Impaired motor cortex inhibition in patients with amyotrophic lateral sclerosis. Evidence from paired transcranial magnetic stimulation

Physiological Biomarkers of Upper Motor Neuron Dysfunction in ALS

Upper motor neuron (UMN) dysfunction is an important feature of amyotrophic lateral sclerosis (ALS) for the diagnosis and understanding of pathogenesis. The identification of UMN signs forms the basis of ALS diagnosis, although may be difficult to discern, especially in the setting of severe muscle weakness. Transcranial magnetic stimulation (TMS) techniques have yielded objective physiological biomarkers of UMN dysfunction in ALS, enabling the interrogation of cortical and subcortical neuronal networks with diagnostic, pathophysiological, and prognostic implications. Transcranial magnetic stimulation techniques have provided pertinent pathogenic insights and yielded novel diagnostic and prognostic biomarkers. Cortical hyperexcitability, as heralded by a reduction in short interval intracortical inhibition (SICI) and an increase in short interval intracortical facilitation (SICF), has been associated with lower motor neuron degeneration, patterns of disease evolution, as well as the development of specific ALS clinical features including the split hand phenomenon. Reduction in SICI has also emerged as a potential diagnostic aid in ALS. More recently, physiological distinct inhibitory and facilitatory cortical interneuronal circuits have been identified, which have been shown to contribute to ALS pathogenesis. The triple stimulation technique (TST) was shown to enhance the diagnostic utility of conventional TMS measures in detecting UMN dysfunction. Resting-state EEG is a novel neurophysiological technique developed for directly interrogating cortical neuronal networks in ALS, that have yielded potentially useful physiological biomarkers of UMN dysfunction. The present review discusses physiological biomarkers of UMN dysfunction in ALS, encompassing conventional and novel TMS techniques developed to interrogate the functional integrity of the corticomotoneuronal system, focusing on pathogenic, diagnostic, and prognostic utility.

Distinct neuronal circuits mediate cortical hyperexcitability in amyotrophic lateral sclerosis

Cortical hyperexcitability is an important pathophysiological mechanism in amyotrophic lateral sclerosis (ALS), reflecting a complex interaction of inhibitory and facilitatory interneuronal processes that evolves in the degenerating brain. The advances in physiological techniques have made it possible to interrogate progressive changes in the motor cortex. Specifically, the direction of transcranial magnetic stimulation (TMS) stimulus within the primary motor cortex can be utilized to influence descending corticospinal volleys and to thereby provide information about distinct interneuronal circuits. Cortical motor function and cognition was assessed in 29 ALS patients with results compared to healthy volunteers. Cortical dysfunction was assessed using threshold-tracking TMS to explore alterations in short interval intracortical inhibition (SICI), short interval intracortical facilitation (SICF), the index of excitation and stimulus response curves using a figure-of-eight coil with the coil oriented relative to the primary motor cortex in a posterior-anterior, lateral-medial and anterior-posterior direction. Mean SICI, between interstimulus interval of 1-7 ms, was significantly reduced in ALS patients compared to healthy controls when assessed with the coil oriented in posterior-anterior (P = 0.044) and lateral-medial (P = 0.005) but not the anterior-posterior (P = 0.08) directions. A significant correlation between mean SICI oriented in a posterior-anterior direction and the total Edinburgh Cognitive and Behavioural ALS Screen score (Rho = 0.389, P = 0.037) was evident. In addition, the mean SICF, between interstimulus interval 1-5 ms, was significantly increased in ALS patients when recorded with TMS coil oriented in posterior-anterior (P = 0.035) and lateral-medial (P < 0.001) directions. In contrast, SICF recorded with TMS coil oriented in the anterior-posterior direction was comparable between ALS and controls (P = 0.482). The index of excitation was significantly increased in ALS patients when recorded with the TMS coil oriented in posterior-anterior (P = 0.041) and lateral-medial (P = 0.003) directions. In ALS patients, a significant increase in the stimulus response curve gradient was evident compared to controls when recorded with TMS coil oriented in posterior-anterior (P < 0.001), lateral-medial (P < 0.001) and anterior-posterior (P = 0.002) directions. The present study has established that dysfunction of distinct interneuronal circuits mediates the development of cortical hyperexcitability in ALS. Specifically, complex interplay between inhibitory circuits and facilitatory interneuronal populations, that are preferentially activated by stimulation in posterior-to-anterior or lateral-to-medial directions, promotes cortical hyperexcitability in ALS. Mechanisms that underlie dysfunction of these specific cortical neuronal circuits will enhance understanding of the pathophysiological processes in ALS, with the potential to uncover focussed therapeutic targets.

Novel approaches to assessing upper motor neuron dysfunction in motor neuron disease/amyotrophic lateral sclerosis: IFCN handbook chapter

Identifying upper motor neuron (UMN) dysfunction is fundamental to the diagnosis and understanding of disease pathogenesis in motor neuron disease (MND). The clinical assessment of UMN dysfunction may be difficult, particularly in the setting of severe muscle weakness. From a physiological perspective, transcranial magnetic stimulation (TMS) techniques provide objective biomarkers of UMN dysfunction in MND and may also be useful to interrogate cortical and network function. Single, paired- and triple pulse TMS techniques have yielded novel diagnostic and prognostic biomarkers in MND, and have provided important pathogenic insights, particularly pertaining to site of disease onset. Cortical hyperexcitability, as heralded by reduced short interval intracortical inhibition (SICI) and increased short interval intracortical facilitation, has been associated with the onset of lower motor neuron degeneration, along with patterns of disease spread, development of specific clinical features such as the split hand phenomenon, and may provide an indication about the rate of disease progression. Additionally, reduction of SICI has emerged as a potential diagnostic aid in MND. The triple stimulation technique (TST) was shown to enhance the diagnostic utility of conventional TMS measures in detecting UMN dysfunction in MND. Separately, sophisticated brain imaging techniques have uncovered novel biomarkers of neurodegeneration that have bene associated with progression. The present review will discuss the utility of TMS and brain neuroimaging derived biomarkers of UMN dysfunction in MND, focusing on recently developed TMS techniques and advanced neuroimaging modalities that interrogate structural and functional integrity of the corticomotoneuronal system, with an emphasis on pathogenic, diagnostic, and prognostic utility.

Reduction in short interval intracortical inhibition from the early stage reflects the pathophysiology in amyotrophic lateral sclerosis: A meta-analysis study

BACKGROUND AND PURPOSE

Cortical hyperexcitability has been identified as a diagnostic and pathogenic biomarker of amyotrophic lateral sclerosis (ALS). Cortical excitability is assessed by transcranial magnetic stimulation (TMS), a non-invasive neurophysiological technique. The TMS biomarkers exhibiting highest sensitivity for cortical hyperexcitability in ALS remain to be elucidated. A meta-analysis was performed to determine the TMS biomarkers exhibiting the highest sensitivity for cortical hyperexcitability in ALS.

METHODS

A systematic literature review was conducted of all relevant studies published in the English language by searching PubMed, MEDLINE, Embase and Scopus electronic databases from 1 January 2006 to 28 February 2023. Inclusion criteria included studies reporting the utility of threshold tracking TMS (serial ascending method) in ALS and controls.

