How to understand an enlarged Meckel's cave? An anatomical study

Purpose: Dilatation of the trigeminal cavum, or Meckel's cave (MC), is usually considered a radiological sign of idiopathic intracranial hypertension. However, the normal size of the trigeminal cavum is poorly characterized. In this study, we describe the anatomy of this meningeal structure.

Methods: We dissected 18 MCs and measured the length and width of the arachnoid web and its extension along the trigeminal nerve.

Results: Arachnoid cysts were clearly attached to the ophthalmic (V1) and maxillary (V2) branches until they entered the cavernous sinus and foramen rotundum, respectively, without extension to the skull base. Arachnoid cysts were close to the mandibular branch toward the foramen ovale, with a median anteromedial extension of 2.5 [2.0-3.0] mm, lateral extension of 4.5 [3.0-6.0] mm, and posterior extension of 4.0 [3.2-6.0] mm. The trigeminal cavum arachnoid had a total width of 20.0 [17.5-25.0] mm and length of 24.5 [22.5-29.0] mm.

Conclusion: Our anatomical study revealed variable arachnoid extension, which may explain the variability in size of the trigeminal cavum in images and calls into question the value of this structure as a sign of idiopathic intracranial hypertension. The arachnoid web extends beyond the limits described previously, reaching almost double the radiological size of the cavum, particularly at the level of V3 afference of the trigeminal nerve. It is possible that strong adhesion of the arachnoid to the nerve elements prevents the formation of a true subarachnoid space that can be visualized by magnetic resonance imaging.