Symmetrical proximal muscle weakness with insidious onset
Generally painless (though 30% have myalgia)
Difficulty with kneeling, climbing stairs, combing hair, or rising from a seated position
Weak neck extensors causing difficulty of holding head up
Associated arthralgias
Polymyositis
Infection:
Dermatomyositis
Viral infection 
Lambert-Eaton myasthenic syndrome
Bacterial infection
Amyotrophic lateral sclerosis
Spirochete infection 