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The effects of mexiletine on excitability properties of human median motor axons
Clinical Neurophysiology 116 (2005) 284–289 www.elsevier.com/locate/clinph
The effects of mexiletine on excitability properties of human median motor axons Satoshi Kuwabara*, Sonoko Misawa, Noriko Tamura, Kazuaki Kanai, Akiyuki Hiraga, Kazue Ogawara, Miho Nakata, Takamichi Hattori Department of Neurology, Chiba University School of Medicine, 1-8-1 Inohana, Chuo-ku, Chiba 260-8670, Japan Accepted 30 August 2004 Available online 29 September 2004
Abstract Objective: To investigate the effects of mexiletine, an analog of lidocaine, on excitability of human axons in vivo. Methods: Threshold tracking was used to measure multiple excitability indices (strength-duration time constant, rheobase, refractoriness, supernormality, and threshold electrotonus) in median motor axons of 20 patients with neuropathic pain or muscle cramping, before and 3 months after treatment with oral 300 mg mexiletine per day. Results: After treatment, there was a reduction in pain/muscle cramps, associated with decreased strength-duration time constants (PZ0.01), increased rheobasic currents (PZ0.06), and lower refractoriness (PZ0.02), all of which were consistent with reduced nodal NaC currents. Supernormality and threshold electrotonus did not change significantly. The changes in strength-duration properties suggest a decrease in persistent NaC conductance. The lowered refractoriness after treatment might result from reduced transient NaC currents, but the lack of change in supernormality and threshold electrotonus was not consistent with this hypothesis. Conclusions: Oral mexiletine in a dosage of 300 mg daily suppresses persistent NaC currents in human motor axons. Significance: Measurements of the excitability indices can be used for non-invasive assessment and monitoring of the effects of mexiletine in patients with neuropathic pain or muscle cramps. q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Mexiletine; Sodium channel; Persistent sodium channel; Neuropathy; Axonal excitability; Threshold tracking
1. Introduction Mexiletine is an orally active local anesthetic agent, which is structurally related to lidocaine, and has been used for alleviating neuropathic pain (Dejgard et al., 1988; Oskarsson et al., 1997; Stracke et al., 1992), and occasionally for muscle cramp (Kanai et al., 2003). Neuropathic pain and muscle cramping partly arise from axonal hyperexcitability, which leads to abnormal spontaneous firing associated with increased NaC channel expression (Waxman et al., 1999). The mechanism of action of mexiletine is a blockage of NaC channels (Jarvis and Coukell, 1998), and this agent would decrease axonal * Corresponding author. Tel.C81 43 222 7171x5414; fax: C81 43 226 2160. E-mail address: [email protected] (S. Kuwabara).
excitability by reducing nodal NaC currents. However, this has rarely been demonstrated in human axons, presumably because of the lack of appropriate tools to assess axonal ionic conductances in human subjects. In the 1990s, the threshold tracking technique was developed to measure a number of axonal excitability indices such as strength-duration properties, refractoriness, and threshold electrotonus, non-invasively in human subjects (Bostock et al., 1998; Burke et al., 2001; Kiernan et al., 2000; Kuwabara et al., 2002). These indices depend on the biophysical properties of the axonal membrane at the site of stimulation, and can provide an insight into NaC and KC conductances (Bostock et al., 1998; Burke et al., 2001). We have used this technique to investigate whether mexiletine administration is associated with specific changes in ionic conductances in human axons.
1388-2457/$30.00 q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2004.08.014
S. Kuwabara et al. / Clinical Neurophysiology 116 (2005) 284–289
2. Methods 2.1. Subjects Twenty patients (8 men and 12 women), who received mexiletine treatment for neuropathic pain or severe muscle cramping, were studied (Table 1). Their age ranged from 20 to 74 years (mean 49 years). Of the 20 patients, 6 patients had painful neuropathy caused by diabetes, systemic vasculitis, alcohol abuse, or a demyelinating form of Guillain–Barre´ syndrome. The remaining 14 patients received mexiletine treatment for their severe muscle cramping; postulated causes of muscle cramps included spinal muscular atrophy, Machado–Joseph disease, and axonal Guillain–Barre´ syndrome. Three patients had no
Age/sex
Neuropathic pain 1 20/F 2
54/F
3
74/F
4
26/M
5
44/M
6
58/M
Muscle cramp 7 25/M 8
52/F
9
53/F
10
53/M
11
65/F
12
61/M
13
62/M
14
49/F
15
62/M
16
53/F
17
60/F
18 19 20
51/F 52/F 56/F
a b
Diagnosis
Diabetic neuropathy Vasculitic neuropathy Vasculitic neuropathy Guillain–Barre´ syndrome Guillain–Barre´ syndrome Alcoholic neuropathy Guillain–Barre´ syndrome Spinal muscular atrophy Spinal muscular atrophy Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Idiopathic Idiopathic Idiopathic
obvious neurological disease, and were diagnosed as having idiopathic muscle cramp. A part of data from patients with Machado–Joseph disease was described elsewhere (Kanai et al., 2003). The normal control data of excitability testing with threshold tracking were obtained from 54 healthy subjects (35 male and 19 female; aged 23–84 years; mean age 44 years). All patients and normal subjects gave informed consent to the experimental procedures, which have been approved by the Ethics Committee of the Chiba University School of Medicine. 2.2. Clinical assessment of pain/muscle cramp
Before treatment
After treatmentb
A pain/cramp disability score (Kanai et al., 2003) was used to evaluate the extent of disability in performing daily activities: 0, no symptom; 1, a complaint but no disability; 2, a chief complaint, sometimes disturbing work or sleep; 3, a chief complaint, disturbing work or sleep daily. All the 20 patients included in this study had the score 2 or 3.
