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Neurodegeneration
Research paper
Motor axonal excitability properties are strong predictors for survival in amyotrophic lateral sclerosis
  1. Kazuaki Kanai1,
  2. Kazumoto Shibuya1,
  3. Yasunori Sato2,
  4. Sonoko Misawa1,
  5. Saiko Nasu1,
  6. Yukari Sekiguchi1,
  7. Satsuki Mitsuma1,
  8. Sagiri Isose1,
  9. Yumi Fujimaki1,
  10. Shigeki Ohmori1,
  11. Shunsuke Koga1,
  12. Satoshi Kuwabara1
  1. 1Department of Neurology, Graduate School of Medicine, Chiba University, Chiba, Japan
  2. 2Clinical Research Center, Chiba University Hospital, Chiba, Japan
  1. Correspondence to Dr Professor Satoshi Kuwabara, Department of Neurology, Graduate School of Medicine, Chiba University, 1-8-1 Inohana, Chuo-ku, Chiba, 260-8670, Japan; kuwabara-s{at}faculty.chiba-u.jp

Abstract

Objective The aim of this study was to investigate whether axonal excitability indices are associated with survival in patients with amyotrophic lateral sclerosis (ALS). Previous nerve excitability studies suggested increased persistent sodium currents in motor axons of patients with ALS, which lead to axonal hyperexcitability and potentially enhance neuronal death.

Methods 112 patients with sporadic ALS were followed up until endpoint (death or tracheostomy). Multivariate analyses were performed using the Cox proportional hazard model. Threshold tracking was used to measure multiple axonal excitability indices in median motor axons, such as strength–duration time constant (SDTC; a measure of nodal persistent sodium current). Latent addition was also used to estimate the magnitude of persistent sodium currents.

Results The overall median tracheostomy-free survival from onset was 37 months. Prolonged SDTC was strongly associated with shorter survival (adjusted HR 4.07; 95% CI 1.7 to 9.8; p=0.0018) compared with older onset age (>60 years; HR=1.80) and bulbar onset (HR=1.80). Estimated median survival was 34 months in the longer SDTC group and 51 months in the shorter SDTC group. This index was highly statistically significant even after multiple testing adjustments with age and site of onset (bulbar or limb). Latent addition study results were consistent with these findings.

Conclusions Axonal persistent sodium currents, estimated by SDTC and latent addition, are strong and independent predictors for shorter survival in patients with ALS. Membrane hyperexcitability is possibly associated with motor neuronal death, and modulation of excessive sodium currents could be a novel therapeutic option for ALS.

  • Neuromuscular
  • EMG (single fibre)
  • HMSN (Charcot-Marie-Tooth)
  • immunology
  • Lambert-Eaton syndrome
  • motor neuron disease
  • neurophysiology
  • channels
  • dystonia
  • channels
  • motor neuron disease
  • neuropathy
  • neurobiology

https://doi.org/10.1136/jnnp-2011-301782

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    Introduction

    Amyotrophic lateral sclerosis (ALS) is a progressive fatal disorder involving both upper and lower motor neurons. Fasciculation is a characteristic feature of ALS resulting from ectopic firing of motor neurons or axons, indicating a widespread disturbance in membrane properties.1 2 Previous excitability studies have suggested two types of abnormalities of axonal ionic currents in ALS: increased persistent sodium currents and reduced potassium currents in motor axons, both of which lead to axonal hyperexcitability and thereby generation of fasciculations.3–8 However, whether the altered membrane properties are related to progression and prognosis of the disease has never been investigated.

    So far, many epidemiological studies have shown factors related to survival in ALS.9 Older age and bulbar onset are consistently reported to have a worse outcome.9 10 The rate of symptom progression is revealed to be an independent prognostic factor. Psychosocial factors, dementia, nutritional status and respiratory function are also related to outcome.11 Some studies reported that the presence of widespread fasciculations could be one of prognostic factors.12

    The phenomenon of ‘split hand syndrome’ in ALS was increasingly recognised.13–15 In patients with ALS, the abductor pollicis brevis (APB) and first dorsal interosseous (FDI) muscles are much more severely involved than abductor digiti minimi (ADM), despite the fact that all three muscles are innervated by the same spinal segments (C8 and T1), and FDI and ADM have the same ulnar nerve supply. ALS is essentially a heterogeneous disorder,16 and patients develop the disease on the basis of different genetic and external environmental factors. However, ‘split hand’ syndrome develops in people with common genetic and environmental factors, and therefore represents vulnerability of specific motor neurons to the ALS pathology.

