The AN usually arises within the nerve trunk of the vestibular part of the 8th nerve. It gradually grows out of the nerve as it increases in size and assumes a peripheral position. The AN's usually arise halfway along the length of the vestibular nerve, which corresponds to the transition zone of the nerve structure. The typical microscopic appearance of AN's has two distinct features of arrangement of the cells-either tightly packed (Antoni A) or loosely packed (Antoni B) fibers. The distinction of these two cell types are of no clinical importance. Indeed, regions of Antoni A and B may coexist in the same tumor. As the tumor grows, it follows the direction of least resistance, usually towards the brain (cerebellopontine angle) and may reach considerable size. Thus, most tumors consist of 2 parts: a stalk or stem within the internal audiory canal (IAC) and another portion near the brain region. Microscopic investigations into the effect of AN's on adjacent facial or 7th nerve have shown tumor involvement in some cases. This involvement may not be recognized by the surgeon during removal of the AN.
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This picture shows the microscopic appearance of a normal vestibular (8th) nerve passing through the internal auditory canal (IAC) to supply the organ of balance. The facial (7th) nerve runs along with the 8th nerve in the IAC. The organ of hearing (cochlea) is also seen in this picture.
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This picture shows an AN tumor arising from the 8th nerve, within the IAC.
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This is a higher magnification of the above picture showing the junction of the tumor and the VII nerve. The arrows indicate the sheath (covering layer) of the tumor.
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This picture is a high magnification of the same tumor showing the arrangement of the fibers within the AN. The arrow indicates a whorled appearance of the fibers while the upper part of the tumor shows loosely packed (Antoni B) fibers.
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We would like to acknowledge the Human Temporal Bone (Otopathology) Laboratory at the Department of Otolaryngology, University of Minnesota, for allowing us to use these sections from their collection.
Written and designed by Vikram J. Jaisinghani, M.D., Department of Otolaryngology, University of Minnesota
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