3
1
2.3. Excitability testing using threshold tracking
3
2
2
2
2
1
2
1
2
2
2
0
3
0
3
1
3
0
3
0
Multiple excitability measurements were performed using a protocol designed to measure rapidly a number of different nerve excitability parameters (inw10 min), which uses a computerized program (QTRAC version 4.3 with multiple excitability protocol TRONDHM; copyright, Prof. Hugh Bostock, Institute of Neurology, London, UK) as described elsewhere (Kiernan et al., 2000; Kuwabara et al., 2002). The compound muscle action potential (CMAP) was recorded from the abductor pollicis brevis after stimulation of the median nerve at the wrist. Skin temperature was measured near the stimulating site, and maintained above 31.5 8C. The protocol began with the measurement of stimulus response curves using test stimuli of duration 0.2 and 1.0 ms. From these curves, strength-duration time constant (SDTC) were calculated using the formula (Kanai et al., 2003; Kiernan et al., 2000):
3
1
3
1
3
0
2
0
2
0
2
1
2 2 3
1 1 1
Table 1 Clinical profiles of patients treated with mexiletine Patient
285
Disability scorea
See the text. P!0.01, compared with scores before treatment.
SDTC Z 0:2ðI0:2 K I1:0 Þ=ðI1:0 K 0:2I0:2 Þ where I0.2 and I1.0 are the respective threshold currents for test stimuli of 0.2 and 1.0 ms duration. SDTC is defined as ratio between the minimum charge threshold and the rheobase, and equates to chronaxie (Bostock et al., 1998; Burke et al., 2001). In the following measurements, the current required to produce a CMAP that was 40% of the maximum was tracked. The recovery cycle of axonal excitability following a single supramaximal stimulus, was recorded from the test stimulation delivered at different intervals after the conditioning stimulus. The relative refractory period was determined as the time of the first intercept of the recovery cycle curve to the X-axis, and refractoriness was defined as the threshold increase during the relative refractory period.
286
S. Kuwabara et al. / Clinical Neurophysiology 116 (2005) 284–289
Threshold changes were measured at the 2.0 and 2.5 ms intervals. Supernormality was defined as the maximal decrease of threshold current during the supernormal period. In the threshold electrotonus studies, membrane potential was altered by the use of subthreshold DC polarizing currents that were 40% of the unconditioned threshold. Depolarizing and hyperpolarizing currents were used, each lasting 100 ms, and threshold changes for the test CMAP were measured. 2.4. Mexiletine treatment After their clinical and electrophysiologic evaluations, patients received oral mexiletine hydrochloride. The initial dose was 150 mg daily for a month, increasing to 300 mg daily from the next month. Follow-up clinical and electrophysiologic assessments were made 3 months after the start of treatment. 2.5. Statistical analyses Values of all the excitability indices except for refractoriness showed the Gaussian distribution, and the paired t test was used to compare patients’ parameters before and after treatment for statistical analysis. Values of refractoriness at the 2.0 ms-interval did not show the normative distribution, and Mann–Whitney U test was used.
3. Results 3.1. Clinical assessment All 20 patients had pain in the limbs or muscle cramping disturbing their daily activities or sleep before treatment. Table 1 shows clinical effects of mexiletine treatment. After 2–3 months from the initiation of mexiletine treatment, the mean pain/muscle cramp disability score decreased (P! 0.01); 6 of the 8 patients with neuropathic pain experienced lessening of pain, whereas the remaining two had no improvement. All 14 patients suffering muscle cramps had nearly complete relief from muscle cramps. Median motor nerve conduction studies were performed in 14 patients before and after mexiletine treatment. There were no significant changes in distal latencies, conduction velocities, and CMAP amplitudes. 3.2. Changes in axonal excitability indices 3.2.1. Strength-duration properties Fig. 1 shows changes in SDTC before and after treatment. There were significant decreases in SDTC after treatment (PZ0.01), and consistent with this, rheobasic currents (PZ0.06) and currents required to produce 50% of maximal CMAP (PZ0.09) tended to increase after treatment (Table 2). These findings indicated that mexiletine
Fig. 1. Strength-duration time constant (SDTC) of median motor axons before and after mexiletine treatment (nZ20). There was a significant decrease in SDTC after treatment (PZ0.01). Error bars indicate SEM.
