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

Abstract

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.

Keywords

Spinal muscular atrophy walking muscle weakness walk test strength

INTRODUCTION

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease that causes deficiency in the survival motor neuron (SMN) protein. The pathologic hallmark of SMA is degeneration of motor neurons of the anterior horn in the spinal cord, which leads to skeletal muscle weakness and progressive loss of motor function. Individuals with SMA III achieve the ability to walk without support during their life but may not retain that ability due to progressive muscle weakness, gait impairments and fatigue [1 –3]. Although SMA, like other neuromuscular disorders, affects proximal muscles more than distal, there is a characteristic pattern of selective weakness that has yet to be explained [3, 4]. This study aims to identify neuroanatomical models that may help explain the selective muscle weakness seen in ambulatory individuals with SMA and assess the relationship of these models to function.

It has been suggested that the characteristic pattern of selective muscle weakness indicates that subsets of motor neurons, or the circuits that control their activity, are particularly vulnerable in SMA [5]. Two neuroanatomical theories have been proposed to help explain the patterns of muscle weakness observed in SMA: 1) motor neuron pool location contributes to patterns of weakness [5, 6] and 2) segmental innervation of muscles contribute to patterns of weakness [4, 7]. Other potential explanations for the patterns of weakness include the characteristic muscle fiber type distribution seen in SMA [8, 9], changes at the neuromuscular junction [10, 11] and impaired mitochondrial function [12, 13].

In the lower extremity, muscles with preserved strength commonly include hip extensors, hip abductors, knee flexors, ankle dorsiflexors and ankle plantar flexors, while vulnerable muscles include hip flexors, hip adductors and knee extensors [3 , 15]. Functionally, these patterns of weakness result in difficulty rising from the floor or a seated position, negotiating stairs, walking and running. Previous work has shown that hip abductors and knee flexors play a key role in compensatory gait patterns characteristic to SMA [14]. Understanding the progression of muscle weakness based on neuroanatomical models may help clinicians focus their efforts on specific muscle groups that may contribute to improving function and help prolong ambulatory status.

Disease modifying therapies are now available for SMA that improve survival and contribute to clinically meaningful improvements in motor function [16 –18], where optimal outcomes are seen with early treatment [19]. Despite the notable success of these treatments a cure remains elusive; weakness and functional limits persist in most treated patients. As such, the need to better understand mechanisms of selective proximal muscle weakness in SMA has become increasingly important to support the breakthroughs that have been made with drug therapy.

MATERIALS AND METHODS

Data were obtained from 23 individuals with SMA ages 10–56 years old (78.3% male) participating in two clinical trial studies, NCT01166022 and NCT022895789. All participants were able to walk without support. Participant characteristics are listed in Table 1. Informed consent was obtained for their respective studies. Data from the baseline assessment were used for analysis in this study. Strength of sixteen lower extremity muscle groups was assessed using the Medical Research Council (MRC) score. Ambulatory function was measured by distance walked on the six-minute walk test (6 MWT).

Table 1

Participant Characteristics

Variable	Mean (SEM)	Range
Age (years)	28.0 (16.5)	10–56
Age at symptom onset (years)	7.34 (5.1)	1–16
Disease Duration (years)	19.52 (15.4)	5–55
Gender (% male)	78.3%
Spinraza (%)	34.7%
Six minute walk test (6MWT) distance (meters)	349 (23.4)	117–557
Hip extension MMT^**	7.7 (0.6)	4–13
Hip flexors MMT	6.6 (0.8)	4–15
Hip abductors MMT	10.5 (0.8)	4–16
Hip adductors MMT	4.3 (0.3)	1–8
Knee extensors MMT	9.3 (1.0)	0–18
Knee flexors MMT	13.3 (0.7)	4–18
Ankle dorsiflexors MMT	19.0 (0.4)	14–20
Ankle plantar flexors MMT	19.0 (0.7)	4–20
LE Composite^*** MMT	89.7 (3.7)	46–123
LE Composite without plantar flexors MMT	70.6 (3.4)	42–103

^*Age of symptom onset and disease duration missing for 1 participant. ^**MMT was scored using the 10-point Medical Research Council Scale (MRC). MMT score was the sum of both left and right lower extremities (maximal possible score = 20). ^***Lower extremity (LE) Composite is the sum of all 16 lower extremity muscles included. The maximum possible score = 160. SEM = Standard error of the mean.

