Arachnoid granulations, also known as Pacchionian granulations, are projections of the arachnoid membrane (villi) into the dural sinuses that allow CSF to pass from the subarachnoid space into the venous system.

Epidemiology

They increase in size and number with age and are seen in approximately two-thirds of patients.

Pathology

Location

They most frequently occur in a parasagittal location with the transverse and superior sagittal sinuses being the most common locations. The granulations typically occur next to the entrance of a superficial draining cortical vein into a sinus (similar to colonic diverticula occurring next to penetrating vessels).

Radiographic features

They are most commonly encountered in radiological practice as incidental osteolytic, sharply circumscribed indolent-appearing lucencies on skull CT or x-rays, or a filling defect in dural venous sinuses, which can be mistaken for dural venous thrombosis.

CT

The granulations are typically of CSF density and protrude into the calvaria or a dural venous sinus causing a filling defect. They may simulate a dural venous sinus thrombosis but are usually easy differentiated given their round well-defined shape and classic location.

MRI

Signal characteristics are generally those of CSF ²:

• T1: low signal intensity

• T2: high signal intensity; iso- or even slightly hyperintense to CSF

• FLAIR: should attenuate

• T1 C+ (Gd): no enhancement

Giant granulations (>10 mm) may show atypical MRI signal characteristics, with higher T1 and T2 signal than CSF and incomplete FLAIR signal suppression ⁵. Rarely these may cause sinus obstruction ⁶.

History and etymology

Arachnoid granulations are named after Antonio Pacchioni (1665-1726), an Italian physician, who wrote extensively on the anatomy of the dura mater, and provided the first written description of his eponymous granulations in 1705 in the Dissertatio Epistolaris de Glandulis Conglobatis Durae Meningis Humanae, one of his monographs ¹.

Differential diagnosis

Considerations include

• dural venous thrombosis

• extradural mass

On CT if there is lytic erosion of the bone also consider

• small calvarial metastases/myeloma/plasmacytoma etc.

or rarely

• intradiploic epidermoid - often centred towards the mid to outer margins of the skull

See also

• aberrant arachnoid granulations