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Higher proportion of fast‐twitch (type II) muscle fibres in idiopathic inflammatory myopathies – evident in chronic but not in untreated newly diagnosed patients
Author(s) -
Loell I.,
Helmers S. B.,
Dastmalchi M.,
Alexanderson H.,
Munters L. A.,
Nennesmo I.,
Lindroos E.,
Borg K.,
Lundberg I. E.,
Esbjörnsson M.
Publication year - 2011
Publication title -
clinical physiology and functional imaging
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.608
H-Index - 67
eISSN - 1475-097X
pISSN - 1475-0961
DOI - 10.1111/j.1475-097x.2010.00973.x
Subject(s) - medicine , myositis , inflammation , cardiology , gastroenterology , pathology
Summary 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.