The Human Cochlear Aqueduct and Accessory Canals: a Micro-CT Analysis Using a 3D Reconstruction Paradigm.

OBJECTIVE: We sought to study the anatomic variations of the cochlear aqueduct and its accessory canals in human temporal bones using micro-CT and a 3D reconstruction paradigm. More knowledge about the anatomic variations of these structures, particularly at the basal turn of the cochlea and round window niche, may be important to better preserve residual hearing as well as the neural supply during cochlear implant surgery. METHODS: An archival collection of 30 human temporal bones underwent micro-CT and 3D reconstruction. A surface enhancement paradigm was applied. The application displays reconstructed slices as a 3D object with realistic 3D visualization of scanned objects. Virtual sectioning or "cropping" of the petrous bone presented subsequent areas. Thereby, the bony canals could be followed from inside the basal turn of cochlea and middle ear to the jugular foramen. RESULTS: The cochlear aqueduct was always paralleled by an accessory canal containing the inferior cochlear vein. It ran from the basal turn of the cochlea and exited laterally in the jugular foramen. In 70% of the cases, a secondary accessory canal was observed and it derived mostly from a depression or infundibulum located in the floor of the round window niche. This canal also exited in the jugular foramen. The secondary accessory canal occasionally anastomosed with the primary accessory canal suggesting that it contains a vein that drains middle ear blood to the cranial sinus. CONCLUSION: Micro-CT with 3D surface reconstruction paradigm offers new possibilities to study the topographic anatomy of minor details in the human inner ear. The technique creates simulated transparent "castings" of the labyrinth with a coinciding surface view through enhancement of contrast between boundaries. Accessory canals that drain blood from the cochlea, spiral ganglion, and middle ear could be characterized three-dimensionally.

Vascular patterns of the saccule of the rat: a microcorrosion cast study.

A scanning electron microscope study of corrosion cast preparations of the vessels of the saccule was carried out in adult rats. This method shows the microvasculature of the saccule in the three-dimensional model. As a result of this technique we have been able to demonstrate a particular microvasculature of the saccule in the rat's inner ear. The main blood supply of the saccule consists of the arterioles coming from the vestibulocochlear artery entering the anterior margin of the macula sacculi. A few branches of the anterior vestibular artery enter the macula sacculi from the superior margin. The saccular vein drains the venous blood from a dense capillary area underneath the striola to the posterior vestibular vein which eventually ends in the vein of the cochlear aqueduct.

The influence of acute venous congestion on the guinea pig cochlea.

The effects of sudden occlusion of the vein of the cochlear aqueduct (VCAQ) on the cochlear blood flow, endocochlear potential (EP), endolymphatic and perilymphatic fluid pressures (PE and PP) were studied in the guinea pig. Cochlear blood flow showed a sudden decrease, and EP began to drop within 90 s, ranging from 50 to 70 mV in 5 of 11 animals studied, recovering to normal levels when the animals were placed on a continuous inhalation of carbogen. The PE and PP increased simultaneously (max. PE = 3.4 +/- 1.4 mm Hg; max. PP = 2.5 +/- 1.0 mm Hg) and returned to their initial values after 5 min. The EP was sustained within the normal range, even when there was an apparent decrease in cochlear blood flow (61.4 +/- 8.4%). We believe that variations in EP following VCAQ occlusion were due to anatomical differences in collateral venous communications among the animals studied. Carbogen inhalation produced uniform recovery patterns, indicating that individual collateral vessel responses were eliminated.

Scanning electron microscopic studies of the capillaries crossing the scala tympani.

Scanning electron microscopic examinations were carried out on the perilymphatic space bordering the round window in guinea pigs. A number of small vessels were found crossing free through the lumen of the scala tympani in this area. The larger number (5-10) of these capillaries are suspended between the bony cochlear wall and the terminal net of fibrocytes which covers the cochlear opening of the cochlear aqueduct and spreads onto the basal third of the round window membrane. Other capillaries (3-5) cross the lumen of the scala tympani from the outer cochlear wall to the modiolus. All of these capillaries have a thin endothelium and a very thin pericytic covering. These findings suggest that the capillaries crossing the perilymphatic space may give rise to a different chemical composition of the perilymph bordering the round window in comparison with the more upper parts of the scala tympani.

Venous communications of the cochlea after acute occlusion of the vein of the cochlear aqueduct.

The vein of the cochlear aqueduct (VCAQ) is the principal drainage vein of the cochlea in the guinea pig. Morphological observations of the VCAQ and its adjacent structures were made by studying serial sections of the cochlea. We detected the presence of two collateral vessels from the mucoperiosteal veins of the middle ear which communicated with the VCAQ. Following acute occlusion of the VCAQ, marked dilatations of these vessels were observed in corrosion cast preparations. Our findings suggest that these vessels act as collateral veins following acute venous congestion of the inner ear.

The effect of round window membrane rupture on endolymphatic and perilymphatic pressures.

We used a guinea pig model to investigate the effect of round window membrane rupture on endolymphatic and perilymphatic pressures under conditions known to increase these pressures: anoxia, hypercapnia, increased intracranial pressure, and occlusion of the vein of the cochlear aqueduct. When the round window membrane was not ruptured, increases in endolymphatic pressure paralleled the perilymphatic pressure following exposure to each of the experimental conditions. After the round window membrane had been ruptured, however, no increases in perilymphatic pressure or endolymphatic pressure were seen. These results suggest that endolymphatic pressure is dependent on perilymphatic pressure, even when the round window membrane is ruptured.

The early postnatal development of the cochlear vasculature in the gerbil.

The blood vessels of the cochlea were studied from birth up to adult age in the gerbil. Even at birth the principal vascular arrangement could be identified in the external wall. However, the early circulatory mainstream appeared even more predominantly radial apico-basal over radiating arterioles-arteriovenous anastomoses-collecting venules than at later stages. At birth, only small sections of capillaries could be observed in the stria vascularis. Stria vascularis achieved its adult vascular appearance rapidly between 8-10 days after birth (DAB). The rapid development of stria vascularis immediately preceded the development of cochlear function and may have been related to the development of the ionic composition of endolymph. At birth the vessel of the basilar membrane showed many large vascular connections with the collecting venules in the scala tympani of the external wall but very few supplying and draining ramifications on the medial side of the vessel. This suggests that the vessel of the basilar membrane originates from the external wall vessels, not from the spiral lamina vessels as has previously been supposed. The peripheral vascular connections degenerated rapidly after birth. The vessel, being very large at birth, degenerated completely by 15 to 20 DAB, ultimately disappearing in the basal turn. The degeneration suggests that its major importance may be during embryonic stages, with great probability for the development of the organ of Corti.

The blood-perilymph barrier.

Freeze fracture replicas of the guinea pig inner ear were studied under the electron microscope to define the blood-perilymph barrier morphologically. This barrier is represented basically by the continuous endothelium of the inner ear capillaries. The endothelial cells contain only a few micropinocytotic vesicles in contrast to those of the stria vascularis vessels. The cochlear plexus also exhibits some special differences. Tight junctions of the continuous mesothelial type connect the endothelial cells of the inner ear capillaries. Cell membranes of these cells possess a lower particle density than those of the stria vascularis vessels. In general, the blood-perilymph barrier is morphologically similar to the blood-brain barrier.