    Importantly, later studies have shown that in normal subjects, APB or FDI motor axons have physiologically much higher excitability than ADM axons because of greater persistent sodium conductances,17 suggesting that hyperexcitability of motor axons is associated with earlier motor neuronal death in ALS. These findings led us to study survival of patients with ALS who underwent axonal excitability testing in the past 10 years. The aim of this study was therefore to determine whether axonal excitability indices correlate with survival in patients with ALS.

    Methods

    Patients

    A total of 112 patients with sporadic ALS who were seen at Chiba University Hospital from 2001 to 2010 and received nerve excitability testing were included in the study. Their condition fulfilled the revised El Escorial criteria for definite or probable ALS or fulfilled the criteria for possible ALS at the time of initial examination and reached endpoint (death or tracheostomy) at follow-up. The clinical courses of these patients were consistent with ALS. Patients with familial ALS were excluded. Patients with cervical spondylosis, systemic disease such as malignancy and advanced diabetic complications, severe respiratory failure (vital capacity <25%) or unrecordable median motor responses were also excluded. The age range of the included patients was 48–80 years (median 65 years), and there were 54 men and 58 women. After the diagnosis, 104 of the 112 patients were treated with riluzole. The median duration between disease onset and examination was 11 months (range 2–63 months; IQR 7–20 months).

    Control data for excitability testing were obtained from 26 age-matched control subjects (age 53–77 years, median 64 years; p=0.23). All patients and control subjects gave informed consent, and the study was approved by the Ethics Committee of Chiba University School of Medicine.

    Excitability testing

    Multiple excitability measurements were performed in the median nerve at the wrist using a computerised program (QTRAC with multiple excitability protocol, TRONDXM2, Institute of Neurology, London, UK). The compound muscle action potential (CMAP) was recorded from the APB after median nerve stimulation at the wrist, as previously described.5 6 18 Excitability testing was performed after sufficient skin scraping to reduce skin impedance. During testing, the skin temperature near the stimulus site was maintained at >32°C. The following excitability indices were included: strength–duration time constant (SDTC; a measure of nodal persistent sodium currents18–20), threshold electrotonus and refractoriness, supernormality, and late subnormality of the recovery cycle of axonal excitability with a single supramaximal conditioning stimulus.

    To further estimate the extent of nodal persistent sodium conductance, the technique of latent addition was performed in 80 of the patients with ALS using a computerised program (QTRAC with multiple excitability protocol, LA99SD, Institute of Neurology, London, UK), as described elsewhere.19 20 The test stimulus was conditioned by a hyperpolarising stimulus from 0.02 to 0.5 ms inter-stimulus test interval, fixed at −90% of the threshold current. The increase in threshold current measured at the 0.2 ms interval (a time when the influence of passive membrane properties had decayed almost to 0) was used as an indicator of nodal persistent sodium currents.

    Statistical analyses

    Overall survival was defined as the time from symptom onset until death or tracheostomy. For time-to-event outcomes, the lengths of time to a first event were compared using the log-rank test, while the Kaplan–Meier method was used to estimate the absolute risk of each event for each group. HRs and 95% CIs were estimated by the Cox proportional hazard model. To identify prognostic factors associated with tracheostomy-free survival, multivariate analysis was performed using the Cox proportional hazard model with a stepwise selection procedure. The stepwise procedure was set to a threshold of 0.15 for inclusion and 0.05 for exclusion.

    All comparisons were planned and the tests were two sided. A p value of <0.05 was considered to be statistically significant. In addition, the more conservative Bonferroni method was used to address the multiple testing problems. All statistical analyses were performed by one of the authors (YS) using SAS V.9.2.

