Clinical Trials in Spinal Muscular Atrophy

Natural history of respiratory muscle strength in spinal muscular atrophy: a prospective national cohort study

BACKGROUND

Respiratory complications are the most important cause of morbidity and mortality in spinal muscular atrophy (SMA). Respiratory muscle weakness results in impaired cough, recurrent respiratory tract infections and eventually can cause respiratory failure. We assessed longitudinal patterns of respiratory muscle strength in a national cohort of treatment-naïve children and adults with SMA, hypothesizing a continued decline throughout life.

METHODS

We measured maximal expiratory and inspiratory pressure (PEmax and PImax), Sniff Nasal inspiratory pressure (SNIP), peak expiratory flow (PEF), and peak cough flow (PCF) in treatment-naïve patients with SMA. We used mixed-models to analyze natural history patterns.

RESULTS

We included 2172 measurements of respiratory muscle function from 80 treatment-naïve patients with SMA types 1c-3b. All outcomes were lower in the more severe phenotypes. Significant differences in PEF were present between SMA types from early ages onwards. PEF decline was linear (1-2%/year). PEF reached values below 80% during early childhood in types 1c-2, and during adolescence in type 3a. PEmax and PImax were severely lowered in most patients throughout life, with PEmax values abnormally low (i.e. < 80 cmH2O) in virtually all patients. The PEmax/PImax ratio was < 1 throughout life in all SMA types, indicating that expiratory muscles were most affected. All but SMA type 3b patients had a lowered PCF. Patients with types 2b and 3a had PCF levels between 160 and 270 L/min, those with type 2a around 160 L/min and patients with type 1c well below 160 L/min. Finally, SNIP was low in nearly all patients, most pronounced in more severely affected patients.

CONCLUSIONS

There are clear differences in respiratory muscle strength and its progressive decline between SMA types. We observed lower outcomes in more severe SMA types. Particularly PEF may be a suitable outcome measure for the follow-up of respiratory strength in patients with SMA. PEF declines in a rather linear pattern in all SMA types, with clear differences at baseline. These natural history data may serve as a reference for longer-term treatment efficacy assessments.

Natural history of lung function in spinal muscular atrophy

BACKGROUND

Respiratory muscle weakness is an important feature of spinal muscular atrophy (SMA). Progressive lung function decline is the most important cause of mortality and morbidity in patients. The natural history of lung function in SMA has, however, not been studied in much detail.

RESULTS

We analysed 2098 measurements of lung function from 170 treatment-naïve patients with SMA types 1c-4, aged 4-74 years. All patients are participating in an ongoing population-based prevalence cohort study. We measured Forced Expiratory Volume in 1 s (FEV₁), Forced Vital Capacity (FVC), and Vital Capacity (VC). Longitudinal patterns of lung function were analysed using linear mixed-effects and non-linear models. Additionally, we also assessed postural effects on results of FEV₁ and FVC tests. In early-onset SMA types (1c-3a), we observed a progressive decline of lung function at younger ages with relative stabilisation during adulthood. Estimated baseline values were significantly lower in more severely affected patients: %FEV₁ ranged from 42% in SMA type 1c to 100% in type 3b, %FVC 50 to 109%, and %VC 44 to 96%. Average annual decline rates also differed significantly between SMA types, ranging from - 0.1% to - 1.4% for FEV₁, - 0.2% to - 1.4% for FVC, and + 0.2% to - 1.7% for VC. In contrast to SMA types 1c-3a, we found normal values for all outcomes in later-onset SMA types 3b and 4 throughout life, although with some exceptions and based on limited available data. Finally, we found no important differences in FVC or FEV₁ values measured in either sitting or supine position.

CONCLUSIONS

Our data illustrate the longitudinal course of lung function in patients with SMA, which is characterised by a progressive decline in childhood and stabilisation in early adulthood. The data do not support an additional benefit of measuring FEV₁ or FVC in both sitting and supine position. These data may serve as a reference to assess longer-term outcomes in clinical trials.

Reliability of home-based, motor function measure in hereditary neuromuscular diseases

Objective

To evaluate the reliability of the motor function measure (MFM) scale in the assessment of disease severity and progression when administered at home and clinic and assess its correlation with the Paediatric Outcomes Data Collection Instrument (PODCI).

Methods

In this prospective study, two assessors rated children with hereditary neuromuscular diseases (HNMDs) using the MFM at the clinic and then 2 weeks later at the patients’ home. Intraclass correlation coefficient (ICC) was calculated for the reliability of the MFM and its domains. The reliability of each item was assessed and the correlation between MFM and three domains of PODCI was evaluated.

Results

A total of 48 children (5–17 years of age) were assessed in both locations and the MFM scale demonstrated excellent inter-rater reliability (ICC, 0.98). Weighted kappa ranged from excellent to poor. Correlation of the home-based MFM with the PODCI domain ‘basic mobility and transfers’ was excellent, with the ‘upper extremity’ domain was moderate, but there was no correlation with the ‘happiness’ domain.

Conclusion

The MFM is a reliable tool for assessing patients with HNMD when used in a home-based setting.

