Cortical inexcitability defines an adverse clinical profile in amyotrophic lateral sclerosis

Neurophysiological assessment of cortical motor function: A direct comparison of methodologies

OBJECTIVE

Assessment of cortical function with threshold tracking transcranial magnetic stimulation (TT-TMS) has developed as a biomarker to inform disease pathophysiology, particularly in neurodegenerative disease and dementia. At present, a fully integrated testing system does not exist. To advance clinical utility, and to streamline software design to integrate with diagnostic approaches in an outpatient setting, the present series of studies assessed the effects of altering diagnostic paradigms to measure interstimulus interval (ISI) including serial ascending [T-SICIs] and parallel [T-SICIp] methodologies as measures of cortical motor function (the MagXite software).

METHODS

Cortical excitability was assessed in 30 healthy controls with a figure-of-eight coil, using an integrated approach compared to previously established experimental paradigms. Motor evoked responses were recorded over the contralateral abductor pollicis brevis muscle. Short interval intracortical inhibition (SICI) was recorded with each testing paradigm and validated in a healthy control cohort.

RESULTS

The integrated system determined a robust measure of T-SICIs between ISI 1-to-7 ms (16.6 ± 2.2 %) that was comparable to previously established testing paradigms (P = 0.34), but greater than T-SICIp (MagXite 10.7 ± 1.5 %, P = 0.016; Sydney TT-TMS 8.7 ± 1.4 %, P = 0.03). SICI peaks at ISI 1 and 2.5-to-3 ms were evident with both protocols. Significant correlations were evident between mean T-SICIs-MagXite and T-SICIp-MagXite (R = 0.599, P < 0.001).

CONCLUSION

The present series validates a fully integrated motor cortical functional assessment to provide reproducible measures of SICI, with data obtained for intracortical inhibition that is more prominent when assessed using the method of serial ascending order.

SIGNIFICANCE

An integrated system for transcranial magnetic stimulation of the human motor system has been validated for clinical practice, suitable for the assessment of cortical function in neurological disease in an outpatient clinic setting.

Prognostic Value of Systemic Inflammation, Nutritional Status and Sarcopenia in Patients With Amyotrophic Lateral Sclerosis

BACKGROUND

Nutritional status, systemic inflammatory responses and muscle mass are associated with the prognosis of patients with amyotrophic lateral sclerosis (ALS). However, the optimal biomarker for predicting prognosis remains unclear. This study aimed to identify the optimal indicators of survival among the nutrition-based, inflammation-based and muscle mass-related markers for ALS patients.

METHODS

We enrolled ALS patients from January 2014 to December 2019. Experienced neurologists followed up with the participants until January 2022. This study included a total of 17 nutritional, systemic inflammatory or muscle mass-related indicators. Maximally selected rank statistics determined the cut-off points for these indicators. Kaplan-Meier estimation was used to assess survival. Uni- and multivariate Cox proportional hazards models were used to determine the effects of indicators on survival. Finally, time-dependent receiver operating characteristic (time-ROC) curves and the C-index were calculated to evaluate the predictive efficacy of different indicators.

RESULTS

A total of 506 patients with ALS were enrolled in this study, including 288 males (56.9%) and 218 females (43.1%), with a mean age of 54.2 ± 10.5 years. Among these ALS patients, 334 cases (68.0%) either died or underwent tracheotomy. In univariate Cox proportional hazards regression, 11 indicators were significantly associated with ALS survival (p < 0.05). And systemic immune inflammation (SII), platelet-to-lymphocyte ratio (PLR), modified geriatric nutritional risk index (mGNRI), creatinine and sarcopenia index (SI, (creatinine/cystatin C) × 100) were determined as independent predictors (p < 0.05) in multivariate Cox proportional hazards regression. A higher SI predicted longer survival (hazard ratio, 0.59; 95% confidence interval [CI], 0.46-0.76; p < 0.001). The results of time-ROC and C-index analyses indicated that SI had the best predictive efficacy for ALS survival, with a C-index of 0.65 (95% CI, 0.54-0.75) for 1-year, 0.61 (95% CI, 0.57-0.65) for 3-year and 0.59 (95% CI, 0.55-0.62) for 5-year survival. Across different subgroups, SI had the highest C-index in men and women, limb onset and aged < 60 year ALS patients, compared with other indicators. However, cystatin C was the best indicator for predicting the survival of ALS patients with bulbar onset, whereas the prognostic nutritional index (PNI) was the best for those aged ≥60 years.