    Results

    The median follow-up time among censored observations was 11 months (range 1–65 months; IQR 5–18 months) and at March 2011, 64 (57%) of the 112 patients had died. The overall median tracheostomy-free survival from onset was 37 months (IQR 19–52 months).

    Univariate analyses

    The results of univariate Cox regression analysis are shown in table 1. Older onset age tended to be associated with shorter survival (p=0.06). Bulbar onset (p=0.025) and decreased vital capacity (p=0.017) were significantly related to a worse prognosis. CMAP amplitude in median motor nerve conduction studies had no significant association with survival. There were no significant gender differences in excitability indices, and no significant differences in percentage of patients treated with riluzole in the subgroups divided by excitability testing results.

    View this table:
    Table 1

    Univariate analysis of prognostic factors in amyotrophic lateral sclerosis

    For excitability indices, the cut-off values were determined as the mean values of the control subjects, and patients with ALS were divided into two subgroups with a higher and lower value than the normal mean of each excitability index. A number of indices were significantly related to survival time. In particular, longer SDTC (p=0.0056) and smaller (being less negative) supernormality (p=0.0041) were strongly associated with a shorter survival. The results were statistically significant even after multiple testing adjustments. Smaller depolarising threshold electrotonus (TEd) (10–30 ms) was also associated with a worse prognosis.

    Figure 1 shows Kaplan–Meier curves for survival of the patient subgroups divided by the values of SDTC and supernormality. The median tracheostomy-free survival was 34 months for patients with longer SDTC (IQR 18–44 months) and 51 months for those with shorter SDTC (IQR 35–70 months). SDTC values were not significantly different for the subgroups of patients with ALS, divided according to age, gender and site of onset (bulbar vs limb onset). The median tracheostomy-free survival was 21 months for patients with smaller (less negative) supernormality and 41 months for patients with greater supernormality.

    Figure 1
    Figure 1

    Kaplan–Meier plots of survival probabilities according to strength–duration time constant (SDTC) (A) and supernormality (B). Patients were divided into two groups according to higher or lower than normal mean value. (A) Higher (longer) SDTC was associated with shorter survival. Estimated median survival was 34 months in the higher (longer) SDTC group and 51 months in the lower (shorter) SDTC group. (B) Estimated median survival was 21 months in patients with higher (less negative) supernormality and 41 months in patients with lower (more negative) supernormaliity.

    Of the 112 patients, 19 had complications (hypercapnia or diabetes not fulfilling exclusion criteria described in the Methods) that could affect axonal excitability. On excluding these patients from the subgroup analyses, SDTC and supernormality were again shown to be strong predictors for survival (figure 2A).

    Figure 2
    Figure 2

    Kaplan–Meier plots of survival probabilities according to indices of latent addition studies. Patients with hypercapnea or diabetes were excluded. Strength–duration time constant (SDTC) was calculated using different software (LA99SD) from that used in figure 1 (TrondXM2) (see the main text). (A) Higher (longer) SDTC was correlated to shorter survival. Estimated median survival was 29 months in patients with higher (longer) SDTC and 51 months in patients with lower (shorter) SDTC. (B) Thresholds change at 0.2 ms which reflects persistent sodium currents. Estimated median survival was 29 months in patients with higher (greater) threshold changes and 52 months in patients with lower (smaller) threshold changes.

    Multivariate analyses using Cox progression hazard model

    Table 2 shows the results of multivariate analyses. We included 102 patients in whom all multiple excitability indices were stably recorded because in the remaining 10 patients, at least one excitability index was not available due to severely decreased CMAP amplitude. The Cox proportional hazard model included the following seven variables: gender, age at onset, site of onset, SDTC, refractoriness, supernormality and late subnormality. Older onset age again tended to relate to shorter survival (p=0.056), but there was no significant relation of bulbar onset with prognosis after adjustment. Vital capacity was measured only in 76 patients and not included in the multivariate analyses; multivariate analysis of the 76 patients showed that vital capacity (<75%) was not significantly related to shorter survival (p=0.645).

