Evidence for a Biological Shift behind the Increasing Incidence of Sporadic Vestibular Schwannoma

Background: The incidence of sporadic vestibular schwannoma (VS) has significantly increased over the last several decades. Most often, this increase is attributed to the increasingly widespread use of magnetic resonance imaging (MRI) and screening protocols for asymmetrical hearing loss. However, apart from the recognition that incidence rates have risen in the post-MRI era, no study to date has directly investigated this supposed etiology.

Objective: The objective of the current work was to characterize the incidence of head MRIs over the previous two decades in Olmsted County, Minnesota and compare this trend to the incidence of asymptomatic, incidentally diagnosed VS over the same time period.

Methods: Procedure codes for head MRIs among residents of Olmsted County, Minnesota between January 1, 1995 and December 31, 2016 were retrieved using the Rochester Epidemiology Project—a unique medical records-linkage system covering the complete population of Olmsted County supported by the National Institute on Aging. Incidence rates of VS were obtained following review of each resident of Olmsted County who was diagnosed with VS over the same time interval. Incidence rates of head MRI and VS were calculated on a per-year basis and directly standardized to the total United States population from the 2000 United States Decennial Census. Trends in tumor size over time were evaluated using Spearman rank correlation coefficients.

Results: A total of 43,561 head MRIs among 30,002 residents of Olmsted County were identified from 1995 through 2016. The incidence of head MRI significantly increased between 1995 and 2003 (p < 0.001), but remained stable between 2004 and 2016 (p = 0.14). Over the same time interval, 25 cases of incidentally diagnosed VS were identified. The incidence of asymptomatic VS increased over time from 0.72 per 100,000 person-years between 1995 and 1999 to 1.29 between 2012 and 2016 (p = 0.058). No plateauing of incidence rates was observed in incidental tumors over the study period. The size of incidentally diagnosed tumors did not change over the study period (p = 0.93), suggesting that the increasing incidence of asymptomatic tumors is not explained by improved diagnostic capability of more recent MRI studies.

Conclusion: Despite the plateauing of head MRI incidence rates after 2004, the incidence of asymptotic, incidentally diagnosed VS continued to increase. Therefore, these data provide evidence that there may exist a true biological shift behind the rising incidence of VS beyond simply greater detection alone.

Acknowledgments: This study was made possible using the resources of the Rochester Epidemiology Project, which is supported by the National Institute on Aging of the National Institutes of Health under Award Number R01AG034676. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.