CONCLUSIONS

The serum SI demonstrates superior prognostic ability compared to other inflammation-based, nutrition-based and muscle mass-related indicators for patients with ALS. Given its simplicity and availability, it is well suited for clinical use in evaluating the prognosis of ALS patients.

Cortical inexcitability in ALS: correlating a clinical phenotype

BACKGROUND

Cortical inexcitability, a less studied feature of upper motor neuron (UMN) dysfunction in amyotrophic lateral sclerosis (ALS), was identified in a large cross-sectional cohort of ALS patients and their demographic and clinical characteristics were contrasted with normal or hyperexcitable ALS cohorts to assess the impact of cortical inexcitability on ALS phenotype and survival.

METHODS

Threshold-tracking transcranial magnetic stimulation (TMS) technique with measurement of mean short interval intracortical inhibition (SICI) differentiated ALS patients into three groups (1) inexcitable (no TMS response at maximal stimulator output in the setting of preserved lower motor neuron (LMN) function), (2) hyperexcitable (SICI≤5.5%) and (3) normal cortical excitability (SICI>5.5%). Clinical phenotyping and neurophysiological assessment of LMN function were undertaken, and survival was recorded in the entire cohort.

RESULTS

417 ALS patients were recruited, of whom 26.4% exhibited cortical inexcitability. Cortical inexcitability was associated with a younger age of disease onset (p<0.05), advanced Awaji criteria (p<0.01) and Kings stage (p<0.01) scores. Additionally, patients with cortical inexcitability had higher UMN score (p<0.01), lower revised ALS Functional Rating Scale score (p<0.01) and reduced upper limb strength score (MRC UL, p<0.01). Patient survival (p=0.398) was comparable across the groups, despite lower riluzole use in the cortical inexcitability patient group (p<0.05).

CONCLUSION

The present study established that cortical inexcitability was associated with a phenotype characterised by prominent UMN signs, greater motor and functional decline, and a younger age of onset. The present findings inform patient management and could improve patient stratification in clinical trials.

Abnormalities of cortical stimulation strength-duration time constant in amyotrophic lateral sclerosis

OBJECTIVES

Strength-duration time constant (SDTC) may now be determined for cortical motor neurones, with activity mediated by transient Na+ conductances. The present study determined whether cortical SDTC is abnormal and linked to the pathogenesis of amyotrophic lateral sclerosis.

METHODS

Cortical SDTC and rheobase were estimated from 17 ALS patients using a controllable pulse parameter transcranial magnetic stimulation (cTMS) device. Resting motor thresholds (RMTs) were determined at pulse widths (PW) of 30, 45, 60, 90 and 120 µs and M-ratio of 0.1, using a figure-of-eight coil applied to the primary motor cortex.

RESULTS

SDTC was significantly reduced in ALS patients (150.58 ± 9.98 µs; controls 205.94 ± 13.7 µs, P < 0.01). The reduced SDTC correlated with a rate of disease progression (Rho = -0.440, P < 0.05), ALS functional rating score (ALSFRS-R) score (Rho = 0.446, P < 0.05), and disease duration (R = 0.428, P < 0.05). The degree of change in SDTC was greater in patients with cognitive abnormalities as manifested by an abnormal total Edinburgh Cognitive ALS Screen score (140.5 ± 28.7 µs, P < 0.001) and ALS-specific subscore (141.7 ± 33.2 µs, P = 0.003).

