What is the primary cause of adult-onset myopathy?

The primary cause of adult-onset myopathy is myotonic dystrophy. This condition is characterized by progressive muscle weakness and wasting, along with myotonia, which is a delayed relaxation of muscles after contraction. Myotonic dystrophy often presents in adulthood and is associated with genetic mutations, specifically in the DMPK gene for type 1 and the CNBP gene for type 2.

Myotonic dystrophy's late onset and multisystem effects, including cardiac and endocrine issues, further distinguish it as a significant cause of myopathy manifesting in adults. The hereditary, progressive nature of the disease often leads to a diagnosis later in life as symptoms develop and can result from both maternal and paternal transmission.

Conditions like Duchenne and Becker muscular dystrophies primarily affect children and are caused by mutations in the dystrophin gene. These disorders are not typically associated with adult-onset myopathy since they generally manifest in early childhood. Spinal muscular atrophy also predominantly affects younger individuals and results from motor neuron degeneration rather than primary muscle pathology.

Understanding the distinct characteristics and genetic backgrounds of these conditions highlights why myotonic dystrophy stands out as the primary cause of adult-onset myopathy.