Segmental distribution of muscle weakness in SMA III: implications for deterioration in muscle strength with time

Brain magnetic resonance imaging of patients with spinal muscular atrophy type 2 and 3

BACKGROUND AND OBJECTIVE

Proximal spinal muscular atrophy (SMA) is caused by deficiency of the ubiquitously expressed survival motor neuron protein. Although primarily a hereditary lower motor neuron disease, it is probably also characterized by abnormalities in other organs. Brain abnormalities and cognitive impairment have been reported in severe SMA. We aimed to systematically investigate brain structure in SMA using MRI.

METHODS

We acquired high-resolution T1-weighted images of treatment-naive patients with SMA, age- and sex-matched healthy and disease controls with other neuromuscular diseases, on a 3 T MRI scanner. We performed vertex-wise whole brain analysis and region of interest analysis of cortical thickness (CT), and volumetric analysis of the thalamus and compared findings in patients and controls using multiple linear regression models and Wald test. We correlated structural abnormalities with motor function as assessed by the Hammersmith Functional Motor Scale Expanded (HFMSE) and SMA Functional Rating Scale (SMA-FRS).

RESULTS

We included 30 patients, 12-70 years old, with SMA type 2 and 3, 30 age- and sex-matched healthy controls and 17 disease controls (with distal SMA, hereditary motor and sensory neuropathy, multifocal motor neuropathy, progressive muscular atrophy and segmental SMA). We found a reduced CT in patients with SMA compared to healthy controls at the precentral, postcentral and medial orbitofrontal gyri and at the temporal pole (mean differences -0.059(p = 0.04); -0.055(p = 0.04), -0.06(p = 0.04); -0.17 mm(p = 0.001)). Differences at the precentral gyrus and temporal pole were most pronounced in SMA type 2 (mean differences -0.07(p = 0.045); -0.26 mm(p < 0.001)) and were also present compared to disease controls (mean differences -0.08(p = 0.048); -0.19 mm(p = 0.003)). There was a positive correlation between CT at the temporal pole with motor function. Compared to healthy controls, we found a reduced volume of the whole thalamus (mean difference -325 mm3(p = 0.03)) and of the anterior, ventral and intralaminar thalamic nuclei (mean differences -9.9(p = 0.02); -157(p = 0.01); -24.2 mm3(p = 0.02) in patients with SMA and a positive correlation between these volumes and motor function.

CONCLUSION

MRI shows structural changes in motor and non-motor regions of the cortex and the thalamus of patients with SMA type 2 and 3, indicating that SMA pathology is not confined to motor neurons.

Clinical decision making around commercial use of gene and genetic therapies for spinal muscular atrophy

Spinal muscular atrophy is no longer a leading cause of inherited infant death in the United States. Since 2016, three genetic therapies have been approved for the treatment of spinal muscular atrophy. Each therapy has been well studied with robust data for both safety and efficacy. However, there are no head-to-head comparator studies to inform clinical decision making. Thus, treatment selection, timing, and combination therapy is largely up to clinician preference and insurance policies. As the natural history of spinal muscular atrophy continues to change, more data is needed to assist in evidence-based and cost-effective clinical decision making.

Dysfunction of proprioceptive sensory synapses is a pathogenic event and therapeutic target in mice and humans with spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease characterized by a varying degree of severity that correlates with the reduction of SMN protein levels. Motor neuron degeneration and skeletal muscle atrophy are hallmarks of SMA, but it is unknown whether other mechanisms contribute to the spectrum of clinical phenotypes. Here, through a combination of physiological and morphological studies in mouse models and SMA patients, we identify dysfunction and loss of proprioceptive sensory synapses as key signatures of SMA pathology. We demonstrate that SMA patients exhibit impaired proprioception, and their proprioceptive sensory synapses are dysfunctional as measured by the neurophysiological test of the Hoffmann reflex (H-reflex). We further show that loss of excitatory afferent synapses and altered potassium channel expression in SMA motor neurons are conserved pathogenic events found in both severely affected patients and mouse models. Lastly, we report that improved motor function and fatigability in ambulatory SMA patients and mouse models treated with SMN-inducing drugs correlate with increased function of sensory-motor circuits that can be accurately captured by the H-reflex assay. Thus, sensory synaptic dysfunction is a clinically relevant event in SMA, and the H-reflex is a suitable assay to monitor disease progression and treatment efficacy of motor circuit pathology.

