Arterial Dissections are tears in the inner most layer of the artery (the intima). This disrupts the normal flow of blood through the artery. Now, blood has two places it can flow: the normal passageway (called the 'true lumen'), in which case it flows like usual. The second passageway is outside the tear-- so blood is flowing inbetween the layers of the blood vessels; the intima is on one side while the rest of the blood vessel (made up of the media and adventitia) are on the other side.
Arterial dissections cause a variety of symptoms or can be entirely asymptomatic. The symptoms depend on the anatomic location of the dissection and the extent of the tear in the blood vessel wall. A risk of dissections is a compromise of blood flow to the organ which that blood vessel is meant to provide blood. For example, dissection of the splenic artery can compromise blood flow the the spleen. Meanwhile a dissection of the renal artery (which carries blood to the kidney) can cause a compromise of blood flow to the kidney. One final complication of dissections can be the subsequent formation of an aneurysm within the area of the dissection.