Rapid progression of a posterior fossa arachnoid cyst in a child following minor head trauma: a case report.

Posterior fossa arachnoid cysts are usually congenital and asymptomatic. Rapid enlargement after minor trauma is rare. Here, we report a case of a 1-year-old boy who developed a giant posterior fossa arachnoid cyst with obstructive hydrocephalus 1 month after a minor head injury, despite an initially normal CT scan. Follow-up CTs showed a giant arachnoid cyst in the right posterior fossa causing obstructive hydrocephalus and evidence of transtentorial herniation. He underwent surgery, and follow-up imaging confirmed cyst decompression, resolution of hydrocephalus, and full neurological recovery. To our knowledge, no previous reports have described a case in which initial neuroimaging was normal after trauma, but a giant arachnoid cyst subsequently formed within a short period, necessitating urgent surgery.