- Email: [email protected]
Altered motor axonal excitability in patients with cervical spondylotic amyotrophy
Clinical Neurophysiology 129 (2018) 1383–1389
Contents lists available at ScienceDirect
Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph
Altered motor axonal excitability in patients with cervical spondylotic amyotrophy Chaojun Zheng a, Yu Zhu b, Cong Nie a, Feizhou Lu a,c, Dongqing Zhu d, Robert Weber b, Jianyuan Jiang a,⇑ a
Department of Orthopedics, Huashan Hospital, Fudan University, Shanghai 200040, China Department of Physical Medicine and Rehabilitation, Upstate Medical University, State University of New York at Syracuse, Syracuse, NY 10212, USA c Department of Orthopedics, The Fifth People’s Hospital, Fudan University, Shanghai 200240, China d Department of Neurology, Huashan Hospital, Fudan University, Shanghai 200040, China b
a r t i c l e
i n f o
Article history: Accepted 30 March 2018 Available online 20 April 2018 Keywords: Threshold tracking Cervical spondylotic amyotrophy Slow K+ channel Motor axonal excitability
h i g h l i g h t s Motor excitability is different between cervical spondylotic amyotrophy (CSA) cases and controls. +
Evidence for a reduction in slow K conductance was found in both distal- and proximal-type CSA
cases. Abnormal excitability changes may contribute to the increased vulnerability of motor axons in CSA.
a b s t r a c t Objective: To investigate the changes in motor axonal excitability properties in cervical spondylotic amyotrophy (CSA). Methods: Threshold tracking was used to measure the median motor axons in 21 patients with CSA, 10 patients with cervical spondylotic radiculopathy (CSR) and 16 normal controls. Results: Compared with normal controls, patients with distal-type CSA showed increased threshold electrotonus hyperpolarization (TEh [90–100]) and increased superexcitability on the symptomatic side (P < 0.05), which are suggestive of distal motor axonal hyperpolarization, presumably due to motor axonal regeneration. More importantly, compared with normal controls and CSR cases, both distal- and proximal-type CSA cases showed lower accommodation during depolarising currents (reduced S2 accommodation, decreased TEd [undershoot] and/or lower subexcitability) (P < 0.05), indicating that slow K+ conductance may be less active in motor axons in patients with CSA. Conclusions: The present study demonstrated changes in motor axonal excitability in patients with CSA compared with both normal controls and patients with CSR. Significance: Less expression of slow K+ conductance may confer greater instability in membrane potential in CSA, thereby presumably contributing to the increased vulnerability of motor axons in patients with CSA. Ó 2018 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.
1. Introduction Cervical spondylotic amyotrophy (CSA) is a special type of cervical spondylosis that is characterised by severe weakness and amyotrophy in the upper limbs with no or insignificant sensory involvement and no lower extremity symptoms ⇑ Corresponding author at: Department of Orthopedics, Huashan Hospital, Fudan University, 12 Mid-Wulumuqi Road, Shanghai 200040, China. Fax: +86 021 6248 9191. E-mail address: [email protected] (J. Jiang).
(Shinomiya et al., 1994; Sonoo, 2016). According to the distribution of muscular atrophy, CSA is classified as proximal or distal type (Jiang et al., 2011; Zheng et al., 2017). The true pathogenesis of CSA has not been well established. One of the current hypotheses regarding the aetiology of CSA mainly involves the selective damage of ventral nerve roots and/or anterior horn cells by a bony spur or herniated disc (Keegan, 1965; Itoh et al., 1980), and the relatively good results obtained with cervical surgical treatment, especially in proximal-type CSA (Fujiwara et al., 2006; Wang et al., 2014), support this hypothesis. Although the sensory nerve action potentials (SNAPs) recorded in distal
https://doi.org/10.1016/j.clinph.2018.03.044 1388-2457/Ó 2018 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.
1384
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
nerves are normal in cervical spondylosis because of preganglionic injury, most cases with cervical spondylosis have initial symptoms of upper limb radiation pain and/or sensory impairment in clinical practice because sensory fibres are more susceptible to compressive injury than motor fibres are in radiculopathy (Wilbourn and Aminoff, 1998; Alfonsi et al., 2003). Therefore, it is difficult to explain why motor fibres are peculiarly vulnerable to compression in patients with CSA. Previous studies have demonstrated that the different excitability properties of axons may cause different responses to injury or disease (Burke et al., 2001; Kuwabara et al., 2000; Kuwabara et al., 2001; Lin et al., 2000; Lin et al., 2001). For example, there are significant differences in the properties of peroneal and median motor axons; as a result of these differences, peroneal axons have less protection from damage and are more likely to become ectopically active (Kuwabara et al., 2000; Kuwabara et al., 2001). Similar conditions were also reported between median and sural sensory axons, and the inherent differences in nerve excitability between these nerves may result in a greater tendency for dysfunction in sural afferents (Lin et al., 2000; Lin et al., 2001). In clinical practice, Han et al. demonstrated that an ischaemic insult may cause a higher intensity of numbness and paraesthesia in patients with carpal tunnel syndrome (CTS) than observed in healthy subjects because of a preexisting abnormal axon excitability in CTS (Han et al., 2009). Therefore, it may be important to identify the motor axonal excitability in patients with CSA to explore its pathogenesis. Unfortunately, no studies have reported the possibility of changes in motor axonal excitability in patients with CSA because it is not possible to directly measure human motor axon excitability in vivo. The recently developed threshold tracking technique provides a non-invasive and painless way to explore a number of indices of axonal excitability (e.g., strength-duration properties, threshold electrotonus, superexcitability, late subexcitability and refractoriness) (Bostock et al., 1998; Burke et al., 2001). These indices shed light on both the sodium (Na+) and potassium (K+) channels, the properties of axon membranes, and membrane potential (Bostock et al., 1998; Burke et al., 2001). Using this nerve excitability testing, the aim of our study was to investigate the changes in axonal excitability properties in patients with CSA and analyse the correlation between the altered motor axonal excitability and increased vulnerability of motor axons in CSA. 2. Materials and methods 2.1. Subjects Twenty-one patients with CSA (distal-type vs. proximal-type: 16 vs. 5) (Fig. 1), 16 healthy subjects and 10 patients with cervical spondylotic radiculopathy (CSR) were included in this study (Table 1). All patients were recruited at Huashan Hospital between February 2016 and September 2017. The Human Ethics Committee of Huashan Hospital at Fudan University in China granted ethical committee approval, and each subject provided informed consent. The subjects in the control and both the CSA and CSR patient groups were selected according to the inclusion and exclusion criteria that have been described previously (Zheng et al., 2014; Zheng et al., 2017). 2.2. Testing methods As previously described (Sawai et al., 2011; Klein et al., 2018), nerve excitability testing was performed by a protocol using a computer program (QTRAC version 4.3 with multiple excitability protocol TRONDF; Institute of Neurology, London, UK). The tests
