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Modulation of sensitized C-fibers by adrenergic stimulation in human neuropathic pain
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European Journal of Pain 12 (2008) 517–524 www.EuropeanJournalPain.com
Modulation of sensitized C-fibers by adrenergic stimulation in human neuropathic pain Jo¨rn Schattschneider *, Marco Scarano, Andreas Binder, Gunnar Wasner, Ralf Baron Sektion fu¨r Neurologische Schmerzforschung und – therapie, Klinik fu¨r Neurologie, Universita¨tsklinikum Schleswig-Holstein, Campus Kiel, Schittenhelmstr. 10, 24103 Kiel, Germany Received 22 January 2007; received in revised form 6 August 2007; accepted 7 August 2007 Available online 20 September 2007
Abstract The chronic constriction injury model is widely used in studying mechanisms of neuropathic pain. In this model neuropathic pain can be influenced by sympathetic interventions. It is assumed that similar mechanisms as in animals are responsible for pain arising from nerve entrapment syndromes in humans. The aim of the present study was to investigate if in patients with nerve entrapment nociceptive afferents can be modulated by adrenergic stimulation. Methods: Twenty patients with pain due to a unilateral entrapment of the median nerve and 10 controls were included in the study. Spontaneous pain, mechanical and thermal evoked pain were assessed within the innervation territory of the lesioned nerve and the corresponding contralateral segment in patients and on the right hand side in healthy volunteers. The examinations were performed at baseline, during whole body cooling (sympathetic activation) and whole body warming (sympathetic inhibition), and after norepinephrine iontophoresis. Results: All patients reported spontaneous pain. Mechanical allodynia, punctate hyperalgesia and cold allodynia was not found. According to side-to-side differences in heat pain thresholds, patients were separated in patients with (n = 10) and without (n = 10) heat hyperalgesia. Adrenergic stimulation did not induce or enhance spontaneous or mechanical evoked pain in any patient or control subject. However in patients with pre-existing heat hyperalgesia sympathetic stimulation aggravated heat hyperalgesia significantly. Further in these patients the decrease in heat pain thresholds observed after norepinephrine iontophoresis was significantly higher compared to patients without pre-existing heat hyperalgesia. Conclusion: Sympathetic–afferent interaction does not play a major role in pain generation due to nerve entrapment. Nevertheless in a subgroup of patients nociceptive afferents show sensitivity to physiological and pharmacological sympathetic stimulation. This finding is important because it emphasises that despite there is no clinical detectable effect on pain sympathetic afferent interaction can be found. 2007 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. Keywords: Neuropathic pain; Nociceptor sensitization; Sympathetic–afferent coupling
1. Introduction *
Corresponding author. Tel.: +49 431 597 8510; fax: +49 431 597 8502. E-mail address: [email protected] (J. Schattschneider).
Several behavioural animal models of neuropathic pain have been used to determine the contribution of the sympathetic nervous system to neuropathic pain. In the spinal nerve ligation model it has be reported that
1090-3801/$34 2007 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ejpain.2007.08.004
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surgical sympathectomy reversed mechanical as well as thermal hyperalgesia (Kim et al., 1993), whereas others failed to demonstrate an effect of sympathectomy on pain behaviour in this model (Ringkamp et al., 1999). In the chronic constriction injury model surgical sympathectomy reduced thermal hyperalgesia whereas mechanical hyperalgesia and spontaneous pain behaviour were unchanged (Bennett and Xie, 1988; Desmeules et al., 1995). These experiments demonstrated that the nature of the nerve lesion determines that only distinct components of neuropathic pain behaviour are reduced after surgical sympathectomy. Furthermore it is possible that in a neuropathic pain condition one symptom for example heat hyperalgesia, is sympathetically maintained whereas another symptom, for example mechanical hyperalgesia or spontaneous pain, is sympathetically independent. It is assumed that the chronic constriction injury (CCI) model mirrors the clinical situation of nerve entrapment syndromes like carpal tunnel syndrome in an experimental set up (Bennett and Xie, 1988; Neil et al., 1991; Desmeules et al., 1995). In this model loose ligation of the sciatic nerve is followed by an occlusion of the epineural vasculature. A swelling of the nerve and an extensive demyelinisation of large diameter fibers can be observed (Bennett and Xie, 1988). The morphological changes are accompanied by an inflammatory response (Sommer et al., 1995; Wagner et al., 1998). After the nerve lesion rats develop mechanical and thermal hyperalgesic behaviour. It has been demonstrated that thermal hyperalgesia responds to surgical sympathectomy or depletion of sympathetic transmitters by guanethidine (Neil et al., 1991; Desmeules et al., 1995). In humans nerve entrapment syndromes are associated with a microcirculatory venous stasis leading to an increased interstitial fluid pressure in the peripheral nerve and the development of progressive oedema (Sunderland, 1976; Seiler et al., 1989; Freeland et al., 2002). Extraneural and intraneural nerve blood flow and axonal transport are progressively compromised leading to tissue ischemia and the accumulation of inflammatory cytokines which may lead to nociceptor sensitization (Freeland et al., 2002). Electrophysiological studies in patients with carpal tunnel syndrome demonstrated demyelinisation and functional impairment of large diameter sensory and motor fibers, which leads to a reduction of sensory and motor conduction velocity (Goadsby and Burke, 1994), whereas unmyelinated and small myelinated nerve fiber function is preserved in the majority of patients (Borg and Lindblom, 1988). Although pain is a dominant symptom of patients with nerve entrapment syndromes, it is not known if the sympathetic system contributes to the generation of spontaneous pain and especially heat hyperalgesia in these patients.
2. Methods 2.1. Subjects Patients with a lesion of the median nerve due to chronic nerve compression attending an orthopaedic clinic or the department of neurology of the university hospital Schleswig-Holstein for treatment were asked to participate in the study. Patients with an electrophysiological confirmed unilateral nerve lesion were included if they showed symptoms of neuropathic pain which were restricted to the innervation territory of the affected nerve. Patients with clinical or electrophysiological evidence for an additional neurological disorder (e.g. damage of the contralateral nerve) or who underwent prior surgical treatment related to the peripheral nerve lesion were excluded from the study. If patients were treated with steroids or NSAID the treatment was stopped 1 week before the patients were included in the study. Patients were allowed to continue with other forms of analgesic medication until 24 h before the experiment. Ten healthy age matched volunteers served as controls. The nature of the investigations was explained according to the Helsinki Declaration. Patients and volunteers gave their informed written consent to participate in the study. The investigations were approved by the local ethical committee. 2.2. Nerve conduction studies The examinations were performed in a warm room. Hand temperature was measured and maintained >34 C using a heating device. Antidromic sensory action potentials were recorded from the index finger on both sides using ring electrodes after stimulation of the median nerve at the wrist. The responses (10–30) were averaged. The stimulus duration was 0.2 ms delivered at 2 Hz from a constant voltage source. The amplitude and nerve conduction velocity were determined. Orthodromic motor conduction was measured on both sides by recording from the Musculus abductor pollicis brevis using surface electrodes after stimulation of the median nerve at the wrist (distance 6.5 cm). The stimulus duration was 0.2 ms. Amplitude and distal motor latency were assessed. The results were evaluated on the basis of our laboratory standard (Table 1). 2.3. Quantitative sensory testing (QST) For quantitative assessment of thermal perception (function of C- and; A-delta fibers) we used the thermotest device (Somedic AB, Sweden). The thermo-stimulator operates on the Peltier principle (Fruhstorfer et al., 1976). The probe (stimulating surface 5 cm2) was applied to the palmar aspect of the index finger. Measurements were taken on both sides in patients and on
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Table 1 Results of nerve conduction studies in patients with a median nerve entrapment syndrome Pat. no.