Multi-parametric quantitative magnetic resonance imaging of the upper arm muscles of patients with spinal muscular atrophy

Quantitative magnetic resonance imaging (qMRI) is frequently used to map the disease state and disease progression in the lower extremity muscles of patients with spinal muscular atrophy (SMA). This is in stark contrast to the almost complete lack of data on the upper extremity muscles, which are essential for carrying out daily activities. The aim of this study was therefore to assess the disease state in the upper arm muscles of patients with SMA in comparison with healthy controls by quantitative assessment of fat fraction, diffusion indices, and water T2 relaxation times, and to relate these measures to muscle force. We evaluated 13 patients with SMA and 15 healthy controls with a 3-T MRI protocol consisting of DIXON, diffusion tensor imaging, and T2 sequences. qMRI measures were compared between groups and related to muscle force measured with quantitative myometry. Fat fraction was significantly increased in all upper arm muscles of the patients with SMA compared with healthy controls and correlated negatively with muscle force. Additionally, fat fraction was heterogeneously distributed within the triceps brachii (TB) and brachialis muscle, but not in the biceps brachii muscle. Diffusion indices and water T2 relaxation times were similar between patients with SMA and healthy controls, but we did find a slightly reduced mean diffusivity (MD), λ1, and λ3 in the TB of patients with SMA. Furthermore, MD was positively correlated with muscle force in the TB of patients with SMA. The variation in fat fraction further substantiates the selective vulnerability of muscles. The reduced diffusion tensor imaging indices, along with the positive correlation of MD with muscle force, point to myofiber atrophy. Our results show the feasibility of qMRI to map the disease state in the upper arm muscles of patients with SMA. Longitudinal data in a larger cohort are needed to further explore qMRI to map disease progression and to capture the possible effects of therapeutic interventions.

Neuroanatomical Models of Muscle Strength and Relationship to Ambulatory Function in Spinal Muscular Atrophy

BACKGROUND

Individuals with spinal muscular atrophy (SMA) III walk independently, but experience muscle weakness, gait impairments, and fatigue. Although SMA affects proximal more than distal muscles, the characteristic pattern of selective muscle weakness has not been explained. Two theories have been proposed: 1) location of spinal motor neurons; and 2) differences in segmental innervation.

OBJECTIVE

To identify neuroanatomical models that explain the selective muscle weakness in individuals with SMA and assess the relationship of these models to ambulatory function.

METHODS

Data from 23 ambulatory SMA participants (78.2% male), ages 10-56 years, enrolled in two clinical studies (NCT01166022, NCT02895789) were included. Strength was assessed using the Medical Research Council (MRC) score; ambulatory function was measured by distance walked on the 6-minute walk test (6 MWT). Three models were identified, and relationships assessed using Pearson correlation coefficients and linear regression.

RESULTS

All models demonstrated a positive association between strength and function, (p < 0.02). Linear regression revealed that Model 3B, consisting of muscles innervated by lower lumbar and sacral segments, explained 67% of the variability observed in 6 MWT performance (β= 0.670, p = 0.003).

CONCLUSIONS

Muscles innervated by lower lumbar and sacral segments, i.e. hip extensors, hip abductors, knee flexors and ankle dorsiflexors, correlated with and predicted greater ambulatory function. The neuroanatomical patterns of muscle weakness may contribute to a better understanding of disease mechanisms and enable delivery of targeted therapies.

Protocol for a phase II, monocentre, double-blind, placebo-controlled, cross-over trial to assess efficacy of pyridostigmine in patients with spinal muscular atrophy types 2-4 (SPACE trial)

INTRODUCTION

Hereditary proximal spinal muscular atrophy (SMA) is caused by homozygous loss of function of the survival motor neuron 1 gene. The main characteristic of SMA is degeneration of alpha motor neurons in the anterior horn of the spinal cord, but recent studies in animal models and patients have shown additional anatomical abnormalities and dysfunction of the neuromuscular junction (NMJ). NMJ dysfunction could contribute to symptoms of weakness and fatigability in patients with SMA. We hypothesise that pyridostigmine, an acetylcholinesterase inhibitor that improves neuromuscular transmission, could improve NMJ function and thereby muscle strength and fatigability in patients with SMA.