RESULTS

In total, more than 2500 participants, incorporating 1530 ALS patients and 1102 controls (healthy, 907; neuromuscular, 195) were assessed with threshold tracking TMS across 25 studies. Significant reduction of mean short interval intracortical inhibition (interstimulus interval 1-7 ms) exhibited the highest standardized mean difference with moderate heterogeneity (-0.994, 95% confidence interval -1.12 to -0.873, p < 0.001; Q = 38.61, p < 0.05; I2 = 40%). The reduction of cortical silent period duration along with an increase in motor evoked potential amplitude and intracortical facilitation also exhibited significant, albeit smaller, standardized mean differences.

CONCLUSION

This large meta-analysis study disclosed that mean short interval intracortical inhibition reduction exhibited the highest sensitivity for cortical hyperexcitability in ALS. Combined findings from this meta-analysis suggest that research strategies aimed at understanding the cause of inhibitory interneuronal circuit dysfunction could enhance understanding of ALS pathogenesis.

Contribution of neurophysiology to the diagnosis and monitoring of ALS

This chapter describes the role of neurophysiological techniques in diagnosing and monitoring amyotrophic lateral sclerosis (ALS). Despite many advances, electromyography (EMG) remains a keystone investigation from which to build support for a diagnosis of ALS, demonstrating the pathophysiological processes of motor unit hyperexcitability, denervation and reinnervation. We consider development of the different diagnostic criteria and the role of EMG therein. While not formally recognised by established diagnostic criteria, we discuss the pioneering studies that have demonstrated the diagnostic potential of transcranial magnetic stimulation (TMS) of the motor cortex and highlight the growing evidence for TMS in the diagnostic process. Finally, accurately monitoring disease progression is crucial for the successful implementation of clinical trials. Neurophysiological measures of disease state have been incorporated into clinical trials for over 20 years and we review prominent techniques for assessing disease progression.

Current perspectives on neuromodulation in ALS patients: A systematic review and meta-analysis

Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disease that affects motor neurons, resulting in muscle weakness, paralysis, and eventually patient mortality. In recent years, neuromodulation techniques have emerged as promising potential therapeutic approaches to slow disease progression and improve the quality of life of ALS patients. A systematic review was conducted until August 8, 2023, to evaluate the neuromodulation methods used and their potential in the treatment of ALS. The search strategy was applied in the Cochrane Central database, incorporating results from other databases such as PubMed, Embase, CTgov, CINAHL, and ICTRP. Following the exclusion of papers that did not fulfil the inclusion criteria, a total of 2090 records were found, leaving a total of 10 studies. R software was used to conduct meta-analyses based on the effect sizes between the experimental and control groups. This revealed differences in muscle stretch measures with manual muscle testing (p = 0.012) and resting motor threshold (p = 0.0457), but not with voluntary isometric contraction (p = 0.1883). The functionality of ALS was also different (p = 0.007), but not the quality of life. Although intracortical facilitation was not seen in motor cortex 1 (M1) (p = 0.1338), short-interval intracortical inhibition of M1 was significant (p = 0.0001). BDNF showed no differences that were statistically significant (p = 0.2297). Neuromodulation-based treatments are proposed as a promising therapeutic approach for ALS that can produce effects on muscle function, spasticity, and intracortical connections through electrical, magnetic, and photonic stimulation. Photobiomodulation stands out as an innovative approach that uses specific wavelengths to influence mitochondria, with the aim of improving mitochondrial function and reducing excitotoxicity. The lack of reliable placebo controls and the variation in stimulation frequency are some of the drawbacks of neuromodulation.

Examining short interval intracortical inhibition with different transcranial magnetic stimulation-induced current directions in ALS

Objective

To establish if induced current direction across the motor cortex alters the sensitivity of transcranial magnetic stimulation (TMS)-evoked short-interval intracortical inhibition (SICI) as an ALS biomarker.

Methods

Threshold tracking-TMS was undertaken in 35 people with ALS and 39 controls. Using a coil orientation which induces posterior-anterior (PA)-directed current across the motor cortex, SICI (1 ms and 3 ms interstimulus intervals) and intracortical facilitation (ICF, 10 ms interstimulus interval) were recorded. SICI3ms was also recorded using a coil orientation which induces anterior-posterior (AP)-directed current across the motor cortex.

Results

At group level, SICI3ms-PA (AUROC = 0.7), SICI3ms-AP (AUROC = 0.8) and SICI1ms (AUROC = 0.66) were substantially lower in those with ALS, although there was considerable interindividual heterogeneity. Averaging across interstimulus intervals (ISIs) marginally improved SICIPA sensitivity (AUROC = 0.76). Averaging SICI values across ISIs and orientations into a single SICI measure did not substantially improve sensitivity (AUROC = 0.81) compared to SICI3ms-AP alone. SICI3ms-AP and SICI3ms-PA did not significantly correlate (rho = 0.19, p = 0.313), while SICI1ms-PA and SICI3ms-PA did (rho = 0.37, p = 0.006). Further, those with ALS with the lowest SICI3ms-PA were not those with the lowest SICI3ms-AP. ICF was similar between groups (AUROC = 0.50).

Conclusions

SICIPA and SICIAP are uncorrelated measures of motor cortical inhibitory functions which are useful as distinct, unequally affected, measures of disinhibition in ALS.

Significance

Examining both SICIPA and SICIAP may facilitate more comprehensive characterisation of motor cortical disinhibition in ALS.

Caffeine and cortical excitability, as measured with paired-pulse transcranial magnetic stimulation

INTRODUCTION/AIMS

The transcranial magnetic stimulation tests of short-interval intracortical inhibition (SICI) by both conventional amplitude measurements (A-SICI) and threshold-tracking (T-SICI) are important methods to investigate intracortical inhibitory circuits, and T-SICI has been proposed to aid the diagnosis of amyotrophic lateral sclerosis. Beverages containing caffeine are widely consumed, and caffeine has been reported to affect cortical excitability. The aim of this study was to determine whether these SICI tests are affected by caffeine.

METHODS

Twenty-four healthy subjects (13 females, 11 males, aged from 19 to 31, mean: 26.2 ± 2.4 years) were studied in a single fixed-dose randomized double-blind placebo-controlled cross-over trial of 200 mg caffeine or placebo ingested as chewing gum. A-SICI and T-SICI, using parallel tracking (T-SICIp), were performed before and after chewing gum.

RESULTS

There was no significant change in SICI parameters after placebo in A-SICI (p > .10) or T-SICIp (p > .30), and no significant effect of caffeine was found on A-SICI (p > .10) or T-SICIp (p > .50) for any of the interstimulus intervals.

DISCUSSION

There is no need for caffeine abstention before measurements of SICI by either the T-SICI or A-SICI measurements.

Novel approaches to motoneuron disease/ALS treatment using non-invasive brain and spinal stimulation: IFCN handbook chapter

Non-invasive brain stimulation techniques have been exploited in motor neuron disease (MND) with multifold objectives: to support the diagnosis, to get insights in the pathophysiology of these disorders and, more recently, to slow down disease progression. In this review, we consider how neuromodulation can now be employed to treat MND, with specific attention to amyotrophic lateral sclerosis (ALS), the most common form with upper motoneuron (UMN) involvement, taking into account electrophysiological abnormalities revealed by human and animal studies that can be targeted by neuromodulation techniques. This review article encompasses repetitive transcranial magnetic stimulation methods (including low-frequency, high-frequency, and pattern stimulation paradigms), transcranial direct current stimulation as well as experimental findings with the newer approach of trans-spinal direct current stimulation. We also survey and discuss the trials that have been performed, and future perspectives.

