Motor Unit Number Index (MUNIX) of hand muscles is a disease biomarker for adult spinal muscular atrophy

Pattern of pareses in 5q-spinal muscular atrophy

Background

This prospective study investigates the pattern of pareses in 5q-associated spinal muscular atrophy (SMA) to identify disease-specific characteristics and potential differences from amyotrophic lateral sclerosis (ALS) and spinobulbar muscular atrophy (SBMA). Detailed knowledge about pareses patterns in SMA facilitates differential diagnosis and supports therapeutic monitoring.

Methods

Between January 2021, and June 2021, 66 SMA patients (59.1% male, aged 33.6 ± 15.2 years) were included in the study. Most patients had SMA type II (n = 28) or SMA type III (n = 28), seven patients had SMA type I, and three patients had SMA type IV. We analyzed the pattern of pareses using the UK Medical Research Council (MRC) scoring system.

Results

In both, upper and lower limbs muscle weakness was less pronounced in distal (upper limbs: MRC median 3.0 (interquartile range 1.5-3.5); lower limbs: 1.5 (0.5-3.0)) compared to proximal muscle groups (upper limbs: 2.0 (1.5-2.6); p < 0.001; lower limbs: 0.5 (0.5-1.5); p < 0.001). Thenar muscles were stronger than other small hand muscles (3.0 (2.0-3.5) vs 3.0 (1.5-3.5); p = 0.004). Muscles had more strength in upper (2.3 (1.5-3.1)) compared to lower limbs (1.1 (0.5-2.3); p < 0.001) and in flexors compared to extensors.

Conclusion

We identified a specific pattern of muscle paresis in SMA which is different from the pattern of paresis in ALS and SBMA. As a rule of thumb, the pattern of pareses is similar, but not identical to ALS in distal, but different in proximal muscle groups.

Neurophysiological and imaging biomarkers of lower motor neuron dysfunction in motor neuron diseases/amyotrophic lateral sclerosis: IFCN handbook chapter

This chapter discusses comprehensive neurophysiological biomarkers utilised in motor neuron disease (MND) and, in particular, its commonest form, amyotrophic lateral sclerosis (ALS). These encompass the conventional techniques including nerve conduction studies (NCS), needle and high-density surface electromyography (EMG) and H-reflex studies as well as novel techniques. In the last two decades, new methods of assessing the loss of motor units in a muscle have been developed, that are more convenient than earlier methods of motor unit number estimation (MUNE),and may use either electrical stimulation (e.g. MScanFit MUNE) or voluntary activation (MUNIX). Electrical impedance myography (EIM) is another novel approach for the evaluation that relies upon the application and measurement of high-frequency, low-intensity electrical current. Nerve excitability techniques (NET) also provide insights into the function of an axon and reflect the changes in resting membrane potential, ion channel dysfunction and the structural integrity of the axon and myelin sheath. Furthermore, imaging ultrasound techniques as well as magnetic resonance imaging are capable of detecting the constituents of morphological changes in the nerve and muscle. The chapter provides a critical description of the ability of each technique to provide neurophysiological insight into the complex pathophysiology of MND/ALS. However, it is important to recognise the strengths and limitations of each approach in order to clarify utility. These neurophysiological biomarkers have demonstrated reliability, specificity and provide additional information to validate and assess lower motor neuron dysfunction. Their use has expanded the knowledge about MND/ALS and enhanced our understanding of the relationship between motor units, axons, reflexes and other neural circuits in relation to clinical features of patients with MND/ALS at different stages of the disease. Taken together, the ultimate goal is to aid early diagnosis, distinguish potential disease mimics, monitor and stage disease progression, quantify response to treatment and develop potential therapeutic interventions.

'Reading the palm' - A pilot study of grip and finger flexion strength as an outcome measure in 5q spinal muscular atrophy

BACKGROUND

Innovative RNA modifying and gene replacement therapies are currently revolutionizing the therapeutic landscape in 5q-associated spinal muscular atrophy (SMA). In order to provide individual recommendations for choice of treatment and therapy (dis-) continuation, objective outcome measures are needed. The purpose of this study was to determine whether maximum isometric voluntary grip and finger flexion strength is a useful sensitive outcome measure in children and adult patients with SMA.

METHODS

In this non-interventional, longitudinal pilot study, we assessed grip and finger flexion strength on 39 patients with SMA II and III (n = 16 children, mean age = 10.0; n = 23 adults, mean age = 38.4) using the Weber hand and finger dynamometer HFD 200. Grip and finger flexion strength, clinical examinations and motor function scores (Revised Upper Limb Module, Hammersmith Functional Motor Scale Expanded) were assessed over a 12-month treatment period concurrent with the nusinersen treatment.

RESULTS

Grip and finger flexion strength was highly associated with motor function and disease severity, SMA type and SMN2 copy number. During nusinersen treatment, grip and finger flexion strength significantly increased in children and adults with SMA.

CONCLUSION

Grip and finger flexion strength measured with the HFD 200 is a promising sensitive outcome measure for SMA.

Neuromuscular disorders in the omics era

Neuromuscular disorders encompass a spectrum of conditions characterized by primary lesions within the peripheral nervous system, which include the anterior horn cell, peripheral nerve, neuromuscular junction, and muscle. In pediatrics, most of these disorders are linked to genetic causes. Despite the considerable progress, the diagnosis of these disorders remains a challenging due to wide clinical presentation, disease heterogeneity and rarity. It is noteworthy that certain neuromuscular disorders, once deemed untreatable, can now be effectively managed through novel therapies. Biomarkers emerge as indispensable tools, serving as objective measures that not only refine diagnostic accuracy but also provide guidance for therapeutic decision-making and the ongoing monitoring of long-term outcomes. Herein a comprehensive review of biomarkers in neuromuscular disorders is provided. We highlight the role of omics-based technologies that further characterize neuromuscular pathophysiology as well as identify potential therapeutic targets to guide treatment strategies.

