Clinical and genetic study of chronic (types II and III) childhood onset spinal muscular atrophy

Systematic Literature Review of the Natural History of Spinal Muscular Atrophy: Motor Function, Scoliosis, and Contractures

BACKGROUND AND OBJECTIVES

Spinal muscular atrophy (SMA) is a progressive neuromuscular disorder associated with continuous motor function loss and complications, such as scoliosis and contractures. Understanding the natural history of SMA is key to demonstrating the long-term outcomes of SMA treatments. This study reviews the natural history of motor function, scoliosis, and contractures in patients with SMA.

METHODS

Electronic databases were searched from inception to June 27, 2022 (Embase, MEDLINE, and Evidence-Based Medicine Reviews). Observational studies, case-control studies, cross-sectional studies, and case series reporting on motor function (i.e., sitting, standing, and walking ability), scoliosis, and contracture outcomes in patients with types 1-3 SMA were included. Data on study design, baseline characteristics, and treatment outcomes were extracted. Data sets were generated from studies that reported Kaplan-Meier (KM) curves and pooled to generate overall KM curves.

RESULTS

Ninety-three publications were included, of which 68 reported on motor function. Of these, 10 reported KM curves (3 on the probability of sitting in patients with types 2 and 3 SMA and 8 on the probability of walking/ambulation in patients with type 3 SMA). The median time to loss of sitting (95% CI) was 14.5 years (14.1-31.5) for the type 2 SMA sitter population (their maximum ability was independent sitting). The median time to loss of ambulation (95% CI) was 13.4 years (12.5-14.5) for type 3a SMA (disease onset at age younger than 3 years) and 44.2 years (43.0-49.4) for type 3b SMA (disease onset at age 3 years or older). Studies including scoliosis and contracture outcomes mostly reported non-time-to-event data.

DISCUSSION

The results demonstrate that a high degree of motor function loss is inevitable, affecting patients of all ages. In addition, data suggest that untreated patients with types 2 and 3 SMA remain at risk of losing motor milestones during late adulthood, and patients with types 3a and 3b SMA are at risk of loss of ambulation over time. These findings support the importance of stabilization of motor function development even at older ages. Natural history data are key for the evaluation of SMA treatments as they contextualize the assessment of long-term outcomes.

Spinal Muscular Atrophy: A (Now) Treatable Neurodegenerative Disease

Spinal muscular atrophy (SMA) is a progressive disease of the lower motor neurons associated with recessive loss of the SMN1 gene, and which leads to worsening weakness and disability, and is fatal in its most severe forms. Over the past six years, three treatments have emerged, two drugs that modify exon splicing and one gene therapy, which have transformed the management of this disease. When treated pre-symptomatically, many children show normal early motor development, and the benefits extend from the newborn period to adulthood. Similar treatment approaches are now under investigation for rare types of SMA associated with genes beyond SMN1.

Quantification of disease progression in spinal muscular atrophy with muscle MRI-a pilot study

OBJECTIVES

Quantitative MRI (qMRI) of muscles is a promising tool to measure disease progression or to assess therapeutic effects in neuromuscular diseases. Longitudinal imaging studies are needed to show sensitivity of qMRI in detecting disease progression in spinal muscular atrophy (SMA). In this pilot study we therefore studied one-year changes in quantitative MR parameters in relation to clinical scores.

METHODS

We repeated quantitative 3 T MR analysis of thigh muscles and clinical testing one year after baseline in 10 treatment-naïve patients with SMA, 5 with Type 2 (21.6 ± 7.0 years) and 5 with Type 3 (33.4 ± 11.9 years). MR protocol consisted of Dixon, T₂ mapping and diffusion tensor imaging (DTI). The temporal relation of parameters was examined with a mixed model.

RESULTS

We detected a significant increase in fat fraction (baseline, 38.2% SE 0.6; follow-up, 39.5% SE 0.6; +1.3%, p = 0.001) in all muscles. Muscles with moderate to high fat infiltration at baseline show a larger increase over time (+1.6%, p < 0.001). We did not find any changes in DTI parameters except for low fat-infiltration muscles (m. adductor longus and m. biceps femoris (short head)). The T₂ of muscles decreased from 28.2 ms to 28.0 ms (p = 0.07). Muscle strength and motor function scores were not significantly different between follow-up and baseline.

CONCLUSION

Longitudinal imaging data show slow disease progression in skeletal muscle of the thigh of (young-) adult patients with SMA despite stable strength and motor function scores. Quantitative muscle imaging demonstrates potential as a biomarker for disease activity and monitoring of therapy response.

Muscle strength and motor function in adolescents and adults with spinal muscular atrophy

OBJECTIVE

To assess longitudinal patterns of muscle strength, motor function, and maximal compound muscle action potential amplitudes (CMAP_MAX) in older patients with spinal muscular atrophy (SMA), hypothesizing a continued decline of motor function parameters throughout life.

