NORD gratefully acknowledges Anne M. Connolly, MD, Professor Neurology and Pediatrics, Department of Neurology, Washington University School of Medicine, for assistance in the preparation of this report.
Myofibrillar myopathies are a group of rare genetic neuromuscular disorders that may be diagnosed in childhood but most often appear after 40 years of age. These conditions are highly variable but are characterized by a slowly progressive muscle weakness that can involve skeletal and smooth muscle. Skeletal muscle weakness can be present in the muscles close to the center of the body (proximal) as well as the distal muscles. A weakening of the heart muscle (cardiomyopathy) is common and may manifest as arrhythmia, conduction defects or congestive heart failure.
Most affected individuals with mofibrillar myopathy secondary to desmin mutations present with a slowly progressive muscle weakness. Distal muscle weakness is more common than proximal weakness but variable presentation within the same family occurs. Some also have muscle stiffness, aching, cramps or decreased muscle mass (atrophy). Pain, loss of sensation and inability to control muscles may also occur in one form (Filamin C protein mutations). Cardiomyopathy is sometimes the presenting symptom and may manifest as arrhythmia, conduction defects or congestive heart failure.
Children with desminopthies may present with cardiomyopathy. Children with BAG3-related myofibrillar myopathies may present in the first or second decade with proximal weakness, respiratory failure and restrictive cardiomyopathy and are frequently rapidly progressive and fatal.
Myofibrillar myopathies are usually inherited as adult autosomal dominant genetic conditions though children with recessive presentation of Desmin mutation do occur and in this case presentation can be in early childhood. It is not always possible to determine the mode of inheritance in families because some mildly affected individuals remain undiagnosed and others are diagnosed late in life.
Dominant genetic disorders occur when only a single copy of an abnormal gene is necessary to cause a particular disease. The abnormal gene can be inherited from either parent or can be the result of a new mutation (gene change) in the affected individual. Approximately 25% of affected individuals have an affected parent. The risk of passing the abnormal gene from affected parent to offspring is 50% for each pregnancy. The risk is the same for males and females.
The genes responsible for myofibrillar myopathies have been identified in approximately 20% of affected individuals. These disorders have been categorized by the gene involved. The gene abnormality results in excess amounts of a particular protein in muscle.
Desminopathy—(onset 20-30) DES gene/desmin protein
Alpha-B crystallinopathy—(onset 20-40 years)CRYAB gene/a-B crystallin protein
Myotilinopathy—(Onset 27-77) Titin immunoglobulin domain protein TTID gene/myotilin protein
Filaminopathy—(onset 37-57) FLNC gene/filamin C protein
BAG3-related myofibrillar myopathy—(onset childhood) BCL2-associated athanogene 3/ BaG3 protein
Zaspopathy—(onset 44-73 years)LDB3 (ZASP) gene/LIM domain-binding protein 3
The frequency of myofibrillar myopathies has not been estimated. It is likely that these conditions are unrecognized and underdiagnosed.
A diagnosis of myofibrillar myopathies is made based on clinical findings, electromyography, nerve conduction studies and muscle biopsy. Molecular genetic testing for the DES, CRYAB, MYOT, LDB3 and ZASP genes is available to confirm the diagnosis. Molecular genetic testing for the BAG3 gene is available on a research basis only.
Individuals affected with cardiomyopathy may consider implantation of a mechanical device to regulate heartbeat (pacemaker) and cardioverter defibrillator (ICD). Heart transplantation may be considered if the cardiomyopathy is progressive or life threatening. Respiratory therapy and physical therapy may be helpful for those with advanced muscle weakness. Orthotics may be helpful if foot drop develops.
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FROM THE INTERNET
Selcen, D and Engel, AG, (Updated 2/2/10). Myofibrillar Myopathy. In: GeneReviews at GeneTests: Medical Genetics Information Resource (database online). Copyright, University of Washington, Seattle. 1997-2010, Available at http://www.genetests.org. Accessed 3/10.
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