Strength assessment

Strength of 16 lower extremity muscle groups (8 on each lower extremity) was assessed by two physical therapists. These muscle groups included: hip flexors, hip extensors, hip abductors, hip adductors, knee flexors, knee extensors, ankle dorsiflexors and ankle plantar flexors. Muscle strength was assessed bilaterally and graded on the 10-point MRC scale in order to calculate composite scores for analysis. Strength was calculated as the sum of MRC scores for the muscles in each model.

Six-minute walk test (6 MWT)

The 6 MWT measures the distance walked in six minutes on a 25-meter course. It has been shown to be a valid and reliable measure of functional exercise capacity and ambulatory function in SMA [20]. It has also been associated with motor function and strength in SMA [21]. All participants completed the 6MWT successfully, walking without support and without seated rest breaks over the duration of six minutes. In this study, the distance walked in meters on the 6 MWT served as an indicator of ambulatory function.

Model development

Three models were tested to examine the relationship of strength and function. Model 1, included hip abductors and knee flexors, and was based on prior biomechanical studies that identified these groups as key muscles for the compensatory gait pattern seen in ambulatory individuals with SMA [14]. Model 1 was included to investigate previously identified muscle groups (hip abductors and knee flexors) as predictors of function. Models 2 and 3 were developed with a neuroanatomical framework. Model 2 was developed to analyze muscle groups based on the hypothesized location of motor neuron pools within the spinal cord [5, 6]. Models 2A and 2B included muscle groups with presumed medially and laterally located motor neuron pools, respectively. Model 3 was developed to analyze muscle groups based on their segmental innervation [4, 15]. Model 3A included muscle groups innervated predominantly by L1–L3 and Model 3B included muscles innervated predominantly by L4–S2. Muscle groups included in each model are described in Table 2.

Table 2

Description of Neuroanatomical Models

Model 1: muscle groups suggested to be key for compensatory gait
Hip abductors
Knee flexors
Model 2: motor neuron location
Model 2A: muscle groups with presumed medial motor neuron pools	Model 2B: muscle groups with presumed lateral motor neuron pools
Hip flexors	Knee flexors
Hip extensors	Knee extensors
Hip adductors	Ankle dorsiflexors
Hip abductors	Ankle plantar flexors^*
Model 3: segmental innervations
Model 3A: muscle groups innervated predominantly by L1–L3	Model 3B: muscle groups innervated predominantly by L4–S2
Hip flexors	Hip extensors
Hip adductors	Hip abductors
Knee extensors	Knee flexors
	Ankle dorsiflexors
	Ankle plantar flexors^*

^*Plantar flexors were not included in statistical analyses.

Statistical methods

A composite score of muscle strength was used to assess the relationship to function for each model. Muscles displaying full strength (19–20/20) in greater than 90% of participants were excluded from analyses. Plantar flexors, demonstrating full strength in all but 8% of participants, were excluded to allow focus on other muscles whose variance in strength may better explain differences in function. Pearson correlation coefficients were used to determine the relationship between strength and function. Linear regression was used to determine which models were predictors of ambulatory function.

RESULTS

Average MRC scores, presented in Table 1, demonstrated a pattern of strength with increasing weakness in distal to proximal fashion. The strongest muscle group was the plantar flexors with greatest weakness observed in the hip flexors and hip adductors. Generally, we observed greater strength of plantar flexors followed by dorsiflexors, knee flexors, hip abductors, knee extensors, hip extensors, hip flexors and hip adductors. Similar patterns of weakness have been demonstrated in both ambulatory and non-ambulatory individuals [3].

