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. The basilar artery, which arises from the union of the two vertebral arteries and forms a wide conduit at the base of the brain, was of particular concern. It feeds such structures as the pons (which plays a role in respiration), the cerebellum (which coordinates the muscles), the occipital lobe of the outer brain (which turns eye impulses into images) and the thalamus (which relays sensory messages to the outer brain). Reductions in blood flow to the basilar artery are known to produce a variety of strokes. These rarely affect language and conscious thinking (often said to be located in the frontal cortex) but can severely damage the body’s core machinery and sometimes be fatal. The majority of patients suffering such a stroke do recover most functions. But in some cases headaches, imbalance, dizziness and difficulty in making fine movements persist for years.

Russell also worried that when strokes hit yoga practitioners, doctors might fail to trace their cause. The cerebral damage, he wrote, “may be delayed, perhaps to appear during the night following, and this delay of some hours distracts attention from the earlier precipitating factor.”

In 1973, a year after Russell’s paper was published, Willibald Nagler, a renowned authority on spinal rehabilitation at Cornell University Medical College, published a paper on a strange case. 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. Given the lack of advanced imaging technologies at the time, an exploratory operation was conducted to get a clearer sense of her injuries. The surgeons who opened her skull found that the left hemisphere of her cerebellum suffered a major failure of blood supply that resulted in much dead tissue and that the site was seeped in secondary hemorrhages.

The patient began an intensive program of rehabilitation. Two years later, she was able to walk, Nagler reported, “with [a] broad-based gait.” But her left arm continued to wander and her left eye continued to show Horner’s syndrome. Nagler concluded that such injuries appeared to be rare but served as a warning about the hazards of “forceful hyperextension of the neck.” He urged caution in recommending such postures, particularly to individuals of middle age.

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” — in other words, no blood could get through to the brain.