Bethlem myopathy, named after the Dutch doctor who first described it in 1976, is a rare and progressive form of muscular dystrophy. The condition mainly affects skeletal muscles and, to an extent, connective tissues.
Less than 100 cases of Bethlem myopathy have been reported in the literature, making it a very rare form of muscular dystrophy.
Bethlem myopathy has not been associated with reduced lifespan. However, since joint mobility is progressively affected, patients over the age of 50 usually require mobility aids such as crutches or wheelchairs.
Bethlem myopathy is caused by mutations in any of the three collagen type 6-encoding genes, including COL6A1, COL6A2, and COL6A3. Each gene produces one component of the whole collagen type 6, which is an important protein found in the extracellular matrix (ECM, the space between adjacent cells in a tissue) of various muscle types. It provides a supportive function by forming a mesh-like structure between cells.
Mutations in any of the collagen type 6-producing genes can result in abnormal forms or reduced amounts of collagen type 6. This weakens the supportive function of the protein in the ECM, leading to progressive loss of muscle strength, and to joint contractures and other symptoms.
Symptoms of Bethlem myopathy can occur at any age and vary from person to person. They can start before birth, with the fetus showing decreased movements in the mother’s womb.
In infants with Bethlem myopathy, hypotonia (low muscle tone or floppiness) and torticollis (stiff neck) are observed. Children with the condition usually experience delayed development, learning to sit or walk later than what’s considered normal. Progressive muscle weakness and contractures can also start appearing well into adulthood.
Contractures in the hands are usually observed in the fingers while those in the feet are observed in the Achilles tendon (connective tissue in the heel). Respiratory muscles, on the other hand, are mildly affected and breathing problems are usually rare in Bethlem myopathy patients.
About 80% to 99% of the Bethlem myopathy patients show cachexia (wasting and weakness of body muscles associated with weight loss). In some patients (about 5% to 29% of all cases), follicular hyperkeratosis over the joints is seen. This is the thickening of hair follicles on the outer skin due to excess keratin, which results in rough, cone-shaped elevations. Scarring may also be observed in some patients.
Many diagnostic approaches are used to confirm Bethlem myopathy. A blood test can be used to measure the levels of serum creatine kinase levels, which can be slightly elevated in people with the condition.
A muscle biopsy, usually taken from the thigh, can help detect changes in muscle structure and determine the amount of collagen type 6 protein produced.
Imaging tests such as muscle MRI can also help detect changes in muscle structure or connective tissue.
The health of the muscles can be further evaluated using electromyography which involves detecting the electrical activity of muscles during movement.
Genetic testing of the COL6A1, COL6A2, and COL6A3 genes can confirm the occurrence of mutations that lead to Bethlem myopathy. However, given the size of these genes, genetic testing can be expensive, and the results may take a long time to become available.
Currently, no cure for Bethlem myopathy is available, but there are treatment options to manage symptoms and provide support to improve patients’ quality of life.
Patients in their 50s often require equipment such as canes, crutches, or wheelchairs. Some patients might also require respiratory therapy to help them breathe at night.
Surgery to correct Achilles-tendon defects might help to improve standing and walking.
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