administration resulted in altered strength-duration properties and a decrease in excitability of motor axons. 3.2.2. Refractoriness and refractory periods Fig. 2A shows recovery cycle of axonal excitability after a single supramaximal conditioning stimulus at the interstimulus intervals of 2–6 ms. Because refractoriness is particularly sensitive to temperature, 6 patients, in whom skin temperatures at the tested site were different by O1.0 8C between the two studies, were excluded. One patient (no. 7 in Table 1) who had markedly high refractoriness at 2.0 ms (1024%), presumably due to conduction block of the second (test) impulse, rather than true refractoriness (Kuwabara et al., 2002), was also excluded. Therefore, findings of the remaining 13 patients were analyzed. After treatment, there was a decrease in refractoriness. Fig. 2B shows the change in refractoriness at the 2.0 ms interval in individual patients (PZ0.02). The higher refractoriness before treatment tended to be associated the greater decrease in refractoriness. Refractory periods were shorter after treatment (Table 2). Supernormality (the decrease in threshold current during the supernormal period) was similar before and after mexiletine administration. These findings indicated that mexiletine
S. Kuwabara et al. / Clinical Neurophysiology 116 (2005) 284–289
287
Table 2 Axonal excitability indices before and after mexiletine treatment Before Strength-duration property Strength-duration time constant (ms) Rheobase (mA) Current for 50% CMAP (mA) Recovery cycle Refractoriness (2.0 ms) (%) Refractoriness (2.5 ms) (%) Relative refractory period (ms) Supernormality (%) Threshold electrotonus TEd (10–30 ms) (%) TEd (90–100 ms) (%) TEh (90–100 ms) (%)
0.50 (0.02)* 3.5 (0.3) 5.1 (0.5) 60.1 (10.8) 10.6 (4.2) 2.97 (0.10) 23.8 (2.0) 68.0 (1.0) 45.5 (1.2) 120.0 (5.8)
After
P value
Normal
0.46 (0.02) 4.3 (0.6) 6.0 (0.6)
0.01 0.06 0.09
0.40 (0.01) 4.0 (0.2) 5.6 (0.3)
39.7 (4.9) 5.2 (2.3) 2.70 (0.07) 23.7 (2.6)
0.02 0.09 0.02 NS
54.9 (4.2) 11.6 (2.5) 2.88 (0.06) 25.9 (1.0)
68.0 (1.5) 46.2 (1.6) 119.0 (5.3)
NS NS NS
65.5 (0.7) 46.8 (0.7) 126.1 (3.5)
TEd (10–30 ms), TEd (90–100 ms)Zthreshold change between the specified latencies after the start of depolarizing current; TEh (90–100 ms)Zthreshold change between the specified latencies after the start of hyperpolarizing current. Data are given as mean (SEM); *P!0.05, compared with normal values. NS, not significant.
treatment resulted in lower refractoriness and shorter refractory periods of median motor axons. 3.2.3. Threshold electrotonus Fig. 3 compares the threshold electrotonus curves of patients before and after mexiletine treatment. The curves were almost identical. Threshold changes to both depolarizing and hyperpolarizing currents were similar to those of normal controls (Table 2). 3.2.4. Changes in excitability indices in patients with pain and those with muscle cramp When patients were divided into the ‘pain’ and ‘muscle cramp’ subgroups, the changes in strength-duration properties, refractoriness, and threshold electrotonus were similar to the findings described above. The changes in excitability indices were not significantly different between the patient subgroups with pain and muscle cramp, and were not significantly associated with the clinical effects.
4. Discussion Our results showed that oral 300 mg mexiletine administration results in substantial decreases in SDTC and refractoriness in motor axons of patients with neuropathic pain or muscle cramps. This is the first study to systemically demonstrate the effects of mexiletine on excitability properties of human axons in vivo. Assuming that the SDTC partly reflects persistent NaC conductance, and that refractoriness depends on inactivation of transient NaC channels caused by prior depolarization (the influx of NaC) (Bostock et al., 1998; Burke et al., 2001), changes in these indices suggest a decrease in persistent and transient NaC conductances resulting from mexiletine administration. Moreover, the lack of changes in threshold electrotonus
Fig. 2. Changes in refractoriness of median motor axons after mexiletine treatment (nZ13). (A) Recovery cycle of axonal excitability at the conditioning-test intervals of 2.0–6.0 ms. Refractoriness was defined as the threshold increase during the relative refractory period (see data points at the 2.0 and 2.5 ms-intervals). Error bars indicate SEM. (B) Changes in refractoriness at 2.0 ms in individual patients. There was a significant decrease in refractoriness after mexiletine treatment (PZ0.02).
