[A Case of Multiple Brain Lesions Associated with Hydrophilic Polymer Allergy after Coil Embolization that Showed Repeated Progression and Regression].

A 41-year-old woman underwent coil embolization for subarachnoid hemorrhage associated with a ruptured anterior cerebral artery(A1)aneurysm. Approximately 3 weeks later, MRI revealed right cerebral white matter changes with extensive edema and enhancement lesions. Even though she was asymptomatic, we suspected an allergic reaction to the hydrophilic coating polymer and initiated steroid treatment. After tapering and discontinuing the steroid treatment, follow-up MRI revealed development of white matter lesions;thus, steroid treatment was reinitiated. Progression and regression of the lesions occurred repeatedly, and she was radiologically stable at almost 1 year after coiling. We speculated that these white matter lesions were foreign body granulomas that reacted to the hydrophilic coating of the endovascular device. Overall, an allergic reaction to hydrophilic coating polymer could occur as a delayed complication after coil embolization and that progression and regression of the lesions could repeatedly occur in rare cases.