Age
Sex
DML (ms) affected side
MSAP (mV)
SNCV (ms)
SNAP (lV)
Heat pain (C)
DML (ms) unaffected side
MSAP (mV)
SNCV (ms)
SNAP (lV)
Heat pain (C)
1 2 3 4 5 6 7 8 9 10
32 38 39 41 42 46 47 47 49 58
M F F M M M F M M M
4.8 4.5 4.6 5.2 5.3 4.1 4.0 – 5.2 6.1
5.4 9.7 7.9 5.3 6.2 7.6 9.5 – 5.5 5.2
40.4 39.3 40.7 39.1 40.7 41.2 38.9 37.1 38.7 39.7
14.8 12.4 18.2 7.1 14.1 12.1 11.2 9.9 15.1 17.3
39.7 38.4 40.9 42.0 39.1 41.2 40.5 44.6 40.5 40.4
3.6 3.8 3.6 3.8 3.7 3.7 3.7 – 3.8 3.9
10 15 18 9 14 10 9 – 11 12
51 54 51 47 44 46 46 42 44 54
27 23 28 16 18 16 17 13 21 19
44.3 43.7 45.2 43.9 45.9 43.0 46.4 46.3 44.7 46.0
11 12 13 14 15 16 17 18 19 20
28 31 32 41 45 58 62 63 64 73
M M F F M F F M F M
6.4 4.8 4.1 6 6.2 4.8 – 4.5 5.8 5.3
4.9 6.6 8.8 4.3 5.3 5.7 – 4.7 5.3 5.7
36.1 44.3 43.5 42.4 41.4 38.0 37.2 38.1 40.8 39.7
10.7 21.2 15.2 13.1 14.3 10.4 8.9 10.4 13.2 12.3
44.2 50.0 45.2 47.1 47.5 49.6 44.9 46.6 42.8 44.8
3.8 3.7 3.5 3.8 3.6 3.9 – 4.0 4.0 4.1
13 16 17 15 13 9 – 10 8 7
49 57 52 54 47 45 41 46 43 41
32 31 29 22 19 18 13 14 18 16
42.0 45.0 45.9 41.8 44.8 47.4 44.2 44.7 43.1 42.8
Patients are grouped in patients with hyperalgesia (patients 1–10) and patients without hyperalgesia (patients 11–20). Normal values: DML (range 25–75 years: 3.9–4.06 ms), MSAP (range 25–75 years: 6.4–5.0 mV), SNCV (range 25–75 years: 46.3–37.0 ms), SNAP (range 25–75 years: 6.6–3.7 lV).
the right side in control subjects. To determine warm and cold perception thresholds patients were instructed to press a button as soon as they perceived a change in temperature (rate of temperature change 1 C/s) respectively if they noticed a painful sensation for determination of cold and heat pain thresholds (rate of temperature change 1 C/s, 20 s interval). The mean of five single tests defined perception thresholds. The median value of side-to-side differences in heat pain thresholds of all patients was calculated and patients were subdivided along this value in patients with and without heat hyperalgesia. The testing was performed at a baseline temperature of 32 C at the second and third visit. The baseline temperature of the thermode during the whole body cooling and warming procedure was adjusted to 35 C, because skin temperature at the affected side was kept constant at 35 C during the experiment. 2.4. Mechano-sensory testing and pain ratings To assess the incidence of mechanical evoked pain the area of punctate hyperalgesia (von Frey filament; Aesthesiometer, Somedic, Stockholm, Sweden, nominal force 3.3 g) and mechanical allodynia (soft brush) were investigated by repetitive mechanical stimulation of the skin. Stimulation was started outside the innervation territory of the median nerve moving in steps of 0.5 cm to a centre within the innervation territory from three different directions. If the subjects reported a pain sensation, pain was rated on a numeric rating scale
((NRS) 0–10). The examination was performed on the affected and contralateral side in patients and on the right side in the control subjects. Spontaneous pain was rated on a NRS (0–10). 2.5. Modulation of sympathetic vasoconstrictor outflow to the limb The activity of cutaneous sympathetic vasoconstrictor neurons projecting to the affected extremity was changed under controlled conditions. Thermoregulatory reflexes were performed to induce a physiological tonic stimulation of sympathetic activity (Baron et al., 2002). This was achieved by changing environmental temperature. The individuals were lying in a thermal suit, in which circulating water of 12 C or 50 C (inflow temperature) was used to cool or warm the whole body respectively. The effect of sympathetic activation, i.e. the amount of cutaneous vasoconstriction, was assessed by measuring cutaneous blood flow (in relative perfusion units, PU) at the index finger on both sides (Laser Doppler perfusion monitor, PF 4001, integrating probe PF 413, Perimed, Ja¨rfalla, Sweden). Skin temperature was measured by infrared thermography at all fingers on the unaffected side or the left side in controls (not covered by the suit). High sympathetic activity was defined as a drop in skin temperature below 26 C during the cooling session, low sympathetic activity as a rise in skin temperature to values greater than 35 C during the warming session (Wasner et al., 2001). At the
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affected side or the right side in controls the local skin temperature was kept constant at 35 C.
3. Results 3.1. Subjects
2.6. Iontophoresis of norepinephrine and placebo Iontophoresis was performed at the palmar aspect of the index finger. A gauze (5 cm2) was saturated with a solution of norepinephrine (0.1%) or placebo (saline 0.9%). The gauze was covered by a silver electrode. The ground electrode was a silver plate (0.7 cm in diameter) covered with electrode paste. An electric current (0.5 mA) was applied for 120 s to introduce positively charged ions into the skin (Schattschneider et al., 2006).
Twenty patients participated in the study (7 women, 13 men; age 46.3 ± 2.9 years). All patients complained about pain or painful paraesthesias (2.4 ± 0.1 (NRS; 0–10)) in the innervation territory of the median nerve. The mean duration of neuropathic pain was 21.5 ± 3.5 months. The group of volunteers consisted of 5 women and 5 men (age 45 ± 1.8 years) without any history or clinical and electrophysiological signs of neurological disorders.
2.7. Procedures
3.2. Nerve conduction studies
2.7.1. Clinical assessment Clinical examination and electrophysiological recordings were carried out during the first visit. During the second visit pain assessment and quantitative sensory testing were performed within the innervation territory of the lesioned nerve and the corresponding contralateral skin area in patients. In the control subjects the testing was performed within the innervation territory of the median nerve on the right side. The testing area was flagged with a permanent marker. Afterwards participants put on the thermal suit and the sympathetic skin vasoconstrictor neurons were activated by whole body cooling. Changes in spontaneous and evoked pain were assessed during the state of sympathetic activation and sympathetic inhibition. Sympathetic inhibition was achieved by whole body warming. The local skin temperature at the testing site was kept constant at 35 C during clinical assessments and the whole-body temperature change by using a feed-back-controlled heating device. During a third visit iontophoresis was performed. Changes in spontaneous and evoked pain were assessed before and after the iontophoresis. The local skin temperature at the testing site was kept constant at 32 C.
Electrophysiological recordings in patients demonstrated a mono-neuropathy of the distal median nerve in all patients. The results of nerve conduction studies measured on the affected and unaffected side are shown in Table 1.
2.8. Statistical analysis Demographic data and the results of QST, skin blood flow and skin temperature measurements were analysed by basic descriptive statistics. Wilcoxons test was used for comparison of paired data. Mann–Whitney-U test was performed for comparison of unpaired data. One way ANOVA was used for comparison of changes in perception thresholds (change in perception thresholds · group). In case of a significant influence of the group factor on the dependent variables post hoc comparisons were computed according to Bonferroni. Spearman’s rank correlation was calculated for the correlation of clinical data. Values of p < 0.05 were considered to be statistically significant. All data are given as mean ± SEM.
