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It is said that in dermatomyositis(DM) , proximal muscle weakness is seen earlier than distal muscle weakness. It is also said that , DM is due to damage to small blood vessels contributing to muscle injury.( from Robbin's textbook of pathology pg.no: 1238-1239, 10e)


Shouldn't the distal group of muscles being smaller show weakness earlier? Why inclusion body myositis does not follow the pattern as in DM?

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Usefull links: https://www.mda.org/disease/dermatomyositis/causes-inheritance https://emedicine.medscape.com/article/332783-overview#a1

My hypothesis are

1) Since skin, is commonly involved in sunexposed areas, may be enviornmental factors do play a major role. So, Proximal Muscles are more used in day to day work, so are more favourable site for drugs, autoantibodies to act upon.

2)May be, in case of due to drugs or vaccines, proximal muscles may be are preferrential site of action of these, making these group of muscle more suceptible.

By the way correct answer would be: Its under research.

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