Pathophysiologic Basis of Muscle Pain Syndromes: An Update

Comparison of sleep quality deterioration by subgroup of painful temporomandibular disorder based on diagnostic criteria for temporomandibular disorders

Chronic pain conditions, including temporomandibular disorders, are closely related to poor sleep quality. This study investigated whether sleep deterioration in patients with painful temporomandibular disorder differed depending on the origin of pain, and also analyzed which clinical disease characteristics and whether psychological distress affected sleep quality. A total of 337 consecutive patients (215 women; mean age, 33.01 ± 13.01 years) with painful temporomandibular disorder (myalgia [n=120], temporomandibular joint arthralgia [n=62], mixed joint–muscle temporomandibular disorder pain [n=155]), who were assessed and classified based on the diagnostic criteria for temporomandibular disorder (DC/TMD), were enrolled. They completed a battery of standardized reports on clinical sign and symptoms, and answered questions on sleep quality, excessive daytime sleepiness, and patients’ psychological status. The mean global Pittsburgh Sleep Quality Index scores were significantly higher in the mixed temporomandibular disorder pain group (6.97 ± 3.38) and myalgia group (6.40 ± 3.22) than in the arthralgia group (5.16 ± 2.94) (p=0.001). Poor sleepers were significantly more prevalent in the mixed temporomandibular disorder pain group (76.8%) and myalgia group (71.7%) than in the arthralgia group (54.8%) (p=0.006). The presence of psychological distress in the myalgia group (β=1.236, p=0.022), global severity index of the Symptom Checklist-90-Revised in the arthralgia group (β=1.668, p=0.008), and presence of headache (β=1.631, p=0.002) and self-reported sleep problems (β=2.849, p<0.001) in the mixed temporomandibular disorder pain group were associated with an increase in the Pittsburgh Sleep Quality Index score. Ultimately, as the source of pain in painful temporomandibular disorder can affect and determine sleep quality and contributing factors, and as the complex interplay between sleep and pain can vary, a comprehensive treatment approach is necessary because good sleep is required by patients.

Visceral and somatic disorders: tissue softening with frequency-specific microcurrent

Frequency-specific microcurrent (FSM) is an emerging technique for treating many health conditions. Pairs of frequencies of microampere-level electrical stimulation are applied to particular places on the skin of a patient via combinations of conductive graphite gloves, moistened towels, or gel electrode patches. A consistent finding is a profound and palpable tissue softening and warming within seconds of applying frequencies appropriate for treating particular conditions. Similar phenomena are often observed with successful acupuncture, cranial-sacral, and other energy-based techniques. This article explores possible mechanisms involved in tissue softening. In the 1970s, neuroscientist and osteopathic researcher Irvin Korr developed a "γ-loop hypothesis" to explain the persistence of increased systemic muscle tone associated with various somatic dysfunctions. This article summarizes how physiologists, neuroscientists, osteopaths, chiropractors, and fascial researchers have expanded on Korr's ideas by exploring various mechanisms by which injury or disease increase local muscle tension or systemic muscle tone. Following on Korr's hypothesis, it is suggested that most patients actually present with elevated muscle tone or tense areas due to prior traumas or other disorders, and that tissue softening indicates that FSM or other methods are affecting the cause of their pathophysiology. The authors believe this concept and the research it has led to will be of interest to a wide range of energetic, bodywork, and movement therapists.

Influence of sympathetic nervous system on sensorimotor function: whiplash associated disorders (WAD) as a model