were performed on both sides of all subjects in the patient groups and five subjects in the control group, and the remaining 11 healthy subjects underwent unilateral upper limb detection. Compound muscle action potentials (CMAPs) were recorded using a belly-tendon method from the abductor pollicis brevis (APB) to median nerve stimulation at the wrist, and the CMAP amplitude was measured from the baseline to the initial negative peak. Stimulation was delivered by an isolated linear bipolar constant current stimulator (maximal output ±50 mA) (DS5, Digitimer Ltd, Welwyn Garden City, UK) controlled by the QtracS software (Ó Prof. H. Bostock, Institute of Neurology, London, UK). The optimal position of the stimulation cathode was determined in each subject using a hand-held stimulator, prior to the application of surface electrodes (Disposable Silver/Silver Chloride cup electrodes, Digitimer Ltd, Welwyn Garden City, UK) secured with a strap, and the reference electrode was placed 10 cm proximal to the stimulation cathode. Skin temperature was monitored at the stimulation site, a heater was used to maintain the temperature above 32 °C, and both the stimulating and recording sites were prepared by lightly abrading the skin with fine sandpaper. The following measures were performed using the TRONDNF protocol (Burke et al., 2001; Sawai et al., 2011; Murray and Jankelowitz, 2011): (1) stimulus-response curves (S-R curves): the stimulus was manually increased to obtain the maximal CMAP, and the curve was repeated in smaller increments of stimulus by the computer. The current that chouls produce 40% of the maximal CMAP amplitude was defined as the ‘‘threshold”. (2) Strengthduration time constant (SDTC): the SDTC for median motor axons was measured using five different stimulus durations (0.2–1.0 ms) to determine the altered stimulus current required to reach the ‘‘threshold”. Weiss’s formula [SDTC = 0.2 (I0.2 I1.0)/ (I1.0 0.2 I0.2)] was used to measure the STDC. (3) Threshold electrotonus (TE): TE was tested using subthreshold 100-ms polarising currents in the depolarising (TEd; +40%) and hyperpolarising (The; 40%) directions. The threshold change (%) was plotted with the depolarising and hyperpolarising directions at various time intervals. (4) Recovery cycle: the recovery cycle of axonal excitability after a single supramaximal stimulus was recorded by delivering the test stimulus at different intervals (from 2 to 200 ms) after the conditioning stimulus. (5) Current-threshold relationship: the current-threshold (I/V) relationship is analogous to the currentvoltage relationship. The threshold was measured 200 ms after the onset of a polarising current lasting 200 ms. The polarising conditioning current was ramped up from +50% of the control threshold to 100% in the 10% step. Furthermore, all subjects in this study underwent further cervical MRI evaluation, bilateral handgrip strength (HGS) examination (Jamar hydraulic hand dynamometer, Sammons Preston Rolyan, IL, USA) and bilateral electrophysiological detection (Nihon Kohden MEB-9400, Japan), including the sensory and motor conduction of the median, ulnar, peroneal and tibial nerves, and concentric needle electromyogram (EMG) examination. These parameters for each patient were defined as abnormal if the measurements were 2 standard deviations (SDs) above the average values for normal controls for the onset-latency or 2 SDs below the average values for normal controls for the maximal amplitude, nerve conduction velocity and HGS. 2.3. Statistical methods All data were analysed using SPSS version 12.0 (IBM, USA). The Kolmogorov-Smirnov test was used to test normally distributed data. The measurements in patients with distal- or proximal-type CSA, the cases with CSR and the healthy subjects were compared by one-way ANOVA (Bonferroni correction), and independent t-tests were used to evaluate the differences in the excitability
1385
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
Fig. 1. Patients with cervical spondylotic amyotrophy (CSA). A: A 47-year-old male patient with distal-type CSA. Significant amyotrophy of the intrinsic hand muscle is observed on the right side (arrows), and cervical magnetic resonance imaging (MRI) shows cervical compression at the C5-6 and C6-7 segments. B: A 52-year-old male patient with proximal-type CSA. Significant amyotrophy of the deltoid and biceps brachii is observed on the left side (arrows), and cervical MRI shows cervical compression at the C34, C4-5 and C5-6 segments.
Table 1 The patients with distal-type CSA, proximal-type CSA, CSR and normal controls.
Number of cases Age range (years) Height range (cm) male:female duration of symptom (months) Symptomatic side (right: left) Amyotrophy (unilateral: bilateral)
Number of tested limbs Handgrip strength (kg) Median onset-latency (ms) Median CMAP amplitude (mV) Median MNCV (m/s) Ulnar onset-latency (ms) Ulnar CMAP amplitude (mV) Ulnar MNCV-Above elbow (m/s) Ulnar MNCV-Below elbow (m/s)
Distal-type CSA
Proximal-type CSA
CSR
Controls
16 54.1 ± 8.9 169.9 ± 5.6 14:2 24.4 ± 19.6 10:6 11:5
5 51.6 ± 4.7 171.8 ± 2.8 5:0 24.0 ± 21.2 1:4 5:0
10 52.8 ± 7.0 170.4 ± 6.6 7:3 17.2 ± 11.9 3:7 10:0
16 51.6 ± 11.2 169.1 ± 6.8 10:6 / / /
S side
Less-s side
S side
Less-s side
S side
Less-s side
n = 16 25.5 ± 11.6*,# 3.7 ± 0.9*,# 4.9 ± 2.2*,# 58.0 ± 5.3 3.1 ± 0.8*,# 4.7 ± 2.5*,# 61.9 ± 5.3 60.8 ± 5.3
n = 16 39.3 ± 6.0*,# 3.2 ± 0.7* 7.8 ± 1.3* 58.5 ± 4.9 2.6 ± 0.2* 7.6 ± 1.7* 61.5 ± 5.4 60.2 ± 5.4
n=5 43.4 ± 4.5 3.2 ± 0.6 8.1 ± 2.0 58.0 ± 4.3 2.6 ± 0.4 8.0 ± 1.1 62.7 ± 3.5 60.9 ± 2.6
n=5 43.2 ± 5.6 3.2 ± 0.5 8.4 ± 1.9 57.5 ± 4.0 2.6 ± 0.2 8.1 ± 1.6 61.3 ± 4.2 60.4 ± 4.1
n = 10 42.5 ± 3.8 3.3 ± 0.5 8.3 ± 1.5 58.7 ± 4.2 2.7 ± 0.3 8.4 ± 1.7 60.2 ± 7.2 59.3 ± 7.0
n = 10 43.2 ± 3.6 3.3 ± 0.6 8.2 ± 1.6 58.6 ± 4.3 2.6 ± 0.4 8.4 ± 1.7 61.7 ± 6.3 60.7 ± 6.1
n = 32 43.0 ± 4.3 3.1 ± 0.7 8.0 ± 1.4 57.8 ± 4.0 2.6 ± 0.3 8.3 ± 1.6 61.8 ± 4.6 60.1 ± 4.6
CSA: cervical spondylotic amyotrophy; CSR: cervical spondylotic radiculopathy; CMAP: Compound muscle action potential; MNCV: motor nerve conduction velocity; S side: Symptomatic side; Less-s side: Less-symptomatic side. * Significant statistical difference between the symptomatic side and less-symptomatic side (P < 0.05). # Significant statistical difference between the patients and normal controls (P < 0.05).
indices between the less-symptomatic side of distal-type CSA patients with identified unilateral involvement and the healthy subjects. In the patient groups, paired t-tests were used to compare the measurements between the symptomatic side and the lesssymptomatic side. The height and age were compared among the control group, CSA patient group and CSR patient group using one-way ANOVA (Student-Newman-Keuls test), and the same statistical methods were used to analyse disease duration among
patients with proximal-type CSA, distal-type CSA and CSR. P-values <0.05 were considered statistically significant. 3. Results The measurements of excitability indices in 21 patients with CSA, 10 patients with CSR and 16 normal controls are presented in Supplementary Tables 1 and 2, and the measurements of the
1386
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
HGS examinations and both median and ulnar motor conduction studies (NCSs) are listed in Table 1. There were no differences in age or height among the normal controls, CSA patients and CSR patients (P > 0.05), and a similar disease duration was found among patients with proximal-type CSA, distal-type CSA and CSR (P > 0.05).
3.1. Multiple excitability measurements Compared with normal controls, patients with distal-type CSA showed greater TEh (90–100) (P = 0.015; Fig. 2E), increased TEd (90–100) (P = 0.005; Fig. 2E), greater superexcitability (P = 0.047; Fig. 2F) and lower hyperpolarizing I/V slope (P = 0.010; Fig. 2C) on the symptomatic side (Supplementary Table 1). On the lesssymptomatic side of distal-type CSA cases, we compared the excitability indices between patients without clinical and needle EMG abnormalities (n = 11) and normal controls. The former group had decreased subexcitability (P = 0.029; Fig. 2F0 ), reduced S2 accommodation (P = 0.018; Fig. 2E0 ) and a lower hyperpolarising I/V slope (P = 0.026; Fig. 2C0 ). In all patients with proximal-type CSA, neither HGS nor needle EMG abnormalities were found in the distal tested muscles of the bilateral upper limbs. In contrast, these cases showed reduced S2 accommodation (P = 0.019; Fig. 3E), decreased TEd (undershoot) (P = 0.026; Fig. 3E), and lower hyperpolarizing I/V slope (P = 0.046; Fig. 3C) on the symptomatic side, along with a lower hyperpolarising I/V slope (P = 0.027; Fig. 3C0 ) on the lesssymptomatic side (Supplementary Tables 1 and 2). Compared with normal controls, patients with CSR showed similar motor axonal excitability measurements on both sides (P > 0.05) (Supplementary Tables 1 and 2; Fig. 4). The differences in axonal excitability measurements between patients with CSR and patients with distal- or proximal-type CSA are presented in Supplementary Tables 1 and 2.
