How common is a carotid artery dissection?

Uncommon. A dissection is a separation of the inner layer of an artery from the outer layer. The risk aside from direct obstruction is that the internal blood pressure can enter this new pocket and rapidly tear open more of the lining, shutting down the artery or even causing a leak - both of these events are deadly. Any major arteries can be affected; the carotid is less common than the aorta.