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Fig. 3. Threshold electrotonus of median motor axons before and after mexiletine treatment.
suggests that mexiletine does not significantly affect membrane potential, or axonal fast or slow KC currents. In addition to the classical transient NaC channels, different types of NaC channels have been found in mammalian axons. In the central neurons, less than 1% of the total NaC currents are persistent at the resting membrane potentials, termed as persistent NaC currents (Crill, 1996). In human motor axons, 1% of the total NaC currents are reported to be persistent (Baker and Bostock, 1997; Bostock and Rothwell, 1997). Insights into this conductance in human axons can come from studies of SDTC (Mogyoros et al., 1996, 1998). The SDTC of a nerve is a classical measure of axonal excitability, and is partly dependent on a persistent NaC currents (Baker and Bostock, 1997; Mogyoros et al., 1996). In normal subjects, SDTC is longer for sensory axons than for motor ones (Mogyoros et al., 1996), and this difference is caused by greater persistent NaC currents in sensory axons (Bostock and Rothwell, 1997). Our findings showed that SDTC became shorter after mexiletine treatment, and this is consistent with blockade of persistent NaC currents. SDTC is also dependent on other factors such as membrane potential and passive membrane properties (Bostock et al., 1998). However, membrane potential was not likely to be altered by mexiletine, because threshold electrotonus and supernormality, which are particularly sensitive to membrane potential (Kiernan and Bostock, 2000), did not change after mexiletine treatment. It is also unlikely that mexiletine administration resulted in nodal structural changes. In Fig. 1, mexiletine appears to affect SDTC especially when the time constant is abnormally long, and this is consistent with a previous experimental study, which showed that mexiletine at a low dose blocks inactivationdeficient mutant muscle NaC currents at the resting-state (Wang et al., 2000). Moreover, there is no study investigating the effects of mexiletine on axonal NaC
currents, but mexiletine is an analogue of lidocaine, which at low concentrations has a selective action on persistent NaC currents in the primary sensory neurons (Baker, 2000). Refractoriness primarily depends on inactivation of transient NaC channels caused by prior depolarization (Bostock et al., 1998). Previous studies showed that mexiletine stabilizes inactivation of NaC channels (Jarvis and Coukell, 1998; Tanelian and Brose, 1991), and therefore, it is possible that when generating an action potential, nodal transient NaC currents would decrease and thereby lead to lower inactivation of NaC channels (Kuwabara, 2003). However, in Fig. 2B, refractoriness appears to decrease only when it is abnormally high before treatment. Moreover, if the action potential is smaller, supernormality would be smaller, and block of transient NaC currents would affect threshold electrotonus. The lack of changes in supernormality and threshold electrotonus in this study does not support the hypothesis of blockage of transient NaC currents. Further studies are required to determine whether mexiletine at a low dose blocks transient NaC channels. In the present study, axonal excitability of median motor axons was measured in a variety of disorders, and most of the patients had disease affecting lower motor neurons. Neuropathic pain and muscle cramping preferentially involve the distal lower limbs, rather than the upper limbs. We did not measure excitability of sensory axons, or motor axons in the lower limb nerves, and mechanisms of neuropathic pain and muscle cramp can be different. Therefore, the results cannot be compared directly with clinical effects. Furthermore, neuropathic pain presumably arises from C fibers rather than A fibers. These are limitations of this study, and further studies are required to evaluate excitability in each type of axons responsible for clinical symptoms. Nevertheless, the measurements of excitability of the median motor axons provide objective information about the effects of mexiletine on axonal excitability, and significant differences were found after mexiletine treatment. The mexiletine dosage of 300 mg in this study was less than those of previous studies (450–675 mg) (Dejgard et al., 1988; Oskarsson et al., 1997; Stracke et al., 1992), but our findings showed that this dose administration resulted in substantial changes in strength-duration properties that reflect persistent NaC currents. Measurements of excitability indices such as SDTC and refractoriness can be used to assess and monitor the effects of mexiletine on axonal excitability in the treatment for patients with positive sensory and motor symptoms.
References Baker MD. Selective block of late NaC current by local anesthetics in rat large sensory neurons. Br J Pharmacol 2000;129:1617–26.
S. Kuwabara et al. / Clinical Neurophysiology 116 (2005) 284–289 Baker MD, Bostock H. Low-threshold, persistent sodium current in rat large dorsal root ganglion neurons in culture. J Neurophysiol 1997;77: 1503–13. Bostock H, Rothwell JC. Latent addition in motor and sensory fibres of human peripheral nerve. J Physiol 1997;498:277–94. Bostock H, Cikurel K, Burke D. Threshold tracking techniques in the study of human peripheral nerve. Muscle Nerve 1998;21:137–58. Burke D, Kiernan MC, Bostock H. Excitability of human axons. Clin Neurophysiol 2001;112:1575–85. Crill WE. Persistent sodium current in mammalian central neurons. Annu Rev Physiol 1996;58:349–62. Dejgard A, Petersen P, Kastrup J. Mexiletine for treatment of chronic painful diabetic neuropathy. Lancet 1988;8575–8576:9–11. Jarvis B, Coukell AJ. Mexiletine. A review of its therapeutic use in painful diabetic neuropathy. Drugs 1998;56:691–707. Kanai K, Kuwabara S, Arai K, Sung J-Y, Ogawara K, Hattori T. Muscle cramp in Machado–Joseph disease: altered motor axonal excitability properties and mexiletine treatment. Brain 2003;126: 965–73. Kiernan MC, Bostock H. Effects of membrane polarization and ischaemia on the excitability properties of human axons. Brain 2000;123: 2542–51. Kiernan MC, Burke D, Andersen KV, Bostock H. Multiple measures of axonal excitability: a new approach in clinical testing. Muscle Nerve 2000;23:399–409.
