What are the effects of adding azathioprine to corticosteroids in polymyositis?

The treatment of polymyositis is based on corticosteroid therapy, with addition of azathioprine for non responsive cases or as an attempt to diminish corticosteroids requirements. However, there is no clear evidence of its benefit in controlling symptoms. Searching in Epistemonikos database, which is maintained by screening 30 databases, we identified only one systematic review including one pertinent randomized trial. We generated a summary of findings following the GRADE approach. We concluded there is uncertainty if azathioprine improves or not muscular strength in polymyositis because the certainty of the evidence is very low.