4. Quantitative sensory testing In patients (n = 20) warm thresholds were significantly increased on the affected side compared to the unaffected side (36.9 ± 0.4 C vs. 35.1 ± 0.3 C; p < 0.001). No significant side differences could be demonstrated in cold perception thresholds (29.1 ± 0.3 vs. 29.0 ± 0.3) and cold pain thresholds (Table 2). Mechanical allodynia or punctate hyperalgesia was not present in any of the patients. Hypoaesthesia within the innervation territory of the median nerve during von Frey filament stimulation was reported by 12 patients. Eighteen patients reported hypoaesthesia under stimulation with a soft brush in comparison to the unaffected side. The patients were subdivided along the median value of side-to-side differences in heat pain thresholds (median (affected side unaffected side) = 1.2 C) in patients without heat hyperalgesia (n = 10, side-to-side difference 1.9 ± 0.7 C) and patients with heat hyperalgesia (n = 10 side-to-side difference 4.2 ± 0.6 C) (Table 2). The degree of heat hyperalgesia was not correlated to the intensity of spontaneous pain, warm perception thresholds, age, the duration of the disease, or velocity and amplitude of sensory nerve conduction. 4.1. Controlled alteration of the sympathetic activity Activation of sympathetic vasoconstrictor neurons by whole body cooling was followed by a decrease of skin blood flow measured at the index finger on both sides in patients and controls. There were no significant side differences (affected side: 50 ± 7 PU vs. unaffected side 54 ± 9 PU; controls right hand: 89 ± 25 PU vs. controls
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Table 2 Results of quantitative sensory testing (thermotest) in volunteers and patients at a baseline temperature of 32 C
thresholds (C) affected side thresholds (C) unaffected side threshold (C) affected side threshold (C) unaffected side
Patient without hyperalgesia (n = 10)
<10
12.3 ± 1.0 11.9 ± 1.2 40.7 ± 0.5 44.9 ± 0.4
10.8 ± 0.8 10.3 ± 0.3 46.2 ± 0.7 44.1 ± 0.6
46.3 ± 0.6
left side 71 ± 19). The mean time needed for lowering skin temperature to 26 C (unaffected side, left side in controls) was 56 ± 13 min. After performing the pain assessments whole body warming was performed to inhibit sympathetic activity. As a result skin blood flow increased symmetrically (affected side: 330 ± 16 PU vs. unaffected side 311 ± 14 PU; controls right hand: 363 ± 26 PU vs. controls left side 342 ± 17). The mean time needed for reaching a skin temperature of 35 C (unaffected side, left side in controls) was 32 ± 8 min.
A)
4.3. Changes in somato-sensory perception by pharmacological adrenergic stimulation Application of norepinephrine was not followed by changes in spontaneous pain. Mechanical allodynia or punctate hyperalgesia were not observed. Iontophoresis of norepinephrine caused a decrease in heat pain thresholds in both patient groups. However the decrease in heat pain thresholds was significantly higher in patients with pre-existing hyperalgesia (n = 10) compared to patients without pre-existing hyperalgesia (n = 10) ( 1.4 ± 0.5 C vs. 0.6 ± 0.7 C; p = 0.007). Warm thresholds were not significantly changed (36.7 ± 0.4 C vs. 36.5 ± 0.3 (patients with pre-existing hyperalgesia)); (36.6 ± 0.5 C vs. 36.5 ± 0.4 C (patients without pre-existing hyperalgesia)). Saline iontophoresis was
[PU]
300
200
100
0
B)
4.2. Changes in somato-sensory perception by physiological adrenergic stimulation
45 [C ]
40
35
C)
[C ]
47.5 Heat pain thresholds
Whole body cooling did not evoke mechanical allodynia or punctate hyperalgesia. No significant changes in spontaneous pain were reported by the patients (2.5 ± 0.3 vs. 2.4 ± 0.2 (NRS)). Warm perception thresholds were not affected by increased sympathetic skin vasoconstrictor activity in any of the groups (Fig. 1). With regard to heat pain thresholds there was a trend to decreased heat pain thresholds in patients. One way ANOVA demonstrated significant differences between the groups (F(2, 27) = 5.874, p < 0.008) regarding the decrease in heat pain thresholds (patients with pre-existing hyperalgesia, n = 10, ( 1.0 ± 0.2 C) vs. patients without preexisting hyperalgesia, n = 10, ( 0.3 ± 0.1 C); p < 0.05; patients with pre-existing hyperalgesia vs. controls, n = 10, ( 0.4 ± 0.1 C); p < 0.05; patients without preexisting hyperalgesia vs. controls; p = 1.0) (Fig. 1).
400
Skin blood flow
pain pain pain pain
Patient with hyperalgesia (n = 10)
Warm perception thresholds
Cold Cold Heat Heat
Volunteers right side (n = 10)
* 42.5
37.5 Low
High
Low
High
Low
High
Sympathetic activity Volunteers
Patients without hyperalgesia
Patients with hyperalgesia
Fig. 1. Measurements of skin blood flow (index finger) were taken during low (whole body warming) and high (whole body cooling) sympathetic activity. (A) A significant reduction in skin blood flow indicating sympathetic activation during the whole body cooling condition could be observed in volunteers (p < 0.05) and both groups of patients (patients without hyperalgesia: p < 0.05; patients with hyperalgesia: p < 0.05). (B) Sympathetic activation did not show influence on warm perception thresholds in any of the group. (C) Heat pain thresholds did not change significantly in volunteers and patients without heat hyperalgesia whereas a significant decrease (p < 0.05) could be observed in patients with pre-existing heat hyperalgesia.
not followed by any significant changes in heat pain and warm thresholds. Saline iontophoresis did not enhance spontaneous pain or evoke pain.
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5. Discussion The aim of the present investigation was to evaluate the role of sympathetic afferent coupling in a human neuropathic pain condition due to chronic peripheral nerve compression. We found that adrenergic stimulation did not influence spontaneous pain. Overall no significant changes in mechanical and thermal evoked pain were observed. However in a subgroup of patients characterized by the presence of heat hyperalgesia, thermal evoked pain was significantly aggravated by pharmacological adrenergic stimulation and physiological sympathetic stimulation.
