriction of the blood supply to the optic nerve. Despite the term posterior this pathophysiology may be applied to cases where the ischemic damage is anterior, as the condition describes a particular mechanism of visual loss as much as the location of damage in the optic nerve. Anterior ischemic optic neuropathy (AION) is distinguished from PION by the fact that AION occurs spontaneously and unilaterally in patients with predisposing anatomy and cardiovascular risk factors. == Causation == PION is a watershed infarction of the optic nerve that may cause either unilateral or, more often, bilateral blindness. PION typically occurs in two categories of patients: * patients who have undergone non-ocular surgery that is particularly prolonged or is associated with a significant blood loss. * patients who have experienced significant hemorrhaging from an accident or ruptured blood vessels. In these cases, the hematocrit (percentage of blood cells in the blood) is low and often the blood pressure is also low. This combination can produce shock, and PION has sometimes been called ''shock-induced optic neuropathy''. The combination of low blood count and low blood pressure means that the blood is carrying less oxygen to the tissues. Many end-organs may suffer from the lack of oxygen, including the brain, kidneys or heart. What may put the optic nerve at particular disadvantage is the superimposition of a compartment syndrome. When tissues suffer from lack of oxygen, they swell. The optic nerve is, however, confined within a bony canal between the brain and eye. Not being free to swell may cause further increased pressure on the optic nerve, reducing blood flow into the nerve, particularly in combination with low blood pressure. Restricted blood flow can result in permanent damage to the optic nerve with resultant blindness, which is often bilateral, causing some patients to wake up permanently blind after long or difficult surgeries. For technical reasons this occurs more frequently with spinal surgeries. 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 ischemic optic neuropathy」の詳細全文を読む スポンサード リンク
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