METHODS AND ANALYSIS

We designed a monocentre, placebo-controlled, double-blind cross-over trial with pyridostigmine and placebo to investigate the effect and efficacy of pyridostigmine on muscle strength and fatigability in patients with genetically confirmed SMA. We aim to include 45 patients with SMA types 2-4, aged 12 years and older in the Netherlands. Participants receive 8 weeks of treatment with pyridostigmine and 8 weeks of treatment with placebo in a random order separated by a washout period of 1 week. Treatment allocation is double blinded. Treatment dose will gradually be increased from 2 mg/kg/day to the maximum dose of 6 mg/kg/day in four daily doses, in the first week of each treatment period. The primary outcome measures are a change in the Motor Function Measure and repeated nine-hole peg test before and after treatment. Secondary outcome measures are changes in recently developed endurance tests, that is, the endurance shuttle nine-hole peg test, the endurance shuttle box and block test and the endurance shuttle walk test, muscle strength, level of daily functioning, quality of and activity in life, perceived fatigue and fatigability, presence of decrement on repetitive nerve stimulation and adverse events.

ETHICS AND DISSEMINATION

The protocol is approved by the local medical ethical review committee at the University Medical Center Utrecht and by the national Central Committee on Research Involving Human Subjects. Findings will be shared with the academic and medical community, funding and patient organisations in order to contribute to optimisation of medical care and quality of life for patients with SMA.

TRIAL REGISTRATION NUMBER

NCT02941328.

Segmental Spinal Muscular Atrophy Localised to the Lower Limbs: First case from Oman

Spinal muscular atrophy (SMA) is a genetic lower motor neuron disease. It usually involves all of the skeletal muscles innervated by the anterior horn cells of the spinal cord. In rare cases, there is also localised involvement of the spinal cord. We report a 10-year-old boy who presented to the Sultan Qaboos University Hospital, Muscat, Oman, in 2015 with muscle weakness restricted to the lower limbs. The presence of a homozygous deletion within the survival of motor neuron 1 gene confirmed the diagnosis of SMA. To the best of the authors' knowledge, this is the first report of an Omani patient with segmental SMA involving only the lower limbs. Treatment for this rare and relatively benign form of SMA is symptomatic and includes physiotherapy.

Muscle magnetic resonance imaging in spinal muscular atrophy type 3: Selective and progressive involvement

INTRODUCTION

In this study we sought to identify magnetic resonance imaging (MRI) signs of selective muscle involvement and disease progression in patients with spinal muscular atrophy type 3b (SMA3b).

METHODS

Twenty-five patients with genetically confirmed SMA3b underwent MRI on a 1.5-Tesla MR scanner.

RESULTS

MRI showed significantly more severe involvement of the iliopsoas than of the gluteus maximus muscles, and more severe involvement of the triceps brachii than of the biceps brachii muscles. The quadriceps femoris muscles were severely involved. The deltoid, adductor longus, portions of the hamstrings, gracilis, sartorius, and rectus abdominis muscles were well preserved. We found a significant positive correlation between MRI changes and disease duration for gluteus maximus and triceps brachii. Follow-up MRIs of 4 patients showed disease progression.

CONCLUSIONS

This study confirms the pattern of selective muscle involvement suggested by previous studies and further refines muscle MRI changes in SMA3b. Progressive muscle involvement is implicated. Muscle Nerve 55: 651-656, 2017.

Six-minute walk test is reliable and valid in spinal muscular atrophy

INTRODUCTION

The Six-Minute Walk Test (6MWT) was adopted as a clinical outcome measure for ambulatory spinal muscular atrophy (SMA). However, a systematic review of measurement properties reported significant variation among chronic pediatric conditions. Our purpose was to assess the reliability/validity of the 6MWT in SMA.

METHODS

Thirty participants performed assessments, including the 6MWT, strength, and function. Reproducibility was evaluated by intraclass correlation coefficients. Criterion/convergent validity were determined using Pearson correlation coefficients.

RESULTS

Test-retest reliability was excellent. The 6MWT was associated positively with peak oxygen uptake, Hammersmith Functional Motor Scale Expanded (HFMSE), lower extremity manual muscle testing, knee flexion hand-held dynamometry, and inversely with 10-m walk/run. The 6MWT discriminates between disease severity, unlike the HFMSE.

