Editing Inclusion body myositis (section)

===Differential diagnosis===
IBM is often initially misdiagnosed as [[polymyositis]]. A course of [[prednisone]] is typically completed with no improvement and eventually, sIBM is confirmed. sIBM weakness comes on over months or years and progresses steadily, whereas polymyositis has an onset of weeks or months. [[Muscular dystrophy]] (e.g., [[limb girdle muscular dystrophy]]) must be considered as well.{{citation needed|date=February 2021}} sIBM can be mistaken for physical deconditioning.<ref name="Weihl2019Review" />

Hereditary myopathies can mimic sIBM, both in signs and symptoms and in the appearance of muscle biopsies. A small percentage of those initially diagnosed with sIBM are later found to have pathogenic mutations in the genes ''[[Valosin-containing protein|VCP]]'' and ''[[SQSTM1]]'', which are known to cause hIBM.<ref name="Weihl2019Review" />

IBM has a distinctive pattern of muscle involvement that distinguishes it among inflammatory myopathies.<ref name="Greenberg2019Review" /> Characteristic of IBM is weakness of finger flexion, knee extension, and ankle dorsiflexion.<ref name="Greenberg2019Review" /> Other inflammatory myopathies cause a proximal muscle weakness pattern, such as weakness of hip flexion, abduction, and extension, as well as shoulder abduction.<ref name="Greenberg2019Review" /> IBM and other inflammatory myopathies both cause bicep/tricep weakness.<ref name="Greenberg2019Review" />