Higher proportion of fast-twitch (type II) muscle fibres in idiopathic inflammatory myopathies - evident in chronic but not in untreated newly diagnosed patients

Clin Physiol Funct Imaging. 2011 Jan;31(1):18-25. doi: 10.1111/j.1475-097X.2010.00973.x. Epub 2010 Oct 1.


Objective: Polymyositis and dermatomyositis are idiopathic, inflammatory myopathies characterized by proximal muscle fatigue. Conventional immunosuppressive treatment gives a variable response. Biopsies from chronic patients display a low proportion type I and a high proportion of type II muscle fibres. This raised a suspicion that the low proportion of type I fibres might play a role in the muscle fatigue.

Aim: To investigate whether the muscle fibre attributes evident in chronic myositis are characteristic for the polymyositis and dermatomyosistis diseases themselves.

Methods: Muscle biopsies were obtained from thigh muscle from untreated patients (n = 18), treated responders (n = 14) and non-responders (n = 6) and from healthy controls (n = 11), respectively. For clinical evaluations, creatine kinase, functional index of myositis and cumulative dose of cortisone were established.

Results: Chronic patients had a lower proportion of type I fibres and a higher proportion of type II fibres compared to untreated myositis patients and healthy controls. Fibre cross-sectional area (CSA) did not differ between patients and healthy individuals but all women had a 20% smaller type II fibre CSA compared to men.

Conclusions: Untreated polymyositis and dermatomyositis patients and healthy controls have a different fibre type composition than chronic polymyositis and dermatomyositis patients. Fibre CSA did not differ between healthy controls or any of the patient groups. A low proportion of oxidative muscle fibres can therefore be excluded as a contributing factor causing muscle fatigue at disease onset and the gender difference should be taken into consideration when evaluating fibre CSA in myositis.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Biopsy / methods
  • Case-Control Studies
  • Chronic Disease
  • Creatine Kinase / blood
  • Dermatomyositis / drug therapy
  • Dermatomyositis / enzymology
  • Dermatomyositis / pathology*
  • Female
  • Glucocorticoids / therapeutic use
  • Humans
  • Immunosuppressive Agents / therapeutic use
  • Male
  • Middle Aged
  • Muscle Fibers, Fast-Twitch / pathology*
  • Muscle Fibers, Slow-Twitch / pathology*
  • Patients
  • Polymyositis / drug therapy
  • Polymyositis / enzymology
  • Polymyositis / pathology*
  • Prednisolone / therapeutic use


  • Glucocorticoids
  • Immunosuppressive Agents
  • Prednisolone
  • Creatine Kinase