Thanks for the query. Indeed, dermatomyositis
can affect virtually any skeletal muscle, including the muscles of the abdominal wall. Some patients do not respon to the usual oral medications, and sometimes they are good candidates for the injections called "biologicals"; I would advise you to talk to your rheumatologist
about these immune-modulators to see if they are a reasonable option for you.
Hope that helps, wish you the best,
Dr Brenes-Salazar MD
Mayo Clinic MN