The cochlear aqueduct: structural considerations

Relationship Between the Cochlear Aqueduct and Internal Auditory Canal: Surgical Implications for Transcanal Transpromontorial Approaches to the Lateral Skull Base

HYPOTHESIS

The cochlear aqueduct (CA) is subject to considerable anatomical variability. We hypothesize a topographical relationship between the CA and the internal auditory canal (IAC).

BACKGROUND

The CA represents the lower limit of dissection during transcanal transpromontorial approaches to the lateral skull base due to its close relationship to the lower cranial nerves and jugular vein.

METHODS

Three-dimensional models from high-resolution computed tomography scans of normal human temporal bones were created using threshold-based segmentation. The CA was classified into four categories. Five points were determined on the three-dimensional models to measure the surgically relevant relationships.

RESULTS

Segmentation was performed on 26 high-resolution computed tomography scans. The average length of the virtual and visual part of the CA was 6.6 mm (SD ±1.7 mm) and 5.5 mm (SD ±1.3 mm) respectively. The mean distance between the IAC and the medial end of the visual part of the CA was 3.8 mm (±0.7 mm), while the average distance between the IAC and the lateral end was 1.4 mm (±0.6 mm). The distance between the visual part of the CA and the IAC increased by 0.25 mm per from the fundus of the IAC.

CONCLUSION

A close relationship between the CA and the IAC could be established, despite the anatomical variability of the CA. The distance between CA and IAC increases by 0.25 per mm from the fundus to the porus of the IAC. These findings quantify the inferior limit of dissection of the transcanal transpromontorial approach to the lateral skull base.

Fine structure of the human cochlear aqueduct: a light and transmission electron microscopic study of decalcified temporal bones

The morphologic features of the human cochlear aqueduct were examined using both light and electron microscopy. The lumen of the cochlear aqueduct was observed to be filled with dense, irregular connective tissue corresponding to dura mater. At the entrance to the cerebrospinal fluid space, the dense connective tissue in the ductal lumen was covered with a thin layer of a few flattened cells, which was contiguous with the arachnoid membrane of the brain. A simple low cuboidal epithelium also separated the perilymphatic space from the lumen of the duct. Our observations confirm the presence of a barrier membrane at the opening to the perilymphatic space, and suggest that no transport occurs in the human cochlear aqueduct.

A radiologico-anatomical comparative study of the cochlear aqueduct

AIM

A comparative radiologico-anatomical study of the cochlear aqueduct (CA) was performed.

MATERIALS AND METHODS

Eight cadavers and 23 dry temporal bones were studied. High-resolution computed tomography (HRCT) was carried out on each cadaver before microdissection. Microdissection was carried out in a plane parallel to the HRCT sections.

RESULTS

The CA was found to be located an average of 7 mm inferior to the internal acoustic meatus and at the superior edge of the jugular foramen. The external aperture of the CA was triangular in shape in 18 bones (78.3%). The petrosal fossa was located just inferior to the external aperture and housed the glossopharyngeal nerve, which had an incomplete bony canal in four bones (17.4%) and a complete bony canal in three bones (13%). It was possible to demonstrate the petrosal portion of the CA on both coronal and axial HRCT. The otic capsule segment of the CA was impossible to demonstrate on coronal sections.

CONCLUSION

The CA cannot be visualized in only one section of the plane in HRCT. Both the otic capsule and petrosal segments can be demonstrated on axial HRCT.

The cochlear aqueduct: an important landmark in lateral skull base surgery

The cochlear aqueduct (CA) is used as a landmark in lateral skull base surgery. In this study anatomic relationships between the CA and adjacent neurovascular structures were examined by dissecting 32 temporal bones. Observations of the relationship of the external opening (EO) of the CA with the ninth, tenth, and eleventh cranial nerves, inferior petrosal sinus (IPS), and intrapetrous carotid artery (ICA) were noted. In addition to the distance of the EO of the CA to the vertical portion of the ICA, the entire length of the CA and the width of the EO were also measured. The ninth nerve was the only structure lodged at the EO of the CA in 34.4% of bones. However, in 40.6% of bones only the IPS crossed the EO of the CA, although the ninth nerve was situated just anteroinferiorly in the vicinity of the EO. In 15.6% of bones it was possible to observe both the ninth nerve and the IPS crossing the EO. In 9.4% of bones the EO of the CA was found to be occupied by the tenth and eleventh nerves. It was also observed that the ICA was located anteriorly on the same sagittal plane with the EO in 15.6% of bones. It was concluded that although in 90% of cases the EO of the CA was in close relation with the ninth nerve, other structures such as the IPS, the tenth and eleventh cranial nerves, and the ICA were also at risk during drilling in this area because of their intimate relationships with the EO of the CA.