CONCLUSIONS

Cortical SDTC reduction was associated with a more aggressive ALS phenotype, or with more prominent cognitive impairment.

SIGNIFICANCE

An increase in transient Na+ conductances may account for the reduction in SDTC, linked to the pathogenesis of ALS.

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.

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.

Amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a fatal CNS neurodegenerative disease. Despite intensive research, current management of amyotrophic lateral sclerosis remains suboptimal from diagnosis to prognosis. Recognition of the phenotypic heterogeneity of amyotrophic lateral sclerosis, global CNS dysfunction, genetic architecture, and development of novel diagnostic criteria is clarifying the spectrum of clinical presentation and facilitating diagnosis. Insights into the pathophysiology of amyotrophic lateral sclerosis, identification of disease biomarkers and modifiable risks, along with new predictive models, scales, and scoring systems, and a clinical trial pipeline of mechanism-based therapies, are changing the prognostic landscape. Although most recent advances have yet to translate into patient benefit, the idea of amyotrophic lateral sclerosis as a complex syndrome is already having tangible effects in the clinic. This Seminar will outline these insights and discuss the status of the management of amyotrophic lateral sclerosis for the general neurologist, along with future prospects that could improve care and outcomes for patients with amyotrophic lateral sclerosis.

Neurophysiological differentiation of upper motor neuron damage in neurodegenerative disorders

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ALS and MSA presented with similar profiles of upper motor neuron signs.

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Central motor conduction time was more abnormal in ALS than in MSA.

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Different structures may be involved in ALS and MSA along the corticospinal tract.

Objective

Using transcranial magnetic stimulation (TMS) to delineate upper motor neuron (UMN) signs of two neurodegenerative disorders: amyotrophic lateral sclerosis (ALS) and multiple system atrophy (MSA).

Methods

Medical records including clinical signs for UMN damage and TMS results were reviewed retrospectively. The UMN signs were classified into none, mild, and severe based on neurological examination of various reflexes. Then TMS-elicited motor evoked potentials (MEPs) were recorded from a hand and a leg muscle to calculate the central motor conduction time (CMCT), which represents fast, mono-synaptic conduction along the corticospinal tract. Relations between the UMN signs and CMCT were analysed for the two diseases.

Results

Prevalence and severity of the UMN signs for ALS and MSA were comparable for both upper and lower limbs. However, abnormality in CMCT was found more frequently in ALS: CMCT abnormalities were found in upper limbs for 44% in ALS patients but only for 7% in MSA patients; CMCT abnormalities in lower limbs were 55% in ALS and 20% in MSA. Some ALS patients showed abnormal CMCT in limbs without UMN signs, which was not true for most MSA patients.

Conclusions

The abnormalities of CMCT were different in ALS and MSA, even for those who clinically had similar UMN signs. Sometimes, CMCT can reveal UMN damage in the absence of clinical UMN signs. Differences presumably derive from selective degeneration of different fibres in the motor descending pathways. Longitudinal studies must be conducted to accumulate neuroimaging and pathological findings.

Significance

CMCT can be useful to differentiate ALS and MSA.

Recent advances in the diagnosis and prognosis of amyotrophic lateral sclerosis

The diagnosis of amyotrophic lateral sclerosis can be challenging due to its heterogeneity in clinical presentation and overlap with other neurological disorders. Diagnosis early in the disease course can improve outcomes as timely interventions can slow disease progression. An evolving awareness of disease genotypes and phenotypes and new diagnostic criteria, such as the recent Gold Coast criteria, could expedite diagnosis. Improved prognosis, such as that achieved with the survival model from the European Network for the Cure of ALS, could inform the patient and their family about disease course and improve end-of-life planning. Novel staging and scoring systems can help monitor disease progression and might potentially serve as clinical trial outcomes. Lastly, new tools, such as fluid biomarkers, imaging modalities, and neuromuscular electrophysiological measurements, might increase diagnostic and prognostic accuracy.

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.

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.