3.2. Electrophysiological evaluation Eleven patients with distal-type CSA showed reduced maximum CMAP amplitudes of the ulnar (11/16, 68.8%) and/or median (10/16, 62.5%) nerves on the symptomatic side along with prolonged motor onset-latency of the median (4/16, 25.0%) and/or ulnar (4/16, 25.0%) nerves in 5 of these 11 cases, and 3 distaltype CSA cases showed significant difference of motor nerve conduction velocity of median nerves between symptomatic side and less-symptomatic side (50.2 m/s vs. 60.7 m/s; 51 m/s vs. 64.3 m/s; 52.1 m/s vs. 62.5 m/s). One patient with CSR showed a mild reduction in the median CMAP amplitudes (5.03 mV) on the symptomatic side. In the distal-type CSA group, nine cases showed changes in motor unit action potential with or without abnormal spontaneous activity in the symptomatic-side C7, C8 and T1 myotomes; two in the symptomatic-side C8 and T1; and five in bilateral C7, C8 and T1. In the proximal-type CSA group, four cases had needle EMG abnormalities in symptomatic-side C5 and C6 myotomes, as did one case in the symptomatic-side C5. In the CSR group, five cases had denervation in the symptomatic-side C6 and C7 myotomes; two in the symptomatic-side C5, C6 and C7; two in the symptomatic-side C7; and one in the symptomatic-side C7 and C8.
3.3. Magnetic resonance imaging In the distal-type CSA group, eight cases had cervical compression at the C5-6 and C6-7 segments; three at C4-5, C5-6 and C6-7; three at C5-6, C6-7 and C7-T1; one at C6-7 and C7-T1; and one at C4-5, C5-6, C6-7 and C7-T1. In the proximal-type CSA group, three cases had cervical compression at C3-4, C4-5 and C5-6, as did two at C4-5 and C5-6. For the CSR group, T2-weighted MRI showed cervical compression in all ten patients as follows: four at C5-6 and
Fig. 2. Multiple excitability measurements in patients with distal-type cervical spondylotic amyotrophy (CSA) and normal controls. On the symptomatic side, distal-type CSA cases show mild changes in threshold electrotonus (TEd [90–100], TEh [90–100]) (E), superexcitability (F) and hyperpolarising I/V slope (C) (P < 0.05). Compared with 0 0 0 controls, distal-type CSA cases with unilateral involvement show reduced subexcitability (F ), decreased S2 accommodation (E ) and smaller hyperpolarising I/V slope (C ) on the less-symptomatic side (P < 0.05). Data are expressed as the means ± SEM.
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
1387
Fig. 3. Multiple excitability measurements between normal controls and patients with proximal-type cervical spondylotic amyotrophy (CSA). On the symptomatic side, proximal-type CSA cases show mild changes in S2 accommodation (E), TEd (undershoot) (E) and hyperpolarising I/V slope (C) (P < 0.05). On the less-symptomatic side, proximal-type CSA cases show changes in hyperpolarizing I/V slope (C0 ) (P < 0.05). Data are expressed as the means ± SEM.
Fig. 4. Multiple excitability measurements between normal controls and patients with cervical spondylotic radiculopathy (CSR). Compared with normal controls, patients with CSR show similar motor axonal excitability measurements on both sides (P > 0.05). Data are expressed as the means ± SEM.
1388
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
C6-7; two at C6-7; one at C5-6; one at C4-5 and C5-6; one at C4-5, C5-6 and C6-7; and one at C6-7 and C7-T1. 3.4. Handgrip strength examination Significantly reduced HGS was found in 11 patients with distaltype CSA on the symptomatic side compared with that in normal controls, and two of these 11 cases also showed reduced HGS on the less-symptomatic side. All patients with CSR or proximaltype CSA showed normal bilateral HGS. 4. Discussion The results of this study document slight deviations in the excitability of motor axons in patients with CSA compared with those in healthy subjects, and these abnormal changes were not present in cases with CSR. In the present study, the patients with distal-type CSA showed significantly abnormal responses to hyperpolarizing but not to depolarizing TE on the symptomatic side. A similar condition was also reported in the previous study (Moldovan and Krarup, 2004a), and the persistent hyperpolarization in regenerated motor axons may be a possible reason (Moldovan and Krarup, 2004b). Proximal compression of ventral nerve roots and/or anterior horn cells in the patients with CSA may cause Wallerian degeneration of distal axons (Jiang et al., 2011; Sonoo, 2016), and both the long disease duration in the distal-type CSA cases in this study and abnormal needle EMG findings (motor unit changes indicating chronic partial denervation) in all tested muscles on the symptomatic side confirmed the existence of regenerated motor axons in the tested nerve (symptomatic-side median nerve) in these cases. Furthermore, hyperactivity of the Na+-K+ pump in the motor axon adjacent to the lesion may be another possible explanation for this hyperpolarisation, and both Kaji and Kiernan et al. demonstrated this compensatory mechanism of the Na+-K+ pump may cause axonal hyperpolarisation (Kaji, 2003; Kiernan et al., 2002). Although the increased superexcitability and mild reduction in both resting I/V slope and early refractoriness (which did not reach statistical significance) on the symptomatic side supported the concept that the motor axons were hyperpolarized (Bostock et al., 1998; Burke et al., 2001), lack of changes in SDTC and S-R curves, which contradict the hyperpolarization, were found in this study. Similar abnormalities in unchanged SDTC with hyperpolarization were reported in both a human and mouse model in previous studies (Sung et al., 2015; Moldovan et al., 2016), and increased expression of persistent Na+ conductance on the axolemma during motor unit reorganisation was demonstrated to be a possible explanation (Sawai et al., 2008; Sung et al., 2015). A good example of unchanged S-R curves with hyperpolarisation is Fig. 3 in the Bostock et al.’s study, and the curves are identical in this example just shifted to the left with depolarisation and to the right with hyperpolarization (Bostock et al., 1998). This is because the S-R curves in the figures are normalized and do not reveal differences in absolute size or in absolute threshold. In the present study, although there were no significant differences in absolute threshold in Supplementary Table 1, there is a trend for these values of CSA cases to be higher than those of normal controls, and that would be consistent with a degree of hyperpolarisation. Compared to normal controls, significant changes in TEd (90–100) with similar TEd (10–20) on the symptomatic side of the distal-type CSA cases may suggest less accommodation rather than hyperpolarization. This example was further supported by relatively less S2 accommodation. Furthermore, similar less accommodation (decreased subexcitability and/or reduced S2 accommodation)
during depolarization was also found on the less-symptomatic side of distal-type CSA cases with unilateral involvement (n = 11) and the symptomatic side of proximal-type CSA cases in this study. Late subexcitability is mainly ascribed to nodal slow K+ conductance, whereas S2 accommodation to subthreshold depolarisation reflects the activation of nodal and internodal slow K+ channels (Kuwabara et al., 2000). Therefore, the differences in membrane potential between patients with CSA and normal controls may be a possible reason for the difference in both subexcitability and S2 accommodation because slow K+ channels are voltage-dependent. However, this assumption was not supported by the similarity of both refractoriness and the SDTC between the normal controls and both distaltype CSA cases with unilateral involvement (less-symptomatic side) and proximal-type CSA cases (symptomatic side), as both measurements are sensitive to changes in membrane potential (Burke et al., 1998). Furthermore, the similar changes in membrane potential observed in response to the depolarising current and identical superexcitability between normal subjects and these CSA patients suggest that the differences in subexcitability and S2 accommodation cannot be due to a difference in passive internodal properties (Kuwabara et al., 2000). Therefore, less slow K+ conductance in motor axons in CSA may be a more likely reason for the changes in these two excitability indices, and a reduced threshold undershoot in the patients with proximal-type CSA also supported this assumption. Similarly, we also found evidence that slow K+ conductance is less active in motor axons on the less-symptomatic side in both proximal-type and distal-type CSA cases than it is in patients with CSR in this study. Among the control group and these patient groups, depolarisation produced similar changes in membrane potential, and accommodation