Clinical diagnostic utility of transcranial magnetic stimulation in neurological disorders. Updated report of an IFCN committee

The review provides a comprehensive update (previous report: Chen R, Cros D, Curra A, Di Lazzaro V, Lefaucheur JP, Magistris MR, et al. The clinical diagnostic utility of transcranial magnetic stimulation: report of an IFCN committee. Clin Neurophysiol 2008;119(3):504-32) on clinical diagnostic utility of transcranial magnetic stimulation (TMS) in neurological diseases. Most TMS measures rely on stimulation of motor cortex and recording of motor evoked potentials. Paired-pulse TMS techniques, incorporating conventional amplitude-based and threshold tracking, have established clinical utility in neurodegenerative, movement, episodic (epilepsy, migraines), chronic pain and functional diseases. Cortical hyperexcitability has emerged as a diagnostic aid in amyotrophic lateral sclerosis. Single-pulse TMS measures are of utility in stroke, and myelopathy even in the absence of radiological changes. Short-latency afferent inhibition, related to central cholinergic transmission, is reduced in Alzheimer's disease. The triple stimulation technique (TST) may enhance diagnostic utility of conventional TMS measures to detect upper motor neuron involvement. The recording of motor evoked potentials can be used to perform functional mapping of the motor cortex or in preoperative assessment of eloquent brain regions before surgical resection of brain tumors. TMS exhibits utility in assessing lumbosacral/cervical nerve root function, especially in demyelinating neuropathies, and may be of utility in localizing the site of facial nerve palsies. TMS measures also have high sensitivity in detecting subclinical corticospinal lesions in multiple sclerosis. Abnormalities in central motor conduction time or TST correlate with motor impairment and disability in MS. Cerebellar stimulation may detect lesions in the cerebellum or cerebello-dentato-thalamo-motor cortical pathways. Combining TMS with electroencephalography, provides a novel method to measure parameters altered in neurological disorders, including cortical excitability, effective connectivity, and response complexity.

Establishing the Minimum Intensity of Transcranial Magnetic Stimulation Superconditioning Pulses to Effect Inhibition and Facilitation of Motor Evoked Potentials

PURPOSE

Previously, we showed that a three-pulse train of weak transcranial magnetic stimulation (TMS) pulses-a superconditioning (SC) train-when followed by a stronger TMS pulse could enhance the inhibition or facilitation of the resultant motor evoked potential (MEP) compared with that seen with traditional dual-pulse inputs. The purpose of the present study was to establish the relative minimum intensity of SC pulses needed to influence MEP output and whether this differed for upper- versus lower-limb muscles.

METHODS

We examined 33 older adult subjects, targeting abductor pollicis brevis and tibialis anterior muscles. Older subjects were included in the anticipation of using findings from this study to guide further studies in persons with amyotrophic lateral sclerosis. Three-pulse trains of SC inputs of different intensities were delivered either 1 millisecond before (for inhibition) or 10 millisecond before (for facilitation) a stronger TMS test pulse. Motor evoked potential magnitudes for SC +test sets were normalized to test input responses and were compared within and between subjects.

RESULTS AND CONCLUSIONS

For inhibition, the minimum intensity of SC pulses needed to influence the follow-on MEP was found to be 60% of the target muscle's resting three-pulse MEP threshold for most abductor pollicis brevis and tibialis anterior muscles (2-millisecond interpulse intervals). For facilitation, somewhat higher intensities (70%) were typically needed to cause facilitation. Both values of SC pulses for inhibition/facilitation are considerably lower than the intensity of the conditioning pulse-often reported as 80% of the single-pulse threshold-typically used in dual-pulse TMS paradigms. This approach may allow testing of upper motor neuron function using weaker stimulus pulse intensities than are typically employed, improving testing compliance in persons whose thresholds are elevated because of injury or disease.

Different saccadic profile in bulbar versus spinal-onset amyotrophic lateral sclerosis

Two clinical phenotypes characterize the onset of amyotrophic lateral sclerosis (ALS): the spinal variant, with symptoms beginning in the limbs, and the bulbar variant, affecting firstly speech and swallowing. The two variants show some distinct features in the histopathology, localization and prognosis, but to which extent they really differ clinically and pathologically remains to be clarified. Recent neuropathological and neuroimaging studies have suggested a broader spreading of the neurodegenerative process in ALS, extending beyond the motor areas, toward other cortical and deep grey matter regions, many of which are involved in visual processing and saccadic control. Indeed, a wide range of eye movement deficits have been reported in ALS, but they have never been used to distinguish the two ALS variants. Since quantifying eye movements is a very sensitive and specific method for the study of brain networks, we compared different saccadic and visual search behaviours across spinal ALS patients (n = 12), bulbar ALS patients (n = 6) and healthy control subjects (n = 13), along with cognitive and MRI measures, with the aim to define more accurately the two patients subgroups and possibly clarify a different underlying neural impairment. We found separate profiles of visually-guided saccades between spinal (short saccades) and bulbar (slow saccades) ALS, which could result from the pathologic involvement of different pathways. We suggest an early involvement of the parieto-collicular-cerebellar network in spinal ALS and the fronto-brainstem circuit in bulbar ALS. Overall, our data confirm the diagnostic value of the eye movements analysis in ALS and add new insight on the involved neural networks.

Modulation of spontaneous motor unit potentials by a new motor cortical magnetic stimulation method in amyotrophic lateral sclerosis

BACKGROUND

Patients with amyotrophic lateral sclerosis (ALS) show altered cortical excitability. In this study, we measure modulation of spontaneous motor unit potentials (sMUPs) in hand muscles by multifocal cortical stimulation with a newly developed wearable transcranial rotating permanent magnet stimulator (TRPMS).

METHODS

We conducted cross-sectional and longitudinal electromyographic assessments in 40 and 20 ALS patients, respectively, of the stimulation-induced peak increase in the count of sMUPs in two hand muscles modulated by unilateral TRPMS stimulation of the primary motor cortex. We measured peak sMUP counts during several short sessions consisting of 10 stimuli over 60 s and 30 s post-stimulation periods. The longitudinal component involved an initial assessment at an early stage of the disease and up to five follow-up assessments at least 3 months apart.

RESULTS

TRPMS stimulation produced no device-related adverse effects. It showed an inverted V-shaped modulation of the peak sMUP counts as a function of ALS functional rating scale revised scores. The ratios of ALS subjects showing peak sMUP count increases between early and intermediate stages (χ2 = 4.086, df = 1, p = 0.043) and intermediate and late stages (χ2 = 4.29, df = 1, p = 0.038) in cross-sectional data were significantly different. Longitudinal assessment also produced a significant (z = 2.31, p = 0.021) result, with all subjects showing a post-initial visit increase in peak sMUP counts.

CONCLUSIONS

These results are consistent with delayed onset of upper motor neuronal dysfunction with respect to onset of clinical features. However, the above results need to be confirmed in a larger sample of patients and with multiple lines of evidence.

