Rapid formation of a multi-compartment neonatal arachnoid cyst

A 3-day-old term infant born via vaginal delivery presented with irritability and tachypnoea. Cranial ultrasound, CT and MRI demonstrated a massive left temporal arachnoid cyst extending into the anterior and posterior fossa as well as the suprasellar area. In utero ultrasounds at 9.5, 19 and 32 weeks of gestation had been normal. Endoscopic exploration revealed that all cyst components were communicating, and a cystoperitoneal shunt was inserted with rapid resolution of symptoms and cyst decompression. Arachnoid cysts may develop rapidly in utero and may not be associated with underlying brain hypoplasia. Attempts at rapid and maximum decompression may be reasonable in this setting.