Aberrant arachnoid granulations

Aberrant arachnoid granulations (AbAG) 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. Occasionally, they are seen at the posterior temporal bone wall.

Although sometimes an incidental finding, nasal CSF leak is the characteristic clinical presentation when symptomatic.

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 CSF but do not communicate with the venous sinuses. Instead, the pressure of the CSF will cause bone erosion. As they grow, these arachnoid granulations can cause defects in the posterior temporal bone wall or the tegmen tympani. Thus CSF leakage and otorrhoea can occur. Arachnoid granulations in this specific location may also lead to bacterial invasion to the meninges. In this context, it is important to report their presence because they are known to enlarge with age.

CT

Sometimes an incidental finding on head CT-scan, they are characterised by multiple focal bone erosions of CSF density 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 granulation may be outpoutching 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

Imaging differentials of temporal bone aberrant arachnoid granulations include:

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

Neuroanatomy
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Article Information

rID: 24988
Section: Anatomy
Synonyms or Alternate Spellings:
  • Aberrant arachnoid granulation
  • Aberrant arachnoid granulations (AbAG)
  • Ectopic arachnoid granulation

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