Polymyositis, like dermatomyositis, strikes females with greater frequency than males. The skin involvement of dermatomyositis is absent in polymyositis.
Sporadic inclusion body myositis (sIBM): IBM is often confused with (misdiagnosed as) polymyositis and polymyositis that does not respond to treatment is likely IBM. sIBM comes on over months to years, polymyositis comes on over weeks to months. It appears that sIBM and polymyositis share some common features, especially the initial sequence of immune system activation, however, polymyositis does not display the subsequent muscle degeneration and protein abnormalities as seen in IBM. As well, polymyositis tends to respond well to treatments, IBM does not. IBM and polymyositis apparently involve different disease mechanisms than are seen in dermatomyositis.
Marked weakness and/or loss of muscle mass in the proximal musculature, particularly in the shoulder and pelvic girdle. The hip extensors are often severely affected, leading to particular difficulty in ascending stairs and rising from a seated position.Thickening of the skin on the fingers and hands (sclerodactyly) is a frequent feature, although it is non-specific and occurs in other autoimmune disorders of the connective tissues. Dysphagia (difficulty swallowing) and/or other aspects of oesophageal dysmotility occur in as much as 1/3 of patients. Low grade fever and peripheral adenopathy may be present.