[Experience with the use of risdiplam in a familial case of spinal muscular atrophy 5q in patients with a homozygous deletion of the SMN1 gene and the same copy number of the SMN2 gene]

Advances in the treatment of spinal muscular atrophy (SMA) have revolutionized the field. SMA is a rare autosomal recessive neurodegenerative motor neuron disease in which wide phenotypic variability has been described. The rate of increase in neurological deficit and the severity of the disease is mainly determined by the amount of functional SMN (Survival of Motor Neuron) protein. However, the clinical picture may differ significantly in patients carrying homozygous deletions of the SMN1 gene (Survival of Motor Neuron 1) and an identical number of copies of the SMN2 gene (Survival of Motor Neuron 2). A family clinical case of adult patients with spinal muscular atrophy 5q with a homozygous deletion of the SMN1 gene and the same number of copies of the SMN2 gene, having a different clinical picture of the disease, is presented, and the dynamics of the condition against the background of oral pathogenetic therapy is presented.

Evaluation of risdiplam efficacy in 5q spinal muscular atrophy: A systematic comparison of electrophysiologic with clinical outcome measures

BACKGROUND

To assess compound muscle action potential (CMAP) amplitudes as electrophysiologic markers in relation to clinical outcome in adult patients with 5q-linked spinal muscular atrophy (SMA) before and during treatment with risdiplam.

METHODS

In this monocentric longitudinal cohort study, CMAP of 18 adult patients with SMA type 2 or 3 were assessed at baseline (T0 ) and after 10 months (T10 ) of risdiplam treatment. CMAP amplitudes of the median, ulnar, peroneal, and tibial nerves were compared with established clinical outcome scores, and with the course of disease before start of treatment.

RESULTS

During a pharmacotherapy-naive pre-treatment period of 328 ± 46 days, Revised Upper Limb Module (RULM) score and peroneal nerve CMAP amplitudes decreased, while CMAP of tibial and upper limb nerves remained unchanged. CMAP amplitudes positively correlated with clinical scores (Hammersmith Functional Motor Scale-Expanded [HFMSE], RULM) at T0 . During risdiplam treatment, HFMSE and Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND) scores increased, paralleled by marked increase of CMAP amplitudes in both median nerves (T10 -T0 ; right: Δ = 1.4 ± 1.4 mV, p = 0.0003; left: Δ = 1.3 ± 1.4 mV, p = 0.0007), but not in ulnar, peroneal, or tibial nerves. A robust increase of median nerve CMAP amplitudes correlated well with an increase in the HFMSE score (T10 -T0 ). Median nerve CMAP amplitudes at T0 were associated with subsequent risdiplam-related improvement of HFMSE and CHOP INTEND scores at T10 .

CONCLUSIONS

Median nerve CMAP amplitudes increase with risdiplam treatment in adult SMA patients, and should be further evaluated as potential easy-to-use electrophysiologic markers in assessing and monitoring clinical response to therapy.

Role of circulating biomarkers in spinal muscular atrophy: insights from a new treatment era

Spinal muscular atrophy (SMA) is a lower motor neuron disease due to biallelic mutations in the SMN1 gene on chromosome 5. It is characterized by progressive muscle weakness of limbs, bulbar and respiratory muscles. The disease is usually classified in four different phenotypes (1-4) according to age at symptoms onset and maximal motor milestones achieved. Recently, three disease modifying treatments have received approval from the Food and Drug Administration (FDA) and the European Medicines Agency (EMA), while several other innovative drugs are under study. New therapies have been game changing, improving survival and life quality for SMA patients. However, they have also intensified the need for accurate biomarkers to monitor disease progression and treatment efficacy. While clinical and neurophysiological biomarkers are well established and helpful in describing disease progression, there is a great need to develop more robust and sensitive circulating biomarkers, such as proteins, nucleic acids, and other small molecules. Used alone or in combination with clinical biomarkers, they will play a critical role in enhancing patients' stratification for clinical trials and access to approved treatments, as well as in tracking response to therapy, paving the way to the development of individualized therapeutic approaches. In this comprehensive review, we describe the foremost circulating biomarkers of current significance, analyzing existing literature on non-treated and treated patients with a special focus on neurofilaments and circulating miRNA, aiming to identify and examine their role in the follow-up of patients treated with innovative treatments, including gene therapy.

Biomarkers in 5q-associated spinal muscular atrophy-a narrative review

5q-associated spinal muscular atrophy (SMA) is a rare genetic disease caused by mutations in the SMN1 gene, resulting in a loss of functional SMN protein and consecutive degeneration of motor neurons in the ventral horn. The disease is clinically characterized by proximal paralysis and secondary skeletal muscle atrophy. New disease-modifying drugs driving SMN gene expression have been developed in the past decade and have revolutionized SMA treatment. The rise of treatment options led to a concomitant need of biomarkers for therapeutic guidance and an improved disease monitoring. Intensive efforts have been undertaken to develop suitable markers, and numerous candidate biomarkers for diagnostic, prognostic, and predictive values have been identified. The most promising markers include appliance-based measures such as electrophysiological and imaging-based indices as well as molecular markers including SMN-related proteins and markers of neurodegeneration and skeletal muscle integrity. However, none of the proposed biomarkers have been validated for the clinical routine yet. In this narrative review, we discuss the most promising candidate biomarkers for SMA and expand the discussion by addressing the largely unfolded potential of muscle integrity markers, especially in the context of upcoming muscle-targeting therapies. While the discussed candidate biomarkers hold potential as either diagnostic (e.g., SMN-related biomarkers), prognostic (e.g., markers of neurodegeneration, imaging-based markers), predictive (e.g., electrophysiological markers) or response markers (e.g., muscle integrity markers), no single measure seems to be suitable to cover all biomarker categories. Hence, a combination of different biomarkers and clinical assessments appears to be the most expedient solution at the time.