Across all models, strength was associated with function as measured by distance walked on the 6 MWT. Model 1 had a moderate correlation (r = 0.62, p = 0.001), demonstrating that combined strength of hip abductors and knee flexors was associated with increased distance walked during the 6 MWT. Model 2A and 2B both demonstrated a moderate correlation with function (respectively; r = 0.67, p = 0.0005, r = 0.54, p = 0.007). Model 3A had a moderate correlation with function (r = 0.48, p = 0.02), whereas, Model 3B had a strong correlation with function (r = 0.71, p < 0.0001). Linear regression was not applied to Model 1 because it was not based on a neuroanatomical framework. Model 2 was also not assessed because location of motor neuron pools could not be confirmed in this study. Linear regression of Model 3 revealed that Model 3B was a significant predictor of function (β= 0.670, p = 0.003), whereas Model 3A was not (β= 0.073, p = 0.715).

DISCUSSION

Consistent with the theory that segmental innervation favors selective proximal weakness, we observed that individuals with greater strength of hip extensors, hip abductors, knee flexors and ankle dorsiflexors performed better on the 6 MWT. These four muscle groups are innervated primarily by nerve roots arising from lower lumbar and sacral spinal segments. Therefore, strength in muscles innervated predominantly by L4–S2 nerve roots predicted functional mobility in ambulatory individuals with SMA. Combined strength of these muscles accounted for 67% of the variability in 6 MWT performance.

Overall, the pattern of strength, calculated by average MRC scores ranging from strongest to weakest, was plantar flexors followed by dorsiflexors, knee flexors, hip abductors, knee extensors, hip extensors, hip flexors and hip adductors. Wadman et al. demonstrated a similar pattern among 180 individuals with SMA, both non-ambulatory and ambulatory. Patients had their lowest MRC scores for hip flexors and knee extensors, while knee flexors were relatively preserved [3]. In this study, the lowest MRC scores were seen in hip adductors and hip flexors. Nineteen (83%) participants demonstrated weakness in hip adductors and 15 participants (65%) demonstrated weakness in hip flexors (MRC≤5).

The muscle groups identified above are all critically important for gait in healthy individuals and may also play a key role in gait for individuals with SMA. It has been shown that individuals with SMA have a reduced step length when compared to healthy age-matched controls [14, 21]. In normal gait, gastrocnemius strength helps to determine step length during pre-swing and the dorsiflexors play a critical role in foot clearance [22]. These muscle groups are relatively preserved in SMA and may play a similar beneficial role in the gait cycle. Hip extensor strength correlates with the ability to control the limb during the stance phase of gait, thus allowing the individual to maintain forward momentum [22], which may also help to preserve ambulatory function in SMA. Additionally, the knee flexors have been shown to assist with hip extension in stance phase and to slow the acceleration of the tibia in swing phase. Due to weakness in the hip flexors, individuals with SMA use pelvic rotation for forward limb advancement. The hip abductors play a key role in this mechanism by stabilizing the pelvis and trunk during single leg stance [14, 23]. Future biomechanical studies of these muscle groups would help to confirm their role in SMA.

Role of segmental innervation

Individuals with SMA have profound muscle weakness in a typical pattern where proximal muscles are more affected than distal muscles, and the lower extremity is more affected than the upper extremity [24]. It has been proposed that the observed patterns of weakness appear to be segmental in distribution [4, 7]. In the lower extremities, muscles (iliopsoas and hip adductors) innervated by upper lumbar segments have been shown to be more involved compared with muscles (gluteus maximus and hip abductors) innervated by lower lumbar and sacral segments during early stage of disease [3, 4]. Although only the lower extremity was examined in this study, similar patterns are thought to exist in the upper extremity, with muscles innervated by lower cervical segments more vulnerable to weakness [4]. Recent radiographic studies in humans with SMA have shown decreases in the cross-sectional area of lower cervical segments, specifically C7, owing to weakness patterns observed in the upper extremity, with triceps brachii being more affected than biceps brachii [25].

