Physical therapy action mechanisms and effects on pain management*
Artur Padão Gosling
Physical Therapist of the Multidisciplinary Pain Center; Master Student in Medical Clinic, Federal University of Rio de Janeiro. Rio de Janeiro, RJ, Brazil
SUMMARY
BACKGROUND AND OBJECTIVES: Along time, physical therapy has played an important role in the treatment of painful patients. Several neurophysiological mechanisms have been proposed to explain pain modulation via physical and cognitive behavioral approaches; however they have to be further explored by the physical therapist. This study aimed at reviewing major action mechanisms and effects of physical therapy techniques to manage pain.
CONTENTS: There are studies on the action mechanisms and effects of physical therapy during pain processing stages in the nervous system. To organize information, mechanisms and theories were grouped in peripheral and central stations. Effects, however, were grouped in motor, sympathetic and hormonal systems, psychophysiological factors and placebo.
CONCLUSION: Several mechanisms and effects may be influenced by physical therapy physical and cognitive behavioral techniques, thus contributing to the treatment of painful patients and helping the choice of techniques based on identified clinical mechanisms.
CONTENTS: There are studies on the action mechanisms and effects of physical therapy during pain processing stages in the nervous system. To organize information, mechanisms and theories were grouped in peripheral and central stations. Effects, however, were grouped in motor, sympathetic and hormonal systems, psychophysiological factors and placebo.
CONCLUSION: Several mechanisms and effects may be influenced by physical therapy physical and cognitive behavioral techniques, thus contributing to the treatment of painful patients and helping the choice of techniques based on identified clinical mechanisms.
Keywords: Pain, Pain perception, Physical therapy modalities.
INTRODUCTION
Pain may have several clinical manifestations and be associated to different symptoms. So, some authors have suggested that physical therapists treat pain according to clinical peripheral, central and / or associated mechanisms, identified during evaluation1,2. Understanding and identifying such mechanisms help evaluation judgment and clinical reasoning, treatment and prognosis of patients with pain3,4.
However, to choose the treatment based on clinical mechanisms, physical therapists need to have scientific and practical knowledge4. Each technique has a neurophysiological explanation with its own action mechanisms. Physical therapy treatments use the specific knowledge about the effect of the techniques for clinical applicability. It is important to determine predominant pain mechanism for physical therapy to be more effective4.
Among primary objectives, inhibitory pain modulation will occur with the decrease of peripheral and central stimuli which sensitize the nervous system5. In addition, biopsychosocial factors provoking or perpetuating sensitization should be identified by physical therapists6.
Mechanisms and theories have been proposed to explain the effects of physical therapy on pain control and handling. These involve local peripheral tissues, neurophysiological and psychophysiological mechanisms, and the placebo effect. They are currently based on pain processing stages in the nervous system and on neuroplasticity changes. So, we will discuss the action of physical therapy on modulation and perception stages, on peripheral stations, on spinal cord dorsal horn, on descending suppressor system and on cerebral cortex. In addition, techniques effects will be discussed on the motor, sympathetic and hormonal systems by means of psychophysiological and placebo effects. Chart 1 shows theories, action mechanisms and effects of some techniques on the nervous system.
This study aimed at reviewing major action mechanisms and effects of physical therapy techniques for pain handling. For such, scientific articles indexed by Pubmed and Bireme from 2000 to 2011 were queried.
PERIPHERY
Any technique having contact with the skin and mobilizing tissues activates Ab mechanoreceptor fibers, which are faster than C and Aδ fibers, and interneurons of lamina IV in spinal cord dorsal horn, in the so-called competitive inhibition3. Skin touch may be a powerful modulator, and proprioceptive stimuli speed may help the inhibition of painful stimuli in the central nervous system (CNS).
In cases such as complex regional pain syndrome (CRPS) or in other peripheral nerve injuries, physical therapy uses skin stimulation with materials of different textures to increase tolerance to touch, decreasing hyperalgesia and alodinia7. It is known that mechanical and chemical factors are responsible for the sensitization of peripheral nociceptors.
