Inflammatory Myopathies

Overview
  • The inflammatory myopathies are a grouping of disorders characterized by muscle weakness due to immune-mediated destruction of muscle fibers. Based on clinical and pathological findings three relatively distinct disorders have been described. We will discuss their common features first and then delve into unique characteristics of each below.
Pathogenesis and Morphology
  • Overview
    • Predictably, the inflammatory myopathies are thought to be caused by immune-mediated destruction of muscle fibers. Although still highly unclear, the pathogenesis of these diseases appears to be distinct and their histmorphology displays unique features.
  • Polymyositis
    • Polymyositis appears to result from a derangement of cellular immunity with destruction of muscle fibers being mediated by CD8+ T-cells. Muscle biopsies show CD8+ T-cells along with other lymphocytes infiltrating within muscle fascicles and mediating destruction.
  • Dermatomyositis
    • The pathogenesis of dermatomyostitis appears to be more related to derangements of humoral immunity with the aberrant generation of antibodies and immune complex. Notably, the lymphocytic infiltrate is primarily seen surrounding muscle fasciciles with a paucity of inflammatory cells within the endomysium. For the sake of interest it is thought the perimysial inflammation in dermatomyositis destroys the capillaries that feed muscle fibers, thus resulting in their progressive atrophy.
  • Inclusion Body Myositis (IBM)
    • IBM also appears to be caused by a derangement in cellular immunity. The characteristic morphological feature of IBM is the presence of vacuolated muscle fibers some of which may contain amyloid.
Clinical Consequences
  • Overview
    • Inflammatory myopathies present as slow-onset development of muscle weakness. Polymyositis and dermatomyositis are characterized by a symmetric weakness of proximal muscles and patients may complain of difficulty getting up from a chair or fixing their hair. In contrast, Inclusion Body Myositis tends develop as an asymmetric pattern of distal muscle weakness with patients complaining of difficulty performing fine motor movements with their hands and fingers. The pharyngeal muscles are frequently affected in all inflammatory myopathies, causing dysphagia. In general, muscle weakness is associated with atrophy.
  • Polymyositis
    • The key characteristic of polymyositis its lack of cutaneous features. It is relatively rare on its own and is usually associated with another autoimmune or connective tissue disease
  • Dermatomyositis
    • Dermatomyositis is characterized by the presence of distinctive cutaneous findings along with muscle weakness. Skin findings include a characteristic pink-purplish rash of the eyelid, referred to as a "heliotrope rash" with accompanying periorbital edema. Additionally, patient's fingers may display raised violaceous papules on their knuckles, termed "Gottron's Papules". Finally, some patients develop an erythematous rash that worsens with sun-exposure along their neck and shoulders, termed the "Shawl Sign", or along their sun-exposed anterior chest, termed "V-sign"
    • Dermatomyositis occurs more frequently in women but its most important association is with internal malignancy. In nearly 25% of cases, dermatomyositis heralds the presence of an occult internal malignancy and cancer screenings should be performed in affected patients.
  • Inclusion Body Myositis (IBM)
    • As mentioned, the characteristic feature of IBM its tendency to affect the distal musculature in an asymmetric pattern. Additionally, the disease is most frequently encountered in elderly men whereas the other inflammatory myopathies are more frequent in women.