MUNIX repeatability evaluation method based on FastICA demixing

To enhance the reproducibility of motor unit number index (MUNIX) for evaluating neurological disease progression, this paper proposes a negative entropy-based fast independent component analysis (FastICA) demixing method to assess MUNIX reproducibility in the presence of inter-channel mixing of electromyography (EMG) signals acquired by high-density electrodes. First, composite surface EMG (sEMG) signals were obtained using high-density surface electrodes. Second, the FastICA algorithm based on negative entropy was employed to determine the orthogonal projection matrix that minimizes the negative entropy of the projected signal and effectively separates mixed sEMG signals. Finally, the proposed experimental approach was validated by introducing an interrelationship criterion to quantify independence between adjacent channel EMG signals, measuring MUNIX repeatability using coefficient of variation (CV), and determining motor unit number and size through MUNIX. Results analysis shows that the inclusion of the full (128) channel sEMG information leads to a reduction in CV value by $1.5 \pm 0.1$ and a linear decline in CV value with an increase in the number of channels. The correlation between adjacent channels in participants decreases by $0.12 \pm 0.05$ as the number of channels gradually increases. The results demonstrate a significant reduction in the number of interrelationships between sEMG signals following negative entropy-based FastICA processing, compared to the mixed sEMG signals. Moreover, this decrease in interrelationships becomes more pronounced with an increasing number of channels. Additionally, the CV of MUNIX gradually decreases with an increase in the number of channels, thereby optimizing the issue of abnormal MUNIX repeatability patterns and further enhancing the reproducibility of MUNIX based on high-density surface EMG signals.

The Sensitivity and Specificity of Split-Hand Index Using Muscle Sonography

BACKGROUND

The split-hand index (SHI) (first dorsal interosseous (FDI) × abductor pollicis brevis (APB)/abductor digiti minimi muscle (ADM)) has been suggested as a useful measure for amyotrophic lateral sclerosis (ALS) diagnosis, using electrophysiological and sonographic indices. In the present study, we aimed to explore the specificity of SHI derived by muscle ultrasound (MUS) for the diagnosis of ALS and spinal muscular atrophy (SMA).

METHODS

Healthy controls (n = 65) were prospectively recruited at the Prosserman Family Neuromuscular clinic at Toronto General Hospital, from October to December 2018. In addition, 181 patients with ALS (n = 91), SMA (n = 33), polyneuropathy (n = 35), and myopathy (n = 22) were prospectively recruited at the neuromuscular clinic at Tel Aviv Sourasky Medical Center, from December 2018 to December 2020. All subjects underwent quantitative sonographic evaluation of muscle thickness, including the right APB, FDI, and ADM muscles. Area under curve (AUC), sensitivity, and specificity were determined for differentiating between groups.

RESULTS

Although SHI showed good to excellent accuracy for differentiating each patient subgroup from controls (AUC 0.83-0.92), poorer diagnostic accuracy was shown for differentiating between different patient subgroups (AUC 0.54-0.74).

CONCLUSIONS

Sonographic SHI is useful for differentiating patients from healthy controls, but might be not specific for motor neuron disease.

Evaluation method of motor unit number index based on optimal muscle strength combination

Repeatability is an important attribute of motor unit number index (MUNIX) technology. This paper proposes an optimal contraction force combination for MUNIX calculation in an effort to improve the repeatability of this technology. In this study, the surface electromyography (EMG) signals of the biceps brachii muscle of eight healthy subjects were initially recorded with high-density surface electrodes, and the contraction strength was the maximum voluntary contraction force of nine progressive levels. Then, by traversing and comparing the repeatability of MUNIX under various combinations of contraction force, the optimal combination of muscle strength is determined. Finally, calculate MUNIX using the high-density optimal muscle strength weighted average method. The correlation coefficient and the coefficient of variation are utilized to assess repeatability. The results show that when the muscle strength combination is 10, 20, 50 and 70% of the maximum voluntary contraction force, the repeatability of MUNIX is greatest, and the correlation between MUNIX calculated using this combination of muscle strength and conventional methods is high (PCC > 0.99), the repeatability of the MUNIX method improved by 11.5-23.8%. The results indicate that the repeatability of MUNIX differs for various combinations of muscle strength and that MUNIX, which is measured with a smaller number and lower-level contractility, has greater repeatability.

Identifying Biomarkers of Spinal Muscular Atrophy for Further Development

BACKGROUND

Spinal muscular atrophy (SMA) is caused by bi-allelic, recessive mutations of the survival motor neuron 1 (SMN1) gene and reduced expression levels of the survival motor neuron (SMN) protein. Degeneration of alpha motor neurons in the spinal cord causes progressive skeletal muscle weakness. The wide range of disease severities, variable rates of decline, and heterogenous clinical responses to approved disease-modifying treatment remain poorly understood and limit the ability to optimize treatment for patients. Validation of a reliable biomarker(s) with the potential to support early diagnosis, inform disease prognosis and therapeutic suitability, and/or confirm response to treatment(s) represents a significant unmet need in SMA.

OBJECTIVES

The SMA Multidisciplinary Biomarkers Working Group, comprising 11 experts in a variety of relevant fields, sought to determine the most promising candidate biomarker currently available, determine key knowledge gaps, and recommend next steps toward validating that biomarker for SMA.

METHODS

The Working Group engaged in a modified Delphi process to answer questions about candidate SMA biomarkers. Members participated in six rounds of reiterative surveys that were designed to build upon previous discussions.

RESULTS

The Working Group reached a consensus that neurofilament (NF) is the candidate biomarker best poised for further development. Several important knowledge gaps were identified, and the next steps toward filling these gaps were proposed.

CONCLUSIONS

NF is a promising SMA biomarker with the potential for prognostic, predictive, and pharmacodynamic capabilities. The Working Group has identified needed information to continue efforts toward the validation of NF as a biomarker for SMA.