and a2 receptors and is independent of the vasoconstriction associated with the norepinephrine application (Drummond, 1996; Fuchs et al., 2001). However in patients with sensitized nociceptive nerve fibers as indicated by the presence of heat hyperalgesia, the decrease in heat pain was significantly increased in comparison to patients without heat hyperalgesia. Previous studies did not show a general sensitization to catecholamnies in painful neuropathy (Schattschneider et al., 2006). This observation points to an increased sensitivity to adrenergic stimulation in the stage of peripheral sensitization. 5.3. Physiological activation of sympathetic vasoconstrictor neurons
5.1. Comparison of pain characteristics There are striking similarities regarding the pathophysiology of the CCI model and nerve entrapment syndromes in humans. Both are characterized by predominant demyelinisation of large nerve fibers (Bennett and Xie, 1988; Borg and Lindblom, 1988). Further there are several studies pointing to an inflammatory process which is involved in the development of pain in animals and in humans (Sommer et al., 1995; Wagner et al., 1998; Freeland et al., 2002). However clinically there are differences in the presentation of the different pain components. None of the patients reported mechanical allodynia or hyperalgesia which is frequent in animals. Further no significant differences in cold pain thresholds were found in patients (Bennett and Xie, 1988). The main difference of both nerve lesions is that in the CCI model the surgical procedure and the suture material will promote the release of proinflammatory cytokines. It is most likely that the extent of the inflammatory response in humans is much lower and that therefore signs of peripheral sensitization like heat hyperalgesia (Meyer and Campbell, 1981; LaMotte et al., 1992) are less frequent. The discrepancy in occurrence of mechanical allodynia and punctuate hyperalgesia may be attributed to differences in the degree of large fiber damage. The majority of patients presented hypoaesthesia during the sensory testing indicating impairment of myelinated nerve function. Regarding small fiber function, warm perception was mildly affected and cold perception was in contrast to other studies (Lang et al., 1995) preserved in the majority of patients. This finding may indicate a higher resistance of small fibers to pressure in comparison to large myelinated fibers (Borg and Lindblom, 1988). 5.2. Effects of pharmacological adrenergic stimulation Consistent with previous reports we could show that in human skin norepinephrine induces heat hyperalgesia which is a physiological reaction to adrenergic substances. This effect is thought to be transmitted via a1
Controlled thermoregulation as used in the present study exclusively affects the activity in one distinct sympathetic channel, i.e. the cutaneous vasoconstrictor system. Whole-body temperature challenges are the most effective stimuli to modulate cutaneous vasoconstrictor activity in the widest physiological range. Accordingly, microneurographical recordings demonstrated a nearly complete inhibition during body warming and maximal physiological activation of these neurons during cooling (Bini et al., 1980). The effect of whole-body temperature changes on general pain perception in healthy volunteers have been investigated in the present and previous studies without showing significant effects (Baron et al., 1999; Baron et al., 2002). On the other hand activation of sympathetic vasoconstrictor neurons increases spontaneous and evoked pains in CPRS I patients with sympathetically maintained pain (Baron et al., 2002). In the present study the overall effect of sympathetic stimulation on mechanical and thermal evoked pain was not significant. This finding mirrors our clinical experience. However the fact that in a subgroup of patients heat pain thresholds could significantly be changed by sympathetic modulation offers new insights in the pathophysiology of sympathetic afferent interactions. Moreover our findings are in accordance with a recent publication by Jorum et al. (2007) who found that sensitized mechano-insensitive nociceptors can be activated by endogenously released catecholamines. 5.4. The role of peripheral sensitization for the development of adrenergic sensitivity Under physiological conditions heat pain is perceived after stimulation of polymodal nociceptive C-fibers with thermal stimuli higher than 45 C (Handwerker and Neher, 1976). Some of our patients reported a burning pain sensation already below this threshold. The decrease in heat pain thresholds indicated the presence of heat hyperalgesia. There is substantial evidence that heat hyperalgesia is produced by peripheral nociceptor sensitization (Meyer and Campbell, 1981; LaMotte
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et al., 1992). Nociceptor sensitization can be found after exposure to heat stimuli, to irritant chemicals and inflammatory mediators like PGE 2 and TNF a (Meyer and Campbell, 1981; Kress et al., 1992; Sorkin et al., 1997; Chen et al., 1999). After chronic nerve constriction in animals the development of heat hyperalgesia within the innervation territory of the injured nerve is a common phenomenon (Bennett and Xie, 1988). The sensitization of nociceptive C-fibers in this model has been attributed to the release of proinflammatory cytokines (Sommer et al., 1993; Wagner and Myers, 1996; Wagner et al., 1998). In parallel to the animal data increased levels of proinflammatory cytokines have been demonstrated in patients with chronic nerve compression (Freeland et al., 2002) suggesting that the heat hyperalgesia observed in our patients is due to nociceptor sensitization by these substances. In an earlier publication Lang et al. (1995) suggested that hyperalgesia in carpal tunnel syndrome also included mechanisms of central sensitization. However not only whole body cooling but also local application of norepinephrine was followed by a significant increase in heat hyperalgesia, indicating a peripheral site of interaction. In the present study only patients with signs of C-fiber sensitization respond to physiological sympathetic activation. Our finding is in accordance with an earlier study by Wahren et al. (1991), who reported lower heat pain thresholds in patients with a peripheral nerve lesion who respond to regional guanethedine blocks. We propose that after a chronic compression injury of a peripheral nerve sympathetic sensitivity depends on the presence of sensitized nerve fibers. Nevertheless, sensitization of nociceptive C-fibers by capsaicin was not followed by an increased responsiveness of these fibers to activation of sympathetic vasoconstrictor neurons (Baron et al., 1999). This contradiction may be explained by changes in excitation properties of the afferent neuron which involves changes in gene expression like up regulation of a1 adrenoreceptor or TTXR sodium channels and therefore occur only after longstanding sensitization (Drummond et al., 1996; Novakovic et al., 1998). However, until now there is no direct evidence of upregulation of adrenoreceptors in the periphery or that this depends on the presence of inflammatory cytokines. In contrast it has been shown that the development of heat hyperalgesia and the sprouting of noradrenergic collaterals in the DRG depend upon peripherally derived NGF (Ramer and Bisby, 1999). An upregulation of nerve growth factor has been demonstrated in the CCI model. Thus it could be suggested that NGF accounts for the development of hyperalgesia through changes in gene expression and sympathetic afferent coupling also in our patients. However from the present results we would expect that the sympathetic afferent coupling takes place in the periphery and not in the DRG. From the results of norepi-
523
nephrine iontophoresis we further conclude that the coupling is not restricted to the area of the nerve lesion but also involves distal parts of the nerve. Whether NGF is also related to an upregulation of a1 adrenoreceptors in the periphery has to be examined.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG Ba 1921/1-4) and the Network ‘‘Neuropathic Pain’’ the German Ministry of Research and Education, German research Network on Neuropathic Pain (BMBF, 01EM01/04) and an unrestricted educational grant from Pfizer, Germany.
