Perigeniculate arachnoid cysts and CSF fistulae of the fallopian canal: Histopathologic correlates of a rare clinical entity

Abstract

Cerebrospinal fluid (CSF) fistulae originating from the fallopian canal of the facial nerve is hypothesized to arise due to atypical patterns of subarachnoid space extension into the geniculate ganglion or more distal regions along the intratemporal course of the facial nerve, but its pathogenesis remains poorly understood. Although a rare etiology of CSF fistulae of the temporal bone, there are significant clinical ramifications due to the risk of recurrent meningitis, difficulty in identifying the anatomic location of the CSF leak, and technical challenges associated with surgical repair. We present three clinical cases of arachnoid cysts within the geniculate fossa with or without CSF fistulization and provide histopathologic correlates of this rare clinical phenomenon from a human temporal bone collection. The pediatric and adult patients presented suggest differential pathophysiologic mechanisms associated with CSF fistulae. Temporal bone histology reveals atypical patterns of subarachnoid space extension in the fallopian canal that may underlie arachnoid cyst formation and overt CSF leak from the geniculate region.

Keywords: Arachnoid cyst; Cerebrospinal fluid leak; Cerebrospinal fluid otorrhea; Facial nerve; Fallopian canal; Geniculate ganglion; Subarachnoid space.

Fig. 1

Illustration of three patterns of subarachnoid space extension. A: Type I, subarachnoid space limited to the termination of the petrosal fallopian canal. B: Type II, SAS extension into the substance of the geniculate ganglion and surrounding nerve fascicles. C: Type III, extension into the tympanic segment of the fallopian canal.

Fig. 2

Audiogram. Twelve year-old girl presented with right sided asymmetric sensorineural hearing loss over several years, with speech discrimination of 40% and 100% in the right and left ears, respectively.

Fig. 3

Case 1 preoperative imaging. A: Magnetic resonance imaging demonstrates a T2-hyperintense, T1-hypointense, and non-contrast enhancing cystic lesion of the right geniculate ganglion. B-D: Serial axial temporal bone computed tomography views demonstrate expanded labyrinthine segment of the fallopian canal (arrows) with cochlear dehiscence and (E-G) dehiscence of the ampullated end of the superior semicircular canal. H–K: Coronal slices demonstrate intact middle fossa floor (black arrows) and an expansile lesion intrinsic to the geniculate ganglion (∗) with possible cochlear dehiscence (white arrow).

Fig. 4

Case 1 intraoperative view. Intraoperative image of transcanal approach to perigeniculate arachnoid cyst following removal of the incus, demonstrating the malleus, stapes capitulum (∗), cochleariform process (arrowhead), and a clear fluid-filled cyst (arrow) in the regions just distal to the geniculate ganglion.

Fig. 5

Case 2 pre-operative Imaging. A: Axial temporal bone computed tomography demonstrates a smoothly dilated right geniculate fossa (arrow). B: Magnetic resonance imaging shows a fluid signal at the geniculate ganglion suspicious for CSF accumulation (arrow).

Fig. 6

Case 2 intraoperative view. Intraoperative image demonstrating CSF accumulation at the geniculate region (arrow), tympanic segment of facial nerve (∗), cochleariform process (arrowhead), chorda tympani (CT), and stapes superstructure (SS).

Fig. 7

Case 3 imaging. A: Magnetic resonance imaging shows T2-hyperintensity at the right geniculate ganglion indicative of a cystic lesion (arrow). Fluid in the optic nerve sheath complex and empty sella (not pictured) is suggestive of intracranial hypertension. B: Coronal temporal bone computed tomography shows erosion (arrowhead) of the middle fossa floor overlying the geniculate ganglion. C: Right axial CT is notable for an enlarged labyrinthine segment of the fallopian canal (arrow) without involvement of the cochlea. D: Contralateral left-sided axial CT with uninvolved labyrinthine segment for comparison.

Fig. 8

Spectrum of bony dehiscence of middle fossa floor overlying the geniculate fossa. Photomicrographs at the level of the geniculate ganglion (∗) demonstrating spectrum of middle fossa architecture in the geniculate region. A: Intact middle fossa floor overlying geniculate fossa. Specimen 11095 L. B: Focal dehiscence (arrow) containing fibromyxoid tissue contiguous with temporal lobe dura. Specimen 4748 R. C: Complete bony dehiscence with intact dura mater. Specimen 7827 L. Original magnification 2.5×. Co, cochlea.

Fig. 9

Type I pattern. HE stain of a vertically sectioned temporal bone specimen. Representative photomicrographs depict a type 1 arachnoid pattern over the facial nerve and geniculate ganglion (∗). The subarachnoid space is contiguous from the fallopian canal in the labyrinthine segment to its termination (arrows) at the geniculate ganglion. Note the normal interface between the geniculate, labyrinthine segment of the fallopian canal, and basal turn of the cochlea (Co). Original magnification 2.5×. Specimen 11095 R.

Fig. 10

Type II arachnoid pattern. Sequential photomicrographs of HE stained vertical temporal bone sections along the course of the fallopian canal demonstrating a representative type II pattern. A-C: The subarachnoid space does not terminate at the distal extent of the labyrinthine fallopian canal but extends into the substance of the geniculate ganglion (∗) with dissection of nerve fascicles and displacement of the ganglion. D: However, tympanic facial nerve bundle is intact without subarachnoid space extension. Original magnification 2.5×. Co, cochlea. Specimen 4532 R.

Fig. 11

Type II arachnoid pattern and fistulae with adjoining air cells. A: Representative photomicrographs of vertical temporal bone section demonstrating a fibrous ring (arrow) at the terminus of the labyrinthine fallopian canal that typically blocks intrusion of the subarachnoid space from the IAC into the geniculate ganglion. B: On subsequent sections however, a dehiscence in the fibrous ring allows subarachnoid space extension into the geniculate ganglion, forming a type II pattern. Osseous communication with neighboring air cells (arrowheads) then presents a pathway for CSF egress into the temporal bone and leading to clinical CSF leak. Specimen 4872 R. C: Similarly in a different specimen, arachnoid space extension from the IAC into the geniculate fossa and (D) osseous communication with adjacent air cells again suggests a path for CSF fistulization. Specimen 11095 L. Original magnification 2.5×. Co, cochlea.