Split-hand index for amyotrophic lateral sclerosis diagnosis: A frequentist and Bayesian meta-analysis

OBJECTIVE

Preferential wasting of the thenar muscles, the split-hand sign, may be used for early diagnosis of amyotrophic lateral sclerosis (ALS).

METHODS

Electronic databases were searched for studies assessing the split-hand index (SHI) and the compound muscle action potential (CMAP) amplitudes of abductor pollicis brevis (APB), first dorsal interosseous (FDI), and abductor digiti minimi (ADM). The SHI was obtained by multiplying CMAP amplitudes of APB and FDI and dividing the product by the CMAP amplitude of ADM. The Bayesian analysis was used for validation.

RESULTS

In total, 17 studies and 1635 patients were included. Our meta-analysis revealed that ALS patients had significantly decreased SHI (standardized mean difference [SMD], -1.60, P < 0.001), CMAP of the APB (SMD, -1.67, P < 0.001), FDI (SMD, -1.12, P < 0.001), and ADM (SMD, -1.09, P < 0.001). The binormal receiver operating characteristic curve analysis showed a threshold of < 7.4 for SHI, and cutoff values of < 6.4 mV for APB and < 8.4 mV for FDI, respectively. The Bayesian analysis validated decreased SHI in ALS patients (posterior mean difference of - 5.91).

CONCLUSIONS

An SHI of < 7.4 can be used facilitating earlier diagnosis of ALS.

SIGNIFICANCE

SHI can be used as a standard neurophysiological biomarker for early diagnosis.

Split-hand and split-limb phenomena in amyotrophic lateral sclerosis: pathophysiology, electrophysiology and clinical manifestations

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder affecting the upper and lower motor neurons. A key clinical feature of ALS is the absence of accurate, early-stage diagnostic indicators. 'Split-hand syndrome' was first described in ALS at the end of the last century and a considerable body of literature suggests that the split-hand phenomenon may be an important clinical feature of ALS. Considering the published investigations, it is conceivable that the 'split-hand syndrome' results from the associated upper and lower motor neuron degeneration, whose interaction remains to be fully clarified. Additionally, other split syndromes have been described in ALS involving upper or lower limbs, with a nuanced description of clinical and neurophysiological manifestations that may further aid ALS diagnosis. In this review, we endeavour to systematically present the spectrum of the 'split syndromes' in ALS from a clinical and neurophysiology perspective and discuss their diagnostic and pathogenic utility.

Molecular Biomarkers for Spinal Muscular Atrophy: A Systematic Review

Background

There is an unmet need for reliable biomarkers to predict disease severity, prognosis, and treatment effect in patients with spinal muscular atrophy (SMA). The purpose of this review is to evaluate the clinical utility of blood-based biomarkers in patients with SMA.

Methods

A systematic review of MEDLINE, DARE, PEDro, PsycINFO, Cochrane Database, LILACS, OTSeeker, SpeechBITE, CINAHL, Scopus, Science Direct, clinicaltrial.gov, OpenGrey, and Google Scholar was performed with the last search data of June 30, 2019.

Results

Survival motor neuron (SMN)-related biomarkers showed an important interpatient and cell variability with a wide overlap between SMA phenotypes and healthy controls. Several plasma protein analytes correlated with motor scores; however, validation studies are needed to rule out false positives. DNA methylation analysis distinguished between patients with mild/moderate SMA and healthy controls. Plasma phosphorylated neurofilament heavy chain (pNF-H) levels increased with disease severity and declined considerably after nusinersen treatment.

Conclusion

There is no sufficient evidence to support the clinical utility of SMN-related biomarkers to predict disease severity in SMA. pNF-H appears to be a promising biomarker of disease activity and treatment effect in SMA. Further studies should include longitudinal assessments of patients with SMA across functional groups and comparisons with age-matched healthy controls to evaluate the stability of putative biomarkers over time and in response to SMA therapeutics. PROSPERO registration: CRD42019139050.

Update on Biomarkers in Spinal Muscular Atrophy

The availability of disease modifying therapies for spinal muscular atrophy (SMA) has created an urgent need to identify clinically meaningful biomarkers. Biomarkers present a means to measure and evaluate neurological disease across time. Changes in biomarkers provide insight into disease progression and may reveal biologic, physiologic, or pharmacologic phenomena occurring prior to clinical detection. Efforts to identify biomarkers for SMA, a genetic motor neuron disease characterized by motor neuron degeneration and weakness, have culminated in a number of putative molecular and physiologic markers that evaluate biological media (eg, blood and cerebrospinal fluid [CSF]) or nervous system function. Such biomarkers include SMN2 copy number, SMN mRNA and protein levels, neurofilament proteins (NFs), plasma protein analytes, creatine kinase (CK) and creatinine (Crn), and various electrophysiology and imaging measures. SMN2 copy number inversely correlates with disease severity and is the best predictor of clinical outcome in untreated individuals. SMN mRNA and protein are commonly measured in the blood or CSF of patients receiving SMA therapies, particularly those aimed at increasing SMN protein expression, and provide insight into current disease state. NFs have proven to be robust prognostic, disease progression, and pharmacodynamic markers for SMA infants undergoing treatment, but less so for adolescents and adults. Select plasma proteins are altered in SMA individuals and may track response to therapy. CK and Crn from blood correlate with motor function and disease severity status and are useful for predicting which individuals will respond to therapy. Electrophysiology measures comprise the most reliable means for monitoring motor function throughout disease course and are sensitive enough to detect neuromuscular changes before overt clinical manifestation, making them robust predictive and pharmacodynamic biomarkers. Finally, magnetic resonance imaging and muscle ultrasonography are non-invasive techniques for studying muscle structure and physiology and are useful diagnostic tools, but cannot reliably track disease progression. Importantly, biomarkers can provide information about the underlying mechanisms of disease as well as reveal subclinical disease progression, allowing for more appropriate timing and dosing of therapy for individuals with SMA. Recent therapeutic advancements in SMA have shown promising results, though there is still a great need to identify and understand the impact of biomarkers in modulating disease onset and progression.