was lower in patients with CSA than it was in normal subjects and cases with CSR for this similar change in membrane potential. As a result, it is likely that the reduced expression of these accommodating conductances in cases with CSA confers greater instability. In previous studies, both Kuwabara et al. and Lin et al. demonstrated that a reduction in accommodation to depolarising currents could increase the vulnerability of axons to disease or injury (Kuwabara et al., 2000; Kuwabara et al., 2001; Lin et al., 2000; Lin et al., 2001). Therefore, less slow K+ conductance in patients with CSA may be one of the possible factors contribute to the increased vulnerability of peripheral motor axons in CSA. Although the present study demonstrated a less slow K+ conductance on peripheral motor axons in CSA cases, the underlying cause of this difference is unclear. Impaired axoplasmic transport of ion channel proteins and other proteins involved in setting membrane potential along the distal axon caused by mild compression of nerve root at intervertebral foramen may be a possible explanation, and previous studies also demonstrated a proximal lesion along an axon predisposes it to injury at a more distal site along the course through impaired axoplasmic flow (Zheng et al., 2016). However, this mechanism would not explain the differences between the cases with CSA and CSR who have similar cervical compression in MRI; here, it is likely that abnormal changes in channel isoforms may be also factors. Whatever the reasons for the difference in excitability properties, they may result in different responses to injury in CSA, although these are likely to be subtle compared with those resulting from the more substantial difference between motor and sensory fibers (Burke et al., 2001). In the current study, although lower hyperpolarising I/V slopes were found in both distal- and proximal-type CSA, which may presumably be due to less Ih in CSA, the interpretation of reduced hyperpolarizing I/V slopes as an indication for impaired Ih is not straightforward without mathematical modelling or additional
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
longer current electrotonus recordings since Ih was demonstrated to be most variable between subject. Therefore, further study is required to apply these methods in a larger number of cases with CSA. 5. Conclusion Our data confirmed that compared with normal controls and patients with CSR, patients with CSA showed mild changes in motor axonal excitability including the evidence for less slow K+ conductance in motor axons. It is possible that these changes reduce the ability of motor axons to handle damage. Therefore, these changes likely constitute one of the contributing factors to the increased vulnerability of motor axons in CSA. Acknowledgements Financial support from Shanghai City Health System of the Second Batch of Important Diseases Combined Project (2014ZYJ0008), the National Natural Science Foundation of China Youth Science Foundation Project (81501909) and the Scientific Research project supported by Huashan Hospital, Fudan University is gratefully acknowledged. We would like to thank Dr. David Burke at the Department of Neurology, Royal Prince Alfred Hospital, Sydney Medical School, University of Sydney and Dr. Karl Ng at the Department of Neurology and Clinical Neurophysiology, Royal North Shore Hospital, for their helpful review and comments during the manuscript preparation. Declaration of interest None of the authors have potential conflicts of interest to be disclosed. The authors alone are responsible for the content and writing of this paper. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.clinph.2018.03.044. References Alfonsi E, Merlo IM, Clerici AM, Candeloro E, Marchioni AM. Proximal nerve conduction by high-voltage electrical stimulation in S1 radiculopathies and acquired demyelinating neuropathies. Clin Neurophysiol 2003;114:239–47. Bostock H, Cikurel K, Burke D. Threshold tracking techniques in the study of human peripheral nerve. Muscle Nerve 1998;21:137–58. Burke D, Mogyoros I, Vagg R, Kiernan MC. Quantitative description of the voltage dependence of axonal excitability in human cutaneous afferents. Brain 1998;121:1975–83. Burke D, Kiernan MC, Bostock H. Excitability of human axons. Clin Neurophysiol 2001;112:1575–85.
1389
Fujiwara Y, Tanaka N, Fujimoto Y, Nakanishi K, Kamei N, Ochi M. Surgical outcome of posterior decompression for cervical spondylosis with unilateral upper extremity amyotrophy. Spine (Phila Pa 1976) 2006;31:E728–32. Han SE, Boland RA, Krishnan AV, Vucic S, Lin CS, Kiernan MC. Ischaemic sensitivity of axons in carpal tunnel syndrome. J Peripher Nerv Syst 2009;14:190–200. Itoh T, Tsuji H, Tamaki T, Miyasaka H, Toyoda A. The clinical consideration of the dissociated motor loss syndrome (Keegan) in diseases of the cervical spine (author’s transl) [Article in Japanese]. Nihon Seikeigeka Gakkai Zasshi 1980;54:135–51. Jiang SD, Jiang LS, Dai LY. Cervical spondylotic amyotrophy. Eur Spine J 2011;20:351–7. Keegan JJ. The cause of dissociated motor loss in the upper extremity with cervical spondylosis. J Neurosurg 1965;23:5236–85. Kiernan MC, Guglielmi JM, Kaji R, Murray NM, Bostock H. Evidence for axonal membrane hyperpolarization in multifocal motor neuropathy with conduction block. Brain 2002;125:664–75. Kaji R. Physiology of conduction block in multifocal motor neuropathy and other demyelinating neuropathies. Muscle Nerve 2003;27:285–96. Kuwabara S, Cappelen-Smith C, Lin CS, Mogyoros I, Bostock H, Burke D. Excitability properties of median and peroneal motor axons. Muscle Nerve 2000;23:1365–73. Kuwabara S, Cappelen-Smith C, Lin CS, Mogyoros I, Burke D. Differences in accommodative properties of median and peroneal motor axons. J Neurol Neurosurg Psychiatry 2001;70:372–6. Klein CS, Zhao CN, Liu H, Zhou P. Differences in excitability properties between medial gastrocnemius, tibialis anterior, and abductor pollicis brevis motor axons. Muscle Nerve 2018;57. E60-9. Lin CS, Mogyoros I, Kuwabara S, Cappelen-Smith C, Burke D. Accommodation to depolarizing and hyperpolarizing currents in cutaneous afferents of the human median and sural nerves. J Physiol 2000;529(Pt 2):483–92. Lin CS, Mogyoros I, Kuwabara S, Cappelen-Smith C, Burke D. Differences in responses of cutaneous afferents in the human median and sural nerves to ischemia. Muscle Nerve 2001;24:1503–9. Moldovan M, Krarup C. Persistent abnormalities of membrane excitability in regenerated mature motor axons in cat. J Physiol 2004a;560:795–806. Moldovan M, Krarup C. Mechanisms of hyperpolarization in regenerated mature motor axons in cat. J Physiol 2004b;560:807–19. Moldovan M, Alvarez S, Rosberg MR, Krarup C. Persistent alterations in active and passive electrical membrane properties of regenerated nerve fibers of man and mice. Eur J Neurosci 2016;43:388–403. Murray JE, Jankelowitz SK. A comparison of the excitability of motor axons innervating the APB and ADM muscles. Clin Neurophysiol 2011;122:2290–3. Shinomiya K, Komori H, Matsuoka T, Mutoh N, Furuya K. Neuroradiologic and electrophysiologic assessment of cervical spondylotic amyotrophy. Spine (Phila Pa 1976) 1994;19:21–5. Sawai S, Kanai K, Nakata M, Hiraga A, Misawa S, Isose S, et al. Changes in excitability properties associated with axonal regeneration in human neuropathy and mouse Wallerian degeneration. Clin Neurophysiol 2008;119:1097–105. Sawai S, Misawa S, Kanai K, Isose S, Shibuya K, Noto Y, et al. Altered axonal excitability properties in juvenile muscular atrophy of distal upper extremity (Hirayama disease). Clin Neurophysiol 2011;122:205–9. Sung JY, Tani J, Hung KS, Lui TN, Lin CS. Sensory axonal dysfunction in cervical radiculopathy. J Neurol Neurosurg Psychiatry 2015;86:640–5. Sonoo M. Cervical spondylotic amyotrophy. Brain Nerve 2016;68:509–19. Wilbourn AJ, Aminoff MJ. AAEM minimonograph 32: the electrodiagnostic examination in patients with radiculopathies. American Association of Electrodiagnostic Medicine. Muscle Nerve 1998;21:1612–31. Wang H, Li H, Jiang J, Lu F, Chen W, Ma X. Evaluation of characteristics and surgical outcomes in cervical spondylotic amyotrophy. Indian J Orthop 2014;48:511–7. Zheng C, Zhu Y, Lu F, Ma X, Xia X, Wang L, et al. Abnormal flexor carpi radialis Hreflex as a specific indicator of C7 as compared with C6 radiculopathy. J Clin Neurophysiol 2014;31:529–34. Zheng C, Zhu Y, Jiang J, Ma X, Lu F, Jin X, et al. The prevalence of tarsal tunnel syndrome in patients with lumbosacral radiculopathy. Eur Spine J 2016;25:895–905. Zheng C, Jin X, Zhu Y, Lu F, Jiang J, Xia X. Repetitive nerve stimulation as a diagnostic aid for distinguishing cervical spondylotic amyotrophy from amyotrophic lateral sclerosis. Eur Spine J 2017;26:1929–36.
