Rare Case of Misdiagnosed Primary Intramuscular Hydatid Cyst of the Deltoid Muscle

This case report describes a rare parasitic cyst called a hydatid cyst located in the shoulder muscle (deltoid) of a 29-year-old male. The patient initially presented with a painless, gradually enlarging swelling over the left shoulder. It was misdiagnosed as a synovial cyst and treated with corticosteroids, which caused local inflammation and purulent discharge. Blood tests for Echinococcus antibodies were negative. Ultrasound and MRI revealed a well-defined intramuscular cyst confined to the deltoid muscle. Surgical excision confirmed the diagnosis, and the patient received postoperative albendazole therapy for three months. Follow-up showed complete healing, restoration of shoulder function, and no recurrence. This case highlights the importance of considering hydatid disease in unusual muscle locations, particularly in endemic areas, to avoid misdiagnosis and ensure proper treatment.