Increased chitotriosidase 1 concentration following nusinersen treatment in spinal muscular atrophy

Background

Studies regarding the impact of (neuro)inflammation and inflammatory response following repetitive, intrathecally administered antisense oligonucleotides (ASO) in 5q-associated spinal muscular atrophy (SMA) are sparse. Increased risk of hydrocephalus in untreated SMA patients and a marginal but significant increase of the serum/CSF albumin ratio (Qalb) with rare cases of communicating hydrocephalus during nusinersen treatment were reported, which confirms the unmet need of an inflammatory biomarker in SMA. The aim of this study was to investigate the (neuro)inflammatory marker chitotriosidase 1 (CHIT1) in SMA patients before and following the treatment with the ASO nusinersen.

Methods

In this prospective, multicenter observational study, we studied CSF CHIT1 concentrations in 58 adult and 21 pediatric patients with SMA type 1, 2 or 3 before treatment initiation in comparison to age- and sex-matched controls and investigated its dynamics during nusinersen treatment. Concurrently, motor performance and disease severity were assessed.

Results

CHIT1 concentrations were elevated in treatment-naïve SMA patients as compared to controls, but less pronounced than described for other neurodegenerative diseases such as amyotrophic lateral sclerosis. CHIT1 concentration did not correlate with disease severity and did not distinguish between clinical subtypes. CHIT1 concentration did show a significant increase during nusinersen treatment that was unrelated to the clinical response to nusinersen therapy.

Conclusions

CHIT1 elevation in treatment-naïve SMA patients indicates the involvement of (neuro)inflammation in SMA. The lacking correlation of CHIT1 concentration with disease severity argues against its use as a marker of disease progression. The observed CHIT1 increase during nusinersen treatment may indicate an immune response-like, off-target reaction. Since antisense oligonucleotides are an establishing approach in the treatment of neurodegenerative diseases, this observation needs to be further evaluated.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-021-01961-8.

Measuring Outcomes in Adults with Spinal Muscular Atrophy - Challenges and Future Directions - Meeting Report

Spinal muscular atrophy (SMA) is a progressive autosomal recessive motor neuron disease which affects 1 in 6,000-10,000 live births, caused by loss of the survival motor neuron 1 gene (SMN1). A major focus of therapeutic developments has been on increasing the full-length SMN protein by increasing the inclusion of exon 7 in SMN2 transcripts, enhancing SMN2 gene expression, stabilizing the SMN protein or replacing the SMN1 gene.In June 2017, FDA and EMA have approved the antisense oligonucleotide Nusinersen as the first treatment for all SMA subtypes without age restriction. While prominent treatment effects have been observed in the earlier stages of the disease and in patients up to 15 years of age, there is only limited data from clinical trials in adult SMA patients. First real-world data from neuromuscular clinical centers suggest a therapeutic benefit of nusinersen with a favourable safety profile also in adult SMA patients: in several cases, relevant improvements of motor function is achieved, which might lead to enhanced autonomy in daily life activities and improved quality of life. Systematic follow-up of the motor status with validated instruments is crucial for an adequate monitoring of the therapeutic effects but most of the widely used scales and scores have been developed and evaluated for the pediatric population only. International neuromuscular experts have met in Frankfurt/Main, Germany in May 2019 to discuss relevant aspects of the diagnostic pathway and patient management in adult SMA. The recommendations and challenges in this patient population are discussed.

Motor unit number index: A potential electrophysiological biomarker for pediatric spinal muscular atrophy

INTRODUCTION/AIMS

In adult spinal muscular atrophy (SMA), the motor unit number index (MUNIX) has been shown to be an useful electrophysiological biomarker. This study evaluated the feasibility and the clinical relevance of using the MUNIX technique for patients with pediatric SMA (Ped-SMA) and correlated MUNIX results with clinical scores.

METHODS

Fourteen patients with type II Ped-SMA (11 females; median age 11 y [interquartile range (IQR), 4.8-17 y]) and 14 controls (nine females; median age 10.75 y [IQR, 6.5-13.4 y]) were enrolled and matched by sex, age, height, weight, and body mass index. Clinical examination included manual muscle testing, dynamometry (grasp and pinch), and motor function measure (MFM). The MUNIX technique was evaluated in the abductor digiti minimi (ADM) and abductor pollicis brevis (APB) on two sides when possible.

RESULTS

In the patients with Ped-SMA, the MUNIX and compound muscle action potential (CMAP) amplitudes were significantly decreased and the motor size unit index (MUSIX) was significantly increased in the ADM and APB when compared to controls. The intraclass correlation coefficient was good for the intrarater variability of the CMAP amplitude, MUNIX, and MUSIX in the ADM (0.95, 0.83, and 0.89, respectively) and the APB (0.98, 0.96, and 0.94, respectively). The total CMAP amplitude correlated with the grasp and pinch scores (P < .05), and the MUNIX measurements correlated with the MFM scores.

DISCUSSION

The MUNIX technique, which accurately estimated lower motor neuron loss and the number of remaining functional motor units, was shown to be a useful electrophysiological biomarker for disease progression and a potential biomarker for treatment response.

Motor unit number index (MUNIX) in children and adults with 5q-spinal muscular atrophy: Variability and clinical correlations

Spinal muscular atrophy (SMA) is a motor neuron disease associated with progressive muscle weakness and motor disability. The motor unit number index (MUNIX) is a biomarker used to assess loss of motor units in later-onset SMA patients. Twenty SMA patients (SMA types 3 and 4), aged between 7 and 41 years, were clinically evaluated through the Hammersmith Motor Functional Scale Expanded and the Spinal Muscular Atrophy-Functional Rating Scale. The patients underwent compound motor action potential (CMAP) and MUNIX studies of the right abductor pollicis brevis, abductor digiti minimi and tibialis anterior (TA) muscles. Age-matched healthy controls (n = 20) were enrolled to obtain normative CMAP and MUNIX values from the same muscles. Compared to healthy controls, SMA patients showed significant reductions in MUNIX values among all muscles studied, whereas CMAP showed reductions only in the weaker muscles (abductor digiti minimi and TA). MUNIX variability was significantly higher in the SMA group than in the control group. MUNIX variability in TA correlated with CMAP variability. Motor functional scores correlated with TA MUNIX. The MUNIX study is feasible in later-onset SMA patients, and TA MUNIX values correlate with disease severity in patients with mild motor impairment.