Contents lists available at ScienceDirect
Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph
Altered motor axonal excitability in patients with cervical spondylotic amyotrophy Chaojun Zheng a, Yu Zhu b, Cong Nie a, Feizhou Lu a,c, Dongqing Zhu d, Robert Weber b, Jianyuan Jiang a,⇑ a
Department of Orthopedics, Huashan Hospital, Fudan University, Shanghai 200040, China Department of Physical Medicine and Rehabilitation, Upstate Medical University, State University of New York at Syracuse, Syracuse, NY 10212, USA c Department of Orthopedics, The Fifth People’s Hospital, Fudan University, Shanghai 200240, China d Department of Neurology, Huashan Hospital, Fudan University, Shanghai 200040, China b
a r t i c l e
i n f o
Article history: Accepted 30 March 2018 Available online 20 April 2018 Keywords: Threshold tracking Cervical spondylotic amyotrophy Slow K+ channel Motor axonal excitability
h i g h l i g h t s Motor excitability is different between cervical spondylotic amyotrophy (CSA) cases and controls. +
Evidence for a reduction in slow K conductance was found in both distal- and proximal-type CSA
cases. Abnormal excitability changes may contribute to the increased vulnerability of motor axons in CSA.
a b s t r a c t Objective: To investigate the changes in motor axonal excitability properties in cervical spondylotic amyotrophy (CSA). Methods: Threshold tracking was used to measure the median motor axons in 21 patients with CSA, 10 patients with cervical spondylotic radiculopathy (CSR) and 16 normal controls. Results: Compared with normal controls, patients with distal-type CSA showed increased threshold electrotonus hyperpolarization (TEh [90–100]) and increased superexcitability on the symptomatic side (P < 0.05), which are suggestive of distal motor axonal hyperpolarization, presumably due to motor axonal regeneration. More importantly, compared with normal controls and CSR cases, both distal- and proximal-type CSA cases showed lower accommodation during depolarising currents (reduced S2 accommodation, decreased TEd [undershoot] and/or lower subexcitability) (P < 0.05), indicating that slow K+ conductance may be less active in motor axons in patients with CSA. Conclusions: The present study demonstrated changes in motor axonal excitability in patients with CSA compared with both normal controls and patients with CSR. Significance: Less expression of slow K+ conductance may confer greater instability in membrane potential in CSA, thereby presumably contributing to the increased vulnerability of motor axons in patients with CSA. Ó 2018 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.
1. Introduction Cervical spondylotic amyotrophy (CSA) is a special type of cervical spondylosis that is characterised by severe weakness and amyotrophy in the upper limbs with no or insignificant sensory involvement and no lower extremity symptoms ⇑ Corresponding author at: Department of Orthopedics, Huashan Hospital, Fudan University, 12 Mid-Wulumuqi Road, Shanghai 200040, China. Fax: +86 021 6248 9191. E-mail address: [email protected] (J. Jiang).
(Shinomiya et al., 1994; Sonoo, 2016). According to the distribution of muscular atrophy, CSA is classified as proximal or distal type (Jiang et al., 2011; Zheng et al., 2017). The true pathogenesis of CSA has not been well established. One of the current hypotheses regarding the aetiology of CSA mainly involves the selective damage of ventral nerve roots and/or anterior horn cells by a bony spur or herniated disc (Keegan, 1965; Itoh et al., 1980), and the relatively good results obtained with cervical surgical treatment, especially in proximal-type CSA (Fujiwara et al., 2006; Wang et al., 2014), support this hypothesis. Although the sensory nerve action potentials (SNAPs) recorded in distal
https://doi.org/10.1016/j.clinph.2018.03.044 1388-2457/Ó 2018 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.
1384
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
nerves are normal in cervical spondylosis because of preganglionic injury, most cases with cervical spondylosis have initial symptoms of upper limb radiation pain and/or sensory impairment in clinical practice because sensory fibres are more susceptible to compressive injury than motor fibres are in radiculopathy (Wilbourn and Aminoff, 1998; Alfonsi et al., 2003). Therefore, it is difficult to explain why motor fibres are peculiarly vulnerable to compression in patients with CSA. Previous studies have demonstrated that the different excitability properties of axons may cause different responses to injury or disease (Burke et al., 2001; Kuwabara et al., 2000; Kuwabara et al., 2001; Lin et al., 2000; Lin et al., 2001). For example, there are significant differences in the properties of peroneal and median motor axons; as a result of these differences, peroneal axons have less protection from damage and are more likely to become ectopically active (Kuwabara et al., 2000; Kuwabara et al., 2001). Similar conditions were also reported between median and sural sensory axons, and the inherent differences in nerve excitability between these nerves may result in a greater tendency for dysfunction in sural afferents (Lin et al., 2000; Lin et al., 2001). In clinical practice, Han et al. demonstrated that an ischaemic insult may cause a higher intensity of numbness and paraesthesia in patients with carpal tunnel syndrome (CTS) than observed in healthy subjects because of a preexisting abnormal axon excitability in CTS (Han et al., 2009). Therefore, it may be important to identify the motor axonal excitability in patients with CSA to explore its pathogenesis. Unfortunately, no studies have reported the possibility of changes in motor axonal excitability in patients with CSA because it is not possible to directly measure human motor axon excitability in vivo. The recently developed threshold tracking technique provides a non-invasive and painless way to explore a number of indices of axonal excitability (e.g., strength-duration properties, threshold electrotonus, superexcitability, late subexcitability and refractoriness) (Bostock et al., 1998; Burke et al., 2001). These indices shed light on both the sodium (Na+) and potassium (K+) channels, the properties of axon membranes, and membrane potential (Bostock et al., 1998; Burke et al., 2001). Using this nerve excitability testing, the aim of our study was to investigate the changes in axonal excitability properties in patients with CSA and analyse the correlation between the altered motor axonal excitability and increased vulnerability of motor axons in CSA. 2. Materials and methods 2.1. Subjects Twenty-one patients with CSA (distal-type vs. proximal-type: 16 vs. 5) (Fig. 1), 16 healthy subjects and 10 patients with cervical spondylotic radiculopathy (CSR) were included in this study (Table 1). All patients were recruited at Huashan Hospital between February 2016 and September 2017. The Human Ethics Committee of Huashan Hospital at Fudan University in China granted ethical committee approval, and each subject provided informed consent. The subjects in the control and both the CSA and CSR patient groups were selected according to the inclusion and exclusion criteria that have been described previously (Zheng et al., 2014; Zheng et al., 2017). 2.2. Testing methods As previously described (Sawai et al., 2011; Klein et al., 2018), nerve excitability testing was performed by a protocol using a computer program (QTRAC version 4.3 with multiple excitability protocol TRONDF; Institute of Neurology, London, UK). The tests
were performed on both sides of all subjects in the patient groups and five subjects in the control group, and the remaining 11 healthy subjects underwent unilateral upper limb detection. Compound muscle action potentials (CMAPs) were recorded using a belly-tendon method from the abductor pollicis brevis (APB) to median nerve stimulation at the wrist, and the CMAP amplitude was measured from the baseline to the initial negative peak. Stimulation was delivered by an isolated linear bipolar constant current stimulator (maximal output ±50 mA) (DS5, Digitimer Ltd, Welwyn Garden City, UK) controlled by the QtracS software (Ó Prof. H. Bostock, Institute of Neurology, London, UK). The optimal position of the stimulation cathode was determined in each subject using a hand-held stimulator, prior to the application of surface electrodes (Disposable Silver/Silver Chloride cup electrodes, Digitimer Ltd, Welwyn Garden City, UK) secured with a strap, and the reference electrode was placed 10 cm proximal to the stimulation cathode. Skin temperature was monitored at the stimulation site, a heater was used to maintain the temperature above 32 °C, and both the stimulating and recording sites were prepared by lightly abrading the skin with fine sandpaper. The following measures were performed using the TRONDNF protocol (Burke et al., 2001; Sawai et al., 2011; Murray and Jankelowitz, 2011): (1) stimulus-response curves (S-R curves): the stimulus was manually increased to obtain the maximal CMAP, and the curve was repeated in smaller increments of stimulus by the computer. The current that chouls produce 40% of the maximal CMAP amplitude was defined as the ‘‘threshold”. (2) Strengthduration time constant (SDTC): the SDTC for median motor axons was measured using five different stimulus durations (0.2–1.0 ms) to determine the altered stimulus current required to reach the ‘‘threshold”. Weiss’s formula [SDTC = 0.2 (I0.2 I1.0)/ (I1.0 0.2 I0.2)] was used to measure the STDC. (3) Threshold electrotonus (TE): TE was tested using subthreshold 100-ms polarising currents in the depolarising (TEd; +40%) and hyperpolarising (The; 40%) directions. The threshold change (%) was plotted with the depolarising and hyperpolarising directions at various time intervals. (4) Recovery cycle: the recovery cycle of axonal excitability after a single supramaximal stimulus was recorded by delivering the test stimulus at different intervals (from 2 to 200 ms) after the conditioning stimulus. (5) Current-threshold relationship: the current-threshold (I/V) relationship is analogous to the currentvoltage relationship. The threshold was measured 200 ms after the onset of a polarising current lasting 200 ms. The polarising conditioning current was ramped up from +50% of the control threshold to 100% in the 10% step. Furthermore, all subjects in this study underwent further cervical MRI evaluation, bilateral handgrip strength (HGS) examination (Jamar hydraulic hand dynamometer, Sammons Preston Rolyan, IL, USA) and bilateral electrophysiological detection (Nihon Kohden MEB-9400, Japan), including the sensory and motor conduction of the median, ulnar, peroneal and tibial nerves, and concentric needle electromyogram (EMG) examination. These parameters for each patient were defined as abnormal if the measurements were 2 standard deviations (SDs) above the average values for normal controls for the onset-latency or 2 SDs below the average values for normal controls for the maximal amplitude, nerve conduction velocity and HGS. 2.3. Statistical methods All data were analysed using SPSS version 12.0 (IBM, USA). The Kolmogorov-Smirnov test was used to test normally distributed data. The measurements in patients with distal- or proximal-type CSA, the cases with CSR and the healthy subjects were compared by one-way ANOVA (Bonferroni correction), and independent t-tests were used to evaluate the differences in the excitability
1385
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
Fig. 1. Patients with cervical spondylotic amyotrophy (CSA). A: A 47-year-old male patient with distal-type CSA. Significant amyotrophy of the intrinsic hand muscle is observed on the right side (arrows), and cervical magnetic resonance imaging (MRI) shows cervical compression at the C5-6 and C6-7 segments. B: A 52-year-old male patient with proximal-type CSA. Significant amyotrophy of the deltoid and biceps brachii is observed on the left side (arrows), and cervical MRI shows cervical compression at the C34, C4-5 and C5-6 segments.
