To understand the origin of much chronic pain, it is essential to appreciate a process known as central sensitization. This may start when there are a number of peripheral (local) areas of constant or frequently recurring pain, possibly involving inflammation or infection.
When, despite appropriate treatment, a pain is constant or recurrent (happening over and over again with only short breaks inbetween), and when there are simultaneously other recurrent or persistent areas of pain in the body, and when this continues for many months or years, the process of sensitization may start. The original peripheral pains might be musculoskeletal, such as neck, back, knee or elbow pains, and/or they might involve headache, toothache, stomach ache or discomfort affecting other organs (such as irritable bowel or painful bladder). Over time the sensitization process makes these pains more severe and wide-ranging – until the brain itself is sensitized and much of the body hurts most of the time.
THE PROCESS OF SENSITIZATION
Sensitization causes all pain to be perceived more strongly, and the area of pain to enlarge.
As sensitization starts, changes occur to features of the pain:
• The nerves that register pain in the peripheral areas may become increasingly sensitive, so that their pain threshold is lowered and they report pain sensations more easily. This is known as hyperalgesia.
• With hyperalgesia peripheral nerves may start to report pain from a wider area, and these pain sensations may continue for longer than they originally did, so that a lesser stimulus produces a stronger and longer pain response.
• Pain may be reported with even extremely light stimuli that previously would not even have been sensed. This is known as hyperesthesia.
• Mild sensations, such as light pressure being applied, that would previously not have been painful, become painful. This is known as allodynia.
• Through a crossover of pain messages, sensations that were felt in an organ, say the stomach, become experienced in muscles, a process known as viscero-muscular hyperalgesia. Similar crossover pain can occur from organ to organ (known as viscerovisceral hyperalgesia).
• These changes can affect the actual function of organs – for example, leading to irritable bowel syndrome.
• Emotional and psychological consequences also result.
• When central sensitization is established, pain transmissions to the brain are exaggerated while, at the same time, the brain’s normal pain-control methods are reduced. In a very real way, the brain itself becomes sensitized.
COMMON FEATURES IN PATIENTS DEVELOPING CENTRAL SENSITIZATION
In one study2, 512 individuals with local back or neck pain were studied over a seven-year period to see how many would develop chronic widespread pain involving central sensitization, and to identify features shared by the group. It was found that 22.6 percent of those with local back or neck pain in 2001/2002 had developed central sensitization by 2007, and that six significant common features could be identified:
1 Pain was of moderate to severe intensity.
2 The majority were female.
3 There was a family history of chronic widespread pain, possibly due to genetic and/or familial environmental influences.
4 The individuals were mainly aerobically unfit as a result of their pain severely interfering with general activities of life, including exercise.
5 The majority displayed one or more central sensitivity syndromes, such as irritable bowel syndrome, irritable bladder syndrome, restless legs syndrome and/or migraines.
6 Numerous pain-management strategies were being used.
The end result of central sensitization can be seen in conditions such as fibromyalgia or myofascial pain syndrome, and in chronic migraine3.
Recent research4 has shown that, because the nervous system and the brain become sensitized, the associated features of this condition include hypersensitivity to many normal stimuli, such as bright light, touch, noise, pesticides, mechanical pressure, medication and high and/or low temperature.
Once central sensitization is established it may become selfperpetuating, even after the peripheral signals stop. However, there is increasing evidence that eliminating the peripheral triggers can reverse the sensitization process.
Other research5 has shown that both local muscle pain – often caused by myofascial trigger points (see pages 99–101) – as well as joint pains can directly influence the development and maintenance of central sensitization, and that successful treatment of these local features reduces sensitization6. An understanding of central sensitization reinforces the need to deal appropriately and safely with minor, peripheral pains, rather than putting up with them.
“Take rest; a field that has rested gives a bountiful crop”
Ovid (43BCE–17CE)