Bethlem myopathy

Bethlem myopathy is a condition that mainly affects skeletal muscles, which are the muscles used for movement. People with this condition experience progressive muscle weakness and develop joint stiffness (contractures) in their fingers, wrists, elbows, and ankles that can restrict movement. Approximately two-thirds of people with Bethlem myopathy over age 50 will need to use a walker or wheelchair.

The features of Bethlem myopathy can appear at any age. Some individuals have signs of the disorder even before birth, most commonly decreased fetal movement. Other affected individuals develop symptoms soon after birth, including low muscle tone and a stiff neck that causes the head to lean to one side (torticollis). People whose symptoms appear in childhood may experience delayed developmental milestones, such as sitting or walking. Others do not develop features of the condition until late adulthood.

Some people with Bethlem myopathy have skin abnormalities such as small bumps called follicular hyperkeratosis that develop around the elbows and knees; soft, velvety skin of the palms and soles; and wounds that split open with little bleeding and widen over time to create shallow scars.

Bethlem myopathy is estimated to occur in 1 in 200,000 individuals.

Mutations in the COL6A1, COL6A2, and COL6A3 genes cause Bethlem myopathy. These genes each provide instructions for making one component of a protein called type VI collagen. This protein plays an important role in muscle, particularly skeletal muscle.

Type VI collagen makes up part of the extracellular matrix that surrounds muscle cells. The extracellular matrix is an intricate lattice that forms in the space between cells and provides structural support. The extracellular matrix that surrounds muscle cells is necessary for muscle cell stability and growth.

Mutations in the type VI collagen genes that cause Bethlem myopathy result in the formation of abnormal type VI collagen or reduce the amount of type VI collagen that is produced. A decrease in normal type VI collagen disrupts the extracellular matrix surrounding muscle cells, leading to progressive muscle weakness and the other signs and symptoms of Bethlem myopathy.

Read more about the COL6A1, COL6A2, and COL6A3 genes.

Bethlem myopathy is typically inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. Most cases result from new mutations in the gene and occur in people with no history of the disorder in their family. In some cases, an affected person inherits the mutation from one affected parent.

Rarely, this condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.