Intralabyrinthine schwannomas: a case series with discussion of the diagnosis and management.

Abstract

In Reply We thank the authors for their comments on the use of ocular vestibular myogenic potentials (OVEMPs) for the diagnosis of intralabyrinthine lesions. They advocate the routine use of OVEMPs in diagnosing intralabyrinthine schwannomas (ILSs) based on their experience and their two cases, one in cochlear and the other in utricular locations. Unilateral deafness was present in both cases, and dizziness has been reported in one case (utricular location). For these two patients, OVEMPs were not found on the affected ear, whereas cervical vestibular myogenic potentials (CVEMPs) were present on the two sides. Their stimulation protocol of OVEMP was carried out by air conduction. The review of the literature shows a growing interest of the VEMPs to explore the otolith functions for approximately 20 years (1). Whereas recording CVEMPs became a routine examination, that of OVEMPs is more recent and confidential. For diagnostics, the OVEMP can provide information on topography of the lesion (utricle, superior vestibular nerve). At chronic stage, they unmask a vestibular deficit that appears as compensated when the examinations explore only the slow phase of the eyes movements (2). The prognostic value of OVEMPs in neuritis and sudden deafness was demonstrated (3,4). Associated with CVEMPs, they also have an interest in following nonoperated schwannomas and can lead to surgery or gamma knife (5). Preoperative chemical labyrinthectomy can be proposed in case of normal otolith function measured by VEMPs to avoid an acute or chronic postoperative syndrome of deafferentation (6). However, several limitations appear because of a lack of specificity: – the origin of OVEMPs is not completely clear (4). They may result from saccular response when air-conducted sounds are used as stimulation (7). – the air or bone stimulation is not physiologic. That induces a lack of specificity of the otolith organ response unlike platform tests using translational movements of the head (8). – a large interindividual variability is caused by the fact that the myogenic potential reflects not only the otolith function but also extravestibular factors as the state of the outer and middle ear, the thickness of the skull, and the magnitude of the muscle activation during the electromyography (8). – there is a variability of equipment and procedures for recording and stimulation protocols that induce inconstant results (2). – during the different stages of Ménière’s disease (4), OVEMP changes do not differentiate from ILS. Moreover, vestibular migraine and Ménière’s disease behave similarly on most VEMPs including OVEMPs (9). In conclusion, VEMP tests are complementary to other vestibular tests but do not have values achieved in isolation. Nevertheless, coupled with video head impulse tests, VEMPs seem to become an indispensable tool in the exploration of the five vestibular end organs. But their diagnostic utility in common vestibular disorders requires more documentation on large series (6). Improved protocols and implementation of their specificity are necessary to include VEMPs and, therefore, OVEMPs in usual practice. Thus, in context of schwannoma screening, they can be integrated into a panel of vestibular additional tests whose results will determine the magnetic resonance imaging request. Pierre Bouchetemblé, M.D. Kate Heathcote, M.D. Jean-Paul Marie, M.D., Ph.D. Department of Otolaryngology-Head and Neck Surgery Rouen University Hospital, Rouen, France E-mail: [email protected]