Incidence of Asymptomatic Vestibular Schwannoma and Origin of the Tumor in Japanese

Abstract

Five small asymptomatic vestibular schwannoma were discovered on routine examination of 973 serially sectioned temporal bones of 521 individuals. One of 5 cases was diagnosed as neurofibromatosis II. The incidence of the tumor including neurofibromatosis II was 0.96%, and that of the tumor excluded the case of neurofibromatosis II was 0.77% of individuals in this series. The mean age of the four individuals except neurofibromatosis II was 70 years, the specific ages being 63, 67, 73, and 77 years. The small size of the vestibular schwannomas and the advanced ages of 4 individuals in this study suggest that schwannomas can either remain inactive for a long period or cease to grow at times. The finding of asymptomatic vestibular schwannoma indicates the high incidence of this tumor in the general population in Japanese as well as in Caucasian. The much lower incidence of symptomatic tumors can only be explained by the conclusion that this tumor exhibits considerable variation in growth characteristics, and usually the tumor enlarges so slowly as to be of no clinical symptoms, but occasionally it grows more rapidly to become symptomatic and require surgical removal. The high incidence of asymptomatic vestibular schwannoma and low incidence of symptomatic one suggest that watchful waiting is a realistic and justifiable alternative to treatment, particularly in patients who have small vestibular schwannoma.
The tumor in 3 cases arose from the inferior division of vestibular nerve, in one from the superior division. Another case of neurofibromatosis II had a multicentric origin from the cochlear nerve and the inferior division of vestibular nerve in unilateral ear. Many vestibular schwannomas originate on the inferior division of vestibular nerve, and this fact suggests that the earlier estimate by caloric test in the evaluation of suspected vestibular schwannomas would be inaccurate.