A 43 year-old woman noted a "popping" sensation in the right neck while on the toilet. Thirty minutes later, she developed an acute left hemiplegia.

MRI - Axial images of the lower brainstem - fat suppression images (i.e., dissection protocol) - notice the normal flow void (black signal in the left internal artery and the bright signal in the right internal carotid area. The bright signal is blood, indicating a dissection of the carotid artery.  On the left image, note that the dissection has spread and created a pseudo-aneurysm.  During a dissection, the inner wall of the artery tears, allowing blood to enter the wall of the vessel, effectively narrowing or occluding the lumen. It may also expand the wall outwards creating a pseudo-aneurysm.  In the case of the carotid artery, as the sympathetic fibers run in the carotid sheath and the carotid expands from the dissection, the sympathetic fibers are disrupted resulting in the ipsilateral Horner's syndrome.  The major morbidity of dissection is stasis of blood flow past the point of dissection, leading to fibrin clot, then distal embolus and stroke.