Physical activity in myotonic dystrophy type 1.

BACKGROUND: Physical inactivity is associated with lifestyle diseases and exercise of moderate intensity seems beneficial in DM1, but knowledge about physical activity and predictors of physical activity in individuals with myotonic dystrophy type 1 (DM1) is limited. The objective of this study is to assess physical activity and predictors of physical activity in individuals with DM1. METHODS: Sixty-seven adults with DM1 and 39 healthy adults were recruited. Physical activity was monitored by accelerometry and assessed using the International Physical Activity Questionnaire. Age, marital status, education, apathy, cognition, fatigue, and muscle strength were assessed as predictors of physical activity in DM1. RESULTS: The individuals with DM1 were on average - 187 min (p < 0.00001) objectively and - 48% (p = 0.001) subjectively less physically active per week compared to healthy controls. Education was the only predictor of physical activity in DM1 (p = 0.02). CONCLUSIONS: Individuals with DM1 are less physically active compared to healthy controls and only half of the patients fulfilled minimum requirements for recommended physical activity. Education is the only predictor of physical activity in DM1. Thus, enhancement of physical activity in individuals with DM1 might be suggested, and especially in the less educated individuals, but RCT studies are needed to guide exact recommendations.

Stable Longitudinal Methylation Levels at the CpG Sites Flanking the CTG Repeat of DMPK in Patients with Myotonic Dystrophy Type 1.

Myotonic dystrophy type 1 (DM1) is an autosomal dominant multisystem disorder mainly characterized by gradual muscle loss, weakness, and delayed relaxation after muscle contraction. It is caused by an expanded CTG repeat in the 3' UTR of DMPK, which is transcribed into a toxic gain-of-function mRNA that affects the splicing of a range of other genes. The repeat is unstable, with a bias towards expansions both in somatic cells and in the germline, which results in a tendency for earlier onset with each generation, as longer repeat lengths generally correlate with earlier onset. Previous studies have found hypermethylation in the regions flanking the repeat in congenital onset DM1 and in some patients with non-congenital DM1. We used pyrosequencing to investigate blood methylation levels in 68 patients with non-congenital DM1, compare the methylation levels between the blood and muscle, and assess whether methylation levels change over time in the blood. We found higher methylation levels in the blood of DM1 patients than in healthy controls and especially in the patients who had inherited the disease allele maternally. The methylation levels remained relatively stable over time and are a strong biomarker of the disease, as well as of the maternal inheritance of the disease.

Muscle contractility in spinobulbar muscular atrophy.

Spinobulbar muscular atrophy (SBMA) is caused by a trinucleotide repeat expansion in the androgen receptor gene on the X chromosome. There is a toxic effect of the mutant receptor on muscle and neurons resulting in muscle weakness and atrophy. The weakness can be explained by wasting due to loss of muscle cells, but it is unknown whether weakness also relates to poor muscle contractility of the remaining musculature. In this study, we investigated the muscle contractility in SBMA. We used stationary dynamometry and quantitative MRI to assess muscle strength and absolute and fat-free, cross-sectional areas. Specific muscle force (strength per cross-sectional area) and contractility (strength per fat-free cross-sectional area) were compared with healthy controls and their relation to walking distance and disease severity was investigated. Specific force was reduced by 14-49% in SBMA patients compared to healthy controls. Contractility was reduced by 22-39% in elbow flexion, knee extension, ankle dorsi- and plantarflexion in SBMA patients. The contractility decreased with increasing muscle fat content in muscles with affected contractility in SBMA. The decreased muscle contractility in SBMA may relate to motor neuron degeneration and changed fibre type distribution and muscle architecture.

Progressive fat replacement of muscle contributes to the disease mechanism of patients with single, large-scale deletions of mitochondrial DNA.

Muscle dysfunction in mitochondrial myopathy is predominantly caused by insufficient generation of energy. We hypothesise that structural changes in muscles could also contribute to their pathophysiology. The aims of this study were to determine fat fractions and strength in selected muscles in patients with chronic progressive external ophthalmoplegia (CPEO), and compare progression of muscle fat fraction with age in individuals with CPEO vs. healthy controls and patients with the m.3243A>G mutation of mitochondrial DNA (mtDNA). Seventeen patients with CPEO and single large-scale deletions of mtDNA, 52 healthy controls, and 12 patients carrying the m.3243A>G mtDNA mutation were included. Muscle fat fractions were measured from cross-sections of paraspinal and leg muscles. Peak muscle strength was assessed from a static dynamometer. There was a direct correlation between age and fat fraction in all muscle groups in CPEO patients and healthy controls (p < 0.05). Analysis of covariance showed a higher progression rate of fat replacement in CPEO patients vs. healthy controls in studied muscle groups (p < 0.05). Patients with the m.3243A>G mutation had slower progression rates of fat replacement. Muscle strength decreased with increasing muscular fat fraction in CPEO patients, no correlation was seen in other groups. This indicates that structural muscle changes contribute to the phenotype of older patients affected by CPEO and large-scale deletions. It should therefore be considered, along with known energy deficiencies, as the cause of exercise intolerance.

Reliability of the 2- and 6-minute walk tests in neuromuscular diseases.

OBJECTIVE: The 2- and 6-minute walk tests are used to evaluate walking capacity, but reliability has been sparsely investigated in patients with neuromuscular diseases. The aim of this study was to investigate the relative and absolute reliability of the 2- and 6-minute walk tests in patients with neuromuscular diseases. DESIGN: Each patient performed a 2- and a 6-minute walk test on 2 test days separated by 1-2 weeks. SUBJECTS: A total of 93 adult patients (mean age 53 years, age range 22-83 years) with 12 different neuromuscular diseases were included. RESULTS: The mean walking distance increased by 4.3 and 11.2 m (p < 0.001) in repeated 2- and 6-minute walk tests, respectively. Intraclass correlation coefficient in the 2- and 6-minute walk tests was 0.99 (p < 0.001). Standard error of measurement was 4.9 m in the 2-minute walk test and 14.0 m in the 6-minute walk test. Minimal detectable difference was 13.7 m in the 2-minute walk test and 38.8 m in the 6-minute walk test. CONCLUSION: These findings show good relative reliability of the 2- and 6-minute walk tests in patients with neuromuscular diseases. However, absolute reliability demonstrated variability in neuromuscular diseases. This should be considered when interpreting a change in walking distance.

Two- and 6-minute walk tests assess walking capability equally in neuromuscular diseases.

OBJECTIVE: This methodologic study investigates if the 2-minute walk test (2MWT) can be a valid alternative to the 6-minute walk test (6MWT) to describe walking capability in patients with neuromuscular diseases. METHODS: Patients (n = 115) with different neuromuscular diseases were invited to participate on 2 test days, each consisting of 1 2MWT and 1 6MWT separated by a minimum 30-minute period of rest. The order of the walk tests was randomly assigned via sealed envelopes. A group of 38 healthy controls completed 1 6MWT. RESULTS: The mean walking distance for the 2MWT was 142.8 meters and for the 6MWT 405.3 meters. The distance walked in the 2MWT was highly correlated to the distance walked in the 6MWT (r = 0.99, p < 0.001). There was a significant decrease in walking speed from the first to last minute in the 6MWT, both among patients and healthy controls, which was not evident in the 2MWT. Results were consistent across diagnoses and levels of disease severity. CONCLUSION: The 2MWT is a potential alternative to the 6MWT to describe walking capability among patients with neuromuscular diseases during clinical trials.