    View this table:
    Table 2

    Multivariate analysis of prognostic factors in amyotrophic lateral sclerosis

    For nerve excitability properties, longer SDTC was strongly associated with shorter survival (adjusted HR 4.07; 95% CI for HR 1.68 to 9.82; p=0.0018), whereas supernormality had no significant association with survival when adjusted with other factors. Thus, multivariate analyses indicated SDTC was a strong predictor of survival of patients with ALS. The analysis of 90 patients, excluding those with hypercapnia or diabetes, showed similar results (online supplementary figure 1).

    Subgroup analyses according to CMAP size

    In the 112 patients with ALS, the APB CMAP size was not related to survival (HR 1.07; 95% CI for HR 0.64 to 1.79; p=0.800; table 1). Our previous report suggested that changes in axonal excitability properties in ALS depended on the size of CMAP.5 Therefore, subgroup analyses were performed according to the CMAP amplitudes. Longer SDTC was significantly associated with shorter survival in the subgroup of patients with preserved CMAP (>5 mV; lower limit of normal in our laboratory, 5.0 mV). In contrast, supernormality affected a prognosis in the subgroup of patients with decreased CMAP (online supplementary figure 2).

    Estimated persistent sodium currents by latent addition

    As described above, SDTC was suggested to be a strong predictor for tracheostomy-free survival. SDTC is a classic measure of axonal excitability and partly depends on persistent sodium conductance and partly on passive membrane properties,18 whereas latent addition is another measure of persistent sodium currents and is, perhaps, a more reliable method to estimate this conductance.19 The major advantage of this method is to separately evaluate passive membrane properties and persistent sodium currents. Kaplan–Meier analyses were performed in 60 patients who underwent latent addition examination. Figure 2B shows that the threshold current measured at the 0.2 ms interval (an indicator of persistent sodium current) had significant effects on survival of patients with ALS; greater threshold change at 0.2 ms (greater persistent sodium currents) was associated with shorter survival (p=0.0019).

    Survival analyses from excitability testing to endpoint

    Survival from excitability testing to endpoint was also analysed with Kaplan–Meier curves (online supplementary figure 3). Patient subgroups showed that the median survival was 13 months for patients with longer SDTC (IQR 0–65 months) and 31 months for those with shorter SDTC (IQR 0–57 months). Similarly, longer SDTC was strongly associated with shorter survival (adjusted HR 1.95; 95% CI for HR 1.09 to 3.51; p=0.014).

    Discussion

    Our results first show that motor nerve excitability properties are strong predictors of survival in patients with ALS. In particular, SDTC and threshold changes at 0.2 ms in latent addition have prominent association with prognosis, and could be novel prognostic biomarkers in ALS. Assuming that these indices reflect the amount of axonal persistent sodium currents,18–20 increased axonal excitability would be associated with motor neuronal death. It may be surprising that results from the only median nerve motor axons determine a prognosis, but this marker was statistically significant even after multiple adjustments of onset age and onset site. Moreover, it has been reported that the neurophysilogical index ((CMAP amplitude/distal latency) × F frequency %) tested in one nerve differentiates rapidly and slowly progressive disease in ALS.21 The findings are potentially of clinical relevance because if an excessive inward sodium conductance enhances motor neuronal death, blocking it may be a novel therapeutic option for ALS.

    There are some major limitations in this study. Clinical evaluation with the ALS functional rating scale was not systematically evaluated, and therefore the speed of clinical progression could not be examined. Vital capacity was also not measured in all the patients and its prognostic value was not tested by multivariate analyses. Nevertheless, the association of SDTC changes and survival time was highly significant, and we believe that the increased currents affected patients' survival.

