**4.1 Metabolites**

The majority of studies concerning the trapezius in chronic trapezius myalgia have reported increases in the interstitial concentrations of lactate and pyruvate (Flodgren et al., 2010; Flodgren et al., 2006; Larsson et al., 2008; Rosendal et al., 2004b; Sjogaard et al., 2010). However, Flodgren et al. (Flodgren et al., 2010; Flodgren et al., 2006) did not compensate for RR and their results might be biased.

There are several possible explanations for the generally higher interstitial levels of pyruvate levels in chronic trapezius myalgia and in the trapezius of patients with FMS. For example, changes in the lactate-pyruvate metabolism via lactate dehydrogenase isoforms may result in higher pyruvate levels (Philip et al., 2005). Another explanation is a reduction in tissue oxygenation in FMS (Bengtsson, 2002) and chronic trapezius myalgia (Larsson et al., 2004), reductions that may result in higher pyruvate and higher lactate concentrations due to a shift towards an anaerobic state. A lower fitness level is a third explanation as a low fitness level means more frequent reliance on anaerobic metabolism. However, it is unknown if a general deconditioning in these two pain conditions involves the postural trapezius. The aerobic capacity of a muscle is largely governed by the number of mitochondria and their enzymes (Weibel & Hoppeler, 2005). The mitochondrial density increases as result of exercise and this increased density affects the level of metabolites (i.e., enhanced aerobic capacity) (Norrbom, 2008). Lower capillary density and/or enzymes associated with aerobic metabolism have been reported in FMS and in chronic trapezius myalgia (Larsson et al., 2004; Lindh et al., 1995). For FMS and chronic trapezius myalgia, the trapezius muscle fibres can appear with alterations in mitochondrial content and distribution, e.g., moth-eaten fibres and ragged red-fibres (Bengtsson, 2002; Bengtsson et al., 1986; Larsson et al., 2000; Larsson et al., 2004).

The role of lactate is complex. Lactate may assist in the detection of exercise stress before tissue damage occurs and can be exchanged rapidly among tissue compartments where it may be oxidized as a fuel or reconverted to form pyruvate or glucose (Gladden, 2004; Kim et al., 2007; Philip et al., 2005; Robergs et al., 2004). Lactate is also involved in peripheral nociception and it appears to facilitate the response of the acid-sensing ion channel 3 (ASIC-3) to low pH (Kim et al., 2007). Such ASIC channels are considered to be molecular transducers for nociception and mechanosensation (Kim et al., 2007).

To summarize, most studies of myalgic trapezius muscles show significant increases in interstitial levels of lactate and pyruvate. These results might be explained by decreased fitness level, reduced tissue oxygenation, increased muscle activation, and/or damaged mitochondria.

#### **4.2 Pain-related substances**

### **4.2.1 Glutamate**

Two of the studies of chronic trapezius myalgia (Larsson et al., 2008; Rosendal et al., 2004b), which are markedly larger than the third study (Flodgren et al., 2005), found significant increases in the interstitial concentrations of glutamate. A possible difference between subjects of these studies may contribute to the inconsistent glutamate finding. The myalgic

Potential Muscle Biomarkers of Chronic Myalgia in Humans –

**4.2.3 Bradykinin (BKN) and Kallidin (KAL)** 

KAL for nociception and pain in patients with chronic pain.

Increased interstitial potassium levels may be related to muscle pain (Graven-Nielsen et al., 1997). Green et al., however, did not find potassium related to acute ischaemic myalgia in

**4.2.4 Potassium** 

A Systematic Review of Microdialysis Studies 121

indicate significantly increased levels of 5-HT in the whole spectrum of severity of chronic trapezius myalgia and these findings agree with other studies of muscle pain conditions such as myalgic masseter muscle (Ernberg et al., 1999) (**Table 4**) and chronic whiplash associated pain including trapezius pain (Gerdle et al., 2008c) (**Table 2**). The finding that there are higher interstitial levels of 5-HT in chronic trapezius myalgia than in controls at rest agrees with other studies that also found that 5-HT is a peripheral pro-nociceptive substance activated by afferents and by the release of other substances (Saria et al., 1990; Sommer, 2004). The results of the present systematic review clearly indicate that 5-HT can

