Are arachnoid cysts localized hydrocephali?

We report 2 cases of arachnoid cysts related to a head trauma. The first case involved a 1-year-old girl who developed an arachnoid cyst 7 months after suffering a head trauma due to a motor vehicle accident, and the second case involved a 1-year-old boy whose arachnoid cyst was first noted 2 months subsequent to his premature birth. We present the serial imaging for both cases. Choi and Kim [Pediatr Neurosurg 1998;29:260-266] postulated that head trauma in infancy may contribute to the pathogenesis of arachnoid cysts in some cases. In presenting our cases, we support this hypothesis, and further postulate that the arachnoid cysts may in fact be localized hydrocephali, triggered either by head trauma or by prematurity.