Serum creatine kinase and creatinine in adult spinal muscular atrophy under nusinersen treatment

OBJECTIVE

To determine whether serum creatine kinase activity (CK) and serum creatinine concentration (Crn) are prognostic and predictive biomarkers for disease severity, disease progression, and nusinersen treatment effects in adult patients with 5q-associated spinal muscular atrophy (SMA).

METHODS

Within this retrospective, multicenter observational study in 206 adult patients with SMA, we determined clinical subtypes (SMA types, ambulatory ability) and repeatedly measured CK and Crn and examined disease severity scores (Hammersmith Functional Motor Scale Expanded, Revised Upper Limb Module, and revised Amyotrophic Lateral Sclerosis Functional Rating Scale). Patients were followed under nusinersen treatment for 18 months.

RESULTS

CK and Crn differed between clinical subtypes and correlated strongly with disease severity scores (e.g., for Hammersmith Functional Motor Scale Expanded: (CK) ρ = 0.786/ (Crn) ρ = 0.558). During the 18 months of nusinersen treatment, CK decreased (∆CK = -17.56%, p < 0.0001), whereas Crn slightly increased (∆Crn = +4.75%, p < 0.05).

INTERPRETATION

Serum creatine kinase activity and serum creatinine concentration reflect disease severity of spinal muscular atrophy and are promising biomarkers to assess patients with spinal muscular atrophy during disease course and to predict treatment response. The decrease of creatine kinase activity, combined with the tendency of creatinine concentration to increase during nusinersen treatment, suggests reduced muscle mass wasting with improved muscle energy metabolism.

Development of new outcome measures for adult SMA type III and IV: a multimodal longitudinal study

OBJECTIVE

The aim of this study was the comprehensive characterisation of longitudinal clinical, electrophysiological and neuroimaging measures in type III and IV adult spinal muscular atrophy (SMA) with a view to propose objective monitoring markers for future clinical trials.

METHODS

Fourteen type III or IV SMA patients underwent standardised assessments including muscle strength testing, functional evaluation (SMAFRS and MFM), MUNIX (abductor pollicis brevis, APB; abductor digiti minimi, ADM; deltoid; tibialis anterior, TA; trapezius) and quantitative cervical spinal cord MRI to appraise segmental grey and white matter atrophy. Patients underwent a follow-up assessment with the same protocol 24 months later. Longitudinal comparisons were conducted using the Wilcoxon-test for matched data. Responsiveness was estimated using standardized response means (SRM) and a composite score was generated based on the three most significant variables.

RESULTS

Significant functional decline was observed based on SMAFRS (p = 0.019), pinch and knee flexion strength (p = 0.030 and 0.027), MUNIX and MUSIX value in the ADM (p = 0.0006 and 0.043) and in TA muscle (p = 0.025). No significant differences were observed based on cervical MRI measures. A significant reduction was detected in the composite score (p = 0.0005, SRM = -1.52), which was the most responsive variable and required a smaller number of patients than single variables in the estimation of sample size for clinical trials.

CONCLUSIONS

Quantitative strength testing, SMAFRS and MUNIX readily capture disease progression in adult SMA patients. Composite multimodal scores increase predictive value and may reduce sample size requirements in clinical trials.

Clinical and radiological profile of patients with spinal muscular atrophy type 4

BACKGROUND AND PURPOSE

Spinal muscular atrophy (SMA) is the most important cause of motor neuron disease in childhood, and continues to represent the leading genetic cause of infant death. Adulthood-onset SMA (SMA type 4) is rare, with few isolated cases reported. The objective of the present study was to describe a cohort of patients with SMA type 4.

METHODS

A cross-sectional study was conducted to characterize clinical, genetic, radiological and neurophysiological features of patients with adulthood-onset SMA. Correlation analysis of functional assessment with genetic, radiological and neurophysiological data was performed.

RESULTS

Twenty patients with SMA type 4 were identified in a Brazilian cohort of 227 patients with SMA. The most common clinical symptom was limb-girdle muscle weakness, observed in 15 patients (75%). The most frequent neurological findings were absent tendon reflexes in 18 (90%) and fasciculations in nine patients (45%). Sixteen patients (80%) had the homozygous deletion of exon 7 in the SMN1 gene, with 12 patients (60%) showing four copies of the SMN2 gene. The functional scales Hammersmith Functional Motor Scale Expanded, Amyotrophic Lateral Sclerosis Functional Rating Scale Revised, Revised Upper Limb Module and Spinal Muscular Atrophy Functional Rating Scale, as well as the six-minute walk and the Time Up and Go tests showed a correlation with duration of disease. Motor Unit Number Index was correlated both with duration of disease and with performance in functional assessment. Radiological studies exhibited a typical pattern, with involvement of biceps femoris short head and gluteus minimus in all patients.

CONCLUSION

This study represents the largest cohort of patients with SMA type 4 and provides functional, genetic, radiological and neurophysiological features that can be used as potential biomarkers for the new specific genetic therapies for SMA.

Split hand and motor axonal hyperexcitability in spinal and bulbar muscular atrophy

OBJECTIVE

The 'split hand' sign refers to preferential wasting of the thenar and first dorsal interosseous muscles with relatively sparing of the hypothenar muscles in amyotrophic lateral sclerosis (ALS) and both cortical and spinal/peripheral excitotoxic mechanisms have been proposed. We aimed to study split hand and axonal excitability in spinal and bulbar muscular atrophy (SBMA) in which cortical motor neurons are intact.

