The course of the internal auditory artery and its branches. Computer-aided three-dimensional reconstructions

A Subset of Intracanalicular Vestibular Schwannomas Demonstrates Minimal Growth Over a 10-Year Period

OBJECTIVE

Vestibular schwannomas (VS) commonly undergo magnetic resonance imaging (MRI) surveillance, but long-term data to support the ideal frequency is limited. Herein, we aim to investigate intracanalicular VS growth predictors and long-term growth rates (GR).

STUDY DESIGN

Retrospective chart review.

SETTING

Two tertiary care centers.

PATIENTS

Sporadic intracanalicular VS with initial conservative management and at least two sequential MRIs.

INTERVENTION

Serial MRI.

MAIN OUTCOME MEASURES

VS were categorized by baseline internal auditory canal tertile sublocalization (fundus, midpoint, porus) and size (≤100, 100-200, >200 mm3). Throughout follow-up, volumetric GR (mm3/yr) were determined (baseline-3 yrs, 3-5 yrs, 5-10 yrs) and treatment rates were assessed.

RESULTS

Ninety-nine intracanalicular VS were identified (mean follow-up of 6.1 ± 4.5 yrs). Mean GR before 5-year follow-up were comparable for baseline tertile involvement and size. After 5-year follow-up, mean GR of VS involving the fundus at baseline were lower than those involving the midpoint and fundus (6.17 ± 21.16 and 119.74 ± 117.57 mm3/yr, respectively; p = 0.034). Mean GR of VS with less than or equal to 100 mm3 at baseline (-7.29 ± 25.44 mm3/yr) were lower than those with 100 to 200 mm3 (86.55 ± 103.99 mm3/yr; p = 0.011) and more than 200 mm3 (45.70 ± 35.71 mm3/yr; p = 0.031). Vestibular schwannomas involving the midpoint and fundus had greater treatment rates compared with VS involving only the fundus (p < 0.001).

CONCLUSIONS

Baseline tertile involvement and size may predict long-term intracanalicular VS growth where fundal tumors or those less than or equal to 100 mm3 exhibit little long-term growth. Extending surveillance after 5-year follow-up may be reasonable for fundal VS.

Topography of the vestibulocochlear nerve

OBJECTIVE

The terms superior vestibular nerve and inferior vestibular nerve have been used in the field of neurosurgery to indicate anatomically the two respective vestibular components of the vestibulocochlear nerve. To reappraise the aptness of this terminology, fascicular patterns and the anatomic relationship of the vestibular and cochlear components were examined.

METHODS

Twenty vestibulocochlear nerve specimens were obtained from cadavers. The nerves were excised, with care taken to sustain their spatial relationships, then embedded in paraffin blocks and cross sectioned in 10-microm-thick slices. Serial cross sections were stained and examined with a light microscope.

RESULTS

The vestibular component was separated into two parts only at the lateral fundus of the internal auditory canal, lateral to the vestibular ganglion. In the internal auditory canal of all specimens, the vestibular component was represented by numerous fascicles. Around the porus acusticus, the fascicular pattern among the specimens was diverse: 13 of the 20 specimens were still polyfascicular, 4 specimens consisted of two large, distinct fascicles, and, in the remaining 3 specimens, a portion of the vestibular fascicles had fused with the cochlear component. Near the root entry zone, all vestibular fascicles fused and merged with the cochlear nerve to form a single trunk.

CONCLUSION

There was no evidence to support the anatomic correctness of specifying the superior and inferior vestibular nerves, except in the lateral fundus of the internal auditory canal.

The vasculature of the peripheral portion of the human eighth cranial nerve

The vasculature of the peripheral portion of the human eighth cranial nerve (VIIIN) was investigated by light and transmission electron microscopy. Arterioles and venules running longitudinally around the VIIIN formed the extrinsic vascular system. The anatomical relationship between these extrinsic vessels and the VIIIN sheath was similar to that between blood vessels on the surface of the brain and the pia mater. In the endoneurium, postcapillary venules and large capillaries were sparsely distributed and longitudinally arranged, and these microvessels formed the intrinsic microvascular system, which was supported by the extrinsic vascular system via anastomosing vessels. The ultrastructural features of the internal auditory artery and its main branches were the same as those of other intracranial arteries. Ultrastructural study also revealed myo-endothelial junctions in anastomosing arterioles, and endothelio-pericytic junctions in extrinsic and anastomosing venules. Microvascular endothelial cells were connected by tight junctions in both the vestibular ganglion and the rest of the VIIIN. These features of the vasculature were considered to be effective for maintenance of the endoneurial fluid and regulation of the circulation in the peripheral portion of the human VIIIN.

Angiogenesis from the eighth cranial nerve to vestibular schwannomas

Vascularization between vestibular schwannomas and the adjacent eighth cranial nerve was examined in order to elucidate the angiogenic effect of these tumours. Specimens from 12 patients were examined by light microscopy and immunohistochemistry (series 1), while specimens from another 17 patients were examined by light and transmission electron microscopy (series 2). Slight to marked angiogenesis was found in 5 patients from series 1 and 7 patients from series 2. Ultrastructural examination indicated that these blood vessels were more fragile than those in the normal eighth cranial nerve. These findings suggested that sudden hearing loss in vestibular schwannoma patients may be related to the disruption of such blood vessels. In addition, surgical injury to these blood vessels may cause postoperative hearing loss despite preservation of the cochlear nerve.

The limitations of hearing preservation in acoustic neuroma surgery: histological study of the interface between the eighth cranial nerve and the tumor

The interface between the eighth cranial nerve and acoustic neuroma was investigated by light microscopy and immunohistochemistry in paraffin sections in 13 patients, and in Epon sections in 19 patients. The cochlear nerve was severely invaded by tumor cells in two of six specimens examined, moderately invaded in two specimens, and not invaded in two specimens. Gliosis was frequently found within the cochlear and vestibular nerve. Hemangioma-like tissue was occasionally found attached to the cochlear and vestibular nerve adjacent to the tumor. These findings were considered to be related to difficulty in hearing preservation in some patients although acoustic neuroma surgery was successful.

The transitional region of the eighth nerve in elderly humans: light and electron microscopic study

To investigate the central-peripheral transitional region (TR) of the eighth nerve in elderly humans and to determine the reasons for its fragility, the eighth nerve root was studied by light and electron microscopy and by immunohistochemistry. Central and peripheral nervous tissue showed complex interdigitations at the TR. Slight proliferation of central glial tissue accompanied with disarranged course of the nerve fibers was also seen in this region. These findings were though to be age-related changes of the eight nerve. The capillaries ran in various directions at the TR and formed anastomoses with capillaries in the peripheral portion of the nerve. The density of capillaries in the peripheral nervous tissue was twice as high as that in the central nervous tissue. These findings indicated that the capillaries at the TR provide a connection between the blood supply to the central and peripheral portions of the eighth nerve. The occurrence of the age-related changes and complex course of the capillaries at the eighth nerve TR appeared to be related with the fragility of this region.