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Soldiers with deep wounds sometimes feel no pain at all for hours, while people without any detectable injury live in chronic physical anguish. How to explain that?

Over drinks in a Boston area bar, Ronald Melzack, a psychologist, and Dr. Patrick Wall, a physiologist, sketched out a diagram on a cocktail napkin that might help explain this and other puzzles of pain perception. The result, once their idea was fully formed, was an electrifying theory that would become the founding document for the field of modern pain studies and establish the career of Melzack, whose subsequent work deepened medicine’s understanding of pain and how it is best measured and treated.

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Dr. Melzack died on Dec. 22 in a hospital near his home in Montreal, where he lived, his daughter, Lauren Melzack, said. He was 90, and had spent most of his professional life as a professor of psychology at McGill University.

When Dr. Melzack and Wall, then at the Massachusetts Institute of Technology, met that day in 1959 or 1960 (accounts of their encounter vary), pain perception was thought to work something like a voltmeter, in which nerves send signals up to the brain that reflect the severity of the injury. But that model failed to explain not only battlefield experience but a host of clinical findings and everyday salves. Most notably, rubbing a wound lessens its sting — and accounting for just that common sensation proved central to the new theory.

Doctors knew that massaging the skin activated so-called large nerve fibers, which are specialized to detect subtle variations of touch, and that deeper, small fibers sounded the alarm of tissue damage. The two researchers reasoned that all these sensations must pass through a “gate” in the spinal cord, which adds up their combined signals before sending a message to the brain. In effect, activating the large fibers blocks signals from the smaller ones by closing the gate.

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Over several years, Wall worked from the bottom up, mapping out the circuitry that was likely to be involved. Dr. Melzack worked from the top down, detailing the various ways the mind moderates and shapes the messages coming through the gate. Injured soldiers often don’t feel pain right away because their brains are focused on bigger threats to survival, like a firefight. Chronic pain without injury is not common but suggests some misfiring in the circuitry.

In a 1965 paper published in the journal Science, “Pain Mechanisms: A New Theory,” the pair rolled out their comprehensive, psycho-biological model for how the body processes pain.

Pain, they concluded, is “a linguistic label for a rich variety of experiences and responses” that are “sequentially reexamined over long periods” by the body.

The new model, called the gate control theory, met with some initial skepticism. But it soon became the touchstone for an emerging, and critical, field of study.

“Finally, for first time, people had a model to try to prove or disprove,” Jeffrey Mogil, a professor of pain studies at McGill, said in a phone interview. “There was no real pain research field before the gate control theory; after it, people started thinking of themselves as pain researchers, and soon an international society was established.”

Doctors loved the theory, and so did their patients, said Dr. Allan Basbaum, chairman of the anatomy department at the University of California, San Francisco. “The key was the gate,” he said in an interview. “Turn it one way and it closes, and the other way it opens. Whether the information that gets to the brain causes pain depends on the balance of activity in small and large fibers coming through the gate.”

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In the 1970s, Dr. Melzack turned to another problem he had been thinking about for years: pain measurement. At the time, doctors had only crude instruments, like simply asking people to rate their pain level on a scale from 1 to 10 (a method still in use). As a young researcher, Dr. Melzack had worked in a chronic pain clinic and befriended a 70-year-old woman with diabetes.

“She was a highly intelligent person with a good vocabulary, and I began to collect her descriptive words about pain, like ‘burning,’ ‘shooting,’ ‘horrible’ and ‘excruciating,’ ” he told McGill Reporter in a 2008 interview.

He continued to build his adjective collection by listening to many patients’ descriptions and, working with a statistician, divided them into 20 categories, each describing a particular kind of pain: “tugging,” “pulling” and “wrenching” in one category, for instance, and “pinching,” “pressing” and “gnawing” in another.

This descriptive catalog, published in the journal Pain in 1975, became the McGill Pain Questionnaire. It soon became a standard measure worldwide, deeply enriching the conversations doctors have with their patients, and in many cases helping with diagnosis.

“Scientists go their entire career trying for one home run, one big finding that makes a difference, and most never get it,” Mogil said. “Ron hit two home runs; it’s not an exaggeration to say he’s the greatest pain researcher who ever lived.”

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Ronald Hyman Melzack was born on July 19, 1929, in Montreal, the youngest of four children of Joseph and Annie (Mandel) Melzack, Jewish immigrants from Poland. His mother worked in the household, and his father, after a variety of jobs, started the store Classic Books, which would later become a national chain in Canada.

Ronald studied psychology as an undergraduate at McGill and went on to complete a PhD there in 1964, under renowned neuroscientist Donald Hebb. He did postdoctoral research under Dr. William Livingston at the University of Oregon Medical School and spent several years at MIT before returning to McGill to finish his career. He married Lucy Birch, an interior designer, in 1960.

His siblings died before him. In addition to his daughter, he is survived by his wife and a son, Joel.

Wall died in 2001.

Dr. Melzack wrote two popular books about the science of pain, “The Puzzle of Pain” (1973) and, with Wall, “The Challenge of Pain” (1996). He became a prominent presence at McGill, having helped establish the McGill Pain Clinic, and received many awards, including the Order of Canada and l’Ordre du Quebec — as well as numerous best-instructor awards, as voted by McGill students.

By all accounts, he wore the honors lightly. “He was gentle with everyone,” Basbaum said. “He’d send me recommendations for his students, and I could never tell which ones he thought were really good. He wrote positive things about all of them.”

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