"Sometimes the damage is immediate and the patient collapses on the chiropractor's table. Sometimes mild symptoms start immediately and progress after the patient leaves the chiropractor's office. Sometimes the tear is a small one and it clots over; then days later the clot breaks loose, travels to the brain and causes a delayed stroke. By this time, the patient may not connect his sudden collapse to the previous visit to the chiropractor..…How often can a stroke be attributed to neck manipulation? We really don't know. Estimates have varied from one in ten million manipulations to one in 40,000. I should clarify that only one specific type of stroke, basilar stroke, has been linked to chiropractic. It has been estimated that about 20% of all basilar strokes are due to spinal manipulations. This would work out to about 1300 a year in the U.S. But we just don't know, because it has not been properly studied. Carotid artery strokes have also been reported after chiropractic treatments. Chiropractors do not follow up on every patient. Patients who have delayed strokes may never see their chiropractor again, so chiropractors would naturally tend to underestimate the risk. Many of these diagnoses are missed because the vertebral arteries are not typically examined on autopsy." Article by Harriet Hall MD, Science Based Medicine (29th April 2008)