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.

Neuroinflammation in amyotrophic lateral sclerosis: pathogenic insights and therapeutic implications.

PURPOSE OF REVIEW: Neuroinflammation appears to be an important pathogenic process in amyotrophic lateral sclerosis (ALS). Dysfunction of central immune pathways, including activation of microglia and astrocytes, and peripherally derived immune cells, initiate noncell autonomous inflammatory mechanisms leading to degeneration. Cell autonomous pathways linked to ALS genetic mutations have been recently identified as contributing mechanism for neurodegeneration. The current review provides insights into the pathogenic importance of central and peripheral inflammatory processes in ALS pathogenesis and appraises their potential as therapeutic targets. RECENT FINDINGS: ALS is a multistep process mediated by a complex interaction of genetic, epigenetic, and environmental factors. Noncell autonomous inflammatory pathways contribute to neurodegeneration in ALS. Activation of microglia and astrocytes, along with central nervous system infiltration of peripherally derived pro-inflammatory innate (NK-cells/monocytes) and adaptive (cell-mediated/humoral) immune cells, are characteristic of ALS. Dysfunction of regulatory T-cells, elevation of pro-inflammatory cytokines and dysbiosis of gut microbiome towards a pro-inflammatory phenotype, have been reported as pathogenic mechanisms in ALS. SUMMARY: Dysregulation of adaptive and innate immunity is pathogenic in ALS, being associated with greater disease burden, more rapid disease course and reduced survival. Strategies aimed at modulating the pro-inflammatory immune components could be of therapeutic 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.

Clinical and neurophysiological biomarkers of disease progression in amyotrophic lateral sclerosis.

INTRODUCTION/AIMS: Rate of disease progression (ΔFS), measured as change in the revised Amyotrophic Lateral Sclerosis Functional Rating Scale (ALSFRS-R) and body mass index (BMI), are predictors of survival in amyotrophic lateral sclerosis (ALS). Our aim in this study was to assess the utility of these clinical biomarkers along with neurophysiological measures, such as the split hand index (SI), in monitoring disease progression. METHODS: Clinical trial data were collected from 107 patients recruited into the Tecfidera in ALS trial. The prognostic utility of clinical and neurophysiological measures, including ΔFS, BMI, SI, and neurophysiological index (NPI), were assessed cross-sectionally and longitudinally (40 weeks). The outcome measures of disease severity and progression included: (i) ALSFRS-R score; (ii) Medical Research Council (MRC) score; and (iii) forced vital capacity and sniff nasal inspiratory pressure. RESULTS: Fast-progressor ALS patients (ΔFS ≥1.1) exhibited significantly lower ALSFRS-R and total MRC scores at baseline. A baseline ΔFS score ≥1.1 was associated with a greater reduction in ALSFRS-R (P = .002) and MRC (P = .002) scores over 40 weeks. Baseline BMI <25 was also associated with faster reduction of ALSFRS-R and MRC scores. SI and NPI were associated with disease severity at baseline, but not with subsequent rate of disease progression. DISCUSSION: Implementation of the assessed clinical and neurophysiological biomarkers may assist in patient management and stratification into clinical trials.

Diagnostic Utility of Gold Coast Criteria in Amyotrophic Lateral Sclerosis.

OBJECTIVE: The diagnosis of amyotrophic lateral sclerosis (ALS) remains problematic, with current diagnostic criteria (revised El Escorial [rEEC] and Awaji) being complex and prone to error. Consequently, the diagnostic utility of the recently proposed Gold Coast criteria was determined in ALS. METHODS: We retrospectively reviewed 506 patients (302 males, 204 females) to compare the diagnostic accuracy of the Gold Coast criteria to that of the Awaji and rEEC criteria (defined by the proportion of patients categorized as definite, probable, or possible ALS) in accordance with standards of reporting of diagnostic accuracy criteria. RESULTS: The sensitivity of Gold Coast criteria (92%, 95% confidence interval [CI] = 88.7-94.6%) was comparable to that of Awaji (90.3%, 95% CI = 86.69-93.2%) and rEEC (88.6, 95% CI = 84.8-91.7%) criteria. Additionally, the Gold Coast criteria sensitivity was maintained across different subgroups, defined by site of onset, disease duration, and functional disability. In atypical ALS phenotypes, the Gold Coast criteria exhibited greater sensitivity and specificity. INTERPRETATION: The present study established the diagnostic utility of the Gold Coast criteria in ALS, with benefits evident in bulbar and limb onset disease patients, as well as atypical phenotypes. The Gold Coast criteria should be considered in clinical practice and therapeutic trials. ANN NEUROL 2021;89:979-986.

Reproducibility of motor unit number index and MScanFit motor unit number estimation across intrinsic hand muscles.

