A dissection is a tear within the wall of a blood vessel, which allows blood to separate the wall layers. Usually, a dissection is an arterial wall dissection, but rarely it may be a vein wall dissection (VWD).By separating a portion of the wall of the artery (a layer of the tunica intima or tunica media), a dissection creates two lumens or passages within the vessel, the native or true lumen, and the "false lumen" created by the new space within the wall of the artery. [Source: Wikipedia 
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