Presented by: Ian Carroll, MD, MS
Assistant Professor of Anesthesia
Stanford University Medical Center
October 17, 2013
The human body contains a complex network of nerve cells that, once activated by an external stimulus, function to transmit the sensation of pain. The nerves remain quiet until the stimulus is strong enough, and the resulting pain is a product of the nervous system functioning properly. This is the kind of pain that arises from hitting your hand with a hammer or by getting kicked in the shin.
Another kind of pain involves damage to the nerve. This condition, called neuropathic pain, is caused by abnormal and continuous firing of the pain-transmitting nerves.
Pain is a complicated process that involves an intricate interplay of chemicals and signaling in the central nervous system (the brain and spinal cord). A painful stimulus activates specialized nerve cells called nociceptors that convert the information into electrochemical messages. The signal for pain is transmitted to the spinal cord, where the message is communicated to secondary neurons that pass it up to the brain. There the message is split into two sections: to the outer cortex, which controls the body’s sense of location and intensity, and to the areas of the brain involved with feelings, emotions, and motivation.
“The sensation of pain is in fact a behavior,” said Ian Carroll, MD, MS, an assistant professor of anesthesiology, who spoke at a presentation sponsored by the Stanford Hospital health Library.
When the nerves are damaged, through trauma, disease, infection, or other reasons, they start to express genes that are not normally active. These genes encode proteins on the cell surface that essentially generates an electric charge. “So instead of serving as a sensor for pain, the nociceptor fires by itself, something like heart pacemaker cells,” said Dr. Carroll. The more intense the pain, the larger the area encompassed.”
The sense of expanded pain can make it difficult for physicians to locate the actual source, since patients cannot tell where the pain is coming from.
Neuropathy symptoms often improve with time, especially if it is caused by an underlying condition that can be treated. A number of medications are used to reduce the pain.
One of the most commonly prescribed drugs is duloxetine, which is also used to treat depression. Dr. Carroll starts patients at a very dose to allow them to acclimate to the drug and to avoid side effects like nausea. He emphasized how important it is to start medications at a very low level—so low that patients sometimes complain that the drug isn’t helping.
The low initial dose helps determine an individual’s metabolic response so that the most effective level can be prescribed—some people respond just as well to a low dose, depending on their metabolism.
Dr. Carroll has found that many of his patients respond better to desipramine, a trycyclic antidepressant. The drug appears to activate pathways in the spinal cord that block pain signals from ascending to the brain. Because its ingredients show up in plasma levels, physicians can easily adjust the dosage for how each patient metabolizes the medication.
“It’s inexpensive, easy to dose, and one of the most effective drugs available for nerve pain,” he said.
Nerve pain can be hard to control, but the proper type and level of medication can help. If one approach doesn’t work, another one might.
About the Speaker
Ian Carroll, MD, is an assistant professor of anesthesiology, perioperative, and pain medicine, with a special interest in neuropathic pain. He received his MD from Columbia University and completed his internship, residency, and fellowship at Stanford, where he became a faculty member in 2003. He is Board Certified in pain medicine and in anesthesiology by the American Board of Anesthesiology.
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