Case Presentation: An 8-year-old female with a 10-day history of intermittent myalgia and fatigue presented to the emergency department after waking with severe, diffuse myalgias. Initial laboratory work-up included CBC, CMP and CPK. Most significant was a CPK of 35,000 units/L. Aggressive fluid repletion was started for presumed viral induced rhabdomyolysis. After 5 days of fluid repletion, CPK level plateaued at 21,000 units/L. Following fluids and pain control, repeat physical exam noted mild proximal muscle weakness of the upper extremities and significant proximal muscle weakness of the lower extremities with positive Gowers’ sign. Further review of the patient’s outpatient medical record revealed an elevated ANA obtained prior to admission. Rheumatologic screening panel including ANA, Anti-DNA antibody, C3/C4, rheumatoid factor, Scl-70 Anti-body, Smith antibody, SS-A/Ro autoantibodies, SS-B/LA autoantibodies & thyroid peroxidase antibodies was obtained. The panel showed elevated ANA (1.96 units) but no other irregularities. MRI without contrast of the lower extremities noted extensive T2 signal abnormality throughout the musculature of the thighs and buttocks consistent with myositis. Since the cause of myositis was indeterminate, a muscle biopsy was performed. Biopsy demonstrated skeletal muscle with perimysial and immunohistochemical abnormalities indicative of Juvenile Polymyositis. The patient was started on high dose steroids with improvement in CPK and resolution of proximal weakness.
Discussion: Due to its rarity (~3 cases per 1,000,000), the initial presentation of Juvenile Polymyositis can be difficult to distinguish from other common causes of myositis, particularly rhabdomyolysis. This condition should be considered when appropriate therapies for other pathologies fail to stabilize laboratory abnormalities. Dermatomyositis should also be included in the differential but can be differentiated from polymyositis by its characteristic cutaneous findings including Gottron’s papules, heliotrope rash, and nailfold abnormalities. Diagnostic approach should begin with less invasive studies such as MRI. Non-contrast MRIs allow for confirmation of muscle inflammation and identification of potential sites for muscle biopsy. Muscle biopsy is generally required in cases of Juvenile Polymyositis due to its lack of other cutaneous findings. Electromyography (EMG) may be used as an alternative to muscle biopsy if available. Treatment for this condition involves high dose steroid therapy for acute episodes. Other immune modulator therapy may be needed for more severe or refractory cases but should be management by a pediatric rheumatologist.
Conclusions: Rhabdomyolysis is the most common cause of acute myositis in children. However, hospitalists should be prepared to explore other possible causes if serum muscle enzyme levels fail to normalize despite adequate therapy. In this case, the hospitalist team included polymyositis on the differential despite it rarity. This led to the appropriate testing and ultimate resolution of symptoms with treatment.