26th January 2024

Background 

Inflammatory myopathies can take many forms. Those relevant to the MRCP examination include mainly polymyositis, dermatomyositis and inclusion body myositis. It is chronic progressive inflammation of muscles, and each form of myositis presents differently. Thus, the management is tailored accordingly. 

Epidemiology 

Inflammatory myopathies are rare- estimated to occur in 1 in 100,000 persons. While dermatomyositis and polymyositis occur more commonly in females, inclusion body myositis is more prevalent in males. The average age of those affected ranges from 40-60 years, with inclusion body myositis mainly affecting those above 50 years of age. 

Pathophysiology 

In polymyositis, lymphocytes such as CD 8+ cytotoxic T cells invade myofibers in the muscle. Triggers for this include viruses such as HIV and parvovirus and drugs such as Phenytoin and Penicillamine. A small percentage (8%) may occur due to malignancies. 

Dermatomyositis results from an exposure to a type 1 interferon, causing capillary, myofiber and keratinocyte injury. It is a humorally mediated process driven by CD4 + T cells, causing capillary damage and ischaemic muscle injury. 

Inclusion body myositis is primarily a degenerative disorder with a likely secondary cytotoxic inflammatory component. The muscles are infiltrated by T cells, myeloid dendritic cells, macrophages and plasma cells, which aggregate in clusters in the perimysial spaces and cause myofiber necrosis.