Anatomy of the normal human cochlear aqueduct with functional implications

There is great variation in published descriptions of the shape, size, and patency of the human cochlear aqueduct. The first part of this paper describes the anatomy of the normal human cochlear aqueduct as determined from a study of 101 temporal bones. Nineteen bones aged 0-1 years and approximately 10 bones per decade of life until age 100 years were examined. The aqueduct was found to have a funnel shaped aperture at the cranial end with a dural sheath extending into it for a varying distance. The rest of the aqueduct was filled with a meshwork of loose connective tissue, often with a central lumen within it. Four types of patencies were noted: central lumen patent throughout length of aqueduct (34%), lumen filled with loose connective tissue (59%), lumen occluded by bone (4%), and obliteration of the aqueduct (3%). The mean value (+/- SD) of the narrowest portion was 138 (+/- 58) microns which occurred 200-300 microns from the cochlear end of the aqueduct. There was no correlation between age and narrowest diameter, or between age and category of patency. In the second part of this paper, we propose quantitative models of aqueduct function, based on measurements of ductal dimensions and known acoustical properties of the inner ear. Our model analyses suggest that in normal ears, the aqueduct (1) cannot support fluid flows large enough to explain stapedectomy gushers, (2) does filter out cardiac- and respiration-induced pulses in CSF and prevents them from affecting cochlear function, and (3) has little effect on normal ossicular transmission of sound for frequencies above 20 Hz. In pathological ears, such as those with ossicular disruption or after a type IV tympanoplasty, a patent aqueduct might affect hearing for frequencies below 150 Hz.

Enlargement of the cochlear aqueduct: fact or fiction?

Enlargement of the cochlear aqueduct (CA) is often mentioned in the otologic literature, usually in its purported association with sensory hearing loss, stapes gusher, and transotic cerebrospinal fluid leak. In CT scans of 100 ears, the diameter of the CA medial aperture was found to be highly variable, ranging from 0 to 11 mm, with a mean of 4.5 mm. In contrast, the otic capsule segment was very narrow in every case. It could be visualized in only 56% of cases, none of which exceeded 2 mm in diameter. Several published reports of supposed CA enlargement presented images of a dilated medial aperture that was well within the range of normal variability according to the present study. In a thorough review of the literature on radiology of the CA, we were unable to find a single published image that convincingly demonstrated enlargement of the otic capsule portion. As radiographic CA enlargement has not been convincingly reported to date, it appears to be an exceedingly rare or perhaps even nonexistent malformation. It is important to recognize than even a radiographically normal CA may be hyperpatent. It is theoretically possible for increased fluid flow to result from either deficiencies in intraluminal membrane baffles or subtle canal enlargement beneath the resolution limits of CT scanning. However, as fluid flow through a tube is regulated by its narrowest point, it is extremely improbable that stapes gusher, transotic CSF leak, and vigorous perilymphatic fistula are generated by the CA when CT scans show any portion of it to be very narrow. A substantial body of evidence points to a deficient partition between the internal auditory canal and inner ear as causative in such cases. We propose that the criteria for the diagnosis of CA enlargement on high-resolution CT scan be a diameter exceeding 2 mm throughout its course from the posterior fossa to the vestibule.

Ultrastructure of the guinea pig cochlear aqueduct. An electron microscopic study of decalcified temporal bones

The ultrastructure of the guinea pig cochlear aqueduct was examined using semi-thin and thin sections. The lumen of the cochlear aqueduct was occupied by a sparse meshwork of fibroblasts and delicate connective tissue trabeculae. The periotic tissue lining the bony wall of the aqueduct was composed of multiple layers of both elongated cells and densely arranged laminae of collagen fibrils. These structures were identical to those of the dura mater and the arachnoid. The opening to the perilymphatic space of the scala tympani also contained connective tissue trabeculae, but the arrangement of fibroblasts was more compact here than in the main part of the duct. These structural features suggest that fluid can move freely through cochlear aqueduct, and that the effects of sudden pressure changes in the CSF may be protected against by the densely and perpendicularly arranged fibroblast at the opening to the perilymphatic space.

Cerebrospinal fluid otorrhea

Cerebrospinal fluid (CSF) otorrhea is a dangerous and potentially life threatening occurrence for which the otolaryngologist is often consulted. CSF otorrhea occurs on the basis of skull fracture, tumor, infections, congenital anomalies, and operative trauma. Forty-three patients with CSF otorrhea of varied etiology are reviewed in this paper. Eight cases are of congenital or labyrinthine origin confirming at surgery the probable connection between the subarachnoid and perilymphatic spaces. Eleven cases had spinal fluid otorrhea due to infection. All cases presented with symptoms of chronic infection: 4 cases had a history of previous surgery for chronic ear disease; 7 cases had temporal lobe abscess; 1 case had a cerebellar abscess; 8 had tegmen defects secondary to cholesteatoma; in 1 case the tegmen defect was due to previous surgery for chronic infection. Nine of 11 cases have serviceable hearing postoperatively. Fourteen cases of spinal fluid otorrhea resulted from trauma: 1 case was due to traumatic stapes footplate fracture in a congenitally malformed ear; 4 were due to transverse temporal bone fracture; and 9 were due to longitudinal temporal bone fractures. All transverse fractures resulted in nonhearing ears. Three cases were due to a combination of temporal bone fracture and infection. In 2 of these cases chronic infection preceded the fracture; in 1 case the fracture led to chronic ear disease with spinal fluid leakage. One patient required 1 surgical procedure for closure of the otorrhea, 1 patient 2 procedures, and 1 patient 3 procedures. Ten cases are due to translabyrinthine acoustic neuroma removal: 7 cases had resolution of the spinal fluid leakage after conservative nonsurgical treatment; and 3 required surgical intervention using muscle, fat and fascia obliteration of the spinal fluid pathway.