Journal of Neurological Sciences (Turkish) 2017 , Vol 34 , Num 1
Single-pulse Transcranial Magnetic Stimulation in Amyotrophic Lateral Sclerosis: Experience of a single institution
F.Gökçem YILDIZ1,Can Ebru Bekircan-KURT2,Kubilay VARLI2
1Hacettepe University, The Institute of Neurological Sciences and Psychiatry, Ankara, Turkey
2Hacettepe University School of Medicine , Department of Neurology, Ankara, Turkey

Summary

Objective: To investigate the assessment of cortical function changes and intrinsic circuitry in Amyotrophic Lateral Sclerosis (ALS) by transcranial magnetic stimulation (TMS).

Methods: We studied 16 ALS and 11 healthy controls by single pulse TMS from abductor digiti minimi muscle (ADM). We evaluated cortical excitability parameters including, resting motor evoked potentials (MEPs) amplitude, resting MEPs threshold , central motor conduction time (CMCT), cortical silent period duration (CSPD) and cortical silent period threshold (CSPT) in ALS.

Results: Resting MEPs threshold, CMCT, CSPT were increased in ALS patients compared to controls. CSPD was significantly shortened in ALS group.

Conclusions: TMS findings reflect the cortical hyperexcitability in ALS.

Introduction

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive, fatal neurodegenerative disease of both upper and lower motor neurons. The clinical presentation of ALS may vary according to the combination of dysfunction in the upper and lower limbs (1). Although ALS is a well-known disease, its underlying pathophysiologic mechanisms remain to be elucidated. The diagnosis is confirmed with clinical and neurophysiologic demonstration of both lower motor neuron and upper neuron involvement. Transcranial magnetic stimulation (TMS) is a kind of non-invasive technique used to assess central motor pathways, networks, and its connections. TMS may trigger both upper and lower motor neuron pathways after activating the motor cortex. Central motor conduction time (CMCT), motor-evoked potential (MEP) thresholds, and silent period measures are the most useful parameters for identifying cortical excitability (2,3). However, silent period threshold detection has not been studied in ALS. In this study, we used TMS measurements as clinical correlates for the assessment of cortical function in patients with ALS.

Methods

1. Subjects:
Sixteen individuals who were diagnosed as having certain sporadic ALS and 11 healthy controls were enrolled in the study. All ALS patients fulfilled the revised Escorial criteria for definite ALS (4). The Amyotrophic Lateral Sclerosis Severity Score (ALSSS) and ALS Functional Rating Scale (ALSFR) were used in the assessments (5,6). A clinical evaluation was performed to all patients for detecting the mean disease duration. None of the patients was under medication. The exclusion criteria included the following: pure upper motor neuron disease, Kennedy's disease, neuroimaging studies showing diagnoses other than definite ALS, and patients receiving medication.

2. Cortical excitability parameters:
Subjects sat in a comfortable chair. Electromyography (EMG) recordings from bilateral abductor digiti minimi (ADM) muscles were acquired using silver-silver chloride surface electrodes, with a muscle belly-tendon set-up. A Keypoint (Denmark) activity monitor was used to collect the signal. Routine needle EMG was performed in all patients for the diagnosis of ALS to detect the lower motor neuron dysfunction.

Cortical excitability was assessed using a 90-mm circular coil oriented to induce current flow in a posterior anterior direction. The current was generated by a Mag Pro (Denmark) stimulator. The single-pulse TMS technique was used to determine the MEP amplitude (mV), MEP onset latency (ms), central motor conduction time (CMCT) (ms), the resting motor-evoked threshold (RMT) (%), cortical silent period duration (CSPD) (ms), and cortical silent period threshold (CSPT) (%).

Resting motor-evoked threshold (RMT): The coil was positioned at the vertex (Cz position) and adjusted in the anterior-posterior position until the optimal position for an evoked response was obtained from the ADM muscle. Records of MEPs were filtered (3Hz - 3 kHz) and amplified using a Keypoint, Denmark activity monitor. The threshold detection TMS technique was used as previously described (7). A target response of 0.2 mV (20%) MEPs were tracked. Resting motor-evoked potential (MEP) was defined as the stimulus intensity required to produce the target MEPs.

Central motor conduction time (CMCT): Central motor conduction time was calculated by subtracting the minimal cervical spinal root stimulation latency of the upper spine from the minimal latency achieved by cortical stimulation.

MEPs amplitudes: Amplitudes were calculated from baseline to the highest negative peak of the MEP responses at each visit for TMS.

Cortical silent period duration: The cortical silent period (CSP) was recorded whilst performing a weak voluntary contraction, and was calculated from the average 5 stimulus of 150% MEP response to the return of electromyography activity (8). The CSP duration was measured from onset of MEP to the return of EMG activity.

