Rheumatology or neurology consultation either in ED or as outpatient depending on severity of symptoms
Very strongly associated with malignancy (~30%), especially:
Ovarian
Lung
Pancreatic
Stomach
Colorectal
Lymphoma
Bladder
Ovarian
Lung
Pancreatic
Stomach
Colorectal
Lymphoma
Bladder
Polymyositis
Infection:
Dermatomyositis
Viral infection 
Lambert-Eaton myasthenic syndrome
Bacterial infection
Amyotrophic lateral sclerosis
Spirochete infection 