CONCLUSIONS

This study documents measurement properties of reproducibility, positive criterion validity, and convergent validity with established clinical assessments and reaffirms the value of the 6MWT as a pivotal outcome measure in SMA clinical trials. Muscle Nerve 54: 836-842, 2016.

Single-Blind, Randomized, Controlled Clinical Trial of Exercise in Ambulatory Spinal Muscular Atrophy: Why are the Results Negative?

BACKGROUND

The benefits of exercise on long-term health and well-being are well established. The possible benefits of exercise in Spinal Muscular Atrophy (SMA) have not been explored in a controlled clinical trial format.

OBJECTIVE

To assess the effects of exercise on measures of function, strength, and exercise capacity in ambulatory SMA patients.

METHODS

Fourteen participants, ages 10-48 years, were randomized to control and exercise cohorts after a 1 month lead-in period. The exercise group received 6 months of intervention. Thereafter, both groups received the intervention for the remaining 12 months. Participants were monitored for a total of 19 months. Exercise included individualized home-based cycling and strengthening. The primary outcome measure was distance walked during the six-minute walk test (6MWT). Secondary outcomes included strength, function, exercise capacity, quality of life and fatigue.

RESULTS

Twelve participants completed the first 7 months of the study, and 9 completed all 19 months. At baseline, the groups were similar on all clinical variables. There were no group changes at any time point in the 6MWT, fatigue, or function. Percent-predicted VO2 max improved 4.9% in all participants in 6 months (p = 0.036) (n = 10).

CONCLUSION

Daily exercise is safe in ambulatory SMA and should be encouraged. We did not uncover any deleterious effects on strength, function, or fatigue. Our study documented a reduction in oxidative capacity and a blunted conditioning response to exercise possibly representing an important insight into underlying pathophysiological mechanisms. These findings also may be linked causally to mitochondrial depletion in SMA and warrant further study.

Performance of the timed "up & go" test in spinal muscular atrophy

INTRODUCTION

The timed "up & go" (TUG) test is a quick measure of balance and mobility. TUG scores correlate with clinical, functional, and strength assessment and decline linearly over time. Reliability and validity have not been tested in spinal muscular atrophy (SMA).

METHODS

Fifteen ambulatory SMA participants performed TUG testing and strength, functional, and clinical assessments. Intraclass correlation coefficients quantified test-retest reliability. Convergent validity was determined using Pearson correlation coefficients.

RESULTS

Test-retest reliability was excellent for all participants. TUG was associated significantly with total leg and knee flexor strength, as well as the Hammersmith Functional Motor Scale Expanded, the 10-meter walk/run, and 6-minute walk tests. TUG findings were not associated with knee extensor strength, pulmonary function, or fatigue.

CONCLUSIONS

In SMA, the TUG test is easily administered, reliable, and correlates with established outcome measures. TUG testing is a potentially useful outcome measure for clinical trials and a measure of disability in ambulatory patients with SMA.

Leg muscle function and fatigue during walking in spinal muscular atrophy type 3

INTRODUCTION

Spinal muscular atrophy (SMA) causes muscle weakness and fatigue. Better understanding of the relationship between weakness and fatigue may help identify potential targets for rehabilitation.

METHODS

Gait and surface electromyography (EMG) from 4 muscle groups were measured during the Six-Minute Walk Test (6MWT) in 10 ambulatory participants, aged 9-49 years. Average root mean square amplitude (RMS) of muscle activity was calculated. Strength was assessed using manual and quantitative methods.

RESULTS

RMS, stride length, and velocity decreased during the 6MWT. Knee flexor and hip abductor strength was associated with fatigue-related changes; overall strength correlated with disease duration; and leg strength was associated with 6MWT distance.

CONCLUSIONS

Clinical measures are valid in assessing fatigue and function in SMA, and these assessments can be enhanced by use of gait analysis and EMG. Disease duration and strength measures may represent further stratification refinements when enrolling patients in clinical trials.

Spinal muscular atrophy type III: trying to understand subtle functional change over time--a case report

Spinal muscular atrophy is a relatively stable chronic disease. Patients may gradually experience declines in muscle strength and motor function over time. However, functional progression is difficult to document, and the mechanism remains poorly understood. An 11-year-old girl was diagnosed at 19 months and took a few steps without assistance at 25 months. She was evaluated for 54 months in a prospective multicenter natural history study. Outcome measures were performed serially. From 6 to 7.5 years, motor function improved. From 7.5 to 11 years, motor function declined with increasing growth. Manual muscle testing scores minimally decreased. Motor unit number estimation studies gradually increased over 4.5 years. Compared to the published natural history of spinal muscular atrophy type III, our patient lost motor function over time. However, she walked with assistance 2 years longer than expected. Our report highlights possible precipitating factors that could affect the natural history of spinal muscular atrophy type III.