Assessment of cortical inhibition depends on inter individual differences in the excitatory neural populations activated by transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is used to probe inhibitory intracortical neurotransmission and has been used to infer the neurobiological dysfunction that may underly several neurological disorders. One technique, short-interval intracortical inhibition (SICI), indexes gamma-aminobutyric acid (GABA) mediated inhibitory activity and is a promising biomarker. However emerging evidence suggests SICI does not exclusively represent GABAergic activity because it may be influenced by inter-individual differences in the specific excitatory neural populations activated by TMS. Here we used the latency of TMS motor evoked potentials (MEPs) to index these inter-individual differences, and found that a significant proportion of the observed variability in SICI magnitude was accounted for by MEP latency, r = − 0.57, r² = 0.33, p = .014. We conclude that SICI is influenced by inter-individual differences in the excitatory neural populations activated by TMS, reducing the precision of this GABAergic probe. Interpreting SICI measures in the context of MEP latency may facilitate a more precise assessment of GABAergic intracortical inhibition. The reduced cortical inhibition observed in some neuropathologies could be influenced by reduced activity in specific excitatory neural populations. Including MEP latency assessment in research investigating SICI in clinical groups could assist in differentiating the cortical circuits impacted by neurological disorders.

Cortical Hyperexcitability in the Driver’s Seat in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by the degeneration of cortical and spinal motor neurons. With no effective treatment available to date, patients face progressive paralysis and eventually succumb to the disease due to respiratory failure within only a few years. Recent research has revealed the multifaceted nature of the mechanisms and cell types involved in motor neuron degeneration, thereby opening up new therapeutic avenues. Intriguingly, two key features present in both ALS patients and rodent models of the disease are cortical hyperexcitability and hyperconnectivity, the mechanisms of which are still not fully understood. We here recapitulate current findings arguing for cell autonomous and non-cell autonomous mechanisms causing cortical excitation and inhibition imbalance, which is involved in the degeneration of motor neurons in ALS. Moreover, we will highlight recent evidence that strongly indicates a cardinal role for the motor cortex as a main driver and source of the disease, thus arguing for a corticofugal trajectory of the pathology.

Short interval intracortical inhibition: Variability of amplitude and threshold-tracking measurements with 6 or 10 stimuli per point

Reduced short-interval intracortical inhibition (SICI) in motor neuron disease has been demonstrated by amplitude changes (A-SICI) and threshold-tracking (T-SICI) using 10 stimuli per inter-stimulus interval (ISI). To test whether fewer stimuli would suffice, A-SICI and T-SICI were recorded twice from 30 healthy subjects using 6 and 10 stimuli per ISI. Using fewer stimuli increased mean A-SICI variances by 23.8% but the 7.3% increase in T-SICI variance was not significant. We conclude that our new parallel threshold-tracking SICI protocol, with 6 stimuli per ISI, can reduce time and stimulus numbers by 40% without appreciable loss of accuracy.

Bradykinesia in motoneuron diseases

OBJECTIVE

Only few studies investigated voluntary movement abnormalities in patients with motoneuron diseases (MNDs) or their neurophysiological correlates. We aimed to kinematically assess finger tapping abnormalities in patients with amyotrophic lateral sclerosis (ALS) and primary lateral sclerosis (PLS), as compared to healthy controls (HCs), and their relationship with motoneuron involvement.

METHODS

Fourteen ALS and 5 PLS patients were enrolled. Finger tapping was assessed by a motion analysis system. Patients underwent a central motor conduction time assessment, a motor nerve conduction study, and needle electromyography. Data were compared to those of 79 HCs using non-parametric tests. Possible relationships between clinical, kinematic, and neurophysiological data were assessed in patients.

RESULTS

As a major finding, ALS and PLS patients performed finger tapping slower than HCs. In both conditions, movement slowness correlated with muscle strength. In ALS, movement slowness also correlated with the amplitude of the compound muscle action potential recorded from the muscles involved in the task and with denervation activity. No correlations were found between slowness, measures of upper motoneuron involvement, and other clinical and neurophysiological data.

CONCLUSIONS

This study provides novel information on voluntary movement abnormalities in MNDs.

SIGNIFICANCE

The results highlight the pathophysiological role of motoneurons in generating movement slowness.

Early diagnosis of amyotrophic lateral sclerosis by threshold tracking and conventional transcranial magnetic stimulation

BACKGROUND AND PURPOSE

Short-interval intracortical inhibition by threshold tracking (T-SICI) has been proposed as a diagnostic tool for amyotrophic lateral sclerosis (ALS) but has not been compared directly with conventional amplitude measurements (A-SICI). This study compared A-SICI and T-SICI for sensitivity and clinical usefulness as biomarkers for ALS.

METHODS

In all, 104 consecutive patients referred with suspicion of ALS were prospectively included and were subsequently divided into 62 patients with motor neuron disease (MND) and 42 patient controls (ALS mimics) by clinical follow-up. T-SICI and A-SICI recorded in the first dorsal interosseus muscle (index test) were compared with recordings from 53 age-matched healthy controls. The reference standard was the Awaji criteria. Clinical scorings, conventional nerve conduction studies and electromyography were also performed on the patients.

RESULTS

Motor neuron disease patients had significantly reduced T-SICI and A-SICI compared with the healthy and patient control groups, which were similar. Sensitivity and specificity for discriminating MND patients from patient controls were high (areas under the receiver operating characteristic curves 0.762 and 0.810 for T-SICI and A-SICI respectively at 1-3.5 ms). Paradoxically, T-SICI was most reduced in MND patients with the fewest upper motor neuron (UMN) signs (Spearman ρ = 0.565, p = 4.3 × 10^-6 ).

CONCLUSIONS

Amplitude-based measure of cortical inhibition and T-SICI are both sensitive measures for the detection of cortical involvement in MND patients and may help early diagnosis of ALS, with T-SICI most abnormal before UMN signs have developed. The gradation in T-SICI from pathological facilitation in patients with minimal UMN signs to inhibition in those with the most UMN signs may be due to progressive degeneration of the subset of UMNs experiencing facilitation.

Enlarged high frequency oscillations of the median nerve somatosensory evoked potential and survival in amyotrophic lateral sclerosis

OBJECTIVE

A large N20 and P25 of the median nerve somatosensory evoked potential (SEP) predicts short survival in amyotrophic lateral sclerosis (ALS). We investigated whether high frequency oscillations (HFOs) over N20 are enlarged and associated with survival in ALS.

METHODS

A total of 145 patients with ALS and 57 healthy subjects were studied. We recorded the median nerve SEP and measured the onset-to-peak amplitude of N20 (N20o-p), and peak-to-peak amplitude between N20 and P25 (N20p-P25p). We obtained early and late HFO potentials by filtering SEP between 500 and 1 kHz, and measured the peak-to-peak amplitude. We followed up patients until endpoints (death or tracheostomy) and analyzed the relationship between SEP or HFO amplitudes and survival using a Cox analysis.

RESULTS

Patients showed larger N20o-p, N20p-P25p, and early and late HFO amplitudes than the control values. N20p-P25p was associated with survival periods (p = 0.0004), while early and late HFO amplitudes showed no significant association with survival (p = 0.4307, and p = 0.6858, respectively).

CONCLUSIONS

The HFO amplitude in ALS is increased, but does not predict survival.

SIGNIFICANCE

The enlarged HFOs in ALS might be a compensatory phenomenon to the hyperexcitability of the sensory cortex pyramidal neurons.