Table 1 The patients with distal-type CSA, proximal-type CSA, CSR and normal controls.
Number of cases Age range (years) Height range (cm) male:female duration of symptom (months) Symptomatic side (right: left) Amyotrophy (unilateral: bilateral)
Number of tested limbs Handgrip strength (kg) Median onset-latency (ms) Median CMAP amplitude (mV) Median MNCV (m/s) Ulnar onset-latency (ms) Ulnar CMAP amplitude (mV) Ulnar MNCV-Above elbow (m/s) Ulnar MNCV-Below elbow (m/s)
Distal-type CSA
Proximal-type CSA
CSR
Controls
16 54.1 ± 8.9 169.9 ± 5.6 14:2 24.4 ± 19.6 10:6 11:5
5 51.6 ± 4.7 171.8 ± 2.8 5:0 24.0 ± 21.2 1:4 5:0
10 52.8 ± 7.0 170.4 ± 6.6 7:3 17.2 ± 11.9 3:7 10:0
16 51.6 ± 11.2 169.1 ± 6.8 10:6 / / /
S side
Less-s side
S side
Less-s side
S side
Less-s side
n = 16 25.5 ± 11.6*,# 3.7 ± 0.9*,# 4.9 ± 2.2*,# 58.0 ± 5.3 3.1 ± 0.8*,# 4.7 ± 2.5*,# 61.9 ± 5.3 60.8 ± 5.3
n = 16 39.3 ± 6.0*,# 3.2 ± 0.7* 7.8 ± 1.3* 58.5 ± 4.9 2.6 ± 0.2* 7.6 ± 1.7* 61.5 ± 5.4 60.2 ± 5.4
n=5 43.4 ± 4.5 3.2 ± 0.6 8.1 ± 2.0 58.0 ± 4.3 2.6 ± 0.4 8.0 ± 1.1 62.7 ± 3.5 60.9 ± 2.6
n=5 43.2 ± 5.6 3.2 ± 0.5 8.4 ± 1.9 57.5 ± 4.0 2.6 ± 0.2 8.1 ± 1.6 61.3 ± 4.2 60.4 ± 4.1
n = 10 42.5 ± 3.8 3.3 ± 0.5 8.3 ± 1.5 58.7 ± 4.2 2.7 ± 0.3 8.4 ± 1.7 60.2 ± 7.2 59.3 ± 7.0
n = 10 43.2 ± 3.6 3.3 ± 0.6 8.2 ± 1.6 58.6 ± 4.3 2.6 ± 0.4 8.4 ± 1.7 61.7 ± 6.3 60.7 ± 6.1
n = 32 43.0 ± 4.3 3.1 ± 0.7 8.0 ± 1.4 57.8 ± 4.0 2.6 ± 0.3 8.3 ± 1.6 61.8 ± 4.6 60.1 ± 4.6
CSA: cervical spondylotic amyotrophy; CSR: cervical spondylotic radiculopathy; CMAP: Compound muscle action potential; MNCV: motor nerve conduction velocity; S side: Symptomatic side; Less-s side: Less-symptomatic side. * Significant statistical difference between the symptomatic side and less-symptomatic side (P < 0.05). # Significant statistical difference between the patients and normal controls (P < 0.05).
indices between the less-symptomatic side of distal-type CSA patients with identified unilateral involvement and the healthy subjects. In the patient groups, paired t-tests were used to compare the measurements between the symptomatic side and the lesssymptomatic side. The height and age were compared among the control group, CSA patient group and CSR patient group using one-way ANOVA (Student-Newman-Keuls test), and the same statistical methods were used to analyse disease duration among
patients with proximal-type CSA, distal-type CSA and CSR. P-values <0.05 were considered statistically significant. 3. Results The measurements of excitability indices in 21 patients with CSA, 10 patients with CSR and 16 normal controls are presented in Supplementary Tables 1 and 2, and the measurements of the
1386
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
HGS examinations and both median and ulnar motor conduction studies (NCSs) are listed in Table 1. There were no differences in age or height among the normal controls, CSA patients and CSR patients (P > 0.05), and a similar disease duration was found among patients with proximal-type CSA, distal-type CSA and CSR (P > 0.05).
3.1. Multiple excitability measurements Compared with normal controls, patients with distal-type CSA showed greater TEh (90–100) (P = 0.015; Fig. 2E), increased TEd (90–100) (P = 0.005; Fig. 2E), greater superexcitability (P = 0.047; Fig. 2F) and lower hyperpolarizing I/V slope (P = 0.010; Fig. 2C) on the symptomatic side (Supplementary Table 1). On the lesssymptomatic side of distal-type CSA cases, we compared the excitability indices between patients without clinical and needle EMG abnormalities (n = 11) and normal controls. The former group had decreased subexcitability (P = 0.029; Fig. 2F0 ), reduced S2 accommodation (P = 0.018; Fig. 2E0 ) and a lower hyperpolarising I/V slope (P = 0.026; Fig. 2C0 ). In all patients with proximal-type CSA, neither HGS nor needle EMG abnormalities were found in the distal tested muscles of the bilateral upper limbs. In contrast, these cases showed reduced S2 accommodation (P = 0.019; Fig. 3E), decreased TEd (undershoot) (P = 0.026; Fig. 3E), and lower hyperpolarizing I/V slope (P = 0.046; Fig. 3C) on the symptomatic side, along with a lower hyperpolarising I/V slope (P = 0.027; Fig. 3C0 ) on the lesssymptomatic side (Supplementary Tables 1 and 2). Compared with normal controls, patients with CSR showed similar motor axonal excitability measurements on both sides (P > 0.05) (Supplementary Tables 1 and 2; Fig. 4). The differences in axonal excitability measurements between patients with CSR and patients with distal- or proximal-type CSA are presented in Supplementary Tables 1 and 2.