There is increasing interest about the possible involvement of the sympathetic nervous system (SNS) in initiation and maintenance of chronic muscle pain syndromes of different aetiology. Epidemiological data show that stresses of different nature, e.g. work-related, psychosocial, etc., typically characterised by SNS activation, may be a co-factor in the development of the pain syndrome and/or negatively affect its time course. In spite of their clear traumatic origin, whiplash associated disorders (WAD) appear to share many common features with other chronic pain syndromes affecting the musculo-skeletal system. These features do not only include symptoms, like type of pain or sensory and motor dysfunctions, but possibly also some of the pathophysiological mechanisms that may concur to establish the chronic pain syndrome. This review focuses on WAD, particular emphasis being devoted to sensorimotor symptoms, and on the actions exerted by the sympathetic system at muscle level. Besides its well-known action on muscle blood flow, the SNS is able to affect the contractility of muscle fibres, to modulate the proprioceptive information arising from the muscle spindle receptors and, under certain conditions, to modulate nociceptive information. Furthermore, the activity of the SNS itself is in turn affected by muscle conditions, such as its current state of activity, fatigue and pain signals originating in the muscle. The possible involvement of the SNS in the development of WAD is discussed in light of the several positive feedback loops in which it is implicated.

The Effect of Intravenous Ketorolac on Capsaicin-Induced Deep Tissue Hyperalgesia

Preclinical and clinical studies have emphasized that persistent small afferent input will induce a state of central facilitation that can be attenuated by systemically administered nonsteroidal antiinflammatory drugs. However, these studies have been performed using cutaneous models of hyperalgesia. In this study we evaluated the effects of IV ketorolac on an experimental model of deep tissue hyperalgesia using IM capsaicin. We used a double-blind, placebo-controlled, crossover design. Ten subjects received 60 mg of ketorolac or placebo in 2 sessions separated by 1 wk. Capsaicin (100 microg in 10 microL) was then injected into the flexor carpi ulnaris muscle of the left forearm. After injection, spontaneous pain scores, pressure pain scores, gripping pain, pain distribution, and pain quality were recorded at 0, 5, 10, 15, 20, and 25 min. Cutaneous allodynia and dysesthesia were then mapped and thermal and mechanical thresholds were measured. The IM injection of capsaicin resulted in a reliable report of pain, hyperalgesia, and referred pain. Ketorolac had no effect on spontaneous pain, elicited pain, pain distribution, or secondary hyperalgesia induced by capsaicin. The findings of this study support the feasibility of further pharmacological studies using the IM capsaicin pain model.

Soft tissue determinants of low back pain

Low back pain is one of the complaints most commonly seen in the clinical setting. Correctly or incorrectly, these patients are often given the diagnosis of fibromyalgia, myofascial pain syndrome, disk herniation, or some other label. It is important to recognize the soft tissue causes of low back pain and understand how they can be most appropriately diagnosed and managed. Nonligamentous disorders of the low back region may occur in isolation or in combination with underlying discogenic, ligamentous, and facet-mediated causes of pain. Therefore, in order to fully evaluate and treat a patient with low back pain, it is necessary to consider and address these soft tissue conditions. This paper reviews soft tissue causes of low back pain and discusses how they are most appropriately diagnosed and managed.

Treatment of Fibromyalgia, Myofascial Pain, and Related Disorders

Chronic muscle pain is a common complaint among patients who seek care for musculoskeletal disorders. A spectrum of clinical presentations exists, ranging from focal or regional complaints that usually represent myofascial pain syndromes to more wide spread pain that may meet criteria for a diagnosis of fibromyalgia. This article addresses the epidemiology, pathophysiology, and clinical management of myofascial pain syndrome and fibromyalgia. These conditions are challenging to treat and require physiatrists to be aware of the wide range of pharmacologic, rehabilitative,and psychosocial interventions that can be helpful.

