With aging, arachnoid villi are increasing in size and may even become mineralized and, as a consequence, they become visible with naked eye, as granulations, known as Pacchionian
During embryogenesis, when arachnoidal cells that are present in the sheaths of nerves or vessels emerge through the skull foramina, the then displaced Pacchionian
bodies are detached, pinched off, or entrapped in an extracranial location.
Cranial morphological age sequencing provided by Krogman: 1) Starting from 25 years of age, muscular markings become increasingly evident particularly on the occipital, temporal and lateral sides of the mandible; 2) Between 35 to 45 years, the surface gives a matted, granular appearance; 3) Around 35 to 45 years, inside of skull shows the Pacchionian
depressions and occur with much more frequency; 4) After 50 years of age, the diploe become less vascular channelled and there is an increasing replacement by bone.
Meningiomas are derived from the arachnoid cap cells, also called pacchionian
Calcium deposition may normally be seen in the choroid plexus, pineal body, falx and Pacchionian
bodies: many pathological conditions may also result in calcification.6 Differential diagnoses of intraparenchymal-calcified lesions include metabolic disorders (hyperparathyroidism, lead poisoning), infectious disorders (cytomegalovirus infection, toxoplasmosis, tuberculomas), neoplasms (ependymoma, oligodendroglioma), and vascular malformation, including Sturge-Weber syndrome and pial arteriovenous malformation.7
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.