A scanning electron microscopic study of the guinea pig cochlear aqueduct. With emphasis on fracture preparation

The relationship of the round window membrane to the cochlear aqueduct shown in three-dimensional imaging

The round window membrane and cochlear aqueduct complex in the guinea pig are reconstructed with 3D-imaging, using orthogonal plane fluorescence optical sectioning (OPFOS). The 3D-images show that the periotic duct and the aqueduct are connected to a pouch-like extension of the round window. The function of this may be regulation of aqueduct flow resistance under the influence of a pressure difference between inner ear fluid and middle ear.

Direct measurement flow resistance of cochlear aqueduct in guinea pigs

OBJECTIVE

The cochlear aqueduct connects the scala tympani to the subarachnoid space and is the main pressure equalization canal for the inner ear. Increases in inner ear volume and pressure are thought to cause clinical symptoms such as vertigo, tinnitus and fluctuating hearing loss. In this study the flow resistance of the cochlear aqueduct was determined and its relation with inner ear pressure was studied.

MATERIAL AND METHODS

Inner ear pressure was measured in the scala tympani through the round window using a micropipette. Through a second micropipette, artificial perilymph was infused into, or withdrawn from, the scala tympani at various constant rates. From the infusion rate and the change in perilymphatic pressure during infusion the flow resistance of the cochlear aqueduct was calculated.

RESULTS

The flow resistance was found not to be constant but to depend on the position of the round window membrane and possibly on the magnitude and direction of fluid flow through the aqueduct. Measured flow resistance values were in the range 11-45 Pa s/nl. For very small flow values the flow resistance averaged over 6 animals was 21 Pa s/nl.

CONCLUSIONS

The flow resistance of the cochlear aqueduct is not a constant value. The cochlear aqueduct is a canal with dynamic properties and may play a role in the complicated process of inner ear pressure regulation.

Change of guinea pig inner ear pressure by square wave middle ear cavity pressure variation

The inner ear fluid pressure of guinea pigs was measured during square wave middle ear cavity pressure variation. Time constants were derived for the slopes of the inner ear pressure recovery curves after middle ear pressure change. A "single exponential" function did not fit well and therefore more complicated functions were used for this purpose. For middle ear pressure increasing from zero to a few centimetres of water, returning to zero again, decreasing from zero to minus a few centimetres of water and then returning to zero again, time constants for the inner ear pressure recovery curves were on average 15.0, 8.6, 2.5 and 2.5 s, respectively. The results could not be described using a linear model with constant window membrane compliance and cochlear aqueduct flow resistance. A possible explanation for the large difference in time constants for positive or negative middle ear pressure changes is a dependence on aqueduct flow resistance or round window membrane position.

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.

Computer-aided three-dimensional reconstruction of guinea pig cochlear aqueduct

A computer-aided method of three-dimensional reconstruction was applied to the determination of the overall spatial configuration of the guinea pig cochlear aqueduct. The rotation function of the reconstructed images was useful in showing the individual small parts of the duct. A semi-translucent display of the segmental reconstruction of the duct demonstrated a difference in the density of the cellular components between the opening to the perilymphatic space and the duct portion. We propose that the cochlear aqueduct serves as a protective mechanism against a sudden change in CSF pressure in the subarachnoid space.

Morphological changes in the cochlear aqueduct following herpes simplex virus inoculation into the subarachnoid space

Type 1 herpes simplex virus (HSV-1) was inoculated into the subarachnoid space through the cisterna magna of guinea pigs to study morphological changes of the inner ear and the ability of the cochlear aqueduct to protect the inner ear. Although most of the animals developed clinical manifestations of meningoencephalitis within a few days after inoculation, Preyer's reflex remained intact. Scanning electron microscopy revealed some significant changes in the cochlear aqueduct. Lymphocytes and macrophages were predominant, with narrowing of reticular tissue spaces caused by the swelling of the periotic duct tissue. The cribriform structure of the internal orifice of the cochlear aqueduct appeared to be completely obstructed, whereas it was normal in the presence of bacterial infection as previously reported (1). The morphological changes were confined to the cochlear aqueduct.

On sources of error in the biochemical study of perilymph (guinea-pig)

Contamination of perilymph with other fluids (cerebrospinal fluid, tissue fluid, blood, endolymph) together with sampling, anaesthesia, surgical intervention or food intake of the animals may considerably affect the analytical result. The numerous possible artefacts seem to be the main reason why varying values are given in the literature for the same chemical component of perilymph. This is also partly true of cerebrospinal fluid and blood. The effect of some sources of error on selected chemical components of perilymph, cerebrospinal fluid and blood is briefly summarized.