Nemaline myopathy belongs to a group of conditions under the umbrella of congenital myopathies which is characterized by the presence of rod-like structures in the muscle cells. This condition primarily affects the skeletal muscles used for the movement. They symptoms include muscle weakness, breathing and feeding problems. Most of the cases become evident at the birth of the infant or early childhood although rare cases don’t occur until adulthood. As the condition continues to progress, the affected individual may need a wheelchair and the severe cases can be life-threatening when the muscle used for breathing also becomes involved. There are six sub-groups of this condition based on the age of onset and the severity of symptoms. This includes severe congenital, Amish, intermediate congenital, typical congenital, childhood-onset and the adult-onset. The severe congenital can be fatal with most of the affected infants not surviving past childhood because of respiratory failure.
Causes
Genetic mutations in one of several genes have been identified to cause nemaline myopathy. Most cases develop because of the mutations in one of two genes; NEB or ACTA1. NEB genetic mutation accounts for around 50% of the cases and the symptoms typically is present at birth or beginning in early childhood. When a mutation in ACTA1 occurs, the severity and the age of onset can vary greatly. The genetic mutations that cause nemaline myopathy are important for the production of proteins that are found in structures called sarcomeres within skeletal muscle cells. The genetic mutation results in the disorganization of proteins in the sarcomeres of skeletal muscles. As the disorganized proteins cannot interact normally, it affects the normal muscle contraction causing the muscle weakness and other features of this condition. Most of the cases are inherited in an autosomal recessive pattern meaning both the copies of the gene in each cell are defective. Less commonly, the condition can be inherited in an autosomal dominant pattern meaning a single copy of the defective gene can cause the condition.
Symptoms
The major feature of this condition is the muscle weakness, hypotonia and reduced reflexes. The muscle weakness can affect the muscles of the face, neck and the proximal muscles which are closest to the center of the body such as the shoulder, pelvis, upper arms and the legs. The muscle weakness can present difficulty speaking, swallowing and breathing. The affected child can also present delays in reaching motor milestones such as sitting up. In addition, they can also experience the restricted movement of the affected area, abnormal curvature of the spine and a sunken chest.
Diagnosis
The diagnosis is done with a muscle biopsy that is examined under the microscope to identify the distinctive pattern of thin thread or rod- like structures in the muscle cells. Molecular genetic testing can also help identify the mutation responsible for developing this condition.
Treatment
There is no effective treatment available to stop the progression of the disease. The treatment focuses on the management of the condition and to prolong the life of the affected individual. Other forms of treatment options include respiratory support, tube-feeding to ensure proper nutritional intake and speech therapy in the case of difficulty talking.
References
https://rarediseases.org/rare-diseases/nemaline-myopathy/
https://ghr.nlm.nih.gov/condition/nemaline-myopathy#inheritance
http://www.musculardystrophyuk.org/app/uploads/2015/09/Nemaline-rod-myopathies_May-2015.pdf
http://muscle.ca/wp-content/uploads/2012/11/Nemaline_E.pdf