3.2. Electrophysiological evaluation Eleven patients with distal-type CSA showed reduced maximum CMAP amplitudes of the ulnar (11/16, 68.8%) and/or median (10/16, 62.5%) nerves on the symptomatic side along with prolonged motor onset-latency of the median (4/16, 25.0%) and/or ulnar (4/16, 25.0%) nerves in 5 of these 11 cases, and 3 distaltype CSA cases showed significant difference of motor nerve conduction velocity of median nerves between symptomatic side and less-symptomatic side (50.2 m/s vs. 60.7 m/s; 51 m/s vs. 64.3 m/s; 52.1 m/s vs. 62.5 m/s). One patient with CSR showed a mild reduction in the median CMAP amplitudes (5.03 mV) on the symptomatic side. In the distal-type CSA group, nine cases showed changes in motor unit action potential with or without abnormal spontaneous activity in the symptomatic-side C7, C8 and T1 myotomes; two in the symptomatic-side C8 and T1; and five in bilateral C7, C8 and T1. In the proximal-type CSA group, four cases had needle EMG abnormalities in symptomatic-side C5 and C6 myotomes, as did one case in the symptomatic-side C5. In the CSR group, five cases had denervation in the symptomatic-side C6 and C7 myotomes; two in the symptomatic-side C5, C6 and C7; two in the symptomatic-side C7; and one in the symptomatic-side C7 and C8.
3.3. Magnetic resonance imaging In the distal-type CSA group, eight cases had cervical compression at the C5-6 and C6-7 segments; three at C4-5, C5-6 and C6-7; three at C5-6, C6-7 and C7-T1; one at C6-7 and C7-T1; and one at C4-5, C5-6, C6-7 and C7-T1. In the proximal-type CSA group, three cases had cervical compression at C3-4, C4-5 and C5-6, as did two at C4-5 and C5-6. For the CSR group, T2-weighted MRI showed cervical compression in all ten patients as follows: four at C5-6 and
Fig. 2. Multiple excitability measurements in patients with distal-type cervical spondylotic amyotrophy (CSA) and normal controls. On the symptomatic side, distal-type CSA cases show mild changes in threshold electrotonus (TEd [90–100], TEh [90–100]) (E), superexcitability (F) and hyperpolarising I/V slope (C) (P < 0.05). Compared with 0 0 0 controls, distal-type CSA cases with unilateral involvement show reduced subexcitability (F ), decreased S2 accommodation (E ) and smaller hyperpolarising I/V slope (C ) on the less-symptomatic side (P < 0.05). Data are expressed as the means ± SEM.
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
1387
Fig. 3. Multiple excitability measurements between normal controls and patients with proximal-type cervical spondylotic amyotrophy (CSA). On the symptomatic side, proximal-type CSA cases show mild changes in S2 accommodation (E), TEd (undershoot) (E) and hyperpolarising I/V slope (C) (P < 0.05). On the less-symptomatic side, proximal-type CSA cases show changes in hyperpolarizing I/V slope (C0 ) (P < 0.05). Data are expressed as the means ± SEM.
Fig. 4. Multiple excitability measurements between normal controls and patients with cervical spondylotic radiculopathy (CSR). Compared with normal controls, patients with CSR show similar motor axonal excitability measurements on both sides (P > 0.05). Data are expressed as the means ± SEM.
1388
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
C6-7; two at C6-7; one at C5-6; one at C4-5 and C5-6; one at C4-5, C5-6 and C6-7; and one at C6-7 and C7-T1. 3.4. Handgrip strength examination Significantly reduced HGS was found in 11 patients with distaltype CSA on the symptomatic side compared with that in normal controls, and two of these 11 cases also showed reduced HGS on the less-symptomatic side. All patients with CSR or proximaltype CSA showed normal bilateral HGS. 4. Discussion The results of this study document slight deviations in the excitability of motor axons in patients with CSA compared with those in healthy subjects, and these abnormal changes were not present in cases with CSR. In the present study, the patients with distal-type CSA showed significantly abnormal responses to hyperpolarizing but not to depolarizing TE on the symptomatic side. A similar condition was also reported in the previous study (Moldovan and Krarup, 2004a), and the persistent hyperpolarization in regenerated motor axons may be a possible reason (Moldovan and Krarup, 2004b). Proximal compression of ventral nerve roots and/or anterior horn cells in the patients with CSA may cause Wallerian degeneration of distal axons (Jiang et al., 2011; Sonoo, 2016), and both the long disease duration in the distal-type CSA cases in this study and abnormal needle EMG findings (motor unit changes indicating chronic partial denervation) in all tested muscles on the symptomatic side confirmed the existence of regenerated motor axons in the tested nerve (symptomatic-side median nerve) in these cases. Furthermore, hyperactivity of the Na+-K+ pump in the motor axon adjacent to the lesion may be another possible explanation for this hyperpolarisation, and both Kaji and Kiernan et al. demonstrated this compensatory mechanism of the Na+-K+ pump may cause axonal hyperpolarisation (Kaji, 2003; Kiernan et al., 2002). Although the increased superexcitability and mild reduction in both resting I/V slope and early refractoriness (which did not reach statistical significance) on the symptomatic side supported the concept that the motor axons were hyperpolarized (Bostock et al., 1998; Burke et al., 2001), lack of changes in SDTC and S-R curves, which contradict the hyperpolarization, were found in this study. Similar abnormalities in unchanged SDTC with hyperpolarization were reported in both a human and mouse model in previous studies (Sung et al., 2015; Moldovan et al., 2016), and increased expression of persistent Na+ conductance on the axolemma during motor unit reorganisation was demonstrated to be a possible explanation (Sawai et al., 2008; Sung et al., 2015). A good example of unchanged S-R curves with hyperpolarisation is Fig. 3 in the Bostock et al.’s study, and the curves are identical in this example just shifted to the left with depolarisation and to the right with hyperpolarization (Bostock et al., 1998). This is because the S-R curves in the figures are normalized and do not reveal differences in absolute size or in absolute threshold. In the present study, although there were no significant differences in absolute threshold in Supplementary Table 1, there is a trend for these values of CSA cases to be higher than those of normal controls, and that would be consistent with a degree of hyperpolarisation. Compared to normal controls, significant changes in TEd (90–100) with similar TEd (10–20) on the symptomatic side of the distal-type CSA cases may suggest less accommodation rather than hyperpolarization. This example was further supported by relatively less S2 accommodation. Furthermore, similar less accommodation (decreased subexcitability and/or reduced S2 accommodation)
during depolarization was also found on the less-symptomatic side of distal-type CSA cases with unilateral involvement (n = 11) and the symptomatic side of proximal-type CSA cases in this study. Late subexcitability is mainly ascribed to nodal slow K+ conductance, whereas S2 accommodation to subthreshold depolarisation reflects the activation of nodal and internodal slow K+ channels (Kuwabara et al., 2000). Therefore, the differences in membrane potential between patients with CSA and normal controls may be a possible reason for the difference in both subexcitability and S2 accommodation because slow K+ channels are voltage-dependent. However, this assumption was not supported by the similarity of both refractoriness and the SDTC between the normal controls and both distaltype CSA cases with unilateral involvement (less-symptomatic side) and proximal-type CSA cases (symptomatic side), as both measurements are sensitive to changes in membrane potential (Burke et al., 1998). Furthermore, the similar changes in membrane potential observed in response to the depolarising current and identical superexcitability between normal subjects and these CSA patients suggest that the differences in subexcitability and S2 accommodation cannot be due to a difference in passive internodal properties (Kuwabara et al., 2000). Therefore, less slow K+ conductance in motor axons in CSA may be a more likely reason for the changes in these two excitability indices, and a reduced threshold undershoot in the patients with proximal-type CSA also supported this assumption. Similarly, we also found evidence that slow K+ conductance is less active in motor axons on the less-symptomatic side in both proximal-type and distal-type CSA cases than it is in patients with CSR in this study. Among the control group and these patient groups, depolarisation produced similar changes in membrane potential, and accommodation was