This is a group of disorders that presents wasting (atrophy) and weakness of the distal skeletal muscles. This can create difficulties with climbing stairs or activities that require arms to be raised. There are different genes detected for the various types of distal myopathies but not all are identified. This condition can develop from infancy to adulthood but generally seems to occur later in life progressing slowly.
Types of Distal myopathies
Udd distal myopathy
This is a slowly progressive muscle weakness affecting the ankles and can involve the muscles of the tibia. This usually affects individuals after the age of 35. As the long extensors of the toes may also become involved, this can present difficulty with walking. When the individual reaches around their mid-70s, the upper leg muscle can also become affected. This is caused by the mutations of the titin (TTN) located on the chromosome 2 (2q24.3) which is inherited in an autosomal dominant trait where a single copy of the defect is sufficient to cause the condition.
Distal myopathy 3
This rare type of distal myopathy affects the distal muscles of the arms or the legs with the onset ranging from 32 – 45 years of age. The muscle weakness results in the clumsiness with the use of their hands and may frequently stumble. This type can progress to include other muscles such as the proximal muscles of the upper leg. This is linked to be caused by the short arm or the long arm of chromosome 8 (8p22-q12) or the chromosome 12 (12q13-q22) which is inherited in an autosomal dominant trait.
Miyoshi distal myopathy
This usually develops between the age of 15 – 30 years that presents weakness and degeneration of the leg muscles, particularly the calf, which may appear abnormally large. This continues to affect the proximal muscles of the upper legs that can present difficulties with various activities such as climbing and walking. The other muscle involvement includes the hands, forearms and the shoulder. These individuals eventually may require a wheelchair. This condition is caused by the mutation in the gene dysferlin.
Nonaka distal myopathy
This begins between the age of 20 and 40 years old individuals. This condition can affect the muscles of the upper arm, leg and the neck. This is caused by the mutation in the gene GNE which alters compounds on cell surfaces that are essential for cellular signal and their adhesion.
Welander distal myopathy
This is a late-onset adult disorder that develops symptoms in the fifth decade of the individual’s life. It can affect certain muscles of the limbs and the digits. This is a slowly progressive condition causing muscle weakness that varies in severity. Individuals affected with this type usually do not have any cardiac involvement. The cause of this is linked to the short arm of chromosome 2p13.
Diagnosis
Various tests are required to diagnose distal myopathy which evaluates the health of the muscle and the nerves. The electromyography involves the insertion of the electrode into the muscle to monitor their activity and the blood test can identify elevated levels of creatine kinase (CK). An MRI can reveal the muscle damage and the muscle biopsy can indicate changes that are characteristic of this condition.
Treatment
There is no cure but treatment focuses on the specific symptoms that are evident in each individual. Depending on the cases, physical therapy can strengthen the muscle and various devices such as braces can assist individuals with walking. Genetic counseling can be beneficial to some of the individuals.
References
https://rarediseases.org/rare-diseases/distal-myopathy/
http://www.nmd-journal.com/article/S0960-8966(98)00008-X/abstract?cc=y=
https://www.med.nyu.edu/neuromuscular/sites/default/files/neuromuscular/ch12a.pdf