The Upper Motor Neuron-Improved Knowledge from ALS and Related Clinical Disorders

Upper motor neuron (UMN) is a term traditionally used for the corticospinal or pyramidal tract neuron synapsing with the lower motor neuron (LMN) in the anterior horns of the spinal cord. The upper motor neuron controls resting muscle tone and helps initiate voluntary movement of the musculoskeletal system by pathways which are not completely understood. Dysfunction of the upper motor neuron causes the classical clinical signs of spasticity, weakness, brisk tendon reflexes and extensor plantar response, which are associated with clinically well-recognised, inherited and acquired disorders of the nervous system. Understanding the pathophysiology of motor system dysfunction in neurological disease has helped promote a greater understanding of the motor system and its complex cortical connections. This review will focus on the pathophysiology underlying progressive dysfunction of the UMN in amyotrophic lateral sclerosis and three other related adult-onset, progressive neurological disorders with prominent UMN signs, namely, primary lateral sclerosis, hereditary spastic paraplegia and primary progressive multiple sclerosis, to help promote better understanding of the human motor system and, by extension, related cortical systems.

Cortical Excitability across the ALS Clinical Motor Phenotypes

Amyotrophic lateral sclerosis (ALS) is characterized by its marked clinical heterogeneity. Although the coexistence of upper and lower motor neuron signs is a common clinical feature for most patients, there is a wide range of atypical motor presentations and clinical trajectories, implying a heterogeneity of underlying pathogenic mechanisms. Corticomotoneuronal dysfunction is increasingly postulated as the harbinger of clinical disease, and neurophysiological exploration of the motor cortex in vivo using transcranial magnetic stimulation (TMS) has suggested that motor cortical hyperexcitability may be a critical pathogenic factor linked to clinical features and survival. Region-specific selective vulnerability at the level of the motor cortex may drive the observed differences of clinical presentation across the ALS motor phenotypes, and thus, further understanding of phenotypic variability in relation to cortical dysfunction may serve as an important guide to underlying disease mechanisms. This review article analyses the cortical excitability profiles across the clinical motor phenotypes, as assessed using TMS, and explores this relationship to clinical patterns and survival. This understanding will remain essential to unravelling central disease pathophysiology and for the development of specific treatment targets across the ALS clinical motor phenotypes.

Comparison of figure-of-8 and circular coils for threshold tracking transcranial magnetic stimulation measurements

OBJECTIVES

The transcranial magnetic stimulation (TMS) technique of threshold-tracking short-interval intracortical inhibition (T-SICI) has been proposed as a diagnostic tool for amyotrophic lateral sclerosis (ALS). Most of these studies have used a circular coil, whereas a figure-of-8 coil is usually recommended for paired-pulse TMS measurements. The aim of this study was to compare figure-of-8 and circular coils for T-SICI in the upper limb, with special attention to reproducibility, and the pain or discomfort experienced by the subjects.

METHODS

Twenty healthy subjects (aged: 45.5 ± 6.7, mean ± SD, 9 females, 11 males) underwent two examinations with each coil, in morning and afternoon sessions on the same day, with T-SICI measured at interstimulus intervals (ISIs) from 1-7 ms. After each examination the subjects rated degree of pain/discomfort from 0 to 10 using a numerical rating scale (NRS).

RESULTS

Mean T-SICI was higher for the figure-of-8 than for the circular coil at ISI of 2 ms (p < 0.05) but did not differ at other ISIs. Intra-subject variability did not differ between coils, but mean inhibition from 1-3.5 ms was less variable between subjects with the figure-of-8 coil (SD 7.2% vs. 11.2% RMT, p < 0.05), and no such recordings were without inhibition (vs. 6 with the circular coil). The subjects experienced less pain/discomfort with the figure-of-8 coil (mean NRS: 1.9 ± 1.28 vs 2.8 ± 1.60, p < 0.005).

DISCUSSION

The figure-of-8 coil may have better applicability in patients, due to the lower incidence of lack of inhibition in healthy subjects, and the lower experience of pain or discomfort.

Effects of mexiletine on hyperexcitability in sporadic amyotrophic lateral sclerosis: Preliminary findings from a small phase II randomized controlled trial

BACKGROUND

To collect preliminary data on the effects of mexiletine on cortical and axonal hyperexcitability in sporadic amyotrophic lateral sclerosis (ALS) in a phase 2 double-blind randomized controlled trial.

METHODS

Twenty ALS subjects were randomized to placebo and mexiletine 300 or 600 mg daily for 4 wk and assessed by transcranial magnetic stimulation and axonal excitability studies. The primary endpoint was change in resting motor threshold (RMT).

RESULTS

RMT was unchanged with 4 wk of mexiletine (combined active therapies) as compared to placebo, which showed a significant increase (P = .039). Reductions of motor evoked potential (MEP) amplitude (P = .013) and accommodation half-time (P = .002), secondary outcome measures of cortical and axonal excitability, respectively, were also evident at 4 wk on mexiletine.

CONCLUSIONS

The relative stabilization of RMT in the treated subjects was unexpected and could be attributed to unaccounted sources of error or chance. However, a possible alternative cause is neuromodulation preventing an increase. The change in MEP amplitude and accommodation half-time supports the reduction of cortical and axonal hyperexcitability with mexiletine.

Age-Dependent Vulnerability to Oxidative Stress of Postnatal Rat Pyramidal Motor Cortex Neurons

Oxidative stress is one of the main proposed mechanisms involved in neuronal degeneration. To evaluate the consequences of oxidative stress on motor cortex pyramidal neurons during postnatal development, rats were classified into three groups: Newborn (P2-P7); infantile (P11-P15); and young adult (P20-P40). Oxidative stress was induced by 10 µM of cumene hydroperoxide (CH) application. In newborn rats, using the whole cell patch-clamp technique in brain slices, no significant modifications in membrane excitability were found. In infantile rats, the input resistance increased and rheobase decreased due to the blockage of GABAergic tonic conductance. Lipid peroxidation induced by CH resulted in a noticeable increase in protein-bound 4-hidroxynonenal in homogenates in only infantile and young adult rat slices. Interestingly, homogenates of newborn rat brain slices showed the highest capacity to respond to oxidative stress by dramatically increasing their glutathione and free thiol content. This increase correlated with a time-dependent increase in the glutathione reductase activity, suggesting a greater buffering capacity of newborn rats to resist oxidative stress. Furthermore, pre-treatment of the slices with glutathione monoethyl ester acted as a neuroprotector in pyramidal neurons of infantile rats. We conclude that during maturation, the vulnerability to oxidative stress in rat motor neurons increases with age.

How Degeneration of Cells Surrounding Motoneurons Contributes to Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder characterized by the progressive degeneration of upper and lower motoneurons. Despite motoneuron death being recognized as the cardinal event of the disease, the loss of glial cells and interneurons in the brain and spinal cord accompanies and even precedes motoneuron elimination. In this review, we provide striking evidence that the degeneration of astrocytes and oligodendrocytes, in addition to inhibitory and modulatory interneurons, disrupt the functionally coherent environment of motoneurons. We discuss the extent to which the degeneration of glial cells and interneurons also contributes to the decline of the motor system. This pathogenic cellular network therefore represents a novel strategic field of therapeutic investigation.

Short-interval intracortical inhibition as a function of inter-stimulus interval: Three methods compared

BACKGROUND

Short-interval intracortical inhibition (SICI), as measured by threshold-tracking as a function of inter-stimulus interval (ISI), has been proposed as a useful biomarker for amyotrophic lateral sclerosis (ALS), but its relationship to conventional amplitude measurements has not been established.