METHODS

We measured muscle strength (Medical Research Council), motor function (Hammersmith Functional Motor Scale Expanded [HFMSE] and Motor Function Measure), and CMAP_MAX in treatment-naive patients. We used both longitudinal and cross-sectional data in mixed models to analyze natural history patterns.

RESULTS

We included 250 patients with SMA types 1c through 4. Median patient age at assessment was 26.8 years, the number of assessments per patient ranged from 1 to 6. Baseline muscle strength and motor function scores differed significantly between SMA types, but annual rates of decline were largely similar and mostly linear. HFMSE floor effects were present for all patients with SMA type 1c, and adolescents and adults with types 2 and 3a. CMAP_MAX differed significantly between SMA types but did not decline significantly with increasing age. Muscle strength correlated very strongly with motor function (τ ≥ 0.8) but only moderately with CMAP_MAX (τ ≈ 0.5-0.6).

CONCLUSION

Muscle strength and motor function decline in older patients with SMA are constant without periods of slower progression or a plateau phase. The floor effects of the HFMSE preclude its use for long-term follow-up of adult patients with SMA types 1c through 3a. Muscle strength sum scores represent an alternative, feasible outcome measure for adolescent and adult patients with SMA.

The Respiratory System

This chapter addresses upper airway physiology for the pediatric intensivist, focusing on functions that affect ventilation, with an emphasis on laryngeal physiology and control in breathing. Effective control of breathing ensures that the airway is protected, maintains volume homeostasis, and provides ventilation. Upper airway structures are effectors for all of these functions that affect the entire airway. Nasal functions include air conditioning and protective reflexes that can be exaggerated and involve circulatory changes. Oral cavity and pharyngeal patency enable airflow and feeding, but during sleep pharyngeal closure can result in apnea. Coordination of breathing with sucking and nutritive swallowing alters during development, while nonnutritive swallowing at all ages limits aspiration. Laryngeal functions in breathing include protection of the subglottic airway, active maintenance of its absolute volume, and control of tidal flow patterns. These are vital functions for normal lung growth in fetal life and during rapid adaptations to breathing challenges from birth through adulthood. Active central control of breathing focuses on the coordination of laryngeal and diaphragmatic activities, which adapts according to the integration of central and peripheral inputs. For the intensivist, knowledge of upper airway physiology can be applied to improve respiratory support. In a second part the mechanical properties of the respiratory system as a critical component of the chain of events that result in translation of the output of the respiratory rhythm generator to ventilation are described. A comprehensive understanding of respiratory mechanics is essential to the delivery of optimized and individualized mechanical ventilation. The basic elements of respiratory mechanics will be described and developmental changes in the airways, lungs, and chest wall that impact on measurement of respiratory mechanics with advancing postnatal age are reviewed. This will be follwowed by two sections, the first on respiratory mechanics in various neonatal pathologies and the second in pediatric pathologies. The latter can be classified in three categories. First, restrictive diseases may be of pulmonary origin, such as chronic interstitial lung diseases or acute lung injury/acute respiratory distress syndrome, which are usually associated with reduced lung compliance. Restrictive diseases may also be due to chest wall abnormalities such as obesity or scoliosis (idiopathic or secondary to neuromuscular diseases), which are associated with a reduction in chest wall compliance. Second, obstructive diseases are represented by asthma and wheezing disorders, cystic fibrosis, long term sequelae of neonatal lung disease and bronchiolitis obliterans following hematopoietic stem cell transplantation. Obstructive diseases are defined by a reduced FEV1/VC ratio. Third, neuromuscular diseases, mainly represented by DMD and SMA, are associated with a decrease in vital capacity linked to respiratory muscle weakness that is better detected by PImax, PEmax and SNIP measurements.

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.

The natural history of infant spinal muscular atrophy in China: a study of 237 patients

The authors retrospectively studied the natural history of 237 patients with infantile spinal muscular atrophy in China. The onset ages (mean ± SD) for types I to III were 3.1 ± 2.7, 8.7 ± 3.8, and 21.1 ± 11.7 months, respectively. The survival probabilities for type I patients at 1, 2, and 5 years were 44.9%, 38.1%, and 29.3%, respectively, and for type II patients, the probabilities were 100%, 100%, and 97%, respectively. All type III patients were alive. Type I patients with onset age after 2 months had significantly increased survival than those with onset before 2 months (P < .05). It should be noticed that survival probability at 2 years in type I patients in our study was close to that in other Asian samples of spinal muscular atrophy, but slightly better than that among whites. Patients accepted minimal proactive interventions other than antibiotics for pulmonary infection, so our study provides reliable baseline data of natural history of spinal muscular atrophy in China.

