Friday, September 01, 2006

Types and Theories of Pain

Types of Pain

Acute pain is typically associated with an active disease state or traumatic injury. When the damaged area heals, the pain typically goes away. Acute pain serves as an important signal identifying that there is damage. With this information, behavior change can be initiated so that further injury might be avoided.

Sometimes pain persists even after injuries have healed. This is usually referred to as chronic pain. Chapman and Bonica (1985) identify three types of chronic pain: (1) pain that lasts after the normal feeling of a disease or an injury; (2) pain associated with a chronic medical condition, such as a degenerative disease or a neurological condition; or (3) pain that develops and persists in the absence of identifiable organic problem.

People's experience with chronic pain also depends on two factors: (1) whether the underlying condition is benign or is malignant and worsening and (2) whether the discomfort exists continuously or occurs in frequent and intense episodes. Using these factors, Dennis Turk, Donald Meichenbaum, and Myles Genest (1983) have described three types of chronic pain:

1. Chronic/recurrent pain stems from benign causes and is characterized by repeated and intense episodes of pain separated by periods without pain. Two examples of chronic/recurrent pain are migraine headaches and muscle-contraction (tension) headaches; another exampie is myofascial pain, a syndrome that typically involves shooting or radiating, but dull, pain in the muscles and connective tissue of the head and neck, and sometimes the back.

2. Chronic/intractable/benign pain refers to discomfort that is typically present all of the time, with varying levels of intensity, and is not related to an underlying malignant condition. Chronic low back pain often has this pattern.

3. Chronic/progressive pain is characterized by continuous discomfort, is associated with a malignant condition, and becomes increasingly intense as the underlying condition worsens. Two of the most prominent malignant conditions that frequently produce chronic/progressive pain are rheumatoid arthritis and cancer.

Theories of Pain

Specificity theory. Von Frey (1895) argued that the body has a separate sensory system for perceiving pain—just as it does for hearing and vision—and this system contains its own special receptors for de:ecting pain stimuli, its own peripheral nerves and pathway to the brain, and its own area of the brain for processing pain signals. But this structure is not correct.

Pattern theory. Goldschneider (1920) proposed that there is no separate system for perceiving pain, and the receptors for pain are shared with other senses, such as of touch. According to this view, people feel pain when certain patterns of neural ctivity occur, such as when appropriate types of activity reach excessively high levels in the brain. These patterns occur only with intense stimulation. Because strong and mild stimuli of the same sense modality produce different patterns of neural activity, being hit hard feels painful, but being caressed does not.

Gate Control Theory. Melzack has proposed a theory of pain that has stimulated considerable interest and debate and has certainly been a vasy improvement on the early theories of pain. According to his theory, pain stimulation is carried by small, slow fibers that enter the dorsal horn of the spinal cord; then other cells transmit the impulses from the spinal cord up to the brain. These fibers are called T-cells. The T-cells can be located in a specific area of the spinal cord, known as the substantial gelatinosa. These fibers can have an impact on the smaller fibers that carry the pain stimulation. In some cases they can inhibit the communication of stimulation, while in other cases they can allow stimulation to be communicated into the central nervous system. For example, large fibers can prohibit the impulses from the small fibers from ever communicating with the brain. In this way, the large fibers create a hypothetical "gate" that can open or close the system to pain stimulation. According to the theory, the gate can sometimes be overwhelmed by a large number of small activated fibers. In other words, the greater the level of pain stimulation, the less adequate the gate in blocking the communication of this information.

There are 3 factors which influence the 'opening and closing' of the gate -

  1. The amount of activity in the pain fibers. Activity in these fibers tends to open the gate. The stronger the noxious stimulation, the more active the pain fibers.
  2. The amount of activity in other peripheral fibers—that is, those fibers that carry information about harmless stimuli or mild irritation, such as touching, rubbing, or lightly scratching the skin. These are large-diameter fibers called A-beta fibers. Activity in A-beta fibers tends to close the gate, inhibiting the perception of pain when noxious stimulation exists. This would explain why gently massaging or applying heat to sore muscles decreases the pain.
  3. Messages that descend from the brain. Neurons in the brainstem and cortex have efferent pathways to the spinal cord, and the impulses they send can open or close the gate. The effects of some brain processes, such as those in anxiety or excitement, probably have a general impact, opening or closing the gate for all inputs from any areas of the body. But the impact of other brain processes may be very specific, applying to only some inputs from certain parts of the body. The idea that brain impulses influence the gating mechanism helps to explain why peopie who are hypnotized or distracted by competing environmental stimuli may not notice the pain of an injury.

Thus we can conclude that our experience of pain is dependent on the condition of 'the gate'. The more the gate is opened the greater the perception of pain. Melzack suggests that several factors can open the gate:

  • Physical factors, such as injury or activation of the large fibres
  • Emotional factors, such as anxiety, worry, tension and depression;
  • Behvioural factors, such as focusing on the pain or boredom.

The gate control theory also suggests that certain factors close the gate.

  • Physical factors, such as medication, stimulation of the small fibres;
  • Emotional factors, such as happiness, optimism or relaxation;
  • Behavioural factors, such as concentration, distraction or involvement in other activities.

12 Comments:

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Blogger Mereoni Ranadi said...

This comment has been removed by the author.

2:36 PM  
Blogger Mereoni Ranadi said...

This comment has been removed by the author.

2:37 PM  
Blogger Mereoni Ranadi said...

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2:38 PM  
Blogger Roman B said...

Very clear and concise explanation about the theories

6:31 PM  
Blogger An Pienyu said...

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