INTRODUCTION: We sought to evaluate the reproducibility of the motor unit number index (MUNIX) and MScanFit motor unit number estimation (MScan) when recording was performed over intrinsic hand muscles. METHODS: The compound muscle action potential (CMAP) amplitude, MUNIX, and MScan were measured from the abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digit minimi (ADM) muscles from 15 healthy volunteers on three different occasions. RESULTS: The reproducibility of CMAP amplitudes was excellent, with intraclass correlation coefficients (ICC) of 0.86 (APB), 0.90 (FDI), and 0.96 (ADM). Motor unit number index (ICCAPB 0.73, ICCFDI 0.85, ICCADM 0.85) and MScan (ICCAPB 0.86, ICCFDI 0.83, ICCADM 0.81) were highly reproducible across the three muscles. There were no significant correlations between MUNIX and MScan coefficients of variation (CV) and CMAP amplitude CVs. DISCUSSION: Reproducibility of MUNIX and MScan was not significantly different across the intrinsic hand muscles and was independent of CMAP amplitude variability.

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.

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.

Cortical inhibition and facilitation are mediated by distinct physiological processes.

A complex interaction of inhibitory and facilitatory interneuronal processes may underlie development of cortical excitability in the human motor cortex. To determine whether distinct interneuronal processes mediated cortical excitability, threshold tracking transcranial magnetic stimulation was utilised to assess cortical excitability, with figure-of-eight coil oriented in posterior-anterior (PA), anterior-posterior (AP) and latero-medial (LM) directions. Motor evoked potential (MEP) responses were recorded over the contralateral abductor pollicis brevis. Resting motor threshold (RMT), short interval intracortical inhibition (SICI), short interval intracortical facilitation (SICF) and intracortical facilitation were recorded. Significant effects of coil orientation were evident on SICI (F = 8.560, P = 0.002) and SICF (F = 7.132, P = 0.003). SICI was greater when recorded with PA (9.7 ± 10.9%, P = 0.029) and AP (13.1 ± 7.0%, P = 0.003) compared to LM (5.2 ± 7.3%) directed currents. SICF was significantly greater with PA (-14.7 ± 8.1%, P = 0.016) and LM (-14.7 ± 8.8%, P = 0.005) compared to AP (-9.1 ± 7.2%) coil orientations. SICI recorded with LM and PA coil orientations were correlated (R = 0.7, P = 0.002), as was SICF recorded with AP vs LM (R = 0.60, P = 0.019) and LM vs PA (R = 0.69, P = 0.002) coil orientations. RMT was significantly smaller with PA compared to AP (P < 0.001) and LM (P = 0.018) stimulation. Recruitment of distinct interneuronal processes with variable cortical orientation and thresholds underlies short interval intracortical inhibition and facilitation.

Utility of split hand index with different motor unit number estimation techniques in ALS.

OBJECTIVE: Utility of the split hand index (SI) in amyotrophic lateral sclerosis (ALS) has been reported when using the compound muscle action potential (CMAP) amplitude method (SICMAP amp). A motor unit number index (MUNIX) based SI method (SIMUNIX) was purported to exhibit higher sensitivity. The present study assessed the clinical utility of SI, derived by CMAP amplitude, MUNIX and MScan-MUNE (SIMScanFit-MUNE) methods, in ALS. METHODS: Sixty-two consecutive patients with neuromuscular symptoms (36 ALS and 26 ALS-mimics) were prospectively recruited. The SI was derived by dividing the product of the CMAP amplitude, MUNIX and MScan-MUNE values recorded over first dorsal interosseous and abductor pollicis brevis by values recorded over abductor digit minimi. RESULTS: SICMAP amp, SIMUNIX and SIMScanFit-MUNE were significantly reduced in ALS, with SICMAP amp (area under curve (AUC) = 0.801) and SIMScanFit-MUNE (AUC = 0.805) exhibiting greater diagnostic utility than SIMUNIX (AUC = 0.713). SICMAP amp and SIMScanFit-MUNE exhibited significant correlations with clinical measures of functional disability and weakness of intrinsic hand muscles. CONCLUSIONS: SI differentiated ALS from mimic disorders, with SICMAP amp and SIMScanFit-MUNE exhibiting greater utility. SIGNIFICANCE: The split hand index represents could serve as a potential diagnostic biomarker in ALS.

Dependence of cortical neuronal strength-duration properties on TMS pulse shape.

