The arachnoid villi are small finger-like projections that consist of the same cellular and septal mesh that forms the structure of the arachnoid, but with less septation and larger spaces. If such villi agglomerate and enlarge, they can form larger spaces (called fossae lacunas) that sometimes communicate with the dural sinus through short channels, in which cerebrospinal fluid (CSF) can be transferred to the venous system. Because of their close proximity to the cranium, such lacunar spaces can create imprints on the internal lamina of the cranium as furrows or holes. 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.1
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.2 Typically, they are asymptomatic and are usually found accidentally on skull roentgenograms or CT scans, with the typical appearance of hypodense lesions in the calvarial bone. On T2-weighted magnetic resonance images, such lesions have a density similar to that of CSF.3-5
These granulations are primarily located in the vicinity of dural venous sinuses and most are close to the sagittal sinus. In this case, the lesions were located in the parasagittal region of the parietal bone anteriorly. The posterior part of the frontal bone is the second most common site.6
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.7 The typical parasagittal location, absence of calcification, and the lack of pain on presentation are distinguishing features. However, diagnostic biopsy is sometimes indicated, especially if the lesion is rapidly increasing in size.
Very rarely, hypertrophic archnoid villi are associated with hydrocephalus, but most cases are asymptomatic and can be observed without treatment.8
Pacchionian granulations are benign overgrowth of arachnoidal villi that have a radiolucent appearance on a skull X-ray. They should be considered in the differential diagnosis for any osteolytic lesions in the parasagittal area.
Bilateral parasagittal pacchionian granulations. Appl Radiol.