Today is Thursday, August 28, 2008
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Nociceptive |
Neuropathic | Phantom Pain |
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What is pain? The most important fact about pain is that pain is subjective - it is what you say it is. Science, however, has provided insight into how pain is generated and therefore how it might be controlled. In general, pain from cancer can result directly from the disease or sometimes even from its treatment. Pain caused by the cancer itself can be divided into two broad categories: nociceptive pain and neuropathic pain. Nociceptive Pain Nociceptive pain is caused by damage to tissue and classified as either somatic or visceral.
Neuropathic Pain Neuropathic pain is caused when there is actual nerve damage. This type of pain is the hardest to treat because it responds poorly to opioids such as morphine. Neuropathic pain has been described as a constant, burning, shooting, or lancinating (sharp, cutting, tearing) pain. It may be due to a tumor pressing on a nerve or on the spinal cord (called “spinal cord compression”). Sometimes, a patient experiences phantom pain. Phantom pain is often associated with an organ or body part that has been removed or amputated. Researchers found the neurons in the brain that used to represent sensation in the lost limb were still functional but now driven by the stimulation of other body parts, usually the part of the body closest to the amputated limb. According to Johns Hopkins University, about 30% of women who have a breast removed experience phantom pain - an aching or itching of tissue no longer there. No standard therapy has been established for combating phantom pain although it's generally accepted that starting regional anesthesia early is important in avoiding the onset of phantom pain. Pain Resulting From Cancer Treatment At times the actual treatment for the cancer can result in pain. For example, pain from surgery may result from a surgical incision. Chronic pain can result if there has been nerve damage or other changes in the body due to the surgery. Chemotherapy can cause pain in several ways. Some chemotherapy drugs, referred to as vesicants, can harm tissues if they leak out of the vein. (Special IV catheters are now used that help reduce the chance of this occurring.) Chemotherapy can also cause sores in the mouth (stomatitis) or lining of the intestines (mucositis). Peripheral neuropathy, a tingling, numbness or pain in the extremities (hands, fingers, toes, and feet) can occur with certain chemotherapy drugs when they are given long-term in high doses. Radiation treatment can also cause pain because it can affect normal cells that surround the cancerous tumor being treated. This can sometimes cause discomfort, dry skin, difficulty swallowing or skin sores. Understanding the Mechanism of Pain Throughout the body there are small nerve cells called nociceptors (from the word noxious, meaning physically harmful or destructive), with extremely fine nerve fibers that are stimulated when pain occurs. They sense pain and then send pain messages along nerve pathways to higher centers in the brain. Conversely, the brain can send messages back to the source of the pain to lessen its “influence” (e.g.,when an athlete does not feel the pain from an injury during intense competition). In the 1960s, a pair of Canadian and English researchers speculated that in addition to such pain pathways, there were also what they termed pain-suppressing “gateways.” They theorized that when pain signals first reach the nervous system they excite activity in a group of small neurons that form a kind of pain “pool.” When the total activity of these neurons reaches a certain minimal level, a hypothetical “gate” opens up that allows the pain signals to be sent to higher brain centers. Chemical Pathways Shortly after the “gate theory” was promulgated, a landmark discovery about pain-suppressing chemicals was made by scientists in Aberdeen, Scotland and at Johns Hopkins in Baltimore. They were curious about how morphine and other opium-derived painkillers, or analgesics, worked. For some time neuroscientists had known that chemicals were important in conducting nerve signals (small bursts of electric current) from cell to cell. In order for the signal from one cell to reach the next in line, the first cell secretes a chemical "neurotransmitter" from the tip of a long fiber that extends from the cell body. The transmitter molecules cross the gap separating the two cells and attach to special receptor sites on the neighboring cell surface. Some neurotransmitters excite the second cell—allowing it to generate an electrical pain signal. Others inhibit the second cell—preventing it from generating a pain signal. When the researchers injected morphine into experimental animals, they found that the morphine molecules fit snugly into receptors on certain brain and spinal cord neurons. They therefore reasoned that there might be naturally occurring brain chemicals that behaved exactly like morphine. Both groups of scientists found not just one pain-suppressing chemical in the brain, but a whole family of such proteins. In time, these larger proteins were isolated and called endorphins, meaning the "morphine within." The discovery of the endorphins lent weight to the general concept of the gate theory. They speculated that endorphins released from brain nerve cells might inhibit spinal cord pain cells' passage through gateways along neural paths from the brain to the spinal cord. Laboratory experiments subsequently confirmed that painful stimulation led to the release of endorphins from nerve cells. Clinical investigators have tested chronic pain patients and found that they often have lower-than-normal levels of endorphins in their spinal fluid. In a few promising studies, clinical investigators have injected an endorphin called beta-endorphin under the membranes surrounding the spinal cord. Patients reported excellent pain relief lasting for many hours. Morphine compounds injected in the same area are similarly effective in producing long-lasting pain relief. But spinal cord injections or other techniques designed to raise the level of endorphins circulating in the brain require surgery and hospitalization. Also, endorphins are involved in other nervous system activities such as controlling blood flow. Increasing the amount of endorphins might therefore have undesirable effects on these and other body activities. Meanwhile, other researchers are synthesizing new analgesics and discovering painkilling virtues in drugs not normally prescribed for pain. Developments in non-drug treatments are also progressing, ranging from new surgical techniques to physical and psychological therapies like exercise, hypnosis, and biofeedback. SOURCE:
This page was last
edited on 05/07/2002 Written by Richard
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