Clinical outcome measures in spinal muscular atrophy

Spinal muscular atrophy is one of the most devastating neurological diseases of childhood. Affected infants and children suffer from often severe muscle weakness caused by degeneration of lower motor neurons in the spinal cord and brainstem. Identification of the causative genetic mutation in most cases has resulted in development of potential treatment strategies. To test these new drugs, clinically feasible outcomes are needed. Several different assessments, validated in spinal muscular atrophy or similar disorders, are being used by national and international research groups; however, their sensitivity to detect change is unknown. Acceptance of a few standardized, easily administered, and functionally meaningful outcomes, applicable to the phenotypic spectrum of spinal muscular atrophy, is needed. Consensus is imperative to facilitate collaboration and explore the ability of these measures to identify the therapeutic effect of disease-modifying agents. Following is an evidence-based review of available clinical outcome measures in spinal muscular atrophy.

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.

Rapidly progressive spinal muscular atrophy in an ambulatory 2-year-old male

A 2-year 9-month-old male was referred for gait disturbances. Main complaints were abnormal gait with frequent falls observed as soon as he began to walk unaided, at 18 months of age. The first neurologic examination revealed symmetric and proximal weakness in the lower limbs with difficulty running and walking upstairs. Deep tendon reflexes were decreased, and generalized hypotonia was observed. Three months later, at 3 years of age, he had lost independent gait, and 1 month later he could not stand unaided. DNA analysis revealed homozygous deletion in exons 7 and 8 of SMN1 gene, confirming the diagnosis of spinal muscular atrophy. According to the current classification, this patient would be classified as spinal muscular atrophy type III. The distinctive feature of this case was the short time elapsed (18 months) between onset of spinal muscular atrophy and the age at which he lost ambulation. This patient reinforces the notion that late onset of symptoms in spinal muscular atrophy and acquisition of independent gait do not exclude a rapidly progressive motor deterioration, which is important when talking with families about outcome. In those rapidly progressive cases, when promptly available, testing for SMN1 gene will prevent unnecessary, invasive, or uncomfortable procedures such as lumbar puncture, electromyography, or spinal cord magnetic resonance imaging.

A comparison of gait in spinal muscular atrophy, type II and Duchenne muscular dystrophy

This study investigated and compared the gait of two patients with spinal muscular atrophy, type II (SMA II) and two patients with Duchenne muscular dystrophy (DMD). These diseases cause a progressive and proximal to distal muscular weakness resulting in the loss of ambulation. The DMD cases had comparable muscle weakness with the SMA II cases on manual muscle testing and patients were assessed using kinematics, kinetics, electromyography and video analysis. SMA II and DMD patients employed different gait strategies for forward movement. SMA II patients used pelvic rotation initiated by the upper body to propel the leg forward and produce the necessary step-length whereas the DMD patients tended to use hip flexion and plantar flexion. Management of SMA II patients would include preservation of hip abductor and flexor strength to maintain mobility.

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.

Stable motor and lung function throughout pregnancy in a patient with infantile spinal muscular atrophy type II

Patients with infantile spinal muscular atrophy rarely decide to have their own children especially if there is major respiratory impairment. We studied prospectively the pregnancy course and outcome of a 34-year-old woman with spinal muscular atrophy type II who delivered a healthy boy. Pregnancy was entirely uneventful, except that for 1-2 weeks after the caesarean section, the patient was extremely weak with dyspnoea and bulbar involvement. Several weeks after delivery her motor function had returned to pre-pregnancy levels. Pulmonary function remained stable throughout pregnancy, and thereafter, at approximately 70% predicted levels for forced vital capacity and for forced expiratory volume in 1 s. Blood gases after midgestation revealed low normal PaO(2) values around 85 mmHg and concomitant hyperventilation resulting in PaCO(2) levels below 30 mmHg.

The predictive value of achieved motor milestones assessed in 441 patients with infantile spinal muscular atrophy types II and III

Proximal spinal muscular atrophy (SMA) is classified into three main subtypes (I-III), defined by age at onset and achieved motor milestones. As age at onset can be very early in SMA II and III (IIIa, onset < 3 years) and does not necessarily correlate with prognosis, the question arises whether the child can be correctly assigned to a specific SMA type at the time of presentation based on the assessment of motor function. Therefore we studied the motor milestones in 175 SMA type II and 266 SMA type III patients. In SMA II, 73% of the patients sat within the normal age range (up to 9 months), the remainder learned to do so at ages between 10 and 30 months. In SMA III, the walking milestone was passed with delay (given an upper normal limit of 18 months) in 10% of all and 16% of SMA IIIa patients (median age 13 months, range 9-53 months). There was a correlation between late sitting and walking in SMA III, since those who sat after 9 months were responsible for the majority of delayed walkers. The median age when becoming chairbound did not differ between early-onset SMA III patients who walked with delay and those who walked within the normal age range (10.2 versus 10.5 years). In conclusion, a significant proportion of patients with early-onset SMA classified as SMA II on the basis of achieved motor function turned out to be SMA III at later follow-up. It is important to reassess a child in the first 2-4 years, to determine whether walking can be achieved with or without aids, as children who start to walk late have a similar favourable outcome for ambulation compared to earlier walkers.