Vestibular Neuronitis and Small Acoustic Neuroma: Electron Microscopic Observations

Vestibular neuritis: an overview using a classical case

Although acute unilateral and/or bilateral vestibular paralysis, known as vestibular neuronitis, is the second most common cause of vertigo (the most common is benign paroxysmal positional vertigo (BPPV), it is fraught with controversies. The clinical symptoms and methods of treatment of vestibular neuronitis are well defined; however, the etiology and pathophysiology of this disorder are still sketchy. Furthermore, there are no specific diagnostic tests available, and unfortunately, there are no animal models for this disorder. The purpose of this paper is to present an overview of those controversies using a classical case of vestibular neuronitis.

The morphology of the vestibular nerve in a patient with normal vestibular function and in patients with Ménière's disease

The morphology of the vestibular nerve was studied in three fresh nerve specimens obtained by surgery from two patients with Ménière's disease and one patient with tinnitus after an attack of sudden deafness. The number of nerve fibres appeared light microscopically normal. The vestibular ganglion cells of the Ménière-specimens showed, as a characteristic finding advanced vesiculation of the cytoplasm. This change was confirmed by electron microscopy and the vacuoles appeared to consist of widened endoplasmic reticulum cisternae. Numerous primary lysosomes and lipofuscin granules were observed in all vestibular ganglion cells but their number appeared increased in the specimens from Ménière-patients. Some of the vestibular ganglion cells of these patients showed a filamentous appearance and the number of glycogen granules seemed reduced in all of them. All vestibular ganglion cells observed were unmyelinated. The degenerative changes observed might explain the typical elevation of the threshold for vestibular stimulation in Ménière's disease.

[The acute vestibular paralysis (author's transl)]

Acute vestibular paralysis may not be considered as a nosologic entity but as a syndrome. Symptomatology (vertigo, spontaneous and provoked vestibular nystagmus, absence of cochlear signs) shows an uniform picture. The results of the caloric test as well as the nystagmic responses induced by galvanic stimulation and the development of central vestibular compensation however indicate that the site of the lesion is not only confined to the labyrinth but may also occur at the level of the peripheral neuron or even the vestibular nuclei. Etiology and pathology are still vague. Our own clinical observations as well as the scarce data in literature about morphological and experimental studies suggest in a way that vascular and infectious disorders are of importance as primary releasing factors. Hypothetically, vestibular loss of function may either be caused by a disturbance of labyrinthine microcirculation, initiated in a great majority of cases by infection, or by a direct lesion of the peripheral neuron as well as the vestibular nuclei. Retrolabyrinthine lesions may be due to menigoencephalitis, caused by a neurotropic virus or other infectious agents such as Toxoplasma gondii. Acute vestibular paralysis should be strictly distinguished from vestibular neuronitis. While vestibular paralysis is a syndrome, vestibular neuronitis must be considered as a nosologic entity, including a lesion of the peripheral neuron as well as evidence of an infectious event.