Explaining Phantom Limbs and Pain
The phantom limb was first proposed by Silas Weir Mitchell, an American physician who tended the wounded at Gettysburg and became intrigued by an epidemic of phantoms. Civil War soldiers' wounded arms and legs often turned gangrenous, and in an age before antibiotics, the only way to save the soldier's life was to amputate the limb before the gangrene spread. Soon amputees began to report that their limbs had returned to haunt them. Mitchell first called these experiences "sensory ghosts," then switched to calling them "phantom limbs." They are often very lively entities. Patients who have lost arms can sometimes feel them gesticulating when they talk, waving hello to friends, or reaching spontaneously for a ringing phone.
Phantom limbs are troubling because they give rise to a chronic "phantom pain" in 95 percent of amputees that often persists for a lifetime. But how do you remove a pain in an organ that isn't there?
The Sherlock Holmes of modern neurology, V. S. Ramachandran, had been curious about phantoms since medical school. Then in 1991 he read the paper by Tim Pons and Edward Taub about the final operations on the Silver Spring monkeys. As you'll recall, Pons mapped the brains of the monkeys who had had all the sensory input from their arms to their brains eliminated by deafferentation and found that the brain map for the arm, instead of wasting away, had become active and now processed input from the face — which might be expected because, as Wilder Penfield had shown, the hand and facial maps are side by side.
Ramachandran immediately thought that plasticity might explain phantom limbs because Taub's monkeys and patients with phantom arms were similar. The brain maps for both the monkeys and the patients had been deprived of stimuli from their limbs. Was it possible that the face maps of amputees had invaded the maps for their missing arms, so that when the amputee was touched on the face, he felt his phantom arm? And where, Ramachandran wondered, did Taub's monkeys feel it when their faces were stroked — on their faces, or in their "deafferented" arm?
Tom Sorenson — a pseudonym — was only seventeen years old when he lost his
arm in an automobile accident. As he was hurled into the air, he looked back and saw his hand, severed from his body, still grabbing the seat cushion. What remained of his arm had to be amputated just above the elbow.
About four weeks later he became aware of a phantom limb that did many of the things his arm used to. It reached out reflexively to break a fall or to pat his younger brother. Tom had other symptoms, including one that really irked him. He had an itch in his phantom hand that he couldn't scratch. Ramachandran heard of Tom's amputation from colleagues and asked to work with him. To test his theory that phantoms were caused by rewired brain maps, he blindfolded Tom. Then he stroked parts of Tom's upper body with a Q-tip, asking Tom what he felt. When he got to Tom's cheek, Tom told him he felt it there but also in his phantom.
When Ramachandran stroked Tom's upper lip, he felt it there but also in the index finger of his phantom. Ramachandran found that when he touched other parts of Tom's face, Tom felt it in other parts of his phantom hand. When Ramachandran put a drop of warm water on Tom's cheek, he felt a warm trickle move down his cheek and also down his phantom limb. Then after some experimentation Tom found that he could finally scratch the un-scratchable itch that had plagued him for so long by scratching his cheek.
After Ramachandran's success with the Q-tip, he went high-tech with a brain scan called an MEG, or magnetoencephalography. When he mapped Tom's arm and hand, the scan confirmed that his hand map was now being used to process facial sensations. His hand and face maps had blurred together.
Ramachandran's finding in the Tom Sorenson case, at first controversial among clinical neurologists who doubted brain maps were plastic, is now widely accepted. Brain scan studies by the German team that Taub works with have also confirmed a correlation between the amount of plastic change and the degree of phantom pain people experience.
Ramachandran strongly suspects that one reason map invasion occurs is that the brain "sprouts" new connections. When a part of the body is lost, he believes, its surviving brain map "hungers" for incoming stimulation and releases nerve growth factors that invite neurons from nearby maps to send little sprouts into them.
Normally these little sprouts link up to similar nerves; nerves for touch link with other nerves for touch. But our skin, of course, conveys far more than touch; it has distinct receptors that detect temperature, vibration, and pain as well, each with its own nerve fibers that travel up to the brain, where they have their own maps, some of which are very near each other. Sometimes after an injury, because the nerves for touch, temperature, and pain are so close together, there can be cross-wiring errors.
Another reason phantoms are so unpredictable and cause so much trouble is that brain maps are dynamic and changing: even under normal circumstances, as Merzenich showed, face maps tend to move around a bit in the brain. Phantom maps move because their input has been so radically changed. Ramachandran and others — Taub and his colleagues among them — have shown with repeated scans of brain maps that the contours of phantoms and their maps are constantly changing. He thinks one reason people get phantom pain is that when a limb is cut off, its map not only shrinks but gets disorganized and stops working properly.
Normally, when the motor command center in the brain sends out an order to move the arm, the brain gets feedback from various senses, confirming that the order has been executed. But the brain of a person without a limb never gets confirmation that the arm has moved, since there are neither arm nor motion sensors in the arm to provide that feedback. Thus, the brain is left with the impression that the arm is frozen. Because the arm had been stuck in a cast or sling for months, the brain map developed a representation of the arm as unmoving. When the arm was removed, there was no new input to alter the brain map, so the mental representation of the limb as fixed became frozen in time — a situation similar to the learned paralysis that Taub discovered in stroke patients.
Ramachandran came to believe that the absence of feedback causes not only frozen phantoms but phantom pain. The brain's motor center might send commands for the hand muscles to contract but, getting no feedback confirming the hand has moved, escalates its command, as if to say: "Clench! You're not clenching enough! You haven't touched the palm yet! Clench as hard as you can!" These patients feel their fingernails are digging into their palms. While actual clenching caused pain when the arm was present, this imaginary clenching evokes pain because maximum contraction and pain are associated in memory.
Excerpted from the book: The Brain That Changes Itself
Stories of Personal Triumph from the Frontiers of Brain Science
By Norman Doidge, MD, Penguin Publishing, December, 2007