Left Shoulder Cystic Lesion Mimicking a Soft Tissue Mass: A Rare Presentation of the Geyser Phenomenon

The geyser phenomenon is a rare complication of chronic rotator cuff disease in which synovial fluid passes from the glenohumeral joint through a rotator cuff defect into the acromioclavicular (AC) joint, resulting in a cystic swelling over the shoulder. Because these lesions can enlarge over time and exhibit atypical imaging features, they may be mistaken for soft tissue tumors. We report the case of a 78-year-old Saudi man with diabetes mellitus, hypertension, and rheumatoid disease who presented with a slowly enlarging left shoulder mass that had been present for 1.5 years. Magnetic resonance imaging (MRI) demonstrated a 5.7-cm complex cyst communicating with the AC joint, associated with massive full-thickness rotator cuff tears and advanced degenerative changes. Follow-up imaging showed interval enlargement of the lesion, hemorrhagic contents, and a new lytic lesion in the lateral clavicle, raising concern for infection or malignancy. The patient underwent surgical excision of the lesion with distal clavicle resection. Histopathological examination revealed a benign inflammatory pseudocyst with no evidence of malignancy. This case illustrates an unusual presentation of the geyser phenomenon that clinically and radiologically mimicked a soft tissue neoplasm. Awareness of this entity may help avoid unnecessary investigations and facilitate the establishment of the correct diagnosis.