Atypical Presentation of Bilateral Acromioclavicular Joint Cysts Extending Medially into the Trapezius Muscles: A Case Report and Literature Review.

Introduction: Acromioclavicular joint cysts represent a relatively rare clinical entity, often manifested as an enlarging mass adjacent to the acromioclavicular joint, which can raise concerns for a potential tumor. These cysts are identified for their association with shoulder pathology, particularly extensive rotator cuff tears, and present a diagnostic and therapeutic challenge for misdiagnosis as neoplastic masses. In this case, we are reporting on an unusual presentation of a patient presenting with a swollen mass in the left trapezius causing neck pain. The aim of this case presentation is to bring awareness to atypical locations of these acromioclavicular joint cysts that can be easily interpreted as a tumoral process in the neck due to the location of the mass. Acromioclavicular cysts remain infrequently reported in the literature.

Case Report: We present the case of an 80-year-old male patient. On clinical examination, a soft and palpable mass over the trapezius was noted, with no signs of infection or inflammation. Plain radiographs showed advanced primary osteoarthritis of the glenohumeral joint with a preserved subacromial space. Magnetic resonance imaging of both shoulders showed that cystic masses were developed on both sides from the acromioclavicular joint and extended medially to the trapezius muscles. It was particularly large on the left side, with a diameter reaching up to 12 cm. The surgical intervention was carried out involving resection of the distal end of the clavicle through an open approach and resection of the neck of the cyst. Through a second incision at the dorsal level on the medial aspect of the trapezius, the cyst sac was identified and its subsequent resection was performed.

Conclusion: The presented case of a rare location of bilateral ACJ cysts, treated successfully with cyst resection and ACJ decompression, illustrates a diagnostic and therapeutic challenge due to the atypical nature of the cysts' presentation. This case aims to bring awareness about this pathology when confronted with a painless lump in the upper shoulder region, although far away from the joint in patients with complex shoulder or neck symptomatology.