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The role of sensitization in musculoskeletal shoulder pain.

Borstad, J., & Woeste, C. (2015). The role of sensitization in musculoskeletal shoulder pain. Brazilian Journal of Physical Therapy, 19(4), 251–7.

For many years the primary pain models suggested that pain was an indicator of tissue damage or injury. If pain was felt in the body, it meant that some tissue damage or dysfunction existed and the body was attempting to call attention to the area. Recent advances in pain science are helping us revise our understanding of pain, along with how it arises and its role in the body. These new findings have very important ramifications for how we might address various pain complaints with massage.

While pain is clearly often caused by tissue injury, we now realize that pain can also exist in the absence of any significant tissue damage or biomechanical disorder. In cases where no significant tissue injury is evident but pain exists, the client may have developed a condition in which their neurological system – either locally or ….has become hypersensitive to a wide variety of stimuli.

Background
Nociceptors are the sensory organs that are responsible for reporting noxious input such as unpleasant mechanical, chemical, or thermal stimuli to the central nervous system. Their primary goal is to alert the nervous system to stimuli that could be dangerous. Consequently they have a threshold set to determine when the stimuli is strong enough to warrant concern. When they become increasingly over-sensitive it is called, sensitization. When sensitization occurs, pain results from something that would ordinarily not cause pain.

There are two different categories of sensitization, peripheral and central, which may cause a variety of different experiences for the client. In peripheral sensitization the receptors at the skin, joints, muscles, and periphery of the body have become increasingly sensitive. An example would be when only moderate pressure to the hand causes a great deal of pain for someone with carpal tunnel syndrome.

Central sensitization occurs when there is an increased degree of sensitivity in the central nervous system overall. Central sensitization often leads to much more widespread body pain. Many researchers have suggested that the constant full body pain felt by patients with conditions such as fibromyalgia or chronic fatigue syndrome may be related to central sensitization.

There are two important terms related to pain sensitivity that may be indicators of peripheral or central sensitization. Hyperalgesia is pain that is disproportionately high compared to the level of stimulus applied. For example, if you were to apply a smooth gliding effleurage stroke and the client reported severe pain with just a moderate to light degree of touch, this would be considered hyperalgesia.

Another important term is allodynia, which is pain felt in response to a previously non-painful stimulus. An example of allodynia is pain that is sometimes reported just from the tactile sensation of clothing dragging on the skin. The presence of allodynia is often an indicator of central sensitization. An understanding of sensitization is very important because prolonged sensitization may make it more difficult for various treatments or interventions to be effective.

In this paper the authors do a literature review on studies that evaluate for the presence of sensitization in conjunction with shoulder injuries. Only six studies were identified. Out of those, all the studies reported peripheral sensitization with unilateral shoulder pain and most reported central sensitization as a result of unilateral shoulder pain.

In this review, the authors note that the rotator cuff muscles, and supraspinatus in particular, are densely populated with nociceptors, and this is likely to contribute to both peripheral and central sensitization. In cases, what starts as an irritation of a local tissue, such as the supraspinatus in an impingement syndrome, can lead to peripheral sensitization. If the condition persists for a long period of time without resolution, peripheral sensitization might evolve into a greater degree of neural disturbance and eventually cause central sensitization.

So what does that mean for us, the massage practitioner?

Shoulder pain such as that which occurs with rotator cuff disorders or chronic shoulder tendinosis may appear to be primarily a result of mechanical tissue damage. However it is possible that sensitization is playing a role in the pain. In these situations direct treatment of the involved tissues may not be sufficient for resolving the complaint.

Massage
Research has not clearly shown that massage can interrupt or slow the progression of neural sensitization. However, that is only because it has not been studied yet. There are good indicators that the pleasurable sensations that arise from massage can decrease neural irritation and may contribute to a reduction in pain.1 Consequently there is a valuable role for massage in decreasing overall neural irritation in some conditions.

This is an exciting area of study that may lead to developing effective treatment strategies for a variety of neural sensitivity issues. There is also great potential for helping to reduce healthcare costs by limiting the extent of pain that develops from various pathologies. Hopefully massage will be further studied as a beneficial intervention for these chronic long-term pain problems.

1. Liptan G, Mist S, Wright C, Arzt A, Jones KD. A pilot study of myofascial release therapy compared to Swedish massage in fibromyalgia. J Bodyw Mov Ther. 2013;17(3):365-370.

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