Aberrant arachnoid granulations

Aberrant arachnoid granulations, also known as arachnoid pits, are arachnoid granulations that penetrated the dura but failed to migrate normally in the venous sinus. They are most often located in the greater wing of the sphenoid bone and may be seen in idiopathic intracranial hypertension. Occasionally, they are seen in the posterior temporal bone wall.

Terminology

Aberrant arachnoid granulations are known by a variety of other names such as sphenoid wing meningoceles (often used in the context of idiopathic intracranial hypertension with or without rhinorrhea), intradiploic arachnoid cysts and arachnoid pits ^5,6. 

Clinical presentation

Although most are incidental finding when associated with elevated intracranial CSF pressures (e.g. idiopathic intracranial hypertension) they may enlarge and present with cerebrospinal fluid rhinorrhea or cephaloceles ⁵. 

Pathology

Aberrant arachnoid granulations can be seen at the floor of the anterior and middle cranial fossa and, less frequently, at the posterior temporal bone wall. They contain cerebrospinal fluid but do not communicate with the dural venous sinuses. Instead, the pressure of the cerebrospinal fluid will cause bone erosion.

As they grow, these arachnoid granulations can cause defects in the posterior temporal bone wall or the tegmen tympani. Thus, cerebrospinal fluid otorrhea can occur. Arachnoid granulations in this specific location may also potentially spread bacterial infection to the meninges. In this context, it is important to report their presence, especially since they are known to enlarge with age.

Radiographic features

CT

Sometimes an incidental finding, they are characterized by multiple CSF isodense focal bone erosions in the sphenoid bone bilaterally. A direct communication with the paranasal sinuses may be seen.

HRCT temporal bone reveals erosions along the posterior wall, without bone spicules and often with a lobulated surface.

Focal dural rim enhancement may be seen on CECT.

If large, the arachnoid granulations may be outpouching in the sphenoid sinus.

MRI

• T1: hypointense on T1 (isointense to CSF)
• T2: hyperintense on T2 (isointense to CSF)
• T1 C+ (Gd): faint dural rim enhancement may be seen

Differential diagnosis

Imaging differentials of temporal bone aberrant arachnoid granulations include:

• endolymphatic sac tumors: it shows heterogeneous signal intensities on T1- and T2-weighted images with typically focal high T1 signal intensities due to hemorrhagic and proteinaceous components and heterogeneously enhancing on postcontrast images
• chordoma
• paraganglioma
• metastasis
• chondromatous tumors ⁴