The Cochlear Aqueduct De Aquaeductibus Auris Humanae: Part II

Abstract

The Cochlear Aqueduct (CA) is a narrow bony canal connecting the cochlear with the posterior cranial fossa in the subarachnoidal space. Narrow in the otic segment, it widens in the vicinity of the petrous bone aperture. The CA contains loose connective tissue making up the periotic duct that in turn derives from primitive extroflexion of the subarachnoidal space. Alongside the CA runs an accessory canal containing the inferior cochlear vein which flows into the inferior petrosal sinus. CT investigations of the petrous bones routinely evidence the CA while the medial aperture is detected at MR. This article describes the typical appearances and provides an in-depth review of the most important studies into CA morphological variability and the possible functional implications. The CA arises in the basal coil of the scala timpani. The isthmus of the duct, located in the otic capsule and not always visible, measures between 0.05 and 0.35 mm. The canal widens funnel-like to 2 – 7 mm as it reaches the superior edge of the jugular foramen. Overall length varies between 9 and 17 mm. In contrast, neonate CA is shorter, straighter and patent. With age, patency is progressively reduced even to the point of obliteration. It has been postulated that the CA provides a direct link between CSF and perilymph in the presence of both normal labyrinth and malformations. While some studies suggest that CA patency sustains conditions involving labyrinthine hypertension, others report complete CA obliteration, thereby excluding any connection between the two districts. An enlarged, patent CA, as evidenced in Mondini type dysplasias, has been considered to enable retrograde flow of CSF into the vestibule and the spread of infection from one compartment to the other and/or induce perilymphatic hydrops with consequent loss of hearing and lesions of the window and possible cerebrospinal fluid otorrhea. More probably, however, the pathway of communication between CSF and perilymph spaces is the fundus of the internal auditory canal. Abnormally dilated CA that may be classified as a true malformation is extremely rare and has been reported in only one case as an isolated anatomic finding. As a result, it has been suggested that the yardstick for diagnosing dilated CA on CT be a diameter of more than 2 mm along the whole duct, from the cochlear end to the posterior fossa. There are also a few documented cases of the CA provide a route for infection between the two spaces. Finally, some cases of tumours invading the CA have been described in which the first clinical signs were cochlear-vestibular symptoms. These findings suggest that even if not a routinely finding, the CA may in certain cases connect the subarachnoidal space and the perilymph and thus affect auditory vestibular function. Better understanding of CA anatomy, morphological variations and their possible functional significance is therefore required since assessment of the cochlear aqueduct should be part of routine imaging diagnostics to investigate labyrinthine disorders.