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Handwerker HO, Neher KD. Characteristics of C-fibre receptors in the cat’s foot responding to stepwise increase of skin temperature ot noxious levels. Pflugers Arch 1976;365(2–3):221–9. Jorum E, Orstavik K, Schmidt R, Namer B, Carr RW, Kvarstein G, et al. Catecholamine-induced excitation of nociceptors in sympathetically maintained pain. Pain 2007;127(3):296–301. Kim SH, Na HS, Sheen K, Chung JM. Effects of sympathectomy on a rat model of peripheral neuropathy. Pain 1993;55(1):85–92. Kress M, Koltzenburg M, Reeh PW, Handwerker HO. Responsiveness and functional attributes of electrically localized terminals of cutaneous C-fibers in vivo and in vitro. J Neurophysiol 1992;68(2):581–95. LaMotte RH, Lundberg LE, Torebjo¨rk HE. Pain, hyperalgesia and activity in nociceptive C units in humans after intradermal injection of capsaicin. J Physiol (Lond) 1992;448:749–64. Lang E, Claus D, Neundorfer B, Handwerker HO. Parameters of thick and thin nerve-fiber functions as predictors of pain in carpal tunnel syndrome. Pain 1995;60(3):295–302. Meyer RA, Campbell JN. Myelinated nociceptive afferents account for the hyperalgesia that follows a burn to the hand. Science 1981;213(4515):1527–9. Neil A, Attal N, Guilbaud G. Effects of guanethidine on sensitization to natural stimuli and self-mutilating behaviour in rats with a peripheral neuropathy. Brain Res 1991;565(2):237–46. Novakovic SD, Tzoumaka E, McGivern JG, Haraguchi M, Sangameswaran L, Gogas KR, et al. Distribution of the tetrodotoxinresistant sodium channel PN3 in rat sensory neurons in normal and neuropathic conditions. J Neurosci 1998;18(6):2174–87. Ramer MS, Bisby MA. Adrenergic innervation of rat sensory ganglia following proximal or distal painful sciatic neuropathy: distinct mechanisms revealed by anti-NGF treatment. Eur J Neurosci 1999;11(3):837–46. Ringkamp M, Eschenfelder S, Grethel EJ, Habler HJ, Meyer RA, Janig W, et al. Lumbar sympathectomy failed to reverse mechan-
ical allodynia- and hyperalgesia-like behavior in rats with L5 spinal nerve injury. Pain 1999;79(2–3):143–53. Schattschneider J, Uphoff J, Binder A, Wasner G, Baron R. No adrenergic sensitization of afferent neurons in painful sensory polyneuropathy. J Neurol 2006;253(3):280–6. Seiler 3rd JG, Milek MA, Carpenter GK, Swiontkowski MF. Intraoperative assessment of median nerve blood flow during carpal tunnel release with laser Doppler flowmetry. J Hand Surg [Am] 1989;14(6):986–91. Sommer C, Galbraith JA, Heckman HM, Myers RR. Pathology of experimental compression neuropathy producing hyperesthesia. J Neuropathol Exp Neurol 1993;52(3):223–33. Sommer C, Lalonde A, Heckmann HM, Rodriguez M, Myers RR. Quantitative neuropathology of a focal nerve injury causing hyperalgesia. J Neuropathol Exp Neurol 1995;54(5): 635–43. Sorkin LS, Xiao W-H, Wagner R, Myers RR. Tumor necrosis factoralpha induces ectopic activity in nociceptive primary afferent fibres. Neurosci 1997;81(1):255–62. Sunderland S. The nerve lesion in the carpal tunnel syndrome. J Neurol Neurosurg Psychiatry 1976;39(7):615–26. Wagner R, Janjigian M, Myers RR. Anti-inflammatory interleukin-10 therapy in CCI neuropathy decreases thermal hyperalgesia, macrophage recruitment, and endoneurial TNF-alpha expression. Pain 1998;74:35–42. Wagner R, Myers RR. Endoneurial injection of TNF-alpha produces neuropathic pain behaviors. Neuroreport 1996;7(18): 2897–901. Wahren LK, Torebjork E, Nystrom B. Quantitative sensory testing before and after regional guanethidine block in patients with neuralgia in the hand. Pain 1991;46(1):23–30. Wasner G, Schattschneider J, Heckmann K, Maier C, Baron R. Vascular abnormalities in reflex sympathetic dystrophy (CRPS I): mechanisms and diagnostic value. Brain 2001;124(Pt 3):587–99.
European Journal of Pain 12 (2008) 517–524 www.EuropeanJournalPain.com
Modulation of sensitized C-fibers by adrenergic stimulation in human neuropathic pain Jo¨rn Schattschneider *, Marco Scarano, Andreas Binder, Gunnar Wasner, Ralf Baron Sektion fu¨r Neurologische Schmerzforschung und – therapie, Klinik fu¨r Neurologie, Universita¨tsklinikum Schleswig-Holstein, Campus Kiel, Schittenhelmstr. 10, 24103 Kiel, Germany Received 22 January 2007; received in revised form 6 August 2007; accepted 7 August 2007 Available online 20 September 2007
Abstract The chronic constriction injury model is widely used in studying mechanisms of neuropathic pain. In this model neuropathic pain can be influenced by sympathetic interventions. It is assumed that similar mechanisms as in animals are responsible for pain arising from nerve entrapment syndromes in humans. The aim of the present study was to investigate if in patients with nerve entrapment nociceptive afferents can be modulated by adrenergic stimulation. Methods: Twenty patients with pain due to a unilateral entrapment of the median nerve and 10 controls were included in the study. Spontaneous pain, mechanical and thermal evoked pain were assessed within the innervation territory of the lesioned nerve and the corresponding contralateral segment in patients and on the right hand side in healthy volunteers. The examinations were performed at baseline, during whole body cooling (sympathetic activation) and whole body warming (sympathetic inhibition), and after norepinephrine iontophoresis. Results: All patients reported spontaneous pain. Mechanical allodynia, punctate hyperalgesia and cold allodynia was not found. According to side-to-side differences in heat pain thresholds, patients were separated in patients with (n = 10) and without (n = 10) heat hyperalgesia. Adrenergic stimulation did not induce or enhance spontaneous or mechanical evoked pain in any patient or control subject. However in patients with pre-existing heat hyperalgesia sympathetic stimulation aggravated heat hyperalgesia significantly. Further in these patients the decrease in heat pain thresholds observed after norepinephrine iontophoresis was significantly higher compared to patients without pre-existing heat hyperalgesia. Conclusion: Sympathetic–afferent interaction does not play a major role in pain generation due to nerve entrapment. Nevertheless in a subgroup of patients nociceptive afferents show sensitivity to physiological and pharmacological sympathetic stimulation. This finding is important because it emphasises that despite there is no clinical detectable effect on pain sympathetic afferent interaction can be found. 2007 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. Keywords: Neuropathic pain; Nociceptor sensitization; Sympathetic–afferent coupling
1. Introduction *
Corresponding author. Tel.: +49 431 597 8510; fax: +49 431 597 8502. E-mail address: [email protected] (J. Schattschneider).
Several behavioural animal models of neuropathic pain have been used to determine the contribution of the sympathetic nervous system to neuropathic pain. In the spinal nerve ligation model it has be reported that
1090-3801/$34 2007 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ejpain.2007.08.004
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surgical sympathectomy reversed mechanical as well as thermal hyperalgesia (Kim et al., 1993), whereas others failed to demonstrate an effect of sympathectomy on pain behaviour in this model (Ringkamp et al., 1999). In the chronic constriction injury model surgical sympathectomy reduced thermal hyperalgesia whereas mechanical hyperalgesia and spontaneous pain behaviour were unchanged (Bennett and Xie, 1988; Desmeules et al., 1995). These experiments demonstrated that the nature of the nerve lesion determines that only distinct components of neuropathic pain behaviour are reduced after surgical sympathectomy. Furthermore it is possible that in a neuropathic pain condition one symptom for example heat hyperalgesia, is sympathetically maintained whereas another symptom, for example mechanical hyperalgesia or spontaneous pain, is sympathetically independent. It is assumed that the chronic constriction injury (CCI) model mirrors the clinical situation of nerve entrapment syndromes like carpal tunnel syndrome in an experimental set up (Bennett and Xie, 1988; Neil et al., 1991; Desmeules et al., 1995). In this model loose ligation of the sciatic nerve is followed by an occlusion of the epineural vasculature. A swelling of the nerve and an extensive demyelinisation of large diameter fibers can be observed (Bennett and Xie, 1988). The morphological changes are accompanied by an inflammatory response (Sommer et al., 1995; Wagner et al., 1998). After the nerve lesion rats develop mechanical and thermal hyperalgesic behaviour. It has been demonstrated that thermal hyperalgesia responds to surgical sympathectomy or depletion of sympathetic transmitters by guanethidine (Neil et al., 1991; Desmeules et al., 1995). In humans nerve entrapment syndromes are associated with a microcirculatory venous stasis leading to an increased interstitial fluid pressure in the peripheral nerve and the development of progressive oedema (Sunderland, 1976; Seiler et al., 1989; Freeland et al., 2002). Extraneural and intraneural nerve blood flow and axonal transport are progressively compromised leading to tissue ischemia and the accumulation of inflammatory cytokines which may lead to nociceptor sensitization (Freeland et al., 2002). Electrophysiological studies in patients with carpal tunnel syndrome demonstrated demyelinisation and functional impairment of large diameter sensory and motor fibers, which leads to a reduction of sensory and motor conduction velocity (Goadsby and Burke, 1994), whereas unmyelinated and small myelinated nerve fiber function is preserved in the majority of patients (Borg and Lindblom, 1988). Although pain is a dominant symptom of patients with nerve entrapment syndromes, it is not known if the sympathetic system contributes to the generation of spontaneous pain and especially heat hyperalgesia in these patients.