So, it is necessary to remove irritating mechanical and chemical agents to decrease nociceptors sensitization and the arrival of noxious stimuli in upper centers8. Examples are manual therapy, muscle contraction techniques, thermotherapy, ultrasound and laser. These techniques are effective for nociceptive pain control, however they should be carefully used for neuropathic pain because they may increase nociceptive sensitization9. Other techniques, such as bandages and orthoses promoting joint alignment minimize nociceptors activation and the effects of hysteresis and tissue cavitation10. Algiogenic substances from the acute inflammatory process, from neurogenic inflammation and edema, make difficult tissue repair and have to be removed to control nociceptive pain8,10.
SPINAL CORD DORSAL HORN
With the strong stimulation of dorsal horn medullar laminae directly from the periphery, interneurons are activated to modulate pain in inhibitory way. According to the gates theory, these interneurons depend on the competition between noxious and proprioceptive stimuli.
Since proprioceptive stimulus is faster, its arrival in the dorsal horn will activate interneurons and help the release of opioid substances and GABA3,4. Techniques such as TENS and acupuncture are still based on this theory, although it does not explain several pain mechanisms. In the counter-irritating theory, a painful irritating mechanical, thermal or chemical stimulus is applied to promote pain relief. Studies have shown that the counter-irritating effect is related to opioid substances release; however this is poorly described in the literature of physical therapy modalities11. Acupuncture, ice spray, manual therapy, miofascial release and massage with deep warming ointments are common examples. Applying techniques aiming at this effect depends on the level of tolerance of patients. For neuropathic pain, increased painful stimuli may have a negative effect on patients9.
DESCENDING SUPPRESSION SYSTEM
Neuronal activation in areas of the descending suppression system, such as thalamus, gray periaquedutal matter and raphe nucleus magnus, cause the release of opioid and non-opioid substances3,12,13. Chronic pain patients have difficulty to activate the suppression system due to structural and anatomic nervous system changes3,14.
So, physical therapy techniques may stimulate descending pathways by the lateral (opioid) and ventrolateral (non-opioid) system, releasing inhibitory neurotransmitters. With TENS, joint manipulation, regular physical exercise, aerobic exercise and acupuncture, studies have shown preference for opioid activation with endorphins release12,13,15-17. With general exercises, transcutaneous electric nerve stimulation, manual joint and muscle therapy, studies have shown activation of non-opioid substances such as serotonin, norepinephrine, dopamine, GABA and growth hormone12,13,17,18.
CEREBRAL CORTEX
Recent studies have shown that patients with fibromyalgia, chronic low back pain, SCDR and phantom pain show major structural and functional corticoid changes, such as cortical atrophy and excessive neuronal hyperactivity20-22. Within painful neuromatrix concepts, there are changes in areas of movement, sensations, memory and emotions5,23.
This negative cortical reorganization in painful conditions causes distorted pain perception, changing body sensory and motor scheme and leading to laterality problems24,26.
It is believed that such changes help the maintenance of pain, of alertness and of central sensitization9,26. Physical therapy may positively reorganize the cortex through active and integrated modalities with cognitive behavioral approaches.
This has been shown in recent studies with functional resonance and transcranial magnetic stimulation of motor and sensory cortex24,26. Treatments such as motor control, biofeedback, imagetics, mirror therapy, distraction tasks, functional tasks training, physical exercise, muscle strengthening and education strategies, in addition to placebo effect, are able to activate relevant cortical areas for positive cortical reorganization16,24,26,27.
EFFECTS ON OTHER SYSTEMS
Motor
When pain is influenced by motor planning, the muscle system may maintain increased activity even at rest, as if it were functioning to fight, flight or protection4,25. Avoidance and fear behaviors, such as kinesiophobia, cause this abnormal pattern, in addition to anxiety and stress28,29. So, it is necessary to look for neuromuscular function balance through physical and cognitive behavioral approaches which will be commented below. Literature shows that manual therapy with joint mobilization at the cervical region decreases superficial muscles activity and helps the activation of deep stabilizing muscles. It is suggested that this response is due to decreased stimuli of alpha and gamma motor neurons, by stimulation of tendinous organs through joint mobilization30,31. However, painful manual therapy causes protective muscle hyperactivity in patients with low tolerance to skin pressure, such as in neuropathic pain3,12.
Sympathetic
Physical therapy techniques applied around sympathetic nervous system (SNS) structures may cause neurovegetative phenomena such as sweating, changes in skin color, tachycardia, anxiety and fear19,31,32. So, painful techniques should be carefully used. In addition, patients with pains associated to SNS activation mechanisms have a constant release of norepinephrine and prostaglandins, which potentiates the action of several organs, including muscles, maintaining constant muscle hyperactivity5.