Reliability and validity of outcome measures of in-hospital and at-home visits in a randomized, double-blind, placebo-controlled trial for spinal muscular atrophy

We used at-home assessments in a clinical trial to relieve the visit burden for participants. A total of 57 patients with type II or III spinal muscular atrophy were enrolled and 10 of them (7 type II and 3 type III) received at-home assessments. The primary end points were Gross Motor Function Measure, Manual Muscle Test, and serum biomarker. The secondary endpoints were Modified Hammersmith Functional Motor Scale and forced vital capacity. The correlation coefficients and analysis of covariance showed good reliability and validity of all outcome measures. Except for Gross Motor Function Measure and Modified Hammersmith Functional Motor Scale, there were no significant differences in measures between in-hospital and at-home groups (intersubject) or among 3 patients who received both at-home and in-hospital visits (intrasubject). We concluded that at-home assessments could provide sufficient reliability in a controlled trial. This modification could help design a successful clinical trial for spinal muscular atrophy.

Hammersmith Functional Motor Scale and Motor Function Measure-20 in non ambulant SMA patients

The aim of this prospective longitudinal multi centric study was to evaluate the correlation between the Hammersmith Functional Motor Scale and the 20 item version of the Motor Function Measure in non ambulant SMA children and adults at baseline and over a 12 month period. Seventy-four non-ambulant patients performed both measures at baseline and 49 also had an assessment 12 month later. At baseline the scores ranged between 0 and 40 on the Hammersmith Motor function Scale and between 3 and 45 on the Motor Function Measure 20. The correlation between the two scales was 0.733. The 12 month changes ranged between -11 and 4 for the Hammersmith and between -11 and 7 for the Motor Function Measure 20. The correlation between changes was 0.48. Our results suggest that both scales provide useful information although they appeared to work differently at the two extremes of the spectrum of abilities. The Hammersmith Motor Function Scale appeared to be more suitable in strong non ambulant patients, while the Motor Function Measures appeared to be more sensitive to capture activities and possible changes in the very weak patients, including more items capturing axial and upper limb activities. The choice of these measures in clinical trials should therefore depend on inclusion criteria and magnitude of expected changes.

Childhood spinal muscular atrophy: controversies and challenges

Spinal muscular atrophy is an autosomal recessive disorder characterised by degeneration of motor neurons in the spinal cord and is caused by mutations of the survival of motor neuron 1 gene SMN1. The severity of spinal muscular atrophy is highly variable and no cure is available at present. Consensus has been reached on several aspects of care, the availability of which can have a substantial effect on prognosis, but controversies remain. The development of standards of care for children with the disorder and the identification of promising treatment strategies have changed the natural history of spinal muscular atrophy, and the prospects are good for further improvements in function, quality of life, and survival. A long-term benefit for patients will be the development of effective interventions (such as antisense oligonucleotides), some of which are in clinical trials. The need to be prepared for clinical trials has been the impetus for a remarkable and unprecedented cooperation between clinicians, scientists, industry, government, and volunteer organisations on an international scale.

Therapeutic developments in spinal muscular atrophy

Spinal muscular atrophy (SMA), a potentially devastating disease marked by progressive weakness and muscle atrophy resulting from the dysfunction and loss of motor neurons of the spinal cord, has emerged in recent years as an attractive target for therapeutic intervention. Caused by a homozygous mutation to the Survival of Motor Neurons 1 (SMN1) gene on chromosome 5q, the severity of the clinical phenotype in SMA is modulated by the function of a related protein, Survival of Motor Neurons 2 (SMN2). SMN2 predominantly produces an unstable SMN transcript lacking exon 7; only about 10% of the transcription product produces a full-length, functional SMN protein. Several therapeutic strategies have targeted this gene with the goal of producing increased full-length SMN transcript, thereby modifying the underlying mechanism. Drugs that have increased SMN2 function, in vitro, are now explored for potential therapeutic benefit in this disease. Alternative approaches, including neuroprotective, muscle anabolic, gene and cell replacement strategies, also hold promise. The recent advances in preclinical research and the development of a wider range of animal models for SMA continue to provide cautious optimism that effective treatments for SMA will eventually emerge.

SMN in spinal muscular atrophy and snRNP biogenesis

Ribonucleoprotein (RNP) complexes function in nearly every facet of cellular activity. The spliceosome is an essential RNP that accurately identifies introns and catalytically removes the intervening sequences, providing exquisite control of spatial, temporal, and developmental gene expressions. U-snRNPs are the building blocks for the spliceosome. A significant amount of insight into the molecular assembly of these essential particles has recently come from a seemingly unexpected area of research: neurodegeneration. Survival motor neuron (SMN) performs an essential role in the maturation of snRNPs, while the homozygous loss of SMN1 results in the development of spinal muscular atrophy (SMA), a devastating neurodegenerative disease. In this review, the function of SMN is examined within the context of snRNP biogenesis and evidence is examined which suggests that the SMN functional defects in snRNP biogenesis may account for the motor neuron pathology observed in SMA.

Living and ageing with spinal muscular atrophy type 2: observations among an unexplored patient population

OBJECTIVE

To study conditions of living, participation and diagnostic perceptions in a national population of adult persons with spinal muscular atrophy type 2 (SMA 2).

METHODS

A mixed method design combining cross-sectional survey data with personal narratives was chosen. Twenty-nine of 37 patients with SMA 2 aged >or=18 years participated in a semi-structured questionnaire survey and three were selected for in-depth journalistic interviews. Socioeconomic data, activities and personal significance of problems were calculated. Journalistic stories were analysed for constructs of personal diagnostic qualities.

RESULTS

Everyone was actively directing their life despite being heavily dependent. Deterioration of physical abilities was a major concern for women. Seventy-six per cent were single, but stated their quality of life as fine. Narratives of living with SMA 2 were associated with positive characteristics, as opposed to the medical diagnostic wording.

CONCLUSION

Female coping needs more research. Narrative method complements medical knowledge.