lower in patients with CSA than it was in normal subjects and cases with CSR for this similar change in membrane potential. As a result, it is likely that the reduced expression of these accommodating conductances in cases with CSA confers greater instability. In previous studies, both Kuwabara et al. and Lin et al. demonstrated that a reduction in accommodation to depolarising currents could increase the vulnerability of axons to disease or injury (Kuwabara et al., 2000; Kuwabara et al., 2001; Lin et al., 2000; Lin et al., 2001). Therefore, less slow K+ conductance in patients with CSA may be one of the possible factors contribute to the increased vulnerability of peripheral motor axons in CSA. Although the present study demonstrated a less slow K+ conductance on peripheral motor axons in CSA cases, the underlying cause of this difference is unclear. Impaired axoplasmic transport of ion channel proteins and other proteins involved in setting membrane potential along the distal axon caused by mild compression of nerve root at intervertebral foramen may be a possible explanation, and previous studies also demonstrated a proximal lesion along an axon predisposes it to injury at a more distal site along the course through impaired axoplasmic flow (Zheng et al., 2016). However, this mechanism would not explain the differences between the cases with CSA and CSR who have similar cervical compression in MRI; here, it is likely that abnormal changes in channel isoforms may be also factors. Whatever the reasons for the difference in excitability properties, they may result in different responses to injury in CSA, although these are likely to be subtle compared with those resulting from the more substantial difference between motor and sensory fibers (Burke et al., 2001). In the current study, although lower hyperpolarising I/V slopes were found in both distal- and proximal-type CSA, which may presumably be due to less Ih in CSA, the interpretation of reduced hyperpolarizing I/V slopes as an indication for impaired Ih is not straightforward without mathematical modelling or additional
C. Zheng et al. / Clinical Neurophysiology 129 (2018) 1383–1389
longer current electrotonus recordings since Ih was demonstrated to be most variable between subject. Therefore, further study is required to apply these methods in a larger number of cases with CSA. 5. Conclusion Our data confirmed that compared with normal controls and patients with CSR, patients with CSA showed mild changes in motor axonal excitability including the evidence for less slow K+ conductance in motor axons. It is possible that these changes reduce the ability of motor axons to handle damage. Therefore, these changes likely constitute one of the contributing factors to the increased vulnerability of motor axons in CSA. Acknowledgements Financial support from Shanghai City Health System of the Second Batch of Important Diseases Combined Project (2014ZYJ0008), the National Natural Science Foundation of China Youth Science Foundation Project (81501909) and the Scientific Research project supported by Huashan Hospital, Fudan University is gratefully acknowledged. We would like to thank Dr. David Burke at the Department of Neurology, Royal Prince Alfred Hospital, Sydney Medical School, University of Sydney and Dr. Karl Ng at the Department of Neurology and Clinical Neurophysiology, Royal North Shore Hospital, for their helpful review and comments during the manuscript preparation. Declaration of interest None of the authors have potential conflicts of interest to be disclosed. The authors alone are responsible for the content and writing of this paper. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.clinph.2018.03.044. References Alfonsi E, Merlo IM, Clerici AM, Candeloro E, Marchioni AM. Proximal nerve conduction by high-voltage electrical stimulation in S1 radiculopathies and acquired demyelinating neuropathies. Clin Neurophysiol 2003;114:239–47. Bostock H, Cikurel K, Burke D. Threshold tracking techniques in the study of human peripheral nerve. Muscle Nerve 1998;21:137–58. Burke D, Mogyoros I, Vagg R, Kiernan MC. Quantitative description of the voltage dependence of axonal excitability in human cutaneous afferents. Brain 1998;121:1975–83. Burke D, Kiernan MC, Bostock H. Excitability of human axons. Clin Neurophysiol 2001;112:1575–85.
1389
Fujiwara Y, Tanaka N, Fujimoto Y, Nakanishi K, Kamei N, Ochi M. Surgical outcome of posterior decompression for cervical spondylosis with unilateral upper extremity amyotrophy. Spine (Phila Pa 1976) 2006;31:E728–32. Han SE, Boland RA, Krishnan AV, Vucic S, Lin CS, Kiernan MC. Ischaemic sensitivity of axons in carpal tunnel syndrome. J Peripher Nerv Syst 2009;14:190–200. Itoh T, Tsuji H, Tamaki T, Miyasaka H, Toyoda A. The clinical consideration of the dissociated motor loss syndrome (Keegan) in diseases of the cervical spine (author’s transl) [Article in Japanese]. Nihon Seikeigeka Gakkai Zasshi 1980;54:135–51. Jiang SD, Jiang LS, Dai LY. Cervical spondylotic amyotrophy. Eur Spine J 2011;20:351–7. Keegan JJ. The cause of dissociated motor loss in the upper extremity with cervical spondylosis. J Neurosurg 1965;23:5236–85. Kiernan MC, Guglielmi JM, Kaji R, Murray NM, Bostock H. Evidence for axonal membrane hyperpolarization in multifocal motor neuropathy with conduction block. Brain 2002;125:664–75. Kaji R. Physiology of conduction block in multifocal motor neuropathy and other demyelinating neuropathies. Muscle Nerve 2003;27:285–96. Kuwabara S, Cappelen-Smith C, Lin CS, Mogyoros I, Bostock H, Burke D. Excitability properties of median and peroneal motor axons. Muscle Nerve 2000;23:1365–73. Kuwabara S, Cappelen-Smith C, Lin CS, Mogyoros I, Burke D. Differences in accommodative properties of median and peroneal motor axons. J Neurol Neurosurg Psychiatry 2001;70:372–6. Klein CS, Zhao CN, Liu H, Zhou P. Differences in excitability properties between medial gastrocnemius, tibialis anterior, and abductor pollicis brevis motor axons. Muscle Nerve 2018;57. E60-9. Lin CS, Mogyoros I, Kuwabara S, Cappelen-Smith C, Burke D. Accommodation to depolarizing and hyperpolarizing currents in cutaneous afferents of the human median and sural nerves. J Physiol 2000;529(Pt 2):483–92. Lin CS, Mogyoros I, Kuwabara S, Cappelen-Smith C, Burke D. Differences in responses of cutaneous afferents in the human median and sural nerves to ischemia. Muscle Nerve 2001;24:1503–9. Moldovan M, Krarup C. Persistent abnormalities of membrane excitability in regenerated mature motor axons in cat. J Physiol 2004a;560:795–806. Moldovan M, Krarup C. Mechanisms of hyperpolarization in regenerated mature motor axons in cat. J Physiol 2004b;560:807–19. Moldovan M, Alvarez S, Rosberg MR, Krarup C. Persistent alterations in active and passive electrical membrane properties of regenerated nerve fibers of man and mice. Eur J Neurosci 2016;43:388–403. Murray JE, Jankelowitz SK. A comparison of the excitability of motor axons innervating the APB and ADM muscles. Clin Neurophysiol 2011;122:2290–3. Shinomiya K, Komori H, Matsuoka T, Mutoh N, Furuya K. Neuroradiologic and electrophysiologic assessment of cervical spondylotic amyotrophy. Spine (Phila Pa 1976) 1994;19:21–5. Sawai S, Kanai K, Nakata M, Hiraga A, Misawa S, Isose S, et al. Changes in excitability properties associated with axonal regeneration in human neuropathy and mouse Wallerian degeneration. Clin Neurophysiol 2008;119:1097–105. Sawai S, Misawa S, Kanai K, Isose S, Shibuya K, Noto Y, et al. Altered axonal excitability properties in juvenile muscular atrophy of distal upper extremity (Hirayama disease). Clin Neurophysiol 2011;122:205–9. Sung JY, Tani J, Hung KS, Lui TN, Lin CS. Sensory axonal dysfunction in cervical radiculopathy. J Neurol Neurosurg Psychiatry 2015;86:640–5. Sonoo M. Cervical spondylotic amyotrophy. Brain Nerve 2016;68:509–19. Wilbourn AJ, Aminoff MJ. AAEM minimonograph 32: the electrodiagnostic examination in patients with radiculopathies. American Association of Electrodiagnostic Medicine. Muscle Nerve 1998;21:1612–31. Wang H, Li H, Jiang J, Lu F, Chen W, Ma X. Evaluation of characteristics and surgical outcomes in cervical spondylotic amyotrophy. Indian J Orthop 2014;48:511–7. Zheng C, Zhu Y, Lu F, Ma X, Xia X, Wang L, et al. Abnormal flexor carpi radialis Hreflex as a specific indicator of C7 as compared with C6 radiculopathy. J Clin Neurophysiol 2014;31:529–34. Zheng C, Zhu Y, Jiang J, Ma X, Lu F, Jin X, et al. The prevalence of tarsal tunnel syndrome in patients with lumbosacral radiculopathy. Eur Spine J 2016;25:895–905. Zheng C, Jin X, Zhu Y, Lu F, Jiang J, Xia X. Repetitive nerve stimulation as a diagnostic aid for distinguishing cervical spondylotic amyotrophy from amyotrophic lateral sclerosis. Eur Spine J 2017;26:1929–36.