METHODS

In 35 patients with genetically confirmed SBMA, 55 with ALS, 158 with other neuromuscular diseases and 90 normal controls; split hand was strictly determined by amplitudes of compound muscle action potentials. Nerve excitability testing of median motor axons was performed in 35 SBMA and 55 patients with ALS and 45 normal controls.

RESULTS

Split hand was as frequently found for patients with SBMA (57%) and ALS (62%), compared with disease (20%) and normal (0%) controls. Excitability testing showed that in both SBMA and ALS, strength-duration time constant was longer, and threshold changes in depolarising threshold electrotonus and superexcitability in the recovery cycle were greater than in normal controls (p<0.01).

CONCLUSIONS

Split hand is not specific to ALS and can be caused by the peripheral mechanism alone in SBMA, whereas the effect of upper motor neuron lesion cannot be excluded in ALS. Our results also suggest that SBMA and ALS share common axonal excitability changes; increased nodal persistent sodium and reduced potassium currents that may accelerate motor neuronal death and differently affect axons-innervating different muscles. Ion channel modulators could be a therapeutic option for both SBMA and ALS.

Motor unit changes in children with symptomatic spinal muscular atrophy treated with nusinersen

OBJECTIVES

To elucidate the motor unit response to intrathecal nusinersen in children with symptomatic spinal muscular atrophy (SMA) using a novel motor unit number estimation technique.

METHODS

MScanFit MUNE studies were sequentially undertaken from the abductor pollicis brevis muscle after stimulation of the median nerve in a prospective cohort of symptomatic children with SMA, undergoing intrathecal treatment with nusinersen at a single neuromuscular centre from June 2017 to August 2019. Electrophysiological measures included compound muscle action potential (CMAP), motor unit number estimation (MUNE), motor unit number contributing to 50%-100% of CMAP (N50) and measures of collateral reinnervation including largest single motor unit potential (LSMUP) and amplitude of the smallest unit contributing to N50 (A50).

RESULTS

Twenty children (median age 99 months, range 4-193) were followed for a median of 13.8 (4-33.5) months. Therapeutic intervention was an independent and significant contributor to an increase in CMAP (p = 0.005), MUNE (p = 0.001) and N50 (p = 0.04). The magnitude of this electrophysiological response was increased in children with shorter disease durations (p<0.05). Electrophysiological changes delineated children who were functionally stable from those who attained clinically significant gains in motor function.

INTERPRETATION

Nusinersen therapy facilitated functional innervation in SMA through recovery of smaller motor units. Delineation of biomechanisms of therapeutic response may be the first step in identifying potential novel targets for disease modification in this and other motor neuropathies. MScanFit MUNE techniques may have a broader role in establishing biomarkers of therapeutic response in similar adult-onset diseases.

Cutaneous silent period in patients with spinal muscular atrophy type 2 and type 3

OBJECTIVE

The aim of this study was to examine cutaneous silent periods (CSPs) in spinal muscular atrophy (SMA) type 2 and type 3 patients.

METHODS

Fourteen patients with SMA and 14 healthy subjects were included. CSPs were recorded from thenar muscles after painful stimulation of the index finger during slight thumb abduction. CSP parameters (including onset latency, duration, index of suppression, long-loop reflex, and post-inhibitory rebound activity) were measured. All parameters were compared between SMA patients and healthy subjects using the Mann-Whitney U test. We then correlated CSP parameters to Hammersmith Functional Motor Scale Expanded (HFMSE) scores in SMA patients by Spearman-rho testing.

RESULTS

No CSP parameter except E3% (i.e., the post-inhibitory rebound activity relative to pre-stimulus baseline) differed significantly between SMA patients and healthy subjects. E3% was significantly smaller in patients than healthy individuals. HFMSE scores of SMA patients correlated negatively with CSP duration and positively with E3%.

DISCUSSION

Smaller E3%, correlating with longer CSP duration, in SMA patients with more severe clinical findings and therefore more marked motoneuron loss suggests that the E3 component following the CSP is directly affected by motoneuron loss.

Split-hand phenomenon in motor neuron diseases: Sonographic assesment of muscle thickness

OBJECTIVE

To explore the diagnostic accuracy of the split-hand index (SHI) for amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) using sonographic assessment of muscle thickness.

METHODS

We performed a prospective sonographic assessment of hand muscle thickness in 59 controls, 87 patients with ALS, and 33 patients with SMA. We determined the diagnostic accuracy of SHI for differentiating patients with ALS and SMA from controls.

RESULTS

Patients with ALS and SMA had significantly lower muscle thickness and SHI values compared with controls. SHI showed excellent diagnostic accuracy for differentiating ALS from controls, and good diagnostic accuracy for differentiating SMA from controls.

CONCLUSIONS

SHI determined by sonographic measurement of hand muscle thickness seems to be a promising tool for the diagnosis of ALS and may be added easily when performing neuromuscular ultrasound.

SIGNIFICANCE

SHI determined by sonographic measurement of hand muscle thickness can differentiate between healthy subjects and patients with ALS and SMA.

Motor unit number index in children with later-onset spinal muscular atrophy

BACKGROUND

Motor unit number index (MUNIX) is a validated electrophysiological biomarker in amyotrophic lateral sclerosis. MUNIX studies in spinal muscular atrophy (SMA) are limited.

METHODS

Later-onset SMA children (n = 13; three SMN2 copy number) were evaluated for Hammersmith Motor Function Scale Expanded (HMFSE) and MUNIX of right abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles. Age-matched healthy controls (n = 8) were enrolled to obtain normative APB and ADM MUNIX values.

RESULTS

Mean APB and ADM MUNIX values in SMA subjects were significantly reduced (P < .001) compared with controls. HMFSE scores strongly correlated with ADM MUNIX (r 0.63).