Painful and non-painful pressure sensations from human skeletal muscle

Painful and non-painful pressure sensations from muscle are generally accepted to exist but the peripheral neural correlate has not been clarified. The aim of the present human study was to assess the non-painful and painful pressure sensitivity with (1) anaesthetised skin, and (2) anaesthetised skin combined with a block of large diameter muscle afferents. The skin was anaesthetised by a topically applied anaesthetic cream and later lidocaine was administrated subcutaneously. The pressure sensitivity was assessed quantitatively by computer-controlled pressure stimulation on the anterior tibial muscle. Thresholds to detection, pain and pain tolerance were assessed. In the first experiment, computer-controlled needle insertion depths evoking touch and pain sensations were used to assess the efficacy of cutaneous anaesthesia. Touch and pain sensations evoked during needle insertions were found to be superficial in intact skin but when anaesthetised, touch sensation was occasionally evoked at depths related to penetration of the fascia. With the skin completely anaesthetised to brush and von Frey hair pinprick stimulation, skin indentation with the strongest von Frey hair caused a sensation described as a deep touch sensation. Simultaneously, pressure detection and pain thresholds increased but it was still possible to elicit non-painful and painful pressure sensation in all subjects. In a second experiment, a differential nerve block of group I and II afferent fibres was obtained by full-leg ischaemia simultaneously with cutaneous anaesthesia. The efficacy of the tourniquet block was continuously assessed by a battery of somatosensory tests (heat, brush, vibration, electrical and movement detection) applied at the foot simultaneously with pressure stimulation on the anterior tibial muscle. After 20 min of ischaemia, group II afferent fibres mediating the sensations of movement detection, vibration and brush on the foot was blocked but the heat pain threshold was not affected. In this condition (anaesthetised skin and block of group I and II fibres from deep tissue) a pressure sensation was evoked in 70% of subjects although the pressure detection threshold was increased. The pressure pain sensitivity was decreased, which, however, might indicate a partial block of group III and IV muscle afferents. In a third experiment, the tactile sensations elicited by electrical stimulation of the tibialis anterior muscle and skin at the lower leg were significantly decreased after 20 min of ischaemia, validating the blocking effects of group I and II nerve fibres. The present data show a marginal contribution of cutaneous afferents to the pressure pain sensation that, however, is relatively more dependent on contributions from deep tissue group III and IV afferents. Moreover, a pressure sensation can be elicited from deep tissue probably mediated by group III and IV afferents involving low-threshold mechanoreceptors.

Agents physiques antalgiques

Analgesic electrotherapy is now based on more consistent scientific data; the biological action of the electric current, of the electromagnetic radiations and of the mechanical vibrations is better approached. But the randomized control trials still provide contradictory results concerning the analgesic efficiency of the cryotherapy, the TENS, the pulsed electro-magnetic fields, the ultrasound and laser therapy, the shock waves; iontophoresis, short waves, microwaves, infrasound vibrations are very few investigated. The analgesic electrotherapy cannot be recommended nor prohibited; physical agents represent only therapeutic options. On the basis of the scientific data and of their personal experience, the therapists can use them. More controlled clinical investigations of higher methodological levels are still required.

The pathogenesis of muscle pain

Nociceptive nerve endings in muscles and other tissues are equipped with a multitude of receptor molecules for endogenous pain-producing and sensitizing agents. Particularly interesting molecules are the purinergic receptors, which can be activated by adenosine triphosphate (ATP), and the vanilloid receptor, which is sensitive to protons (low pH). The purinergic receptors are activated by tissue damage because cell necrosis is associated with the release of ATP. A low pH is present in many pathologic conditions such as ischemia and inflammation. At the spinal and medullar level, painful muscle lesions induce marked neuroplastic changes that result in hyperexcitability and hyperactivity of nociceptive neurons. This central sensitization is the basis for the spontaneous pain and hyperalgesia of patients. The transition from acute to chronic muscle pain is complete when the initially functional changes are transformed into structural ones. Patients with morphologic alterations in their nociceptive system are difficult to treat because the changes need time to normalize.

Possible mechanisms for the effects of botulinum toxin on pain

The therapeutic effects of botulinum toxin are principally, if not exclusively, derived from an alteration in the release of acetylcholine (ACh) at pre-synaptic neurons. The rationale for how these effects could be beneficial in conditions characterized by excessive muscle contraction is clear, but the hypotheses regarding botulinum toxin-induced effects on pain are highly speculative. We explore five possible mechanisms by which botulinum toxin could directly or indirectly alter pain, including: 1) changes in the sensitivity and response patterns of group III and IV muscle nociceptors, 2) diminished activity in the gamma-motor neurons and consequent changes in muscle spindle afferents, 3) alterations in cholinergic control of vascular and autonomic functions, including neurogenic inflammation, 4) induced neuroplastic changes in the processing of afferent somatosensory activity at multiple levels of the neuroaxis, and 5) direct non-cholinergic effects on pain afferents.