2. Methods 2.1. Subjects Patients with a lesion of the median nerve due to chronic nerve compression attending an orthopaedic clinic or the department of neurology of the university hospital Schleswig-Holstein for treatment were asked to participate in the study. Patients with an electrophysiological confirmed unilateral nerve lesion were included if they showed symptoms of neuropathic pain which were restricted to the innervation territory of the affected nerve. Patients with clinical or electrophysiological evidence for an additional neurological disorder (e.g. damage of the contralateral nerve) or who underwent prior surgical treatment related to the peripheral nerve lesion were excluded from the study. If patients were treated with steroids or NSAID the treatment was stopped 1 week before the patients were included in the study. Patients were allowed to continue with other forms of analgesic medication until 24 h before the experiment. Ten healthy age matched volunteers served as controls. The nature of the investigations was explained according to the Helsinki Declaration. Patients and volunteers gave their informed written consent to participate in the study. The investigations were approved by the local ethical committee. 2.2. Nerve conduction studies The examinations were performed in a warm room. Hand temperature was measured and maintained >34 C using a heating device. Antidromic sensory action potentials were recorded from the index finger on both sides using ring electrodes after stimulation of the median nerve at the wrist. The responses (10–30) were averaged. The stimulus duration was 0.2 ms delivered at 2 Hz from a constant voltage source. The amplitude and nerve conduction velocity were determined. Orthodromic motor conduction was measured on both sides by recording from the Musculus abductor pollicis brevis using surface electrodes after stimulation of the median nerve at the wrist (distance 6.5 cm). The stimulus duration was 0.2 ms. Amplitude and distal motor latency were assessed. The results were evaluated on the basis of our laboratory standard (Table 1). 2.3. Quantitative sensory testing (QST) For quantitative assessment of thermal perception (function of C- and; A-delta fibers) we used the thermotest device (Somedic AB, Sweden). The thermo-stimulator operates on the Peltier principle (Fruhstorfer et al., 1976). The probe (stimulating surface 5 cm2) was applied to the palmar aspect of the index finger. Measurements were taken on both sides in patients and on
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Table 1 Results of nerve conduction studies in patients with a median nerve entrapment syndrome Pat. no.
Age
Sex
DML (ms) affected side
MSAP (mV)
SNCV (ms)
SNAP (lV)
Heat pain (C)
DML (ms) unaffected side
MSAP (mV)
SNCV (ms)
SNAP (lV)
Heat pain (C)
1 2 3 4 5 6 7 8 9 10
32 38 39 41 42 46 47 47 49 58
M F F M M M F M M M
4.8 4.5 4.6 5.2 5.3 4.1 4.0 – 5.2 6.1
5.4 9.7 7.9 5.3 6.2 7.6 9.5 – 5.5 5.2
40.4 39.3 40.7 39.1 40.7 41.2 38.9 37.1 38.7 39.7
14.8 12.4 18.2 7.1 14.1 12.1 11.2 9.9 15.1 17.3
39.7 38.4 40.9 42.0 39.1 41.2 40.5 44.6 40.5 40.4
3.6 3.8 3.6 3.8 3.7 3.7 3.7 – 3.8 3.9
10 15 18 9 14 10 9 – 11 12
51 54 51 47 44 46 46 42 44 54
27 23 28 16 18 16 17 13 21 19
44.3 43.7 45.2 43.9 45.9 43.0 46.4 46.3 44.7 46.0
11 12 13 14 15 16 17 18 19 20
28 31 32 41 45 58 62 63 64 73
M M F F M F F M F M
6.4 4.8 4.1 6 6.2 4.8 – 4.5 5.8 5.3
4.9 6.6 8.8 4.3 5.3 5.7 – 4.7 5.3 5.7
36.1 44.3 43.5 42.4 41.4 38.0 37.2 38.1 40.8 39.7
10.7 21.2 15.2 13.1 14.3 10.4 8.9 10.4 13.2 12.3
44.2 50.0 45.2 47.1 47.5 49.6 44.9 46.6 42.8 44.8
3.8 3.7 3.5 3.8 3.6 3.9 – 4.0 4.0 4.1
13 16 17 15 13 9 – 10 8 7
49 57 52 54 47 45 41 46 43 41
32 31 29 22 19 18 13 14 18 16
42.0 45.0 45.9 41.8 44.8 47.4 44.2 44.7 43.1 42.8
Patients are grouped in patients with hyperalgesia (patients 1–10) and patients without hyperalgesia (patients 11–20). Normal values: DML (range 25–75 years: 3.9–4.06 ms), MSAP (range 25–75 years: 6.4–5.0 mV), SNCV (range 25–75 years: 46.3–37.0 ms), SNAP (range 25–75 years: 6.6–3.7 lV).
the right side in control subjects. To determine warm and cold perception thresholds patients were instructed to press a button as soon as they perceived a change in temperature (rate of temperature change 1 C/s) respectively if they noticed a painful sensation for determination of cold and heat pain thresholds (rate of temperature change 1 C/s, 20 s interval). The mean of five single tests defined perception thresholds. The median value of side-to-side differences in heat pain thresholds of all patients was calculated and patients were subdivided along this value in patients with and without heat hyperalgesia. The testing was performed at a baseline temperature of 32 C at the second and third visit. The baseline temperature of the thermode during the whole body cooling and warming procedure was adjusted to 35 C, because skin temperature at the affected side was kept constant at 35 C during the experiment. 2.4. Mechano-sensory testing and pain ratings To assess the incidence of mechanical evoked pain the area of punctate hyperalgesia (von Frey filament; Aesthesiometer, Somedic, Stockholm, Sweden, nominal force 3.3 g) and mechanical allodynia (soft brush) were investigated by repetitive mechanical stimulation of the skin. Stimulation was started outside the innervation territory of the median nerve moving in steps of 0.5 cm to a centre within the innervation territory from three different directions. If the subjects reported a pain sensation, pain was rated on a numeric rating scale
((NRS) 0–10). The examination was performed on the affected and contralateral side in patients and on the right side in the control subjects. Spontaneous pain was rated on a NRS (0–10). 2.5. Modulation of sympathetic vasoconstrictor outflow to the limb The activity of cutaneous sympathetic vasoconstrictor neurons projecting to the affected extremity was changed under controlled conditions. Thermoregulatory reflexes were performed to induce a physiological tonic stimulation of sympathetic activity (Baron et al., 2002). This was achieved by changing environmental temperature. The individuals were lying in a thermal suit, in which circulating water of 12 C or 50 C (inflow temperature) was used to cool or warm the whole body respectively. The effect of sympathetic activation, i.e. the amount of cutaneous vasoconstriction, was assessed by measuring cutaneous blood flow (in relative perfusion units, PU) at the index finger on both sides (Laser Doppler perfusion monitor, PF 4001, integrating probe PF 413, Perimed, Ja¨rfalla, Sweden). Skin temperature was measured by infrared thermography at all fingers on the unaffected side or the left side in controls (not covered by the suit). High sympathetic activity was defined as a drop in skin temperature below 26 C during the cooling session, low sympathetic activity as a rise in skin temperature to values greater than 35 C during the warming session (Wasner et al., 2001). At the
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affected side or the right side in controls the local skin temperature was kept constant at 35 C.
3. Results 3.1. Subjects
2.6. Iontophoresis of norepinephrine and placebo Iontophoresis was performed at the palmar aspect of the index finger. A gauze (5 cm2) was saturated with a solution of norepinephrine (0.1%) or placebo (saline 0.9%). The gauze was covered by a silver electrode. The ground electrode was a silver plate (0.7 cm in diameter) covered with electrode paste. An electric current (0.5 mA) was applied for 120 s to introduce positively charged ions into the skin (Schattschneider et al., 2006).