    Persistent sodium conductance and the pathophysiology of ALS

    Several studies have suggested the significant role of the increased persistent sodium conductance in the pathophysiology of ALS. In addition to evidence of the increased sodium conductance in human ALS,4–6 experimentally, persistent sodium currents were shown to increase in cultured motor neurons derived from the spinal anterior horn in SOD1 G93A mice.22 Furthermore, riluzole is considered to act on the glutamate system, but this drug has other effects including partial blockade of voltage-dependent sodium channels.23 24

    In this study, multivariate analyses showed that the effects of persistent sodium conductance on ALS survival were independent of other previously reported clinical prognostic factors, such as age and onset site. Moreover, we used two independent measures, SDTC and latent addition, to estimate persistent sodium conductances, and the findings of survival analyses were consistent for both measurements.

    The precise mechanisms for increased persistent sodium conductance in ALS are unknown, but the increase in the Na+ influx into the cytoplasm would result in the increase in the intracellular Ca2+ concentration via activation of the Na+/Ca2+ exchanger,25 possibly enhancing motor neuronal death. In addition, a continuous increase in Na+ influx would lead to overload of the Na+-K+ ATPase-dependent pump and excessive consumption of intracellular ATP. Because dysfunction of mitochondrial energy conversion has been supposed to be implicated in motor neuronal degeneration in ALS, such increased Na+ influx may exacerbate the energy failure in the motor neurons of patients with ALS. As a result, axons could be depolarised and further activate sodium channels.

    SDTC values are considerably variable among normal controls, ranging from 0.2 to 0.6 ms, and increase with age.26 Clarification of the genetic background for individual variability of nodal persistent sodium conductance would provide further insights into the pathogenesis of ALS, and may explain the different rate of disease progression among individual patients. At present, it is not known whether an increased persistent sodium current in ALS axons is a primary event or a consequence. One fact supporting the former possibility is the split hand syndrome13–15; motor axons with physiologically higher persistent sodium currents die more readily in patients with ALS.

    Axonal potassium conductances and progression of ALS

    The univariate analyses showed that ‘smaller’ supernormality and TEd (10–30 ms) were related to shorter survival in patients with ALS. The size of both indices is limited by juxta-paranodal fast potassium currents.3 Therefore, our findings suggest that ‘greater’ potassium currents were associated with a worse prognosis. Previous studies reported that in patients with ALS, the mean values of supernormality and TEd (10–30 ms) were ‘greater’,3 5 6 suggesting ‘reduced’ potassium currents, but it was also demonstrated that in the end stage of the disease when CMAP amplitudes were severely decreased, supernormality and TEd (10–30 ms) became ‘smaller’, possibly by membrane depolarisation.5 Thus, the extent of potassium currents would change in association with disease progression. We speculate that in the early phase of the disease, potassium conductances primarily decrease, the less accommodation contributing to generation of fasciculations,3 and then relatively increase by membrane depolarisation as the disease progresses. Most ion channels expressed on the axons, including potassium channels, are voltage dependent, and therefore their currents increase when axons are depolarised.18–20

    Therapeutic implications

    The possibility that our results may reflect that the underlying biology causes changes in sodium conductance and early death cannot be excluded. However, the split hand syndrome in ALS suggests that motor axons with physiologically higher excitability (ie, greater persistent sodium currents) are preferentially affected, and we believe that the positive association between the increased currents and shorter survival are not merely the results.

    Based on the finding that increased persistent currents are associated with shorter survival in ALS, blocking the currents may suppress disease progression. We think that sodium channel blockers could be candidates for novel therapeutic agent for ALS. A clinical trial with topiramate, which is a partial sodium channel blocker, has failed to show beneficial effects on the progression of ALS,27 but topiramate exerts only modest inhibition of transient sodium channels,28 and its effects on persistent sodium channels are not elucidated. More direct sodium channel blockers such as lidocaine or mexiletine may have therapeutic effects, particularly when started in the early stage of the disease.

