For some reason the NYT is all about neck injury lately. In yesterday’s discussion of a possible chiropractic induced injury, Russell asked:
But given all the other stresses people put on their necks, from accidents such as headbumps, from purposeful athletics such as whacking soccer balls, and from just craning one’s head in odd positions when performing various kinds of mechanical labor, it puzzles me that the risk from a chiropractor would be much greater than the risks from these other kinds of use/abuse. Of course, this is not excuse for the chiropractor, who is imposing that risk, likely on those more susceptible to injury, under false pretense or treating disease. It’s more a general lament that we each carry so much haphazard anatomy.
Interesting he should mention this as today the NYT has an article How Yoga Can Wreck Your Body describing many ways that neck hyperextension during this popular exercise can also create similar injuries to the vertebral and carotid arteries.
The mechanism is similar…
From the article, a discussion of a small literature describing yoga induced injuries emphasizes the importance of not taking big risks with your neck:
In 1972 a prominent Oxford neurophysiologist, W. Ritchie Russell, published an article in The British Medical Journal arguing that, while rare, some yoga postures threatened to cause strokes even in relatively young, healthy people. Russell found that brain injuries arose not only from direct trauma to the head but also from quick movements or excessive extensions of the neck, such as occur in whiplash — or certain yoga poses. Normally, the neck can stretch backward 75 degrees, forward 40 degrees and sideways 45 degrees, and it can rotate on its axis about 50 degrees. Yoga practitioners typically move the vertebrae much farther. An intermediate student can easily turn his or her neck 90 degrees — nearly twice the normal rotation.
Hyperflexion of the neck was encouraged by experienced practitioners. Iyengar emphasized that in cobra pose, the head should arch “as far back as possible” and insisted that in the shoulder stand, in which the chin is tucked deep in the chest, the trunk and head forming a right angle, “the body should be in one straight line, perpendicular to the floor.” He called the pose, said to stimulate the thyroid, “one of the greatest boons conferred on humanity by our ancient sages.”
Extreme motions of the head and neck, Russell warned, could wound the vertebral arteries, producing clots, swelling and constriction, and eventually wreak havoc in the brain.
…
A healthy woman of 28 suffered a stroke while doing a yoga position known as the wheel or upward bow, in which the practitioner lies on her back, then lifts her body into a semicircular arc, balancing on hands and feet. An intermediate stage often involves raising the trunk and resting the crown of the head on the floor. While balanced on her head, her neck bent far backward, the woman “suddenly felt a severe throbbing headache.” She had difficulty getting up, and when helped into a standing position, was unable to walk without assistance. The woman was rushed to the hospital. She had no sensation on the right side of her body; her left arm and leg responded poorly to her commands. Her eyes kept glancing involuntarily to the left. And the left side of her face showed a contracted pupil, a drooping upper eyelid and a rising lower lid — a cluster of symptoms known as Horner’s syndrome. Nagler reported that the woman also had a tendency to fall to the left.
Her doctors found that the woman’s left vertebral artery, which runs between the first two cervical vertebrae, had narrowed considerably and that the arteries feeding her cerebellum had undergone severe displacement.
…
The experience of Nagler’s patient was not an isolated incident. A few years later, a 25-year-old man was rushed to Northwestern Memorial Hospital, in Chicago, complaining of blurred vision, difficulty swallowing and controlling the left side of his body. Steven H. Hanus, a medical student at the time, became interested in the case and worked with the chairman of the neurology department to determine the cause (he later published the results with several colleagues). The patient had been in excellent health, practicing yoga every morning for a year and a half. His routine included spinal twists in which he rotated his head far to the left and far to the right. Then he would do a shoulder stand with his neck “maximally flexed against the bare floor,” just as Iyengar had instructed, remaining in the inversion for about five minutes. A series of bruises ran down the man’s lower neck, which, the team wrote in The Archives of Neurology, “resulted from repeated contact with the hard floor surface on which he did yoga exercises.” These were a sign of neck trauma. Diagnostic tests revealed blockages of the left vertebral artery between the c2 and c3 vertebrae; the blood vessel there had suffered “total or nearly complete occlusion”
Two months after his attack, and after much physical therapy, the man was able to walk with a cane. But, the team reported, he “continued to have pronounced difficulty performing fine movements with his left hand.” Hanus and his colleagues concluded that the young man’s condition represented a new kind of danger. Healthy individuals could seriously damage their vertebral arteries, they warned, “by neck movements that exceed physiological tolerance.” Yoga, they stressed, “should be considered as a possible precipitating event.”
The mechanisms of all of these injuries and that of neck hyperextension by a chiropractor are similar, and have to do with an unfortunate quirk of neck anatomy. The vertebral arteries, which feed the posterior portion of the circle of Willis and much of the posterior structures of the brain, travel through a thin bony canal in the spinous processes of your cervical vertebrae. 
(Image via the full wiki)
When the neck is stretched or rotated in the course of normal movements there is plenty of give in the artery and between the sites of fixation of the spinal processes, but with extreme rotation or hyperflexion the artery can be dangerously stretched between two fixed points. The common injury is a dissection – the formation of a flap within the artery from a tear in the wall. It would be unusual for an artery to completely tear open with extravasation of blood, but the flap created from a dissection is dangerous because it can be a source for clots causing embolic strokes, as well as occlusion of the vessel or extension of the dissection with risk of rupture. I have seen these injuries from motor vehicle accidents when there is a whiplash injury of the neck, and they can be quite serious.
The point here isn’t to disparage Yoga which can be healthy exercise, and probably a lot safer than bicycle riding on busy streets or running at night. But people should know the limits of what the body can do, and be very careful about injuring certain areas of the body which can be devastating. Be very careful with your neck and head. Wear helmets when traveling at speed. Wear a running vest at night so you don’t get clocked by a drunk. And when some Yogi comes along and says putting your body weight on the back of your neck is a panacea and will cure your thyroid? Tell them to shove it.
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