Presents with weakness in the large muscles that spares the small muscles. About a third of patients have a rash (dermatomyositis) – the one I always remember is the blue-purple discolouration on the eyelids, but it can also prseent as a flat red rash on the face & upper trunk or as raised purple-red scaly patches on the knuckles.
Check muscle enzymes – CK, LDH, transaminases, aldolase (whatever that is). Muscle biopsy confirms the diagnosis. Diagnose early, as delay in starting treatment leads to further muscle deterioration and reduced chance recovery.