METHODS

Serial tracking of SICI at increasing ISIs from 1 to 7 ms (T-SICIs) was compared in 50 healthy control subjects with the same ISIs tracked in parallel (T-SICIp), and with conventional amplitude measurements (A-SICI). For T-SICIp and A-SICI, pairs of conditioning and test stimuli with different ISIs were pseudo-randomised and interspersed with test-alone stimuli given at regular intervals. Thresholds were estimated by regression of log peak-to-peak amplitude on stimulus.

RESULTS

T-SICIp and A-SICI were closely related: a ten-fold reduction in amplitude corresponding to an approximately 18% increase in threshold. Threshold increases were greater for T-SICIs than for T-SICIp at 3.5-5 ms (P < 0.001). This divergence depended on the initial settings and whether ISIs were progressively increased or decreased, and was attributed to the limitations of the serial tracking protocol. SICI variability between subjects was greatest for T-SICIs estimates and least for A-SICI, and only A-SICI estimates revealed a significant decline in inhibition with age.

CONCLUSIONS

The serial tracking protocol did not accurately show the dependence of inhibition on ISI. Randomising ISIs gives corresponding SICI measures, whether tracking thresholds or measuring amplitude measurements. SICI variability suggested that A-SICI measurements may be the most sensitive to loss of inhibition.

Exciting Complexity: The Role of Motor Circuit Elements in ALS Pathophysiology

Amyotrophic lateral sclerosis (ALS) is a fatal disease, characterized by the degeneration of both upper and lower motor neurons. Despite decades of research, we still to date lack a cure or disease modifying treatment, emphasizing the need for a much-improved insight into disease mechanisms and cell type vulnerability. Altered neuronal excitability is a common phenomenon reported in ALS patients, as well as in animal models of the disease, but the cellular and circuit processes involved, as well as the causal relevance of those observations to molecular alterations and final cell death, remain poorly understood. Here, we review evidence from clinical studies, cell type-specific electrophysiology, genetic manipulations and molecular characterizations in animal models and culture experiments, which argue for a causal involvement of complex alterations of structure, function and connectivity of different neuronal subtypes within the cortical and spinal cord motor circuitries. We also summarize the current knowledge regarding the detrimental role of astrocytes and reassess the frequently proposed hypothesis of glutamate-mediated excitotoxicity with respect to changes in neuronal excitability. Together, these findings suggest multifaceted cell type-, brain area- and disease stage- specific disturbances of the excitation/inhibition balance as a cardinal aspect of ALS pathophysiology.

The Electrophysiological Determinants of Corticospinal Motor Neuron Vulnerability in ALS

The brain is complex and heterogeneous. Even though numerous independent studies indicate cortical hyperexcitability as a potential contributor to amyotrophic lateral sclerosis (ALS) pathology, the mechanisms that are responsible for upper motor neuron (UMN) vulnerability remain elusive. To reveal the electrophysiological determinants of corticospinal motor neuron (CSMN, a.k.a UMN in mice) vulnerability, we investigated the motor cortex of hSOD1^G93A mice at P30 (postnatal day 30), a presymptomatic time point. Glutamate uncaging by laser scanning photostimulation (LSPS) revealed altered dynamics especially within the inhibitory circuitry and more specifically in L2/3 of the motor cortex, whereas the excitatory microcircuits were unchanged. Observed microcircuitry changes were specific to CSMN in the motor column. Electrophysiological evaluation of the intrinsic properties in response to the microcircuit changes, as well as the exon microarray expression profiles of CSMN isolated from hSOD1^G93A and healthy mice at P30, revealed the presence of a very dynamic set of events, ultimately directed to establish, maintain and retain the balance at this early stage. Also, the expression profile of key voltage-gated potassium and sodium channel subunits as well as of the inhibitory GABA receptor subunits and modulatory proteins began to suggest the challenges CSMN face at this early age. Since neurodegeneration is initiated when neurons can no longer maintain balance, the complex cellular events that occur at this critical time point help reveal how CSMN try to cope with the challenges of disease manifestation. This information is critically important for the proper modulation of UMNs and for developing effective treatment strategies.

Cortical Circuit Dysfunction as a Potential Driver of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease that affects selected cortical and spinal neuronal populations, leading to progressive paralysis and death. A growing body of evidences suggests that the disease may originate in the cerebral cortex and propagate in a corticofugal manner. In particular, transcranial magnetic stimulation studies revealed that ALS patients present with early cortical hyperexcitability arising from a combination of increased excitability and decreased inhibition. Here, we discuss the possibility that initial cortical circuit dysfunction might act as the main driver of ALS onset and progression, and review recent functional, imaging and transcriptomic studies conducted on ALS patients, along with electrophysiological, pathological and transcriptomic studies on animal and cellular models of the disease, in order to evaluate the potential cellular and molecular origins of cortical hyperexcitability in ALS.

Cortical hyperexcitability evolves with disease progression in ALS

Objective

Cortical hyperexcitability has been established as an early feature of amyotrophic lateral sclerosis (ALS). The evolution of cortical hyperexcitability with ALS progression remains to be fully elucidated. This study aims to investigate changes in cortical function in ALS with disease progression.

Methods

Cortical function assessed by threshold tracking transcranial magnetic stimulation (TMS) along with clinical phenotyping was prospectively undertaken on 444 patients presenting with suspected ALS (345 ALS; 99 neuromuscular mimics). Disease stage was defined as follows: (1) King’s clinical staging system and (2) proportion of disease duration statistically categorized into tertials.

Results

Cortical hyperexcitability was evident across all ALS stages, being more prominent in later stages of ALS as indicated by increased motor‐evoked potential amplitude (P < 0.05), as well as longer disease duration as reflected by reduced short‐interval intracortical inhibition (P < 0.05). Prolonged central motor conduction time was evident with disease progression. These changes were accompanied by reduction in neurophysiological index (P < 0.001) and compound muscle action potential amplitude (P < 0.01), progressive muscle weakness (P < 0.001), and decline in the ALS functional rating scale (P < 0.001).

Interpretation

This study established an increase in cortical hyperexcitability with increased disease duration in ALS, mediated by cortical disinhibition and direct increase in corticomotoneuronal excitability.

A new parameter to discriminate amyotrophic lateral sclerosis patients from healthy participants by motor cortical excitability changes

INTRODUCTION

We sought a combination of abnormalities to define a more sensitive measure of cortical excitability in amyotrophic lateral sclerosis (ALS).

METHODS

The automatic threshold tracking method was employed to assess the resting motor threshold, intracortical facilitation (ICF), short-interval intracortical inhibition (SICI), and short-interval intracortical faciilitation (SICF) in patients and controls.

RESULTS

SICF at interstimulus intervals (ISI) between 1 and 1.8 ms and 2 and 3 ms as well as average SICI and SICI at ISIs of 1 and 2.5 ms were significantly reduced in ALS. The SICI curve was altered, displaying a solitary peak. Discriminant analysis revealed that the combination of SICI 2.5 ms and the mean SICF between 1 and 1.8 ms ISIs was the most sensitive parameter to distinguish patients with ALS from healthy participants.

DISCUSSION

Along with the reduced SICI and its altered shape, connectivity between motor cortical circuits is changed in ALS. Combination with SICF increases the diagnostic utility of SICI in ALS.

