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Although arachnoid granulations are a normal anatomic structure, if they are large and appear in the dominant sinus or only in the transverse sinus, they could cause venous obstruction and lead to symptoms of venous hypertension

Classic appearance of arachnoid granulations. (a) Photograph from an anatomic dissection of the right transverse sinus demonstrates focal protuberances consistent with arachnoid granulations (arrows). Intrasinus septa (chordae willisii) (arrowheads) also are depicted.

Chordae willisii are structures located in the lumen of the superior sagittal sinus (SSS). It is thought that they act as flow-improving structures within the sinuses.

How could a septum improve the flow?

METHODS: ... The chordae willisii were observed in all examined specimens. Three different types of the cords were found: lamellar, trabecular, and valvelike types. The most common type was the valvelike (mixed) one, which comprised 45.1% of all cords. The chordae willisii were most commonly observed in the parietooccipital region of the SSS.
CONCLUSIONS: Witout disturbing any structural relationships, the use of endoscopy allowed visualization and description of intraluminal structures as they behaved physiologically.

The growth of arachnoid membrane into the dural sinus (glandulae conglobatae) was first described by Pacchioni2) in 1705. These projections are called arachnoid villi or arachnoid granulation, depending on their size. Arachnoid villi are microscopic, whereas granulation is visible to the naked eye11). They function as a passive filtration system for CSF, providing a pathway from subarachnoid space into the venous system. Arachnoid granulations increase in number and enlarge with age in response to increased CSF pressure from the subarachnoid space and are usually quite conspicuous by 4 years of age.