Thrombosed Middle Meningeal Artery Aneurysms Mimicking Aneurysmal Bone Cysts in a Patient with Fibrous Dysplasia

Background The aneurysmal bone cyst (ABC) is a rare osteolytic lesion that is often associated with osseous disease such as fibrous dysplasia (FD). Case Description A 66-year-old woman previously diagnosed with FD presented with suddenly reduced visual acuity. Computed tomography and magnetic resonance imaging revealed a large osteolytic lesion in the middle cranial fossa that had invaded the left orbit. Although its radiological findings were compatible with an ABC, rotational digital subtraction angiography revealed that this osteolytic lesion was a large middle meningeal artery (MMA) aneurysm. To release the compression force on the optic nerve and confirm the diagnosis, we performed endovascular embolization, followed by surgical removal of the lesion. Histologic examination revealed that the resected lesion met the requirements of a thrombosed aneurysm. No new neurologic deficits appeared after the surgery, and no symptomatic recurrence have been noted during the 6-month follow-up. Conclusions This is the first case report to show that ABC-like lesions in patients with FD could be thrombosed MMA aneurysms. It is thus possible that some lesions previously diagnosed as ABCs were thrombosed MMA aneurysms or included a thrombosed MMA aneurysm. It is therefore important to rule out the presence of a MMA aneurysm when we encounter what appears to be an ABC in the presence of FD.