In contrast to our findings, Wadman et al. concluded that weakness was not segmentally distributed. Notably, their study included participants with all types of SMA, while this cohort included only ambulatory individuals. While SMA is a monogenic disease, there is broad phenotypic heterogeneity that might explain observed differences. Further, the influence of musculoskeletal complications associated with the more severe phenotypes, such as limb contractures and scoliosis, may not represent the neuroanatomical basis of the patterns of weakness. In other studies that included a similar distribution of participants, the findings also supported a segmental pattern of involvement [4].

Segmental patterns of weakness have been demonstrated in SMA mouse models. It is speculated that this may reflect selective motor neuron loss originating from the inability of proprioceptive afferents to excite motor neurons at certain spinal cord levels [5]. Specifically, monosynaptic responses emerging from the L1 ventral nerve root are significantly reduced in SMA mice spinal cord [5]. It has also been shown that specific proteins capable of promoting motor neuron death, such as extracellular signal-regulated kinases, appear to be upregulated in certain regions of the lumbar spinal cord, contributing to the observed patterns of muscle weakness [26].

Selective vulnerability of medial motor neurons in SMA

Model 2 examined the theory that muscle weakness is influenced by motor neuron pool location in the spinal cord. Recent MRI studies have shown increasing fatty infiltration and atrophy of muscles innervated by medial motor neuron pools [27]. In ambulant individuals with SMA, the greatest muscle involvement on MRI was in the gluteus maximus, quadriceps, semitendinosus, gracilis and soleus. The sartorius, adductor longus and brevis, peronei and gastrocnemii muscles were relatively spared [27]. In this study, muscle groups were examined without distinguishing between specific muscles. If individual muscles are considered, segmental innervation alone does not appear to account for the increased levels of fatty infiltration in the gluteus maximus, semitendinosus and soleus, which are innervated by lower lumbar and sacral segments. The patterns of muscle involvement seen on MRI indicate greater involvement of muscles with medial motor neuron pools (gluteus maximus, quadriceps, semitendinosus and gracilis) despite segmental innervation level [27]. However, fatty infiltration and atrophy may not necessarily correlate with strength and function. Future work should consider patterns of weakness in individual muscles and degree of fatty infiltration/atrophy in model development to better understand the relationship to function.

In patients with SMA, the most severely affected muscle groups are proximal or axial, which are known to be innervated by medial motor neurons. In SMA mouse models, examination of the L5 spinal segment allows for distinguishing between medial and lateral motor neuron pools. Deafferentation occurs earlier and is more severe in medial compared to lateral motor neuron pools in agreement with the known progression of muscle weakness in model mice and humans [28]. It has also been suggested that SMN expression is differentially regulated in medial versus lateral motor neurons [5]. In a study by Simon et al. it was suggested that nuclear accumulation of p53 is a specific downstream event induced by SMN deficiency that may account for neuronal death in SMA [28]. This study showed increased p53 in both medial and lateral motor columns. Interestingly, there was greater p53 phosphorylation in vulnerable medial motor columns (MMC), in L1 and L5 segments, but not in resistant lateral motor columns. Further, it was indicated that onset of p53 accumulation occurs earlier in L1 MMC-motor neurons than in L5 MMC-motor neurons [28]. Together these findings appear to further support the theory both of motor neuron pool location and segmental innervation influence on the patterns of weakness.

Animal models have provided valuable insight into the development of pathology in SMA [29]. Despite these observations, additional approaches are needed to elucidate in vivo mechanisms that underlie SMA pathogenesis. Model 2 was based on theoretical locations of motor neuron pools; and as such it was difficult for us to determine accurate motor neuron pool location for lower extremity muscle groups. MRI techniques, such as Diffusion Tensor Imaging (DTI), have been shown to be sensitive to random motion of water molecules, and can provide useful information on the microstructural assembling and condition of nervous tissue [25]. Such approaches would be of great value in future studies.