OBJECTIVE: The aim of present study was to explore the effects of different combinations of transcranial magnetic stimulation (TMS) pulse width and pulse shape on cortical strength-duration time constant (SDTC) and rheobase measurements. METHODS: Resting motor thresholds (RMT) at pulse widths (PW) of 30, 45, 60, 90 and 120 µs and M-ratios of 0.2, 0.1 and 0.025 were determined using figure-of-eight coil with initial posterior-to-anterior induced current. The M-ratio indicates the relative phases of the induced current with lower values signifying a more unidirectional stimulus. Strength-duration time constant (SDTC) and rheobase were estimated for each M-ratio and various PW combinations. Simulations of biophysically realistic cortical neuron models assessed underlying neuronal populations and physiological mechanisms mediating pulse shape effects on strength-duration properties. RESULTS: The M-ratio exerted significant effect on SDTC (F(2,44) = 4.386, P = 0.021), which was longer for M-ratio of 0.2 (243.4 ± 61.2 µs) compared to 0.025 (186.7 ± 52.5 µs, P = 0.034). Rheobase was significantly smaller when assessed with M-ratio 0.2 compared to 0.025 (P = 0.026). SDTC and rheobase values were most consistent with pulse width sets of 30/45/60/90/120 µs, 30/60/90/120 µs, and 30/60/120 µs. Simulation studies indicated that isolated pyramidal neurons in layers 2/3, 5, and large basket-cells in layer 4 exhibited SDTCs comparable to experimental results. Further, simulation studies indicated that reducing transient Na+ channel conductance increased SDTC with larger increases for higher M-ratios. CONCLUSIONS: Cortical strength-duration curve properties vary with pulse shape, and the modulating effect of the hyperpolarising pulse phase on cortical axonal transient Na+ conductances could account for these changes, although a shift in the recruited neuronal populations may contribute as well. SIGNIFICANCE: The dependence of the cortical strength-duration curve properties on the TMS pulse shape and pulse width selection underscores the need for consistent measurement methods across studies and the potential to extract information about pathophysiological processes.

The split-elbow index: A biomarker of the split elbow sign in ALS.

OBJECTIVE: The split elbow sign is a clinical feature of amyotrophic lateral sclerosis (ALS), characterised by preferential weakness of biceps brachii muscle compared to triceps. A novel neurophysiological index, termed the split elbow index (SEI), was developed to quantify the split-elbow sign, and assess its utility in ALS. METHODS: Clinical and neurophysiological assessment was prospectively undertaken on 34 ALS patients and 32 ALS mimics. Compound muscle action potential (CMAP) amplitude was recorded from biceps brachii and triceps muscles from which the SEI was calculated using the following formula: SEI = CMAPamplitudeBICEPSBRACHII CMAPamplitudeTRICEPSBRACHII . RESULTS: The split elbow sign was significantly more common in ALS patients when compared to ALS mimic patients (P < 0.05). The SEI was significantly reduced in ALS patients when compared to ALS mimics (P < 0.01). This reduction was evident in spinal and bulbar onset ALS. A SEI cut-off value of ≤0.62 exhibited a sensitivity of 71% and specificity of 61%. CONCLUSIONS: The split elbow sign is significantly more common in ALS patients, and was supported by a reduction in the SEI. SIGNIFICANCE: The SEI may be utilised as a surrogate biomarker of the split elbow sign in future ALS studies.

Electrodiagnostic findings in facial onset sensory motor neuronopathy (FOSMN).

OBJECTIVE: To determine the electrodiagnostic characteristics of facial onset sensory and motor neuronopathy (FOSMN). METHODS: Electrophysiological data from 10 FOSMN patients in Newcastle-upon-Tyne and Sydney were reviewed. Relevant literature was reviewed. RESULTS: Findings on standard electrophysiological assessment were in broad agreement with those published: blink reflexes were abnormal in all but one patient; sensory nerve action potentials were reduced but compound muscle action potentials preserved; mixed acute and chronic neurogenic change was identified on needle electromyography in bulbar and cervico-thoracic muscles in approximately 50% of patients. Upper limb somatosensory evoked potential (SEP) central conduction times were increased (n = 4) and progressed on repeat testing (n = 3). Upper motor neuron dysfunction was revealed by several measures [ipsilateral motor evoked potentials (MEPs) (n = 1); reduced short interval intra-cortical inhibition on threshold-tracking transcranial magnetic stimulation (n = 2); absent beta-band intermuscular coherence (n = 3)]. CONCLUSIONS: Electrodiagnostic investigation of FOSMN should include blink reflex testing, SEPs and tests of upper motor neuron function. The combination of progressive lower motor neuron disease and upper motor neuron disease on neurophysiological investigation provides further support for the contention that FOSMN is a rare variant of motor neurone disease. SIGNIFICANCE: These findings will aid the neurologist and neurophysiologist in making a confident diagnosis of FOSMN, thus expediting appropriate care.

