Quantitative Muscle MRI and Clinical Findings in Women With Pathogenic Dystrophin Gene Variants.

Objective: To explore fat replacement, muscle strength, and clinical features in women heterozygous for a pathogenic DMD variant, we prospectively examined 53 women, assuming that some of these women-despite of the recessive X-linked inheritance-manifested clinical symptoms. Methods: We performed a cross-sectional observational study using MRI and stationary dynamometry of lower extremities, extracted blood muscle biomarkers, and investigated subjective complaints. Results were compared with 19 healthy women. Results: DMD variant carriers were weaker and had higher fat fractions than controls in all investigated muscle groups (p < 0.02). Fat fractions were 18% in carriers vs. 11% in controls in thighs (p = 0.008), and 15 vs. 11% in calf muscles (p = 0.032). Seventy-two percent had fat fractions deviating from controls by two standard deviations (SDs) in one or more of the 16 investigated muscle groups. On strength testing, 40% of the carriers had results deviating from control muscle strength by two SDs in one or more dynamometry assessments. Forty-three carriers (81%) had either reduced muscle strength (<2 SDs from control mean) and/or elevated muscle fat fraction (>2 SDs from control mean). Thirty of these had subjective symptoms. Blood creatine kinase and myoglobin were elevated in 57% of the carriers. Conclusion: Using quantitative methods, this study shows that both clinically symptomatic and asymptomatic women with pathogenic DMD variants show a high prevalence of muscle affection. Longitudinal studies in female carriers of pathogenic DMD variants are needed to follow the evolution of these changes.

Contractile properties are impaired in congenital myopathies.

The ratio between muscle strength and muscle cross-sectional area is called the specific force. Fatty replacement of muscles is seen in many myopathies, affecting the specific force, without necessarily affecting the ability of the remaining muscle fibers to contract. This ability is called the contractility and is the ratio between muscle strength and the lean muscle cross-sectional area, i.e. the contractile cross-sectional area. We hypothesized that contractility is disrupted in patients with congenital myopathy, because of defects in contractile proteins of the sarcomere. Peak torque across ankle and knee joints was measured by isokinetic dynamometry in 16 patients with congenital myopathy and 13 healthy controls. Five patients only participated partially in the dynamometer measurements due to severe muscle weakness. Dixon MRI technique was used to quantify muscle fat fractions and calculate cross-sectional area. Patients with congenital myopathy had lower cross-sectional area in all muscle groups (P<0.01), higher fat fraction (P<0.01) and less strength (P<0.005) in all studied muscle groups. Their fat content was more than doubled and peak torque lower than half that in healthy controls. Muscle contractility was reduced (P<0.01) in three of four patient muscle groups. In conclusion, muscle contractility was reduced in patients with congenital myopathy, across different diagnoses, and was independent of the level of muscle fat fraction, suggesting that intrinsic defects of the myocyte are responsible for reduced contractility.

Growth and differentiation factor 15 as a biomarker for mitochondrial myopathy.

OBJECTIVE: We investigated if Growth and Differentiation Factor 15 (GDF-15) can be used as a biomarker to distinguish patients with mitochondrial myopathy from patients with other myopathies. METHODS: Serum GDF-15 was measured in 28 patients with mitochondrial disease, 24 with metabolic myopathies, 27 with muscular dystrophy and 21 healthy controls. RESULTS AND CONCLUSIONS: Our findings indicate that elevated GDF-15 can distinguish patients with mitochondrial myopathy from other myopathies, including metabolic myopathies. This suggests that increases in GDF-15 is specific to respiratory chain dysfunction rather than general metabolic dysfunction or muscle defect.

Impaired Fat Oxidation During Exercise in Long-Chain Acyl-CoA Dehydrogenase Deficiency Patients and Effect of IV-Glucose.

CONTEXT: Long-chain 3-hydroxyacyl-coenzyme A dehydrogenase deficiency (LCHADD) affects oxidation of long-chain fatty acids (FAO) and is associated with risk of metabolic crises and episodic rhabdomyolysis. CASE DESCRIPTION: We present the cases of two patients with LCHADD. Patient 1 (male, 26 years old) was severely affected by muscle weakness and neuropathy. He was diagnosed at age 20 years and was nonadherent to standard dietary management. MRI revealed significant fat replacement of muscle in both calves. Patient 2 (female, 15 years old) was diagnosed at age 1 year. She had no muscle weakness and was compliant with the recommended diet. Compared with healthy persons, both patients had reduced FAO and palmitate oxidation, measured with indirect calorimetry and stable isotope technique during a submaximal cycle ergometer test. Patient 2 had some residual capacity to increase FAO and a compensatory higher carbohydrate oxidation, which ensured a near-normal exercise capacity. Patient 1 was unable to increase FAO and could only complete 23 minutes of exercise, vs 60 minutes by patient 2 and healthy persons. In both, 10% IV infusion of glucose (IV-glucose) during exercise increased carbohydrate oxidation slightly, but endurance was not improved, which likely relates to the fixed weakness in patient 1 and because the residual FAO was suppressed by the glucose infusion in both. CONCLUSION: The two patients illustrate that FAO is impaired and carbohydrate oxidation is elevated during exercise in patients affected by LCHADD, compared with healthy persons, but IV-glucose has no beneficial effect on exercise tolerance in LCHADD.

Muscle contractility of leg muscles in patients with mitochondrial myopathies.

BACKGROUND: The primary disease mechanism underlying mitochondrial myopathies (MM) is impaired energy generation to support muscle endurance. Little is known about muscle contractility before energy becomes deficient during muscle contractions. We investigated muscle contractility in MM to uncover potentially fixed weakness aspects of the disorders. METHODS: Contractility of calf and thigh muscles was investigated by comparing strength with contractile cross-sectional area (CCSA) of the used muscles, as measured by stationary dynamometry and MRI, respectively. RESULTS AND DISCUSSION: Our findings suggest reduced contractile properties in thigh and calf muscles of patients with MM.