Muscle volume estimation by magnetic resonance imaging in spinal muscular atrophy

Thigh muscle volume was assessed using magnetic resonance imaging in 16 subjects with spinal muscular atrophy. Scans were successful for 14 of 16 subjects (1 type 1, 6 type 2, and 7 type 3) as young as 5.7 years. Muscle volume with normal and abnormal signal was measured using blinded, semiautomated analysis of reconstructed data. Results were compared with segmental lean mass estimated by dual-energy X-ray absorptiometry and correlated with clinical and electrophysiological measures of disease severity. Muscle volume was reduced with abnormal signal quality. Test-retest reliability (r = .99) and correlation with dual-energy X-ray absorptiometry (r = .91) were excellent. Type 2 subjects had lower volume (3.5 ± 1.6 vs 6.3 ± 2.8 mL/cm height; P = .06) and higher percentage of muscle with abnormal signal (68% ± 20% vs 47% ± 27%; P = .14) than type 3. Reproducibility, tolerability, and strong correlation with clinical measures make magnetic resonance imaging a candidate biomarker for clinical research.

Compensatory mechanisms during walking in response to muscle weakness in spinal muscular atrophy, type III

Our knowledge on altered neurological control of walking due to weakness of various muscle groups of the lower extremities is limited. The aim of this study was to assess kinematic, kinetic and electromyographic (EMG) walking patterns in a functionally homogeneous group of seven subjects with spinal muscular atrophy, type III (SMA group) and compare them with normal data obtained from nine healthy subjects (CONTROL group) in order to identify characteristic compensatory changes. Muscle strength at the ankle and knee joints was assessed using isokinetic dynamometry to determine variability in muscle strength: this was found to be similar in the two groups. Kinematic, kinetic and EMG patterns were assessed during walking in the SMA and CONTROL groups. The results showed changes in the activity of ankle plantarflexors and associated control of the center of pressure during loading response and midstance, which facilitated minimization of the external flexion moment acting on the knee and hip in the SMA group. Additionally, we identified distinct and consistent changes in the control of hip rotators that act to rapidly extend the hip early in stance phase and in the control of contralateral hip abductors that act delay weight shift onto the leg entering the stance phase. From these results we can conclude that the most important muscle groups compensating for reduced strength in knee and hip muscles are the ankle plantarflexors, hip rotators and hip abductors. This finding would have direct application in rehabilitation treatment programs.

Motor function-muscle strength relationship in spinal muscular atrophy

The relationship between motor function and muscle strength in patients with spinal muscular atrophy (SMA) is still controversial. In 120 genetically proven SMA patients, aged 5 years or older, we measured muscle strength in the arms and legs by a hand-held dynamometer, forced vital capacity by a spirometer, and the time needed to walk 10 m, arise from the floor, and climb steps. SMA patients had markedly reduced muscle strength, approximating 20% of that predicted from age- and gender-matched normative data. Knee extensors were the weakest muscles in SMA patients. The young ambulant SMA patients performed better than adults in all the timed tests and had greater muscle strength on knee extension. This study shows a good relationship between motor ability and muscle strength in SMA and confirms that age-related loss of function in SMA is due to loss of muscle strength.

Reliability of hand-held dynamometry in spinal muscular atrophy

We have assessed the reliability of hand-held myometry in 33 patients with spinal muscular atrophy (SMA), testing elbow flexion, handgrip, three-point pinch, knee flexion, knee extension, and foot dorsiflexion, and determining intraclass correlation coefficients (ICC). Interrater reliability was high for upper limbs, with an ICC of 0.92 for three-point pinch and 0.98 for elbow flexion and grip. For lower limbs interrater reliability was good with ICC >0.85 for all measures except foot dorsiflexion. Test-retest results were excellent with ICC >0.91 in all instances. Hand-held myometry is easily performed in SMA patients of various ages and muscle strengths, is a reliable measure of limb muscle strength, and can be used in longitudinal studies and clinical trials.