Autonomic nervous system balance depend on factors such as minimizing painful stimuli with physical therapy, minimizing fear and anxiety and the use of techniques to decrease SNS hyperactivity, such as deep relaxation, diaphragmatic breathing training and painless manual therapy.
Hormonal
Studies have shown that hormonal unbalance is a sensitization factor5,9. Hormones, such as growth hormones, have a critical role in pain modulation. Decreased testosterone, estrogen and progesterone have been the subject of still poorly conclusive studies. On the other hand, norepinephrine and cortisol, when constantly released, cause muscle hyperactivity throughout the body and maintenance of alertness5,9. Regular physical exercises, improved sleep quality, adequate feeding and enjoyable activities may be a way to look for the balance of such hormones and decrease central sensitization.
PSYCHOPHYSIOLOGICAL EFFECTS
Physical therapy and psychology go hand in hand. Physical interventions associated to cognitive behavioral approaches promote interesting responses. Both touch and exercise produce motor, autonomic, neuroendocrine, emotional, behavioral and corporal perception responses regulated by the limbic system. Responsible for the organization of somatic expressions of emotional states and experiences, the limbic system is highly relevant for painful patients5,9. Due to major structural and functional nervous system changes and associated to emotional and behavioral factors, some persistent pain patients become intolerant to touch and exercise25, 28.
Passive techniques, that is, applied by the physical therapist, have fast however short analgesic effect, varying from some hours to weeks. In addition to already described neurophysiological effects, responses will directly depend on patients' interpretation of the manual stimulus on their bodies. In patients with pain, body touch may bring enjoyable sensations such as relaxation, well being and relief, but also negative sensations such as fear of pain, muscle tension, irritation, anxiety and worsening of symptoms28,29. For example, although manual therapy has direct action on primary hyperalgesia and indirect action on secondary hyperalgesia and allodynia, skin pressure may be uncomfortable for sensitized patients. So, techniques using touch, such as manual therapy, are not always the best first choice.
Active techniques, on the other hand, that is, those performed by the patient with or without physical therapist help, have long-term analgesic effects. Some patients may take months to feel the analgesic effects. Keeping the example of the exercises, it is known that they are a modulation source, especially in people performing regular exercises16,29. Exercises, in addition to releasing analgesic substances, promote functional autonomy. For patients with major limitations, function gain encourages the change of abnormal pain-related behaviors, increasing self-efficacy, motivation, well-being and satisfaction. On the other hand, the association of the exercise to pain or injury favors behaviors such as avoidance and fear, kinesiophobia, isolation and immobility28,29,34,35.
Physical therapists should be educator therapists, regardless of choosing passive or active techniques27. Education strategies to fight dysfunctional beliefs, abnormal behaviors, negative thoughts and attitudes are able to modulate pain and increase functional capacity of patients27,33. Associated to other cognitive behavioral techniques, such as pacing of activities, they encourage patients to be active participants of their healthcare, being able to understand chronic pain, its impact on health and recover pain control27,33,34.
PLACEBO
All physical therapy techniques have placebo effect. Authors have shown that passive techniques, such as electrotherapy and manual therapy may have variations of 5% to 100% in placebo effect, depending on the study methodology. However, it is believed that approximately 30% of patients have symptoms improvement25,36. Authors believe that the explanations for the analgesic mechanisms of the placebo effect are more related to the classic conditioning role and to psychological factors. Among explanations, there are expectations and positive beliefs about efficacy and symptoms relief, decreased anxiety and desire to improve. Authors discuss the associated opioid substances release25,36. The placebo effect is still a mystery for physical therapists, however patients may benefit for a short period of time. It is important to remember that physical therapists are prohibited to use the placebo effect in their clinical practice.
Attention and concern with patients' health status, combined with the desire to improve, optimism and the way to deal with pain are independent effects of physical therapy techniques26,37. The best result for pain management is expected when there is integration among physical modalities, cognitive behavioral approaches such as education, and with the help of other health professionals27,33,38.
CONCLUSION
Different mechanisms and effects may be influenced by physical therapy physical and cognitive behavioral techniques, contributing for the treatment of painful patients and helping the choice of techniques based on identified clinical mechanisms.
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