Twenty patients participated in the study (7 women, 13 men; age 46.3 ± 2.9 years). All patients complained about pain or painful paraesthesias (2.4 ± 0.1 (NRS; 0–10)) in the innervation territory of the median nerve. The mean duration of neuropathic pain was 21.5 ± 3.5 months. The group of volunteers consisted of 5 women and 5 men (age 45 ± 1.8 years) without any history or clinical and electrophysiological signs of neurological disorders.
2.7. Procedures
3.2. Nerve conduction studies
2.7.1. Clinical assessment Clinical examination and electrophysiological recordings were carried out during the first visit. During the second visit pain assessment and quantitative sensory testing were performed within the innervation territory of the lesioned nerve and the corresponding contralateral skin area in patients. In the control subjects the testing was performed within the innervation territory of the median nerve on the right side. The testing area was flagged with a permanent marker. Afterwards participants put on the thermal suit and the sympathetic skin vasoconstrictor neurons were activated by whole body cooling. Changes in spontaneous and evoked pain were assessed during the state of sympathetic activation and sympathetic inhibition. Sympathetic inhibition was achieved by whole body warming. The local skin temperature at the testing site was kept constant at 35 C during clinical assessments and the whole-body temperature change by using a feed-back-controlled heating device. During a third visit iontophoresis was performed. Changes in spontaneous and evoked pain were assessed before and after the iontophoresis. The local skin temperature at the testing site was kept constant at 32 C.
Electrophysiological recordings in patients demonstrated a mono-neuropathy of the distal median nerve in all patients. The results of nerve conduction studies measured on the affected and unaffected side are shown in Table 1.
2.8. Statistical analysis Demographic data and the results of QST, skin blood flow and skin temperature measurements were analysed by basic descriptive statistics. Wilcoxons test was used for comparison of paired data. Mann–Whitney-U test was performed for comparison of unpaired data. One way ANOVA was used for comparison of changes in perception thresholds (change in perception thresholds · group). In case of a significant influence of the group factor on the dependent variables post hoc comparisons were computed according to Bonferroni. Spearman’s rank correlation was calculated for the correlation of clinical data. Values of p < 0.05 were considered to be statistically significant. All data are given as mean ± SEM.
4. Quantitative sensory testing In patients (n = 20) warm thresholds were significantly increased on the affected side compared to the unaffected side (36.9 ± 0.4 C vs. 35.1 ± 0.3 C; p < 0.001). No significant side differences could be demonstrated in cold perception thresholds (29.1 ± 0.3 vs. 29.0 ± 0.3) and cold pain thresholds (Table 2). Mechanical allodynia or punctate hyperalgesia was not present in any of the patients. Hypoaesthesia within the innervation territory of the median nerve during von Frey filament stimulation was reported by 12 patients. Eighteen patients reported hypoaesthesia under stimulation with a soft brush in comparison to the unaffected side. The patients were subdivided along the median value of side-to-side differences in heat pain thresholds (median (affected side unaffected side) = 1.2 C) in patients without heat hyperalgesia (n = 10, side-to-side difference 1.9 ± 0.7 C) and patients with heat hyperalgesia (n = 10 side-to-side difference 4.2 ± 0.6 C) (Table 2). The degree of heat hyperalgesia was not correlated to the intensity of spontaneous pain, warm perception thresholds, age, the duration of the disease, or velocity and amplitude of sensory nerve conduction. 4.1. Controlled alteration of the sympathetic activity Activation of sympathetic vasoconstrictor neurons by whole body cooling was followed by a decrease of skin blood flow measured at the index finger on both sides in patients and controls. There were no significant side differences (affected side: 50 ± 7 PU vs. unaffected side 54 ± 9 PU; controls right hand: 89 ± 25 PU vs. controls
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Table 2 Results of quantitative sensory testing (thermotest) in volunteers and patients at a baseline temperature of 32 C
thresholds (C) affected side thresholds (C) unaffected side threshold (C) affected side threshold (C) unaffected side
Patient without hyperalgesia (n = 10)
<10
12.3 ± 1.0 11.9 ± 1.2 40.7 ± 0.5 44.9 ± 0.4
10.8 ± 0.8 10.3 ± 0.3 46.2 ± 0.7 44.1 ± 0.6
46.3 ± 0.6
left side 71 ± 19). The mean time needed for lowering skin temperature to 26 C (unaffected side, left side in controls) was 56 ± 13 min. After performing the pain assessments whole body warming was performed to inhibit sympathetic activity. As a result skin blood flow increased symmetrically (affected side: 330 ± 16 PU vs. unaffected side 311 ± 14 PU; controls right hand: 363 ± 26 PU vs. controls left side 342 ± 17). The mean time needed for reaching a skin temperature of 35 C (unaffected side, left side in controls) was 32 ± 8 min.
A)
4.3. Changes in somato-sensory perception by pharmacological adrenergic stimulation Application of norepinephrine was not followed by changes in spontaneous pain. Mechanical allodynia or punctate hyperalgesia were not observed. Iontophoresis of norepinephrine caused a decrease in heat pain thresholds in both patient groups. However the decrease in heat pain thresholds was significantly higher in patients with pre-existing hyperalgesia (n = 10) compared to patients without pre-existing hyperalgesia (n = 10) ( 1.4 ± 0.5 C vs. 0.6 ± 0.7 C; p = 0.007). Warm thresholds were not significantly changed (36.7 ± 0.4 C vs. 36.5 ± 0.3 (patients with pre-existing hyperalgesia)); (36.6 ± 0.5 C vs. 36.5 ± 0.4 C (patients without pre-existing hyperalgesia)). Saline iontophoresis was
[PU]
300
200
100
0
B)
4.2. Changes in somato-sensory perception by physiological adrenergic stimulation
45 [C ]
40
35
C)
[C ]
47.5 Heat pain thresholds
Whole body cooling did not evoke mechanical allodynia or punctate hyperalgesia. No significant changes in spontaneous pain were reported by the patients (2.5 ± 0.3 vs. 2.4 ± 0.2 (NRS)). Warm perception thresholds were not affected by increased sympathetic skin vasoconstrictor activity in any of the groups (Fig. 1). With regard to heat pain thresholds there was a trend to decreased heat pain thresholds in patients. One way ANOVA demonstrated significant differences between the groups (F(2, 27) = 5.874, p < 0.008) regarding the decrease in heat pain thresholds (patients with pre-existing hyperalgesia, n = 10, ( 1.0 ± 0.2 C) vs. patients without preexisting hyperalgesia, n = 10, ( 0.3 ± 0.1 C); p < 0.05; patients with pre-existing hyperalgesia vs. controls, n = 10, ( 0.4 ± 0.1 C); p < 0.05; patients without preexisting hyperalgesia vs. controls; p = 1.0) (Fig. 1).
400
Skin blood flow
pain pain pain pain
Patient with hyperalgesia (n = 10)
Warm perception thresholds
Cold Cold Heat Heat
Volunteers right side (n = 10)
* 42.5
37.5 Low
High
Low
High
Low
High
Sympathetic activity Volunteers
Patients without hyperalgesia
Patients with hyperalgesia
Fig. 1. Measurements of skin blood flow (index finger) were taken during low (whole body warming) and high (whole body cooling) sympathetic activity. (A) A significant reduction in skin blood flow indicating sympathetic activation during the whole body cooling condition could be observed in volunteers (p < 0.05) and both groups of patients (patients without hyperalgesia: p < 0.05; patients with hyperalgesia: p < 0.05). (B) Sympathetic activation did not show influence on warm perception thresholds in any of the group. (C) Heat pain thresholds did not change significantly in volunteers and patients without heat hyperalgesia whereas a significant decrease (p < 0.05) could be observed in patients with pre-existing heat hyperalgesia.
not followed by any significant changes in heat pain and warm thresholds. Saline iontophoresis did not enhance spontaneous pain or evoke pain.