Role of motor and sensory fibers

In SMA, muscle groups containing predominantly type I (slow twitch) muscle fibers are generally less affected [24]. Interestingly, the muscles identified as being contributors to improved performance on the 6 MWT in this study contain predominantly type I fibers in typically developing muscle. In SMA it is unknown if these characteristics and relative distribution of muscle fiber types are akin to that of healthy individuals. Adaptive changes have been shown in SMA, specifically, the hypertrophy of slow twitch muscle fibers and relative regrouping of muscle fibers by type [8, 30]. The role that these changes play in influencing patterns of muscle strength observed in SMA necessitates further exploration.

Further, type Ia sensory afferents contribute a significant amount of synaptic drive that continually modulates motor output, muscle stiffness, and tone, which are helpful for the maintenance of postural control. In humans with SMA, tendon reflexes and H-reflexes are generally absent [31, 32]. The possibility has been raised that a loss in central Ia afferents may be a common characteristic of the disease in both humans and animal models, contributing to the observed patterns of weakness.

Limitations

This study had limitations; including the inability to accurately assess motor neuron pool location within the spinal cord, small sample size, lack of a comparison group, and focus on the lower extremity. Additionally, approximately one third of all participants included were receiving treatment with nusinersen (Spinraza) at the time of strength assessment. However, this fact was not taken into account when analyzing strength and performance. The purpose of this study was to assess patterns of muscle weakness in relation to ambulatory function and it remains unclear how drug therapy, specifically intrathecally delivered SMN augmenting therapy, is impacting strength at the muscular level. Recently it was suggested that SMN protein is intrinsically important to motor function; and restoration of SMN to muscle in mice, even after onset of pathology, can have disease mitigating effects [33]. Future studies should take clinical treatment status into account, as this may influence the difference in the observed patterns of weakness.

Summary

This method of using neuroanatomical models to assess function has not been utilized in other neuromuscular diseases. However, patterns of weakness and their relationship to function have been well studied in other conditions. Among muscular dystrophies, patterns of weakness have been used to distinguish disease types (e.g. Duchenne, Limb Girdle, Emery-Dreifus, etc) and predict prognosis and function [34, 35]. Because the primary pathology of the muscular dystrophies is in skeletal muscle, neuroanatomical models such as the ones used in this study are not applicable. In contrast, the patterns of weakness in Charcot Marie Tooth disease have been well studied showing distal muscles being more affected than proximal, with all muscles and muscle fiber types involved [9, 36]. Although other neuromuscular diseases have well established patterns, further exploration may consider using similar strategies unique to the specific pathogenesis of the disease.

Optimal treatment of individuals requires a multi-disciplinary clinical team [16]. SMA is a challenging disorder that involves complex multi-disciplinary care, with each of the disciplines being intertwined as a team. Since the original consensus statement paper, there has been growing evidence that a proactive approach, including consistent sessions of physical therapy may influence trajectories of progression [16, 37]. Given the emergence of therapeutic agents that have been shown to have a positive effect on the trajectory of SMA, and several more interventions in development, it is imperative from a rehabilitation perspective that targeting vulnerable muscle(s) will yield optimal functional outcomes. Furthermore, the use of combinatorial therapies, including muscle targeted approaches, may influence the observed patterns of weakness. It has recently been shown, for example, that myostatin inhibitors when combined with SMN-restoring antisense therapy, exert additional therapeutic effects in SMA model mice with less severe phenotypes, and result in increased muscle mass and improved motor function [38].

Individuals with SMA present with symptoms of progressive muscular weakness and fatigue [14]. Modest gains in strength have been shown with exercise, such as progressive resistance training. These patients have demonstrated improvements in overall motor function in a safe and feasible manner [39, 40]. As such, the identification and subsequent clinical targeting of specific muscle groups that are important for ambulation would be a critical step towards helping maintain patients’ quality of life. Concurrently, identifying those patients who are more likely to benefit from targeted muscle therapies is of essence [27]. Ultimately, preserving independence and community participation are critical in managing the disease process.

CONFLICT OF INTEREST

The authors have no conflict of interest to report.

Footnotes

ACKNOWLEDGMENTS

This work was supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (K01HD084690), Cure SMA, and the Programs in Physical Therapy at Columbia University Irving Medical Center.

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