    References

    1. ↵
      1. de Carvalho M,
      2. Swash M
      . Fasciculation potentials: a study of amyotrophic lateral sclerosis and other neurogenic disorders. Muscle Nerve 1998;21:336–44.
      OpenUrlCrossRefPubMedWeb of Science
    2. ↵
      1. Kiernan MC,
      2. Vucic S,
      3. Cheah BC,
      4. et al
      . Amyotrophic lateral sclerosis. Lancet 2011;377:942–55.
      OpenUrlCrossRefPubMedWeb of Science
    3. ↵
      1. Bostock H,
      2. Sharief MK,
      3. Reid G,
      4. et al
      . Axonal ion channel dysfunction in amyotrophic lateral sclerosis. Brain 1995;118:217–25.
      OpenUrlAbstract/FREE Full Text
    4. ↵
      1. Mogyoros I,
      2. Kiernan MC,
      3. Burke D,
      4. et al
      . Strength–duration properties of sensory and motor axons in amyotrophic lateral sclerosis. Brain 1998;121:851–9.
      OpenUrlAbstract/FREE Full Text
    5. ↵
      1. Kanai K,
      2. Kuwabara S,
      3. Misawa S,
      4. et al
      . Altered axonal excitability properties in amyotrophic lateral sclerosis: impaired potassium channel function related to disease stage. Brain 2006;129:953–62.
      OpenUrlAbstract/FREE Full Text
    6. ↵
      1. Vucic S,
      2. Kiernan MC
      . Axonal excitability properties in amyotrophic lateral sclerosis. Clin Neurophysiol 2006;117:1458–66.
      OpenUrlCrossRefPubMedWeb of Science
    7. ↵
      1. Nakata M,
      2. Kuwabara S,
      3. Kanai K,
      4. et al
      . Distal excitability changes in motor axons in amyotrophic lateral sclerosis. Clin Neurophysiol 2006;117:1444–8.
      OpenUrlCrossRefPubMedWeb of Science
    8. ↵
      1. Tamura N,
      2. Kuwabara S,
      3. Misawa S,
      4. et al
      . Increased nodal persistent Na+ currents in human neuropathy and motor neuron disease estimated by latent addition. Clin Neurophysiol 2006;117:2451–8.
      OpenUrlCrossRefPubMedWeb of Science
    9. ↵
      1. Chio A,
      2. Logroscino G,
      3. Hardiman O,
      4. et al
      . Prognostic factors in ALS: a critical review. Amyotroph Lateral Scler 2009;10:310–13.
      OpenUrlCrossRefPubMedWeb of Science
    10. ↵
      1. Stambler N,
      2. Charatan M,
      3. Cedarbaum JM
      . Prognostic indicators of survival in ALS. ALS CNTF Treatment Study Group. Neurology 1998;50:66–72.
      OpenUrlAbstract/FREE Full Text
    11. ↵
      1. Traynor BJ,
      2. Alexander M,
      3. Corr B,
      4. et al
      . Effect of a multidisciplinary amyotrophic lateral sclerosis (ALS) clinic on ALS survival: a population based study, 1996–2000. J Neurol Neurosurg Psychiatry 2003;74:1258–61.
      OpenUrlAbstract/FREE Full Text
    12. ↵
      1. Ringel SP,
      2. Murphy JR,
      3. Alderson MK,
      4. et al
      . The natural history of amyotrophic lateral sclerosis. Neurology 1993;43:1316–22.
      OpenUrlAbstract/FREE Full Text
    13. ↵
      1. Wilbourn AJ
      . The ‘split hand syndrome’. Muscle Nerve 2000;23:138.
      OpenUrlPubMed
    14. ↵
      1. Kuwabara S,
      2. Sonoo M,
      3. Komori T,
      4. et al
      . Dissociated small hand muscle atrophy in amyotrophic lateral sclerosis: frequency, extent, and specificity. Muscle Nerve 2008;37:426–30.
      OpenUrlCrossRefPubMedWeb of Science
    15. ↵
      1. Eisen A,
      2. Kuwabara S
      . The split hand syndrome in amyotrophic lateral sclerosis. J Neurol Neurosurg Psychiatry 2012;83:399–403.
      OpenUrlAbstract/FREE Full Text
    16. ↵
      1. Bento-Abreu A,
      2. Van Damme P,
      3. Van Den Bosch L,
      4. et al
      . The neurobiology of amyotrophic lateral sclerosis. Eur J Neurosci 2010;31:2247–65.
