Lateralization during the Weber test: animal experiments.

Abstract

OBJECTIVES/HYPOTHESIS: The objective of this study were to present an assessment of a new theory to explain lateralization during the Weber test using an animal model. This theory is based on the discovery that a major pathway in bone conduction stimulation to the inner ear is through the skull contents (probably the cerebrospinal fluid [CSF]). The placement of a bone vibrator or tuning fork on the skull excites the inner ear by the classic osseous pathway and by the suggested CSF pathway. We assume that there is a phase difference between the stimulation mediated by the ossicular chain (inertial and occlusion mechanisms) and the one mediated by the CSF. The presence of a conductive pathology will decrease the magnitude of the sound energy mediated by the ossicular chain. Thus, the out-of-phase signal arriving through the bony pathways will be decreased, hence increasing the resultant sound intensity stimulating the cochlea. STUDY DESIGN: Prospective animal study. METHODS: The experiment was performed on 10 fat sand rats, which had undergone unilateral cochleostomy and a small craniotomy. The auditory nerve brainstem response (ABR) thresholds were measured to air-conducted stimulation, to stimulation with the bone vibrator applied to the skull, and to stimulation with the bone vibrator applied directly to the brain through the craniotomy. The ossicular chain of the second ear was then fixed to the middle ear walls with cyanoacrylate glue to induce a conductive hearing loss. The ABR thresholds to the same three stimuli were then measured again. RESULTS: After ossicular chain fixation, the ABR threshold to air-conducted stimulation increased, to bone vibrator stimulation on the bone decreased (hearing improvement), and to bone vibrator stimulation directly on the brain remained unchanged. CONCLUSIONS: This experiment confirms the proposed theory. During clinical bone conduction stimulation, there is a phase difference between sound energy reaching the inner ear through the middle ear ossicles and from the CSF. A middle ear conductive pathology removes one of these components, thus increasing the effective sound intensity in the affected ear. On the other hand, when the bone vibrator is applied on the brain, the inner ear is stimulated only through the CSF, so ossicular chain fixation does not change the ABR threshold. Moreover, this study proves that lateralization during the Weber phenomenon is the result, at least in part, of an intensity difference between sound energy reaching the two cochleae.