Cortical silent period threshold (CSPT): The cortical silent period threshold detection was elucidated by single-pulse TMS delivered to the optimal location starting at suprathreshold intensity and then decreasing in steps of 3% of the stimulator output. The cortical silent period threshold was defined as the minimal intensity required to elicit a visible silent period duration in five out of 10 consecutive trials. To achieve sufficient muscle activation, EMG was continuously observed, and the subject was provided with visual feedback. Stimulation intensity influenced the appearance of the silent period. The strength of the muscle contraction was adjusted by EMG monitoring to approximately 1 mV peak-to-peak.

3. Statistical analysis: Statistical analysis was performed using the Statistical Package for the Social Sciences (SPSS 16.0). Demographic data and neurophysiologic data were compared between the two groups using the independent sample t-test for normally distributed data. The Mann-Whitney U test was used to detect group differences in RMT, CSP duration, CSP threshold, CMCT, and CSP between patients and healthy subjects. All significance levels are reported as two-tailed, and the criterion for statistical significance was accepted as P < .05.

Results

1.1. Clinical Characteristics:
Sixteen patients with ALS (4 female, 12 male), with a mean age of 54.1±12.2 years (range, 31-79 years) and 11 (6 female, 5 male) healthy subjects with a mean age of 46.9±8.4 years (range, 33-57 years) with no known history of a neurologic disorder were included in the study. The median disease duration was 15.8±11.9 months (range, 6-48 months). Limb-onset disease was evident in 13 (81.3%) patients, and bulbar-onset disease was evident in 3 (18.7%) patients. The median ALSFRS-R score was 38.8±3.2, and the median ALSSS was 33.6±3.3 (Table 1).

Table 1: Demographic and clinical data of individuals

1.2. Cortical excitability parameters:
All cortical excitability measures are summarized in Table 2.

Table 2: Electrophysiological data of the patients and controls

Resting motor-evoked threshold: The resting motor threshold in patients with ALS was significantly greater than in the control group (56.7±14.3; 40.2±3.5, respectively; p< 0.001).

Cortical silent period duration: Patients with ALS had significantly reduced CSP duration compared with the control group (139.3 ±50.9; 200.6±28.5, respectively; p< 0.001).

Cortical silent period threshold: CSPT was significantly increased in patients with ALS (55.5±13.7; 40.9±2.4, respectively; p< 0.001).

Central motor conduction time (CMCT): There were no significant differences in onset latency (20.2±6.8; 21.01±1.2 respectively; p=0.62) between the groups. CMCT was significantly prolonged in patients with ALS compared with the controls (9.3±1.8; 7.9±1.8, respectively; p=0.019).

MEPs amplitudes: The CMAP amplitude was 1.06±0.8 mV in patients with ALS and 1.2±0.5 mV in the controls (p=0.064). Cortical MEPs were absent in two patients with ALS.

Discussion

Corticomotoneuronal system involvement in ALS seems to have positive signs of cortical hyperexcitability, which may be assessed using TMS techniques (9,2). The novel finding in the present study is that cortical silent period threshold measurements were at lower levels in patients with ALS than in healthy controls. However, previous studies did not use these measurements, which were not designed for cortical silent period threshold detection. Defining this neurophysiologic parameter may be useful for detecting ALS in early stages of the disease and may contribute to diagnostic success. In agreement with previous studies, the most prevalent excitability impairment was the resting MEP threshold, which showed significantly increased intensity in patients with ALS compared with the controls (7). It reflects the excitability of a central core of cortical neurons, and it also depends on the integrity of the corticospinal tract and spinal motor neurons (10). To the best of our knowledge, cortical silent period is absent or shortened in sporadic and familial ALS (11). Although the silent period is mediated by cortical inhibitory neurons, spinal mechanisms also suggested a major role of GABA b receptors in the generation of the silent period(12-16). GABA b receptor agonists and GABA reuptake inhibitors are believed to increase the cortical silent period duration (17,18). Accordingly, CMCT is typically prolonged in ALS (19,3). The attenuation or absence of cortical MEPs has been previously reported in patients with ALS (7). We also found that cortical MEPs were absent in two patients, and CMCT was significantly prolonged when compared with the healthy controls.

A potential limitation of our study is the sample size and disease heterogeneity. Future studies with larger series may demonstrate the specificity and sensitivity of this technique as a diagnostic tool in the early stages of the disease, its implication for the prognosis of the disease, and it may provide diagnostic biomarkers or function as a monitoring tool for physicians.

Received by: 28 July 2016
Revised by: 08 September 2016
Accepted: 13 October 2016

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