CONCLUSIONS

APB and ADM muscle MUNIX studies are feasible in SMA type 2 children. ADM MUNIX correlated with disease severity on motor function testing. MUNIX studies in later-onset SMA could be a potential biomarker of motor neuron loss.

Patient-Reported Prevalence of Non-motor Symptoms Is Low in Adult Patients Suffering From 5q Spinal Muscular Atrophy

Background: 5q spinal muscular atrophy (SMA) is an autosomal recessive lower motoneuron disease caused by deletion or mutations in the survival motor neuron 1 gene (SMN1) which results in reduced expression of full-length SMN protein. The main symptoms are caused by spinal motor neuron demise leading to muscle atrophy, and medical care mostly refers to motor symptoms. However, new insights of recent studies in severe SMA type I revealed disease involvement of several non-motor regions, for example cardiac, vascular, sensory nerve involvement, and thalamic lesions. Non-motor symptoms (NMS) were previously described in many neurodegenerative diseases i.e., Parkinson's disease and, importantly, also amyotrophic lateral sclerosis. Methods: We screened for NMS in 70 adult patients with SMA type II (SMAII) and type III (SMAIII) and 59 age/sex-matched healthy controls (controls) in a multicenter cross-sectional study including 5 different centers with specialized expertise in medical health care of motoneuron diseases. We used a self-rating questionnaire including 30 different items of gastrointestinal, autonomic, neuropsychiatric, and sleep complaints [NMS questionnaire (NMSQuest)], which is a validated tool in Parkinson's disease. Results: Total NMS burden was low in adult SMA (median: 3 items) and not significantly different compared to controls (median: 2 items). Total NMS of SMA patients did not correlate with disease severity scores. However, the items "swallowing difficulties," "falling," and particularly "swelling legs" were significantly more frequently reported in SMA. Neuropsychiatric symptoms were reported in a frequency comparable to controls and were not significantly increased in SMA. Conclusion: Patient-reported prevalence of NMS in adult SMA was low, which does not argue for a clinically relevant multisystemic disorder in SMAII/III. Importantly, adult SMA patients do not seem to suffer more frequently from symptoms of depression or adaptive disorders compared to controls. Our results yield novel information on previously underreported symptoms and will help to improve the medical guidance of these patients.

Far-field potentials in the compound muscle action potential

It has long been believed that the compound muscle action potential (CMAP) in motor-nerve conduction studies (MCSs) records the action potential beneath the active electrode over the muscle belly. However, recent studies have revealed the contribution of the reference electrode to the CMAP, most prominent in the tibial nerve, followed by the ulnar nerve. This "reference electrode potential" is recorded when the conventional reference electrode distal to the muscle belly is connected to a proximal reference. It must be a far-field potential (FFP) considering its distribution, although the precise mechanism of its generation has not been clarified. The conventional theory of termination of the action potential at the muscle-tendon junction is insufficient. Regarding the ulnar CMAP, interosseous muscles mostly contribute to the FFPs. New understanding of CMAP based on the FFP theory may provide new insights into the interpretation of MCSs and related techniques, including motor unit number estimation.

Biomarkers and the Development of a Personalized Medicine Approach in Spinal Muscular Atrophy

Recent unprecedented advances in treatment for spinal muscular atrophy (SMA) enabled patients to access the first approved disease modifying therapy for the condition. There are however many uncertainties, regarding timing of treatment initiation, response to intervention, treatment effects and long-term outcomes, which are complicated by the evolving phenotypes seen in the post-treatment era for patients with SMA. Biomarkers of disease, with diagnostic, prognostic, predictive, and pharmacodynamic value are thus urgently required, to facilitate a wider understanding in this dynamic landscape. A spectrum of these candidate biomarkers, will be evaluated in this review, including genetic, epigenetic, proteomic, electrophysiological, and imaging measures. Of these, SMN2 appears to be the most significant modifier of phenotype to date, and its use in prognostication shows considerable clinical utility. Longitudinal studies in patients with SMA highlight an emerging role of circulatory markers such as neurofilament, in tracking disease progression and response to treatment. Furthermore, neurophysiological biomarkers such as CMAP and MUNE values show considerable promise in the real word setting, in following the dynamic response and output of the motor unit to therapeutic intervention. The specific value for these possible biomarkers across diagnosis, prognosis, prediction of treatment response, efficacy, and safety will be central to guide future patient-targeted treatments, the design of clinical trials, and understanding of the pathophysiological mechanisms of disease and intervention.

Neurochemical markers in CSF of adolescent and adult SMA patients undergoing nusinersen treatment

Background:

There is limited information on neurochemical markers being used to support and monitor the affection of motoneurons in patients with spinal muscular atrophy (SMA). The objective of this study was to examine neurochemical markers in cerebrospinal fluid (CSF) under treatment with the antisense-oligonucleotide (ASO), nusinersen.

Methods:

We measured markers of axonal degeneration [neurofilament light chain (NfL) and phosphorylated neurofilament heavy chain (pNfH)] along with basic CSF parameters in 25 adolescent and adult SMA type 2 and 3 patients at baseline and after four intrathecal injections of nusinersen. Neurochemical markers were compared with controls. In addition, neurochemical markers in SMA patients were related to the Hammersmith Functional Rating Scale Expanded (HFMSE).

Results:

No significant difference in neurofilament (Nf) values was observed between SMA and control group, neither at baseline nor after four injections of nusinersen. NfL, protein and quotients of albumin (Q_alb) increased slightly in SMA patients after the fourth injection. The slight increase of NfL could be related to the development of mild CSF flow change. No relations were observed between changes in Nf and HFMSE.

Conclusion:

We assume that Nf levels in CSF in these patients may result from slow disease progression in this stage of disease, pre-existing loss of motoneurons due to long disease duration besides affection of the LMN only. Therefore, we conclude that Nf levels in CSF do not seem useful as diagnostic and monitoring markers in adolescent and adult SMA type 2 and 3 patients.