BKN and KAL are kinins – a group of structurally related 9-11 amino acid peptides that are produced by kallikrein-mediated enzymatic cleavage of kininogen (Coutaux et al., 2005; Riedel & Neeck, 2001; Wang et al., 2006). Kinins mediate their effects via two different G protein coupled receptors, B1 and B2, that provoke an increase in intracellular Ca2+ (Meyer et al., 2006; Zubakova et al., 2008). In normal tissue in the acute situation, BKN and KAL act via the B2 receptor. In the chronic phase of the response of tissue injury and infection, B1 receptors are expressed by BKN and KAL via this receptor (Calixto et al., 2004; Coutaux et al., 2005; Couture et al., 2001; Graven-Nielsen & Mense, 2001; McMahon et al., 2006). Interstitial muscle BKN and KAL have been suggested as algesic kinins involved in muscle pain. BKN was the first inflammatory mediator recognized to have potent hyperalgesic properties (Levine & Reichling, 1999). BKN induces pain and modifies the receptive fields of dorsal horn neurons to noxious stimuli in humans when administered in different ways (Boix et al., 2005; Meyer et al., 2006). BKN and cytokines are central factors in the link between tissue damage and inflammatory responses (Coutaux et al., 2005). Moreover, BKN is a potent vasodilatator and is increased in the interstitium of muscle during exercise (Clifford & Hellsten, 2004; Schmelz et al., 2003; Stewart & Rittweger, 2006). Animal studies have shown that BKN can both excite (i.e., algogenic) and sensitize nociceptors (Levine & Reichling, 1999; Wang et al., 2006). The present review identified four studies investigating the interstitial concentrations of BKN and/or Kallidin (Gerdle et al., 2008b; Larsson et al., 2008; Shah et al., 2008; Shah et al., 2005). The relatively small studies conducted by Shah et al. (Shah et al., 2008; Shah et al., 2005) clearly indicated that BKN was involved since increased levels of BKN in subjects with active trigger points and the levels were higher in the trapezius (with pain) than in a pain-free distant muscle. In contrast no significant differences in BKN were found between patients and controls in a field study and a laboratory study, (Gerdle et al., 2008b; Larsson et al., 2008) The difference in results between the above mentioned studies could be due to the fact that alterations in BKN might be very localized (i.e., in the trigger points) and not generally found in the aching trapezius muscle. KAL was only investigated in one study and increased in chronic trapezius myalgia but not in the trapezius of chronic WAD compared to controls (Gerdle et al., 2008b). Clearly, more pathophysiological *in vivo* studies are necessary in order to understand the roles of BKN and

be a potential biomarker of different types and severity of chronic myalgia.

subjects studied by Larsson et al. and Rosendal et al. (Larsson et al., 2008; Rosendal et al., 2004b) comprised subjects reporting considerable pain and had distinct current muscular signs confirmed at clinical examination. The pain history, the present pain, and clinical muscular neck status of the subjects are very sparsely presented in the Flodgren study (Flodgren et al., 2005). Moreover, one study found the painful masseter was significantly associated with increased glutamate (Castrillon et al., 2010).

Glutamate, a pain modulator in the human central nervous system, acts via the N-methyl-Daspartate (NMDA) receptor (Coggeshall & Carlton, 1997) (Hudspith, 1997) and influences peripheral pain processing (Carlton, 2001; Varney & Gereau, 2002 ), e.g., muscle inflammation and delayed onset muscle soreness (Cairns et al., 2001a; Cairns et al., 2001b; Cairns et al., 2003; Svensson et al., 2003; Svensson et al., 2005; Tegeder et al., 2002 ). Glutamate is released from peripheral afferent nerve terminals (Miller et al., 2011). Studies of animals have shown that glutamate receptors are located on the peripheral ends of smalldiameter primary afferents in several tissues such as muscle (Coggeshall & Carlton, 1998). Inflammatory animal models reveal increased levels of glutamate in peripheral tissues and nociceptive behaviours (Miller et al., 2011). Several studies have demonstrated that injections of glutamate increase pain intensity (Cairns et al., 2003; Gazerani et al., 2006). A review from 2008 concluded that elevation of interstitial glutamate in skeletal muscle alters pain sensitivity in healthy humans and is associated with pain symptoms in some chronic non-inflammatory muscle pain conditions (Cairns & Dong, 2008), which are probably mediated through activation of peripheral excitatory amino acid receptors located on the terminal ends of nociceptors. The present review mainly supports the conclusions of that review. However, the interstitial concentrations of glutamate were not increased in the trapezius of patients with chronic WAD (Gerdle et al., 2008c) or in patients with fibromyalgia (Gerdle et al., 2010). One difference between chronic trapezius myalgia and the two other conditions might be the more widespread (spatial) hyperalgesia in the two latter conditions (Arendt-Nielsen & Graven-Nielsen, 2003; Wallin et al., 2011).

#### **4.2.2 Serotonin**

Serotonin (5-hydroxtryptamine, 5-HT) is involved in the central and peripheral modulation of nociceptive pain and hyperalgesia (Sommer, 2004). 5-HT is synthesized in brain neurons from the essential amino acid tryptophan and released from platelets and mast cells in the periphery due to tissue damage (Mense, 1993). Whether 5-HT has an analgesic or hyperalgesic action depends on the cell type and type of receptor it targets. Currently, seven receptor families have been identified. These receptors are cell-surface proteins that bind 5- HT and trigger intracellular changes that influence the behaviour of cells and explain the broad physiological actions and distribution of this biochemical mediator. According to animal studies, the 5-HT receptor classes currently identified, the 5-HT1 and the 5-HT2, may be involved in mainly chemical and thermal hyperalgesia (Ernberg, 2008). 5-HT activates the descending endogenous pain system and this inhibits centrally mediated pain transduction (Sommer, 2006; Suzuki et al., 2004). In the periphery, 5-HT sensitizes afferent nerve fibres, contributing to hyperalgesia in inflammation and nerve injury (Giordano & Rogers, 1989; Sommer, 2004; Taiwo & Levine, 1992). Intramuscular administration of 5-HT into the human masseter muscle has been demonstrated to induce pain (Ernberg et al., 2006). The studies identified in this systematic review concerning chronic trapezius myalgia (**Table 1**) clearly

