Background: Arachnoid cysts are intra-arachnoidal cerebrospinal fluid collections that are usually asymptomatic, however, they may become acutely symptomatic due to enlargement of the cyst or the presence of hemorrhage.
Materials and methods: We report a case of a child presenting with a 3-week history of headaches, nausea, and vision problems. There was no history of trauma. MRI clearly demonstrated a left middle cranial fossa arachnoid cyst with associated subacute intracystic and subdural hematoma that was causing mass effect and required surgery.
Results: This lesion was isodense to the brain on CT.
Conclusion: We focus on the importance of MR imaging in the differentiation of these subacute/early chronic hemorrhagic collections that may be overlooked with CT.