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Kuwabara S. Shortened refractory periods in human diabetic neuropathy. Clin Neurophysiol 2003;114:169–70. Kuwabara S, Ogawara K, Sung JY, Mori M, Kanai K, Hattori T, Yuki N, Lin CS, Burke D, Bostock H. Differences in membrane properties of axonal and demyelinating Guillain–Barre syndromes. Ann Neurol 2002;52:180–7. Mogyoros I, Kiernan MC, Burke D. Strength-duration properties of human peripheral nerve. Brain 1996;119:439–47. Mogyoros I, Kiernan MC, Burke D, Bostock H. Strength-duration properties of sensory and motor axons in amyotrophic lateral sclerosis. Brain 1998;121:851–9. Oskarsson P, Ljunggren JG, Lins PE. Efficacy and safety of mexiletine in the treatment of painful diabetic neuropathy. The Mexiletine Study Group. Diabetes Care 1997;20:1594–7. Stracke H, Meyer UE, Schumacher HE, Federlin K. Mexiletine in the treatment of diabetic neuropathy. Diabetes Care 1992;15:1550–5. Tanelian DL, Brose WG. Neuropathic pain can be relieved by drugs that are use-dependent sodium channel blockers: lidocaine, carbamazepine, and mexiletine. Anesthesiology 1991;74:949–51. Wang GK, Russell C, Wang SY. Mexiletine block of wild-type and inactivation-deficient human skeletal muscle hNav1.4 NaC channels. J Physiol (Lond) 2003;554:621–33. Waxman SG, Dib-Haji S, Cummins TR, Black JA. Sodium channel and pain. Proc Natl Acad Sci USA 1999;96:7635–9.
The effects of mexiletine on excitability properties of human median motor axons Satoshi Kuwabara*, Sonoko Misawa, Noriko Tamura, Kazuaki Kanai, Akiyuki Hiraga, Kazue Ogawara, Miho Nakata, Takamichi Hattori Department of Neurology, Chiba University School of Medicine, 1-8-1 Inohana, Chuo-ku, Chiba 260-8670, Japan Accepted 30 August 2004 Available online 29 September 2004
Abstract Objective: To investigate the effects of mexiletine, an analog of lidocaine, on excitability of human axons in vivo. Methods: Threshold tracking was used to measure multiple excitability indices (strength-duration time constant, rheobase, refractoriness, supernormality, and threshold electrotonus) in median motor axons of 20 patients with neuropathic pain or muscle cramping, before and 3 months after treatment with oral 300 mg mexiletine per day. Results: After treatment, there was a reduction in pain/muscle cramps, associated with decreased strength-duration time constants (PZ0.01), increased rheobasic currents (PZ0.06), and lower refractoriness (PZ0.02), all of which were consistent with reduced nodal NaC currents. Supernormality and threshold electrotonus did not change significantly. The changes in strength-duration properties suggest a decrease in persistent NaC conductance. The lowered refractoriness after treatment might result from reduced transient NaC currents, but the lack of change in supernormality and threshold electrotonus was not consistent with this hypothesis. Conclusions: Oral mexiletine in a dosage of 300 mg daily suppresses persistent NaC currents in human motor axons. Significance: Measurements of the excitability indices can be used for non-invasive assessment and monitoring of the effects of mexiletine in patients with neuropathic pain or muscle cramps. q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Mexiletine; Sodium channel; Persistent sodium channel; Neuropathy; Axonal excitability; Threshold tracking
1. Introduction Mexiletine is an orally active local anesthetic agent, which is structurally related to lidocaine, and has been used for alleviating neuropathic pain (Dejgard et al., 1988; Oskarsson et al., 1997; Stracke et al., 1992), and occasionally for muscle cramp (Kanai et al., 2003). Neuropathic pain and muscle cramping partly arise from axonal hyperexcitability, which leads to abnormal spontaneous firing associated with increased NaC channel expression (Waxman et al., 1999). The mechanism of action of mexiletine is a blockage of NaC channels (Jarvis and Coukell, 1998), and this agent would decrease axonal * Corresponding author. Tel.C81 43 222 7171x5414; fax: C81 43 226 2160. E-mail address: [email protected] (S. Kuwabara).
excitability by reducing nodal NaC currents. However, this has rarely been demonstrated in human axons, presumably because of the lack of appropriate tools to assess axonal ionic conductances in human subjects. In the 1990s, the threshold tracking technique was developed to measure a number of axonal excitability indices such as strength-duration properties, refractoriness, and threshold electrotonus, non-invasively in human subjects (Bostock et al., 1998; Burke et al., 2001; Kiernan et al., 2000; Kuwabara et al., 2002). These indices depend on the biophysical properties of the axonal membrane at the site of stimulation, and can provide an insight into NaC and KC conductances (Bostock et al., 1998; Burke et al., 2001). We have used this technique to investigate whether mexiletine administration is associated with specific changes in ionic conductances in human axons.