subjects studied by Larsson et al. and Rosendal et al. (Larsson et al., 2008; Rosendal et al., 2004b) comprised subjects reporting considerable pain and had distinct current muscular signs confirmed at clinical examination. The pain history, the present pain, and clinical muscular neck status of the subjects are very sparsely presented in the Flodgren study (Flodgren et al., 2005). Moreover, one study found the painful masseter was significantly

Glutamate, a pain modulator in the human central nervous system, acts via the N-methyl-Daspartate (NMDA) receptor (Coggeshall & Carlton, 1997) (Hudspith, 1997) and influences peripheral pain processing (Carlton, 2001; Varney & Gereau, 2002 ), e.g., muscle inflammation and delayed onset muscle soreness (Cairns et al., 2001a; Cairns et al., 2001b; Cairns et al., 2003; Svensson et al., 2003; Svensson et al., 2005; Tegeder et al., 2002 ). Glutamate is released from peripheral afferent nerve terminals (Miller et al., 2011). Studies of animals have shown that glutamate receptors are located on the peripheral ends of smalldiameter primary afferents in several tissues such as muscle (Coggeshall & Carlton, 1998). Inflammatory animal models reveal increased levels of glutamate in peripheral tissues and nociceptive behaviours (Miller et al., 2011). Several studies have demonstrated that injections of glutamate increase pain intensity (Cairns et al., 2003; Gazerani et al., 2006). A review from 2008 concluded that elevation of interstitial glutamate in skeletal muscle alters pain sensitivity in healthy humans and is associated with pain symptoms in some chronic non-inflammatory muscle pain conditions (Cairns & Dong, 2008), which are probably mediated through activation of peripheral excitatory amino acid receptors located on the terminal ends of nociceptors. The present review mainly supports the conclusions of that review. However, the interstitial concentrations of glutamate were not increased in the trapezius of patients with chronic WAD (Gerdle et al., 2008c) or in patients with fibromyalgia (Gerdle et al., 2010). One difference between chronic trapezius myalgia and the two other conditions might be the more widespread (spatial) hyperalgesia in the two latter

associated with increased glutamate (Castrillon et al., 2010).

conditions (Arendt-Nielsen & Graven-Nielsen, 2003; Wallin et al., 2011).

Serotonin (5-hydroxtryptamine, 5-HT) is involved in the central and peripheral modulation of nociceptive pain and hyperalgesia (Sommer, 2004). 5-HT is synthesized in brain neurons from the essential amino acid tryptophan and released from platelets and mast cells in the periphery due to tissue damage (Mense, 1993). Whether 5-HT has an analgesic or hyperalgesic action depends on the cell type and type of receptor it targets. Currently, seven receptor families have been identified. These receptors are cell-surface proteins that bind 5- HT and trigger intracellular changes that influence the behaviour of cells and explain the broad physiological actions and distribution of this biochemical mediator. According to animal studies, the 5-HT receptor classes currently identified, the 5-HT1 and the 5-HT2, may be involved in mainly chemical and thermal hyperalgesia (Ernberg, 2008). 5-HT activates the descending endogenous pain system and this inhibits centrally mediated pain transduction (Sommer, 2006; Suzuki et al., 2004). In the periphery, 5-HT sensitizes afferent nerve fibres, contributing to hyperalgesia in inflammation and nerve injury (Giordano & Rogers, 1989; Sommer, 2004; Taiwo & Levine, 1992). Intramuscular administration of 5-HT into the human masseter muscle has been demonstrated to induce pain (Ernberg et al., 2006). The studies identified in this systematic review concerning chronic trapezius myalgia (**Table 1**) clearly

**4.2.2 Serotonin** 

indicate significantly increased levels of 5-HT in the whole spectrum of severity of chronic trapezius myalgia and these findings agree with other studies of muscle pain conditions such as myalgic masseter muscle (Ernberg et al., 1999) (**Table 4**) and chronic whiplash associated pain including trapezius pain (Gerdle et al., 2008c) (**Table 2**). The finding that there are higher interstitial levels of 5-HT in chronic trapezius myalgia than in controls at rest agrees with other studies that also found that 5-HT is a peripheral pro-nociceptive substance activated by afferents and by the release of other substances (Saria et al., 1990; Sommer, 2004). The results of the present systematic review clearly indicate that 5-HT can be a potential biomarker of different types and severity of chronic myalgia.
