Introduction and importance: Hydatid disease, caused by the larval stage of Echinococcus granulosus, is a common parasitic infection of humans and herbivores. Although livers and lungs are the most commonly affected organ, hydatid cysts may develop in any body part. Primary muscular hydatid cyst is extremely rare.
Case presentation: We reported the case of a 40-year-old-woman with the presentation of a soft, mobile, and non-tender lump in the dorsal part of her left upper arm (triceps brachii), which emerged one year ago. Her past medical history was unremarkable. The arm sonography revealed a single uniloculated cystic mass (6.5 cm × 5.5 cm) with a thick wall containing cystic lesions. It suggested the diagnosis of echinococcosis. The patient underwent surgery, and the hydatid cyst was excised. Histopathological examination confirmed hydatidosis.
Clinical discussion: Hydatid cysts occur rarely (about 4 %) in muscles even in endemic regions. The study is the first case of hydatidosis found in triceps brachii in Fars province, Iran. In endemic regions, considering the hydatid cyst possibility is very important because it presents with many diversities. As it clinically presents a painless slow-growing mass, may be misdiagnosed with benign soft tissue tumors.
Conclusion: Although muscular hydatidosis is extremely rare, it should be considered a differential diagnosis of any growing subcutaneous or muscular masses or tumors. Imaging modalities and blood tests are highly relevant for diagnosis. Surgical excision, a choice of treatment, should be done with cautions and is combined with anthelmintic therapy to reduce the risk of recurrence.
Keywords: Case report; Echinococcosis; Iran; Skeletal muscle.
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