Primary pyomyositis in an infant following one-day afebrile upper limb monoplegia

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Background : Pyomyositis is a subacute bacterial infection of the skeletal muscles. Its most common features are fever, muscle swelling, and focal pain. There have been insufficient data regarding pyomyositis in healthy infants. Case report : A one-month-old boy presented with an impairment of his left arm movement. He was well-nourished and not under any apparent distress. His vital signs were within the normal limits but neurological examination revealed left forearm paralysis. Physical examination showed no abnormal findings in the region from the left shoulder joint to the fingertips. Considering these factors, an intracranial pathology was initially suspected. However, he developed a fever, regular tachycardia, and swelling in the left forearm. Magnetic resonance imaging revealed inflammation in the left forearm muscles. He was diagnosed with bacterial myositis and started on intravenous antibiotics. On the 17th day, he was discharged with oral antibiotic treatment, which was completed over 25 days without any sequelae nor relapse. Conclusion : Here we report the case of Japanese primary pyomyositis following one-day afebrile upper limb monoplegia in an infant. Even when infants exhibit afebrile symptoms, a bacterial infection should be suspected. J. Med. Invest. 68 : 372-375, August, 2021