Single-pulse transcranial magnetic stimulation in amyotrophic lateral sclerosis

INTRODUCTION

Transcranial magnetic stimulation (TMS) is one of the best methods to identify changes in the corticospinal tract. We used single-pulse TMS at the beginning of the disease and in the follow-up in a group of patients with amyotrophic lateral sclerosis (ALS).

METHODS

We evaluated the corticospinal tract in the bulbar, upper, and lower regions in 55 patients with ALS, and we monitored them for a period of 24 months. Data were correlated with clinical scales.

RESULTS

An increase of central motor conduction time (CMCT) was the most sensitive marker of upper motor neuron involvement. The resting motor threshold, CMCT, and the central silent period increased linearly with disease duration and upper/lower motor neuron involvement.

DISCUSSION

Transcranial magnetic stimulation could be an essential neurophysiological technique in the early phase of ALS because it has been shown to be useful in detecting subclinical upper motor neuron involvement. Multiple evaluations of several regions increase TMS sensitivity.

Amyotrophic lateral sclerosis: Origins traced to impaired balance between neural excitation and inhibition in the neonatal period

Amyotrophic lateral sclerosis (ALS) is an adult onset disease but with an increasingly recognized preclinical prodrome. A wide spectrum of investigative approaches has identified loss of inhibitory function at the heart of ALS. In developing an explanation for the onset of ALS, it remains a consideration that ALS has its origins in neonatal derangement of the γ-aminobutyric acid (GABA)-ergic system, with delayed conversion from excitatory to mature inhibitory GABA and impaired excitation/inhibition balance. If this is so, the resulting chronic excitotoxicity could marginalize cortical network functioning very early in life, laying the path for neurodegeneration. The possibility that adult-onset neurodegenerative conditions might have their roots in early developmental derangements is worthy of consideration, particularly in relation to current models of disease pathogenesis. Unraveling the very early molecular events will be crucial in developing a better understanding of ALS and other adult neurodegenerative disorders. Muscle Nerve, 2019.

Superconditioning TMS for examining upper motor neuron function in MND

We used transcranial magnetic stimulation (TMS) of motor cortex, including a novel four-pulse superconditioning (TMSsc) paradigm, in repeated examinations of motor-evoked potentials (MEPs) in eight subjects with motor neuron disease (MND), including seven with amyotrophic lateral sclerosis (ALS). The goals were: (1) to look for evidence of cortical hyperexcitability, including a reduction in short-interval intracortical inhibition (SICI); and (2) to examine the utility of using TMSsc for quantifying upper motor neuron function during MND progression. Testing of abductor pollicis brevis (APB) and tibialis anterior (TA) muscles bilaterally was carried out every 3 months in MND subjects for up to 2 years; results were compared to those from a cohort of 15 control subjects. Measures of SICI were not significantly different between control and MND subjects for either APB or TA muscles. Other measures of cortical excitability, including TMS threshold and MEP amplitude, were consistent with lowered cortical excitability in MND subjects. Certain combinations of superconditioning TMS were capable of causing stronger inhibition or facilitation of MEPs compared to dual-pulse TMS, for both APB and TA target muscles. Moreover, there were multiple cases in which target muscles unresponsive to strong single-pulse TMS, whether at rest or when tested with an active contraction, showed an MEP in response to TMSsc optimized for facilitation. Our findings suggest that a multi-faceted neurophysiologic protocol for examining upper motor neuron function in MND subjects might benefit from inclusion of TMSsc testing.

Excitability in somatosensory cortex correlates with motoric impairment in amyotrophic lateral sclerosis

Objective: Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative motoneuron disease. As previous studies reported alterations in motor cortex excitability, we evaluate excitability changes in somatosensory system. Methods: We examined 15 ALS patients and 15 healthy controls. Cortical excitability was assessed using paired somatosensory evoked potentials of median nerves. To determine disease severity and functional impairment, we assessed muscle strength and revised ALS-Functional Rating Scale (ALSFRS-R). Results: We found significantly reduced bilateral paired-stimulation inhibition in the ALS-group (both p < 0.05). Additionally, paired-stimulation ratios significantly correlated with ALSFRS-R (left somatosensory cortex: r= -orte; right somatosensory cortex: r= -ort4; both p < 0.05) and contralateral muscle strength (left somatosensory cortex: r= -orte, p = 0.007; right somatosensory cortex: r= -ortex p = 0.003). Conclusions: The results indicate disinhibition of the somatosensory cortex in ALS. It remains open if central somatosensory disinhibition is a primary characteristic of ALS as one element of a multisystem neurodegenerative disorder or a compensatory up-regulation due to functional motoric impairment. Longitudinal studies are necessary to categorize these findings.

Functional Biomarkers for Amyotrophic Lateral Sclerosis

The clinical diagnosis of amyotrophic lateral sclerosis (ALS) relies on determination of progressive dysfunction of both cortical as well as spinal and bulbar motor neurons. However, the variable mix of upper and lower motor neuron signs result in the clinical heterogeneity of patients with ALS, resulting frequently in delay of diagnosis as well as difficulty in monitoring disease progression and treatment outcomes particularly in a clinical trial setting. As such, the present review provides an overview of recently developed novel non-invasive electrophysiological techniques that may serve as biomarkers to assess UMN and LMN dysfunction in ALS patients.

Utility of threshold tracking transcranial magnetic stimulation in ALS

Upper motor neuron [UMN] and lower motor neuron [LMN] dysfunction, in the absence of sensory features, is a pathognomonic feature of amyotrophic lateral sclerosis [ALS]. Although the precise mechanisms have yet to be elucidated, one leading hypothesis is that UMN precede LMN dysfunction, which is induced by anterograde glutamatergic excitotoxicity. Transcranial magnetic stimulation (TMS) is a neurophysiological tool that provides a non-invasive and painless assessment of cortical function. Threshold tracking methodologies have been recently adopted for TMS, whereby changes in threshold rather than motor evoked potential (MEP) amplitude serve as outcome measures. This technique is reliable and provides a rapid assessment of cortical function in ALS. Utilisng the threshold tracking TMS technique, cortical hyperexcitability was demonstrated as an early feature in sporadic ALS preceding the onset of LMN dysfunction and possibly contributing to disease spread. Separately, cortical hyperexcitability was reported to precede the clinical onset of familial ALS. Of further relevance, the threshold tracking TMS technique was proven to reliably distinguish ALS from mimicking disorders, even in the presence of a comparable degree of LMN dysfunction, suggesting a diagnostic utility of TMS. Taken in total, threshold tracking TMS has provided support for a cortical involvement at the earliest detectable stages of ALS, underscoring the utility of the technique for probing the underlying pathophysiology. The present review will discuss the physiological processes underlying TMS parameters, while further evaluating the pathophysiological and diagnostic utility of threshold tracking TMS in ALS.

