Pacchionian granulations are hypertrophic arachnoidal villi, which can erode through cranial bone, causing lytic lesions on skull roentgenograms or computed tomography (CT) scans.
If hypertrophic, such arachnoid villi are then called pacchionian granulations, which can erode even further, reaching the outer table of the cranium and causing a punched-out lesion on roentgenograms.
Pacchionian granulations are reported more frequently among adults, but earlier stages of hypertrophic arachnoid villi can be present as early as 18 months of age.
Because of the osteolytic appearance of pacchionian granulations on roentgenograms, the differential diagnosis includes all other osteolytic lesions of the calvarial bone, including dermoid/epidermoid cyst, osteoma, enchondroma, fibroma, fibrous dysplasia, and calvarial hemangioma.
Pacchionian granulations are benign overgrowth of arachnoidal villi that have a radiolucent appearance on a skull X-ray.