1388-2457/$30.00 q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2004.08.014
S. Kuwabara et al. / Clinical Neurophysiology 116 (2005) 284–289
2. Methods 2.1. Subjects Twenty patients (8 men and 12 women), who received mexiletine treatment for neuropathic pain or severe muscle cramping, were studied (Table 1). Their age ranged from 20 to 74 years (mean 49 years). Of the 20 patients, 6 patients had painful neuropathy caused by diabetes, systemic vasculitis, alcohol abuse, or a demyelinating form of Guillain–Barre´ syndrome. The remaining 14 patients received mexiletine treatment for their severe muscle cramping; postulated causes of muscle cramps included spinal muscular atrophy, Machado–Joseph disease, and axonal Guillain–Barre´ syndrome. Three patients had no
Age/sex
Neuropathic pain 1 20/F 2
54/F
3
74/F
4
26/M
5
44/M
6
58/M
Muscle cramp 7 25/M 8
52/F
9
53/F
10
53/M
11
65/F
12
61/M
13
62/M
14
49/F
15
62/M
16
53/F
17
60/F
18 19 20
51/F 52/F 56/F
a b
Diagnosis
Diabetic neuropathy Vasculitic neuropathy Vasculitic neuropathy Guillain–Barre´ syndrome Guillain–Barre´ syndrome Alcoholic neuropathy Guillain–Barre´ syndrome Spinal muscular atrophy Spinal muscular atrophy Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Machado–Joseph disease Idiopathic Idiopathic Idiopathic
obvious neurological disease, and were diagnosed as having idiopathic muscle cramp. A part of data from patients with Machado–Joseph disease was described elsewhere (Kanai et al., 2003). The normal control data of excitability testing with threshold tracking were obtained from 54 healthy subjects (35 male and 19 female; aged 23–84 years; mean age 44 years). All patients and normal subjects gave informed consent to the experimental procedures, which have been approved by the Ethics Committee of the Chiba University School of Medicine. 2.2. Clinical assessment of pain/muscle cramp
Before treatment
After treatmentb
A pain/cramp disability score (Kanai et al., 2003) was used to evaluate the extent of disability in performing daily activities: 0, no symptom; 1, a complaint but no disability; 2, a chief complaint, sometimes disturbing work or sleep; 3, a chief complaint, disturbing work or sleep daily. All the 20 patients included in this study had the score 2 or 3.
3
1
2.3. Excitability testing using threshold tracking
3
2
2
2
2
1
2
1
2
2
2
0
3
0
3
1
3
0
3
0
Multiple excitability measurements were performed using a protocol designed to measure rapidly a number of different nerve excitability parameters (inw10 min), which uses a computerized program (QTRAC version 4.3 with multiple excitability protocol TRONDHM; copyright, Prof. Hugh Bostock, Institute of Neurology, London, UK) as described elsewhere (Kiernan et al., 2000; Kuwabara et al., 2002). The compound muscle action potential (CMAP) was recorded from the abductor pollicis brevis after stimulation of the median nerve at the wrist. Skin temperature was measured near the stimulating site, and maintained above 31.5 8C. The protocol began with the measurement of stimulus response curves using test stimuli of duration 0.2 and 1.0 ms. From these curves, strength-duration time constant (SDTC) were calculated using the formula (Kanai et al., 2003; Kiernan et al., 2000):
3
1
3
1
3
0
2
0
2
0
2
1
2 2 3
1 1 1
Table 1 Clinical profiles of patients treated with mexiletine Patient
285
Disability scorea
See the text. P!0.01, compared with scores before treatment.
SDTC Z 0:2ðI0:2 K I1:0 Þ=ðI1:0 K 0:2I0:2 Þ where I0.2 and I1.0 are the respective threshold currents for test stimuli of 0.2 and 1.0 ms duration. SDTC is defined as ratio between the minimum charge threshold and the rheobase, and equates to chronaxie (Bostock et al., 1998; Burke et al., 2001). In the following measurements, the current required to produce a CMAP that was 40% of the maximum was tracked. The recovery cycle of axonal excitability following a single supramaximal stimulus, was recorded from the test stimulation delivered at different intervals after the conditioning stimulus. The relative refractory period was determined as the time of the first intercept of the recovery cycle curve to the X-axis, and refractoriness was defined as the threshold increase during the relative refractory period.
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Threshold changes were measured at the 2.0 and 2.5 ms intervals. Supernormality was defined as the maximal decrease of threshold current during the supernormal period. In the threshold electrotonus studies, membrane potential was altered by the use of subthreshold DC polarizing currents that were 40% of the unconditioned threshold. Depolarizing and hyperpolarizing currents were used, each lasting 100 ms, and threshold changes for the test CMAP were measured. 2.4. Mexiletine treatment After their clinical and electrophysiologic evaluations, patients received oral mexiletine hydrochloride. The initial dose was 150 mg daily for a month, increasing to 300 mg daily from the next month. Follow-up clinical and electrophysiologic assessments were made 3 months after the start of treatment. 2.5. Statistical analyses Values of all the excitability indices except for refractoriness showed the Gaussian distribution, and the paired t test was used to compare patients’ parameters before and after treatment for statistical analysis. Values of refractoriness at the 2.0 ms-interval did not show the normative distribution, and Mann–Whitney U test was used.
