Magnetic resonance imaging of abnormal shoulder pain following influenza vaccination

The influenza vaccine is increasingly available to the general public and mandated by many employers in the United States. The prevalence of post-vaccination complications is likely on the rise. Complications are well known to general clinicians, but are under-reported in the imaging literature. We present four cases of post-vaccination shoulder pain with magnetic resonance imaging (MRI) findings. An intrasubstance fluid-like signal in deep muscular and/or tendinous structures was the most common finding on MRI of these four cases. Focal bone marrow signal within the humeral head and inflammatory changes in the subacromial/subdeltoid bursa were also observed. The most likely reason for a humeral intraosseous edema-like signal was presumed injection of vaccine substance directly into osseous structures that might lead to focal osteitis. In the published literature, there is little emphasis on the imaging of local injection site complications accompanying influenza vaccination. We intended to increase familiarity of MRI findings in the setting of prolonged or severe clinical symptoms following influenza vaccination through the imaging findings of these four cases.