Case Report: Misdiagnosed Primary Intramuscular Hydatid Cyst of the Deltoid Muscle in a Male Patient

This report presents a rare case of a primary intramuscular hydatid cyst confined to the deltoid muscle in a 29-year-old male. The patient had a seven-month history of a gradually enlarging swelling in the left shoulder. Initially misdiagnosed as a synovial cyst, he received corticosteroid injection, which led to pain, erythema, and purulent discharge. Physical examination revealed a 10 cm fluctuant mass over the posterolateral deltoid. Serology for Echinococcus antibodies was negative. Ultrasound showed a well-defined, anechoic intramuscular cyst measuring 7 × 8 × 10 cm. MRI confirmed a fluid-filled cyst confined to the deltoid muscle, without involvement of bone, tendon, or joint structures. Under local anesthesia, the cyst was completely excised, and the cavity was irrigated with 10% povidone-iodine. Histopathology confirmed a non-fertile hydatid cyst (Echinococcus granulosus) without malignancy. Postoperative therapy included albendazole 400 mg daily for three months. Follow-up at six weeks showed complete wound healing, full shoulder function, and no recurrence. This case underscores the diagnostic challenges of intramuscular hydatid cysts, especially with negative serology, and highlights the importance of imaging, histopathology, and careful surgical management in endemic regions.