3. Results 3.1. Clinical assessment All 20 patients had pain in the limbs or muscle cramping disturbing their daily activities or sleep before treatment. Table 1 shows clinical effects of mexiletine treatment. After 2–3 months from the initiation of mexiletine treatment, the mean pain/muscle cramp disability score decreased (P! 0.01); 6 of the 8 patients with neuropathic pain experienced lessening of pain, whereas the remaining two had no improvement. All 14 patients suffering muscle cramps had nearly complete relief from muscle cramps. Median motor nerve conduction studies were performed in 14 patients before and after mexiletine treatment. There were no significant changes in distal latencies, conduction velocities, and CMAP amplitudes. 3.2. Changes in axonal excitability indices 3.2.1. Strength-duration properties Fig. 1 shows changes in SDTC before and after treatment. There were significant decreases in SDTC after treatment (PZ0.01), and consistent with this, rheobasic currents (PZ0.06) and currents required to produce 50% of maximal CMAP (PZ0.09) tended to increase after treatment (Table 2). These findings indicated that mexiletine
Fig. 1. Strength-duration time constant (SDTC) of median motor axons before and after mexiletine treatment (nZ20). There was a significant decrease in SDTC after treatment (PZ0.01). Error bars indicate SEM.
administration resulted in altered strength-duration properties and a decrease in excitability of motor axons. 3.2.2. Refractoriness and refractory periods Fig. 2A shows recovery cycle of axonal excitability after a single supramaximal conditioning stimulus at the interstimulus intervals of 2–6 ms. Because refractoriness is particularly sensitive to temperature, 6 patients, in whom skin temperatures at the tested site were different by O1.0 8C between the two studies, were excluded. One patient (no. 7 in Table 1) who had markedly high refractoriness at 2.0 ms (1024%), presumably due to conduction block of the second (test) impulse, rather than true refractoriness (Kuwabara et al., 2002), was also excluded. Therefore, findings of the remaining 13 patients were analyzed. After treatment, there was a decrease in refractoriness. Fig. 2B shows the change in refractoriness at the 2.0 ms interval in individual patients (PZ0.02). The higher refractoriness before treatment tended to be associated the greater decrease in refractoriness. Refractory periods were shorter after treatment (Table 2). Supernormality (the decrease in threshold current during the supernormal period) was similar before and after mexiletine administration. These findings indicated that mexiletine
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Table 2 Axonal excitability indices before and after mexiletine treatment Before Strength-duration property Strength-duration time constant (ms) Rheobase (mA) Current for 50% CMAP (mA) Recovery cycle Refractoriness (2.0 ms) (%) Refractoriness (2.5 ms) (%) Relative refractory period (ms) Supernormality (%) Threshold electrotonus TEd (10–30 ms) (%) TEd (90–100 ms) (%) TEh (90–100 ms) (%)
0.50 (0.02)* 3.5 (0.3) 5.1 (0.5) 60.1 (10.8) 10.6 (4.2) 2.97 (0.10) 23.8 (2.0) 68.0 (1.0) 45.5 (1.2) 120.0 (5.8)
After
P value
Normal
0.46 (0.02) 4.3 (0.6) 6.0 (0.6)
0.01 0.06 0.09
0.40 (0.01) 4.0 (0.2) 5.6 (0.3)
39.7 (4.9) 5.2 (2.3) 2.70 (0.07) 23.7 (2.6)
0.02 0.09 0.02 NS
54.9 (4.2) 11.6 (2.5) 2.88 (0.06) 25.9 (1.0)
68.0 (1.5) 46.2 (1.6) 119.0 (5.3)
NS NS NS
65.5 (0.7) 46.8 (0.7) 126.1 (3.5)
TEd (10–30 ms), TEd (90–100 ms)Zthreshold change between the specified latencies after the start of depolarizing current; TEh (90–100 ms)Zthreshold change between the specified latencies after the start of hyperpolarizing current. Data are given as mean (SEM); *P!0.05, compared with normal values. NS, not significant.
treatment resulted in lower refractoriness and shorter refractory periods of median motor axons. 3.2.3. Threshold electrotonus Fig. 3 compares the threshold electrotonus curves of patients before and after mexiletine treatment. The curves were almost identical. Threshold changes to both depolarizing and hyperpolarizing currents were similar to those of normal controls (Table 2). 3.2.4. Changes in excitability indices in patients with pain and those with muscle cramp When patients were divided into the ‘pain’ and ‘muscle cramp’ subgroups, the changes in strength-duration properties, refractoriness, and threshold electrotonus were similar to the findings described above. The changes in excitability indices were not significantly different between the patient subgroups with pain and muscle cramp, and were not significantly associated with the clinical effects.
