Painful Intramuscular Lipoma of the Infraspinatus: Unusual Location and Presentation

Intramuscular lipomas are considered a rare type of benign lipomas. They are usually located deeper and are less palpable than subcutaneous lipomas. A painful presentation with no palpable mass will make clinical diagnosis difficult; in these cases, further imaging should be considered. Only a small number of cases of intramuscular lipomas present with pain; these are located in the supraspinatus and deltoid muscles. To the authors' knowledge, there have been no previous reports of painful intramuscular lipomas involving the infraspinatus muscle. This article describes a case of intramuscular lipoma uniquely located in the infraspinatus muscle and presenting with shoulder pain. A 49-year-old woman presented with 2 months of left shoulder pain. There was no history of preceding trauma. Pain was aggravated by lying on the left shoulder and by the hand behind the back similar to Crass position. On the physical examination, her shoulder joint range of motion was slightly decreased. Simple radiography showed no significant abnormality, but ultrasonography revealed a hyperechogenic mass within the infraspinatus muscle. There was focal tenderness over the mass, but definite palpation of the mass was not possible. Magnetic resonance imaging revealed a well-circumscribed, homogeneous lesion measuring 43 x 28 x 16 mm within the infraspinatus muscle, leading to a diagnosis of intramuscular lipoma. Her pain was not improved with medication, suprascapular nerve block, and steroid injections. Finally, surgical intervention was done and intramuscular lipoma was confirmed by specimen. After excision, her shoulder pain was improved and resolved.