Polymyositis and dermatomyositis differential diagnosis

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Template:Aditya, Anmol Pitliya, M.B.B.S. M.D.[2]

Overview

Polymyositis and Dermatomyositis must be differentiated from other inflammatory myopathies which cause muscle weakness and systemic symptoms.

Differentiating Polymyositis and Dermatomyositis from other Inflammatory Myopathies

Organ system	Disease	Symptoms												History	Physical Examination	Diagnosis
		Age of onset	Muscle weakness	Fever	Myalgia	Contractures	Gait abnormality	Neuropathy	Atrophy	Stiffness	Myoglobinuria	Other features	Laboratory Findings			Creatine Kinase	Muscle Biopsy	Electromyogram
Inflammatory/ Rheumatologic	Dermatomyositis	40s−50s Can affect children	Proximal	+	+	−	−	−	−	+	−	Rash Dyspnea Weight loss Cough	Viral infections Cancer	Heliotrope rash on face and hands Telangiectasia Erythema Mechanic's hands Gottron's sign ( violaceous scaly eruption )	↑↑ ESR ↑↑ CRP	↑↑	Perimysial mononuclear infiltrate	Myopathic
	Polymyositis	> 18 years	Proximal	+	+	−	−	−	−	+	−	Similar to dermatomyositis without mucous and skin involvement	N/A	N/A	↑↑ ESR ↑↑ CRP	↑↑	Endomysial mononuclear infiltrate Patchy necrosis
	Inclusion body myositis	50s	Proximal & distal	−	−	−	−	−	−	−	−	Dysphagia Asymmetric weakness	Retrovirus (most common)	N/A	Antibodies to cytoplasmic 5'−nucleotidase	↑↑	Inflammatory cells Invading muscle cells Vacuolar degeneration Inclusions or plaques	Neurogenic
	Fibromyalgia	40−50s	Generalized	−	−	−	−	+	−	−	−	Anxiety or depression features Fatigue Sleep disturbance Numbness Muscle spasms	History of depression	Tenderness in the soft tissue anatomical location	Normal	Normal	Normal	Normal
	Polymyalgia Rheumatica	50s	Diffuse	+	+	−	−	−	−	+	−	Weight loss	History of joints stiffness, worse in the morning	Restricted shoulder motion	↑↑ ESR ↑↑ CRP	Normal	Normal	Normal