Muscle strength and motor function in neuromuscular disorders. A clinical study of children and adolescents with spinal muscular atrophy, myotonic dystrophy, Duchenne muscular dystrophy and amyoplasia

Abstract: Aim: The aims of this study were to investigate muscle strength and motor function in children and adolescents with four neuromuscular disorders; 1) spinal muscular atrophy (SMA), 2) myotonic dystrophy (DM), 3) Duchenne muscular dystrophy (DMD and 4) the amyoplasia form of arthrogryposis multiplex congenita. Further: 1) to analyze compensatory maneuvers due to muscle weakness in individuals with SMA, 2) to correlate motor function in individuals with DM with the size of the mutation, 3) to evaluate the long-term side effects and effects on muscle strength, motor function, vital capacity and development of scoliosis in boys with DMD treated with low-dose prednisone, and 4) to investigate how muscle strength and joint contractures affect motor function in individuals with amyoplasia and to relate current status to joint position at birth. Methods: 14 children and adolescents from western part of Sweden with SMA, 42 children and adolescents from western and southern Sweden with DM, 37 boys from western Sweden with DMD and 35 individuals from Sweden with amyoplasia participated in this study. In study 2, a control group of 42 age and gender-matched healthy children and adolescents was investigated. Medical records were reviewed and a clinical examination was performed. Isometric muscle strength was measured with an electronic hand-held dynamometer, contractures with an ordinary goniometer and motor function was assessed according to a scale designed for children with neuromuscular disorders. Results: 1) In children and adolescents with SMA profound muscle weakness was found in all assessed muscle groups compared to normal strength. Proximal weakness was greater than distal and lower limbs were more affected than upper limbs. Walking, transfer from lying or sitting to the standing position, and climbing stairs were possible in some of the individuals, despite marked reduction of muscle strength. Compensatory maneuvers described were reinforcement by using stronger upper limbs or distal muscle groups. 2) Children and adolescents with DM were significantly weaker than healthy controls in a majority of the assessed muscle groups. There was a great variation where some of the individuals had normal muscle strength. A strong correlation was found between motor function score and size of the mutation (rho=-0.743). Jumping, heel-standing and head-lifting in supine were difficult to perform, but few had difficulty in walking, running and stair-climbing. 3) Low-dose prednisolone treatment in boys with DMD delayed the time of loss of ambulation by at least 1.7 years and postponed development of scoliosis and contractures. There were also beneficial effects on muscle strength, motor function and vital capacity. The side effects were mild and long-term treatment was possible.4) In individuals with amyoplasia muscle strength affected motor function to a greater extent than joint contractures, although the joint contractures and joint position at birth also had an impact. Muscle strength was reduced in most of the investigated individuals. There was, however, heterogeneity and some of the patients had normal muscle strength in some of the investigated muscle groups. More attention should be paid at developing muscle strength, with early stimulation of active movement, and periods of immobilization should be minimized. Conclusions: SMA, DM, DMD and amyoplasia are heterogeneous conditions with a wide spectrum of muscle involvement. This investigation of muscle strength and motor function increases our knowledge concerning pattern and degree of muscle weakness in the different disorders, but also the impact of muscle weakness on motor function. The results have enhanced the possibilities of planning therapeutic interventions and the understanding of the prerequisites for everyday life in the children and adolescents with neuromuscular disorders.