Implications of structural and functional brain changes in amyotrophic lateral sclerosis

INTRODUCTION

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that causes progressive muscle weakness and disability, eventually leading to death. Heterogeneity of disease has become a major barrier to understanding key clinical questions such as prognosis and disease spread, and has disadvantaged clinical trials in search of therapeutic intervention. Patterns of disease have been explored through recent advances in neuroimaging, elucidating structural, molecular and functional changes. Unique brain signatures have emerged that have lent a greater understanding of critical disease mechanisms, offering opportunities to improve diagnosis, guide prognosis, and establish candidate biomarkers to direct future therapeutic strategies. Areas covered: This review explores patterns of cortical and subcortical change in ALS through advanced neuroimaging techniques and discusses the implications of these findings. Expert commentary: Cortical and subcortical signatures and patterns of atrophy are now consistently recognised, providing important pathophysiological insight into this heterogenous disease. The spread of cortical change, particularly involving frontotemporal networks, correlates with cognitive impairment and poorer prognosis. Cortical differences are also evident between ALS phenotypes and genotypes, which may partly explain the heterogeneity of prognosis. Ultimately, multimodal approaches with larger cohorts will be needed to provide sensitive biomarkers of disease spread at the level of the individual patient.

Short-interval intracortical inhibition: Comparison between conventional and threshold-tracking techniques

Background

Short-interval intracortical inhibition (SICI) is conventionally measured as the relative amplitude reduction of motor evoked potentials (MEPs) by subthreshold conditioning stimuli. In threshold-tracking SICI (T-SICI), stimulus intensity is instead adjusted repeatedly to maintain a constant MEP and inhibition is measured as the relative threshold increase. T-SICI is emerging as a useful diagnostic test, but its relationship to conventional amplitude SICI (A-SICI) is unclear.

Objective

To compare T-SICI and its reliability with conventional A-SICI measurements.

Methods

In twelve healthy volunteers (6 men, median age 30 years), conventional and T-SICI were recorded at conditioning stimuli (CS) of 50–80% resting motor threshold (RMT) and interstimulus interval of 2.5 ms. Measurements were repeated on the same day and at least a week later by a single operator.

Results

Across the CS range, mean group T-SICI showed a strong linear relationship to the mean group values measured by conventional technique (y = 29.7–0.3x, R² = 0.99), but there was considerable interindividual variability. At CS 60–80% RMT, T-SICI had excellent intraday (intraclass correlation coefficient, ICC, 0.81–0.92) and adequate-to-excellent interday (ICC 0.61–0.88) reproducibility. Conventional SICI took longer to complete (median of 5.8 vs 3.8 min, p < 0.001) and tended to have poorer reproducibility (ICC 0.17–0.42 intraday, 0.37–0.51 interday). With T-SICI, smaller sample sizes were calculated for equally powered interventional studies.

Conclusion

The close relationship between conventional and T-SICI suggests that both techniques reflect similar cortical inhibitory mechanisms. Threshold-tracking measurements of SICI may be able to improve reproducibility, to shorten acquisition time and to reduce sample sizes for interventional studies compared with the conventional technique.

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Threshold-tracking SICI correlates well with conventional measures.

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It shows a trend towards better reproducibility on a group level.

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Threshold-tracking SICI is potentially quicker to obtain.

•

It may require a smaller sample size in interventional studies.

Four-pulse transcranial magnetic stimulation using multiple conditioning inputs. Normative MEP responses

A four-pulse pattern of transcranial magnetic stimulation (TMS) was compared to traditional dual-pulse TMS for its ability to modulate motor cortical excitability. This novel pattern consisted of a three-pulse train of subthreshold conditioning pulses followed by a suprathreshold test pulse (i.e., SC-T). The intervals between these superconditioning (SC) pulses (1, 3, or 6 ms) and the follow-on test pulse (1, 3, 10, or 25 ms) were varied, and the resultant MEPs were compared to those elicited by: (1) single-pulse TMS; and (2) dual-pulse conditioning-test (C-T) TMS with either short (3 ms) or long (10 ms) intervals to elicit short-interval intracortical inhibition (SICI) or intracortical facilitation (ICF), respectively. Testing included abductor pollicis brevis (APB) and tibialis anterior (TA) in 15 neurologically normal adults. For superconditioning inputs, 10 ms test intervals caused especially strong facilitation of the test MEP, while 1 ms test intervals were particularly effective at causing inhibition of the test response. For both muscles and across all subjects, the most effective of the 12 SC-T inputs tested for causing either facilitation or inhibition was-with rare exception-superior to the dual-pulse TMS input for causing facilitation (i.e., ICF) or inhibition (i.e., SICI), while the overall magnitude of effect was more pronounced in APB compared to TA. Nevertheless, after normalization, the impact of a superconditioning input train on the test MEP was similar in APB and TA muscles, suggesting similar mechanisms of action. Limited findings from a single subject with amyotrophic lateral sclerosis (ALS) are included to further illustrate the potential advantages of using a train of conditioning pulses preceding a TMS test pulse to selectively investigate abnormal motor cortical excitatory and inhibitory circuitry.

Global network modulation during thalamic stimulation for Tourette syndrome

Background and objectives

Deep brain stimulation (DBS) of the thalamus is a promising therapeutic alternative for treating medically refractory Tourette syndrome (TS). However, few human studies have examined its mechanism of action. Therefore, the networks that mediate the therapeutic effects of thalamic DBS remain poorly understood.

Methods

Five participants diagnosed with severe medically refractory TS underwent bilateral thalamic DBS stereotactic surgery. Intraoperative fMRI characterized the blood oxygen level-dependent (BOLD) response evoked by thalamic DBS and determined whether the therapeutic effectiveness of thalamic DBS, as assessed using the Modified Rush Video Rating Scale test, would correlate with evoked BOLD responses in motor and limbic cortical and subcortical regions.

Results

Our results reveal that thalamic stimulation in TS participants has wide-ranging effects that impact the frontostriatal, limbic, and motor networks. Thalamic stimulation induced suppression of motor and insula networks correlated with motor tic reduction, while suppression of frontal and parietal networks correlated with vocal tic reduction. These regions mapped closely to major regions of interest (ROI) identified in a nonhuman primate model of TS.

Conclusions

Overall, these findings suggest that a critical factor in TS treatment should involve modulation of both frontostriatal and motor networks, rather than be treated as a focal disorder of the brain. Using the novel combination of DBS-evoked tic reduction and fMRI in human subjects, we provide new insights into the basal ganglia-cerebellar-thalamo-cortical network-level mechanisms that influence the effects of thalamic DBS. Future translational research should identify whether these network changes are cause or effect of TS symptoms.

Abnormal cortical brain integration of somatosensory afferents in ALS

OBJECTIVES

Infraclinical sensory alterations have been reported at early stages of amyotrophic lateral sclerosis (ALS). While previous studies mainly focused on early somatosensory evoked potentials (SEPs), late SEPs, which reflect on cortical pathways involved in cognitive-motor functions, are relatively underinvestigated. Early and late SEPs were compared to assess their alterations in ALS.

METHODS

Median and ulnar nerves were electrically stimulated at the wrist, at 9 times the perceptual threshold, in 21 ALS patients without clinical evidence of sensory deficits, and 21 age- and gender-matched controls. SEPs were recorded at the Erb point using surface electrodes and using a needle inserted in the scalp, in front of the primary somatosensory area (with reference electrode on the ear lobe).

RESULTS

Compared to controls, ALS patients showed comparable peripheral (N9) and early cortical component (N20, P25, N30) reductions, while the late cortical components (N60, P100) were more depressed than the early ones.

CONCLUSIONS

The peripheral sensory alteration likely contributed to late SEP depression to a lesser extent than that of early SEPs.

SIGNIFICANCE

Late SEPs may provide new insights on abnormal cortical excitability affecting brain areas involved in cognitive-motor functions.