Abstract
Problem: The average of evoked myogenic potentials in sternomastoid muscle elicited by a click has been accepted as a vestibular function test (VEMP). The origin of the VEMP is thought to be the saccule. In this study, the neural pathway of the VEMP was evaluated on the basis of neurotological findings in the patients with small acoustic neuroma (AT).
Methods: Twenty patients with AT who had been operated at the Neurosurgery Department in Toranomon Hospital were the subjects. In ten patients, the origin of the tumor was identified during the operation (inferior vestibular nerve [IV], 5 cases; superior vestibular nerve [SV], 5 cases). Preoperative neurotological findings (VEMP, PTA, ABR, caloric test, and tumor size) were compared between two groups. In fourteen patients, the cut nerve branch was identified during the operation (IV, 9 cases; SV, 9 cases; cochlear nerve [CN], 5 cases). Postoperative VEMP were evaluated in accordance with nerve branch preservation.
Results: In the AT originating from the IV (IVAT), only one patient (ear tumor) showed normal VEMP before the operation. In 2 of 4 patients with abnormal VEMP, both ABR and caloric test were normal. In the AT originating from the SV (SVAT), 3 patients showed normal VEMP. In 2 patients with abnormal VEMP, both the ABR and caloric test were abnormal. After the operation, 3 of 14 patients, whose IV and CN, but not SV, were preserved, showed normal VEMP. But, VEMP were lost when IV was cut or only SV was preserved.
Conclusion: In the IVAT, a majority of patients showed abnormal VEMP. In addition, when IV is cut, VEMP was lost in all patients. These findings suggested that the neural pathway of VEMP is the IV rather than the SV.
Significance: This study demonstrates the retrolabyrin-thine neural pathway of the VEMP.
Support: Supported in part by a grant from JSPS (No. 13671772).