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5. Discussion The aim of the present investigation was to evaluate the role of sympathetic afferent coupling in a human neuropathic pain condition due to chronic peripheral nerve compression. We found that adrenergic stimulation did not influence spontaneous pain. Overall no significant changes in mechanical and thermal evoked pain were observed. However in a subgroup of patients characterized by the presence of heat hyperalgesia, thermal evoked pain was significantly aggravated by pharmacological adrenergic stimulation and physiological sympathetic stimulation.
and a2 receptors and is independent of the vasoconstriction associated with the norepinephrine application (Drummond, 1996; Fuchs et al., 2001). However in patients with sensitized nociceptive nerve fibers as indicated by the presence of heat hyperalgesia, the decrease in heat pain was significantly increased in comparison to patients without heat hyperalgesia. Previous studies did not show a general sensitization to catecholamnies in painful neuropathy (Schattschneider et al., 2006). This observation points to an increased sensitivity to adrenergic stimulation in the stage of peripheral sensitization. 5.3. Physiological activation of sympathetic vasoconstrictor neurons
5.1. Comparison of pain characteristics There are striking similarities regarding the pathophysiology of the CCI model and nerve entrapment syndromes in humans. Both are characterized by predominant demyelinisation of large nerve fibers (Bennett and Xie, 1988; Borg and Lindblom, 1988). Further there are several studies pointing to an inflammatory process which is involved in the development of pain in animals and in humans (Sommer et al., 1995; Wagner et al., 1998; Freeland et al., 2002). However clinically there are differences in the presentation of the different pain components. None of the patients reported mechanical allodynia or hyperalgesia which is frequent in animals. Further no significant differences in cold pain thresholds were found in patients (Bennett and Xie, 1988). The main difference of both nerve lesions is that in the CCI model the surgical procedure and the suture material will promote the release of proinflammatory cytokines. It is most likely that the extent of the inflammatory response in humans is much lower and that therefore signs of peripheral sensitization like heat hyperalgesia (Meyer and Campbell, 1981; LaMotte et al., 1992) are less frequent. The discrepancy in occurrence of mechanical allodynia and punctuate hyperalgesia may be attributed to differences in the degree of large fiber damage. The majority of patients presented hypoaesthesia during the sensory testing indicating impairment of myelinated nerve function. Regarding small fiber function, warm perception was mildly affected and cold perception was in contrast to other studies (Lang et al., 1995) preserved in the majority of patients. This finding may indicate a higher resistance of small fibers to pressure in comparison to large myelinated fibers (Borg and Lindblom, 1988). 5.2. Effects of pharmacological adrenergic stimulation Consistent with previous reports we could show that in human skin norepinephrine induces heat hyperalgesia which is a physiological reaction to adrenergic substances. This effect is thought to be transmitted via a1
Controlled thermoregulation as used in the present study exclusively affects the activity in one distinct sympathetic channel, i.e. the cutaneous vasoconstrictor system. Whole-body temperature challenges are the most effective stimuli to modulate cutaneous vasoconstrictor activity in the widest physiological range. Accordingly, microneurographical recordings demonstrated a nearly complete inhibition during body warming and maximal physiological activation of these neurons during cooling (Bini et al., 1980). The effect of whole-body temperature changes on general pain perception in healthy volunteers have been investigated in the present and previous studies without showing significant effects (Baron et al., 1999; Baron et al., 2002). On the other hand activation of sympathetic vasoconstrictor neurons increases spontaneous and evoked pains in CPRS I patients with sympathetically maintained pain (Baron et al., 2002). In the present study the overall effect of sympathetic stimulation on mechanical and thermal evoked pain was not significant. This finding mirrors our clinical experience. However the fact that in a subgroup of patients heat pain thresholds could significantly be changed by sympathetic modulation offers new insights in the pathophysiology of sympathetic afferent interactions. Moreover our findings are in accordance with a recent publication by Jorum et al. (2007) who found that sensitized mechano-insensitive nociceptors can be activated by endogenously released catecholamines. 5.4. The role of peripheral sensitization for the development of adrenergic sensitivity Under physiological conditions heat pain is perceived after stimulation of polymodal nociceptive C-fibers with thermal stimuli higher than 45 C (Handwerker and Neher, 1976). Some of our patients reported a burning pain sensation already below this threshold. The decrease in heat pain thresholds indicated the presence of heat hyperalgesia. There is substantial evidence that heat hyperalgesia is produced by peripheral nociceptor sensitization (Meyer and Campbell, 1981; LaMotte
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et al., 1992). Nociceptor sensitization can be found after exposure to heat stimuli, to irritant chemicals and inflammatory mediators like PGE 2 and TNF a (Meyer and Campbell, 1981; Kress et al., 1992; Sorkin et al., 1997; Chen et al., 1999). After chronic nerve constriction in animals the development of heat hyperalgesia within the innervation territory of the injured nerve is a common phenomenon (Bennett and Xie, 1988). The sensitization of nociceptive C-fibers in this model has been attributed to the release of proinflammatory cytokines (Sommer et al., 1993; Wagner and Myers, 1996; Wagner et al., 1998). In parallel to the animal data increased levels of proinflammatory cytokines have been demonstrated in patients with chronic nerve compression (Freeland et al., 2002) suggesting that the heat hyperalgesia observed in our patients is due to nociceptor sensitization by these substances. In an earlier publication Lang et al. (1995) suggested that hyperalgesia in carpal tunnel syndrome also included mechanisms of central sensitization. However not only whole body cooling but also local application of norepinephrine was followed by a significant increase in heat hyperalgesia, indicating a peripheral site of interaction. In the present study only patients with signs of C-fiber sensitization respond to physiological sympathetic activation. Our finding is in accordance with an earlier study by Wahren et al. (1991), who reported lower heat pain thresholds in patients with a peripheral nerve lesion who respond to regional guanethedine blocks. We propose that after a chronic compression injury of a peripheral nerve sympathetic sensitivity depends on the presence of sensitized nerve fibers. Nevertheless, sensitization of nociceptive C-fibers by capsaicin was not followed by an increased responsiveness of these fibers to activation of sympathetic vasoconstrictor neurons (Baron et al., 1999). This contradiction may be explained by changes in excitation properties of the afferent neuron which involves changes in gene expression like up regulation of a1 adrenoreceptor or TTXR sodium channels and therefore occur only after longstanding sensitization (Drummond et al., 1996; Novakovic et al., 1998). However, until now there is no direct evidence of upregulation of adrenoreceptors in the periphery or that this depends on the presence of inflammatory cytokines. In contrast it has been shown that the development of heat hyperalgesia and the sprouting of noradrenergic collaterals in the DRG depend upon peripherally derived NGF (Ramer and Bisby, 1999). An upregulation of nerve growth factor has been demonstrated in the CCI model. Thus it could be suggested that NGF accounts for the development of hyperalgesia through changes in gene expression and sympathetic afferent coupling also in our patients. However from the present results we would expect that the sympathetic afferent coupling takes place in the periphery and not in the DRG. From the results of norepi-
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nephrine iontophoresis we further conclude that the coupling is not restricted to the area of the nerve lesion but also involves distal parts of the nerve. Whether NGF is also related to an upregulation of a1 adrenoreceptors in the periphery has to be examined.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG Ba 1921/1-4) and the Network ‘‘Neuropathic Pain’’ the German Ministry of Research and Education, German research Network on Neuropathic Pain (BMBF, 01EM01/04) and an unrestricted educational grant from Pfizer, Germany.
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