Association of Cortical Hyperexcitability and Cognitive Impairment in Patients With Amyotrophic Lateral Sclerosis.

OBJECTIVE: To determine whether cortical hyperexcitability was more prominent in cognitively impaired patients with amyotrophic lateral sclerosis (ALS). METHODS: Threshold tracking transcranial magnetic stimulation (TMS) was used to assess cortical excitability and cognitive function was determined by the Edinburgh Cognitive and Behavioural ALS Screen (ECAS). Cognitive impairment was defined by ECAS < 105. Patients with ALS, defined by the Awaji criteria, were prospectively recruited. Patients unable to undergo TMS, or in whom TMS indices were compromised by coexistent medical conditions, were excluded. Cortical hyperexcitability was defined by reduced short interval intracortical inhibition (SICI) and increased short interval intracortical facilitation (SICF), index of excitability (IE), and motor evoked potential (MEP) amplitude. Student t test determined differences between groups and multivariable regression modeling was used to assess association among cognitive, clinical, and TMS measures. TMS results were compared with those of 42 controls. RESULTS: Cognitive impairment was evident in 36% of the 40 patients with ALS (23 male, mean age 62.1 years). Cortical hyperexcitability was more prominent in cognitively impaired patients as indicated by an increase in SICF (ECAS≥105 -15.3 ± 1.7%, ECAS<105 -20.6 ± 1.2%; p < 0.01), IE (ECAS ≥105 80.9 ± 7.8, ECAS <105 95.0 ± 4.5; p < 0.01), and MEP amplitude (ECAS≥105 28.7 ± 3.3%, ECAS<105 43.1 ± 5.9%; p < 0.05). SICF was independently associated with the ECAS score (ß = 2.410; p < 0.05). Reduced SICI was evident in ALS, being more prominent in patients with reduced executive score (ECASexecutive score>33 6.2 ± 1.3%, ECASexecutive score<33 1.5 ± 2.1%; p < 0.01). CONCLUSION: Cortical hyperexcitability was more prominent in cognitively impaired patients with ALS than in controls. Given that ECAS is a valid predictor of TDP-43 pathology, the increase in cortical hyperexcitability may be associated with TDP-43 accumulation.

Cortical hyperexcitability: Diagnostic and pathogenic biomarker of ALS.

Cortical hyperexcitability is an early and intrinsic feature of both sporadic and familial forms of amyotrophic lateral sclerosis (ALS).. Importantly, cortical hyperexcitability appears to be associated with motor neuron degeneration, possibly via an anterograde glutamate-mediated excitotoxic process, thereby forming a pathogenic basis for ALS. The presence of cortical hyperexcitability in ALS patients may be readily determined by transcranial magnetic stimulation (TMS), a neurophysiological tool that provides a non-invasive and painless method for assessing cortical function. Utilising the threshold tracking TMS technique, cortical hyperexcitability has been established as a robust diagnostic biomarker that distinguished ALS from mimicking disorders at early stages of the disease process. The present review discusses the pathophysiological and diagnostic utility of cortical hyperexcitability in ALS.

Split-hand index: A diagnostic and prognostic marker in amyotrophic lateral sclerosis across varying regions of onset.

OBJECTIVE: The split-hand index (SI), a reliable diagnostic marker of amyotrophic lateral sclerosis (ALS), was prospectively assessed for differences across ALS subtypes and between the onset side of clinical symptoms or the dominant and contralateral sides. In addition, the prognostic utility of the SI was longitudinally assessed. METHODS: Two hundred and forty-five ALS patients underwent measurement of SI on both sides compared with 126 neuromuscular mimic disorders (NMD). A subset of patients (N = 45) underwent longitudinal assessment of SI. RESULTS: The SI was significantly reduced (SI RIGHT ALS 5.47(4.2), SINMD 9.0 (5.0); P < 0.001; SILEFT ALS 5.5 (4.1), SI NMD 9.4 (5.0), P < 0.001) on both sides in all ALS patients with prominent reduction on the onset side in upper limb onset ALS (SI RIGHT P < 0.001; SI LEFT P < 0.05) and in Awaji definite/probable diagnostic category (SI RIGHT P < 0.05; SI LEFT P < 0.05). Longitudinal studies disclosed that the rate of SI decline correlated with the decline in ALSFRS-R (r = 0.21, P < 0.05). CONCLUSION: The SI is reduced in all ALS subtypes most prominently in upper limb onset disease, on the side of clinical onset, and in patients with Awaji definite/probable diagnostic category. SIGNIFICANCE: The split-hand index is a reliable diagnostic and outcome biomarker across ALS subtypes and may have potential utility in a clinical trial setting, although further multicenter studies are required to confirm this.