To understand dissections, we first need to discuss the anatomy of a normal vessel. The normal vessel is made of 3 layers. The outer layer is the strongest and consists of connective tissue called adventitia. The middle layer is a muscle layer which is strong and thick. The inner layer or intimal layer consists of a very thin layer of endothelial cells. These cells make direct contact with the bloodstream.
A cervical artery dissection (both carotid dissection and vertebral dissection) occurs when the inner lining of the artery in either carotid artery or a vertebral artery segment in the neck separates from the rest of the artery. Any injury to the vessel may create a small crack in the intimal layer that will allow blood to penetrate the barrier and to accumulate between the muscle layer and intimal layer. Sometimes, the intimal layer will create a flap within the vessel that can also block the blood flow within the vessel. Both carotid arteries and vertebral arteries are more prone to dissections because they have a fixation point at the base of the skull where they enter. They are thinner than other vessels in the body, so it is easier for torsion or stretching of either carotid artery or vertebral artery against they are fixation point
to create enough tension that can potentially rupture it.
Less frequently, those vessels may get damaged within the skull cavity. At that time, because of their thin walls, the middle and outer layers may also get damaged and cause a leakage of blood outside of the vessel.
Common risk factors for carotid dissections and vertebral dissections include prolonged extension of the neck such as during dental procedures, washing hair at a salon, or painting the ceiling or upper trim. I have seen dissections related to roller coasters, yoga, golf, etc. Chiropractic manipulations has also been associated with dissections but there was no significant evidence of direct correlation between those manipulations and the dissection. Having a history of fibromuscular dysplasia also increases risk of dissections.
If a patient has had a dissection, I strongly recommend against any chiropractic manipulations in the future. It is best to avoid hyperextension of the neck for long periods of time. If a prolonged dental procedure is required, try to divide the procedure into several stages if possible. As for yoga, I just recommend trying to keep the head in a neutral position. If there is a history with dissections, try not to play golf for one to two years afterwards as this can cause another problem.