      OpenUrlCrossRefPubMed
    17. ↵
      1. Bae JS,
      2. Sawai S,
      3. Misawa S,
      4. et al
      . Differences in excitability properties of FDI and ADM motor axons. Muscle Nerve 2009;39:350–4.
      OpenUrlCrossRefPubMed
    18. ↵
      1. Kiernan MC,
      2. Burke D,
      3. Andersen KV,
      4. et al
      . Multiple measures of axonal excitability: a new approach in clinical testing. Muscle Nerve 2000;23:399–409.
      OpenUrlCrossRefPubMedWeb of Science
    19. ↵
      1. Bostock H,
      2. Rothwell JC
      . Latent addition in motor and sensory fibres of human peripheral nerve. J Physiol 1997;498:277–94.
      OpenUrlCrossRefPubMedWeb of Science
    20. ↵
      1. Kuwabara S,
      2. Misawa S,
      3. Tamura N,
      4. et al
      . Latent addition in human motor and sensory axons: different site-dependent changes across the carpal tunnel related to persistent Na+ currents. Clin Neurophysiol 2006;117:810–14.
      OpenUrlCrossRefPubMed
    21. ↵
      1. Swash M,
      2. de Carvalho M
      . The neurophysiological index in ALS. Amyotroph Lateral Scler Other Motor Neuron Disord 2004;5(Suppl 1):S108–10.
      OpenUrlCrossRef
    22. ↵
      1. Kuo JJ,
      2. Siddique T,
      3. Fu R,
      4. et al
      . Increased persistent Na+ current and its effect on excitability in motoneurones cultured from mutant SOD1 mice. J Physiol (Lond) 2005;563:843–54.
      OpenUrlCrossRefPubMedWeb of Science
    23. ↵
      1. Morrison KE
      . Therapies in amyotrophic lateral sclerosis—beyond riluzole. Curr Opin Pharmacol 2002;2:302–9.
      OpenUrlCrossRefPubMedWeb of Science
    24. ↵
      1. Urbani A,
      2. Belluzzi O
      . Riluzole inhibits the persistent sodium current in mammalian CNS neurons. Eur J Neurosci 2000;12:3567–74.
      OpenUrlCrossRefPubMedWeb of Science
    25. ↵
      1. Stys PK,
      2. Waxman SG
      . Ransom BR Na+-Ca2+ exchanger mediates Ca2+ influx during anoxia in mammalian central nervous system white matter. Ann Neurol 1991;30:375–80.
      OpenUrlCrossRefPubMedWeb of Science
    26. ↵
      1. Bae JS,
      2. Sawai S,
      3. Misawa S,
      4. et al
      . Effects of age on excitability properties in human motor axons. Clin Neurophysiol 2008;119:2282–6.
      OpenUrlCrossRefPubMedWeb of Science
    27. ↵
      1. Cudkowicz ME,
      2. Shefner JM,
      3. Schoenfeld DA,
      4. et al
      . A randomized, placebo-controlled trial of topiramate in amyotrophic lateral sclerosis. Neurology 2003;61:456–64.
      OpenUrlAbstract/FREE Full Text
    28. ↵
      1. Vohora D,
      2. Saraogi P,
      3. Yazdani MA,
      4. et al
      . Recent advances in adjunctive therapy for epilepsy: focus on sodium channel blockers as third-generation antiepileptic drugs. Drugs Today (Barc) 2010;46:265–77.
      OpenUrlCrossRefPubMed

    Supplementary materials

    Footnotes

    • KK and KS equally contributed to this study.

    • Funding Drs Shibuya, Sato, Misawa, Nasu, Isose, Sekiguchi, Mitsuma, Fujimaki, Ohmori and Koga report no disclosures. Dr Kanai receives research support from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Dr Kuwabara receives research support from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and Grants-in-Aid from the Research Committee of CNS Degenerative Diseases, the Ministry of Health, Labour and Welfare of Japan.

    • Competing interests None.

    • Ethics approval This study was conducted with the approval of Chiba University School of Medicine.

    • Provenance and peer review Not commissioned; externally peer reviewed