Massage & Bodywork

March/April 2013

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The phenomenon behind Mrs. Smith's chronic pain may be a fairly recently explored dysfunction of central nervous system (CNS) processing called central sensitization. And the reason massage therapists might be interested is that careful manual therapy may have a profoundly positive effect on this situation that is termed by one specialist as a "prevalent, promiscuous pain plasticity."1 In other words, this is a common condition, where pain is perceived out of proportion to the input the system receives. Definition of Central Sensitization In its simplest terms, central sensitization is the result of synaptic changes in the spinal cord that occur after injury or irritation, with the consequence of a persistent, yet possibly reversible, perception of pain. Synapses, the junction point where signals jump from one neuron to the next, can vary greatly in their activity levels and efficiency. Seldom used or unused synapses can degenerate, but highly sensitized ones can become overactive. This ability of synapses to change according to demand is called synaptic plasticity. In central sensitization, a triggering problem or trauma causes sensory neurons to release a flood of pain-promoting chemicals at the synapses in the dorsal horn. The signals transferred by these chemicals can outlast the actual trigger by minutes, hours, or longer. If this environment is prolonged, painful signals are triggered with much less intense stimuli: the pain threshold is lowered, and synaptic plasticity allows the pain-sensing neurons in the spinal cord to become more excitable. At the same time, the inhibitory signals that would ordinarily limit or interrupt the perception of pain are essentially locked out of the loop. Soon, only minimal sensory input (or sometimes none at all) is needed to maintain the perception of damage, and the person enters a persistent state of constantly high reactivity: this is a type of chronic pain. The chronic pain related to central sensitization has traditionally been associated with a CNS injury, like a stroke or a spinal cord injury. This makes sense since damaged neurons, plus inflammation, means the ongoing transmission of massive pain signals. More recently, we have realized that this pain pattern can be initiated by tissue damage or inflammation anywhere in the body. The most consistent characteristics of central sensitization are: • Allodynia: even normally nonpainful stimuli produce a sensation of pain. • Hyperalgesia: normally painful events cause pain that is out of proportion to the stimulus. • Enlarging areas of perceived pain: if pain begins in a foot, for instance, the painful area seems to expand, possibly spreading up the leg; this is sometimes called secondary hyperalgesia. • Wind-up and temporal summation: these are similar phenomena that occur in different CNS locations. The basic principle is that stimuli that are repeated at a consistent intensity lead to progressively more pain perception. Many people who live with central sensitization also experience problems with short-term memory and concentration, and amplification of all senses (light is brighter, odors are stronger, textures are rougher, noises are louder, and so on). These other sensory misunderstandings are termed dysesthesias, which essentially means incorrect perceptions. This suggests that similar synaptic plasticity changes can occur throughout the CNS. Central sensitization is a normal function of the nervous system, in place to help us resolve problems as they develop. This is important, because it means that nothing is really wrong with the nervous system—the situation is reversible. Problems occur, however, when those heightened pain perceptions become ingrained. Some people appear to be especially susceptible to this problem. Genetics may predispose some people, and a prior history of anxiety or depression is also predictive, because the nervous system This is a common condition, where pain is perceived out of proportion to the input the system receives. See what benefits await you. 43

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