Histopathology of the peripheral vestibular system in small vestibular schwannomas

Gadolinium-enhanced magnetic resonance imaging can be used to detect small vestibular schwannomas/acoustic neuromas. Early detection raises the question of the necessity of their surgical removal. Do all tumors induce lesions in the vestibule and to what extent? We thus investigated the ultrastructure of peripheral vestibular systems in grades I and II schwannomas. Vestibular tissues were fixed as soon as they were removed during the resection of tumors, by the translabyrinthine approach, and then processed for transmission electron microscope observations. In neurosensory epithelia, hair cells lost stereocilia, whereas cuticular plates disaggregated. The cytoplasm of hair cells degenerated in either a dense or vacuolated manner, and cytoplasmic blisters extended into the endolymph. In some cases, supporting cells extended processes covering the apical surface of hair cells. Nerve fibers massively disappeared from epithelia, only few nerve fibers contacted type I and type II hair cells, and both afferent and efferent terminals were abnormal. In vestibular nerves, axons degenerated, and myelin sheaths disaggregated. Glycogen was present in both intracellular and extracellular spaces. Luse bodies associated with collagen bundles were found between fibers. Scarpa ganglion neurons contained lysosomes/lipofuscin granules and vacuoles. Tumor cells were found in both the ganglion and the vestibular nerve. Thus small tumors induce extensive degeneration of vestibular tissues. The various hallmarks of schwannomas are already present in small acoustic neuromas. Moreover, different types of degeneration of hair cells and neurons were observed, together with the covering of hair cells by supporting cells and the accumulation of glycogen in the vestibular nerve.