4. Discussion Our results showed that oral 300 mg mexiletine administration results in substantial decreases in SDTC and refractoriness in motor axons of patients with neuropathic pain or muscle cramps. This is the first study to systemically demonstrate the effects of mexiletine on excitability properties of human axons in vivo. Assuming that the SDTC partly reflects persistent NaC conductance, and that refractoriness depends on inactivation of transient NaC channels caused by prior depolarization (the influx of NaC) (Bostock et al., 1998; Burke et al., 2001), changes in these indices suggest a decrease in persistent and transient NaC conductances resulting from mexiletine administration. Moreover, the lack of changes in threshold electrotonus
Fig. 2. Changes in refractoriness of median motor axons after mexiletine treatment (nZ13). (A) Recovery cycle of axonal excitability at the conditioning-test intervals of 2.0–6.0 ms. Refractoriness was defined as the threshold increase during the relative refractory period (see data points at the 2.0 and 2.5 ms-intervals). Error bars indicate SEM. (B) Changes in refractoriness at 2.0 ms in individual patients. There was a significant decrease in refractoriness after mexiletine treatment (PZ0.02).
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Fig. 3. Threshold electrotonus of median motor axons before and after mexiletine treatment.
suggests that mexiletine does not significantly affect membrane potential, or axonal fast or slow KC currents. In addition to the classical transient NaC channels, different types of NaC channels have been found in mammalian axons. In the central neurons, less than 1% of the total NaC currents are persistent at the resting membrane potentials, termed as persistent NaC currents (Crill, 1996). In human motor axons, 1% of the total NaC currents are reported to be persistent (Baker and Bostock, 1997; Bostock and Rothwell, 1997). Insights into this conductance in human axons can come from studies of SDTC (Mogyoros et al., 1996, 1998). The SDTC of a nerve is a classical measure of axonal excitability, and is partly dependent on a persistent NaC currents (Baker and Bostock, 1997; Mogyoros et al., 1996). In normal subjects, SDTC is longer for sensory axons than for motor ones (Mogyoros et al., 1996), and this difference is caused by greater persistent NaC currents in sensory axons (Bostock and Rothwell, 1997). Our findings showed that SDTC became shorter after mexiletine treatment, and this is consistent with blockade of persistent NaC currents. SDTC is also dependent on other factors such as membrane potential and passive membrane properties (Bostock et al., 1998). However, membrane potential was not likely to be altered by mexiletine, because threshold electrotonus and supernormality, which are particularly sensitive to membrane potential (Kiernan and Bostock, 2000), did not change after mexiletine treatment. It is also unlikely that mexiletine administration resulted in nodal structural changes. In Fig. 1, mexiletine appears to affect SDTC especially when the time constant is abnormally long, and this is consistent with a previous experimental study, which showed that mexiletine at a low dose blocks inactivationdeficient mutant muscle NaC currents at the resting-state (Wang et al., 2000). Moreover, there is no study investigating the effects of mexiletine on axonal NaC
currents, but mexiletine is an analogue of lidocaine, which at low concentrations has a selective action on persistent NaC currents in the primary sensory neurons (Baker, 2000). Refractoriness primarily depends on inactivation of transient NaC channels caused by prior depolarization (Bostock et al., 1998). Previous studies showed that mexiletine stabilizes inactivation of NaC channels (Jarvis and Coukell, 1998; Tanelian and Brose, 1991), and therefore, it is possible that when generating an action potential, nodal transient NaC currents would decrease and thereby lead to lower inactivation of NaC channels (Kuwabara, 2003). However, in Fig. 2B, refractoriness appears to decrease only when it is abnormally high before treatment. Moreover, if the action potential is smaller, supernormality would be smaller, and block of transient NaC currents would affect threshold electrotonus. The lack of changes in supernormality and threshold electrotonus in this study does not support the hypothesis of blockage of transient NaC currents. Further studies are required to determine whether mexiletine at a low dose blocks transient NaC channels. In the present study, axonal excitability of median motor axons was measured in a variety of disorders, and most of the patients had disease affecting lower motor neurons. Neuropathic pain and muscle cramping preferentially involve the distal lower limbs, rather than the upper limbs. We did not measure excitability of sensory axons, or motor axons in the lower limb nerves, and mechanisms of neuropathic pain and muscle cramp can be different. Therefore, the results cannot be compared directly with clinical effects. Furthermore, neuropathic pain presumably arises from C fibers rather than A fibers. These are limitations of this study, and further studies are required to evaluate excitability in each type of axons responsible for clinical symptoms. Nevertheless, the measurements of excitability of the median motor axons provide objective information about the effects of mexiletine on axonal excitability, and significant differences were found after mexiletine treatment. The mexiletine dosage of 300 mg in this study was less than those of previous studies (450–675 mg) (Dejgard et al., 1988; Oskarsson et al., 1997; Stracke et al., 1992), but our findings showed that this dose administration resulted in substantial changes in strength-duration properties that reflect persistent NaC currents. Measurements of excitability indices such as SDTC and refractoriness can be used to assess and monitor the effects of mexiletine on axonal excitability in the treatment for patients with positive sensory and motor symptoms.
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