Cervical myofascial pain and headache

Myofascial pain is a common cause of regional chronic pain. Myofascial trigger points can refer pain to the head and face in the cervical region, thus contributing to cervicogenic headache. When identified properly, cervical myofascial pain is a treatable component of headache management. This article reviews current literature on the pathophysiology, diagnosis, and management of cervical myofascial pain.

Focused review: myofascial pain

Myofascial pain is defined as pain that originates from myofascial trigger points in skeletal muscle. It is prevalent in regional musculoskeletal pain syndromes, either alone or in combination with other pain generators. The appropriate evaluation and management of myofascial pain is an important part of musculoskeletal rehabilitation of regional axial and limb pain syndromes. This article reviews the current hypotheses regarding the pathophysiology of myofascial trigger points and muscle pain. A critical evidence-based review of the pharmacologic, nonpharmacologic, alternative medicine, and exercise treatments of myofascial pain is provided, as well as future research directions. OVERALL LEARNING OBJECTIVE: To review critically the state of the art knowledge of myofascial pain, including pathophysiology and comprehensive management. Areas of future research are identified.

The role of the gamma-motor system in increasing muscle tone and muscle pain syndromes: a review of the Johansson/Sojka hypothesis

OBJECTIVES

To review literature that pertained to the Johansson/Sojka hypothesis that positive feedback loops in the gamma-motor system are responsible for chronic muscle pain and increases in muscle tone.

DATA SOURCES

Articles were selected from MEDLINE searches and from manual library searches.

RESULTS

Normal, static, and ischemic muscle contractions and/or chemical mediators of inflammation excite intramuscular groups III and IV chemonociceptors. In groups III and IV, afferent firing stimulates gamma-motorneurons, which causes the firing of Ia and II muscle spindle afferents and increased extrafusal resistance to stretch (muscle tone). Some criticism of the involvement of the gamma-motor system in muscle tone was found to be dated or based on data from noncomparable research. Most of these studies (pro and con) were performed on prepared test animals, and the results may or may not translate to human subjects.

CONCLUSIONS

There exists a sizable body of research that establishes a link between the activation of intramuscular chemonociceptors, increased gamma-motor activity, and increased Ia and II spindle output, as proposed by the hypothesis of Johansson and Sojka. However, because of the lack of sufficient data on human subjects, their hypothesis should not be considered proved. Further research into the effects of metabolites of muscle contraction and their effects on muscle tone is recommended. Research into subluxation/joint dysfunction in light of the Johansson/Sojka hypothesis is recommended.

Contribution of group III and IV muscle afferents to multisensorial spinal motor control in cats

The contribution of group III and IV muscle afferents to multisensorial segmental reflex pathways was investigated by testing for spatial facilitation between these afferents and non-nociceptive segmental afferents from skin, muscles and joints on postsynaptic potentials (PSPs) in alpha-motoneurones recorded in anaemically decapitated high spinal cats. Group III and IV muscle afferents were activated by intraarterial injection of potassium chloride (320 mM) or bradykinin triacetate (81 microM). Skin, joint and group I-II muscle afferents were stimulated by graded electrical stimulation of various nerves. Conditioning by stimulation of group III and IV muscle afferents spatially facilitated the transmission in segmental reflex pathways from low- to medium-threshold cutaneous and joint afferents as well as from lb and group II muscle afferents. Both excitatory and inhibitory pathways from these afferents were facilitated. Monosynaptic excitation and disynaptic antagonistic inhibition from group Ia afferents remained unaffected. It is concluded that the spatial facilitation observed between group III and IV muscle afferents and the other afferents indicate a convergence from group III and IV muscle afferents and the other afferents on common interneurones in segmental flexor reflex pathways. Under physiological conditions they thus contribute to the multisensorial feedback of the flexor reflex pathways. Pathophysiologically, the observed convergence may aggravate muscle weakness and atrophy of muscles induced by group III and IV muscle afferents.