Vestibular Schwannoma–Related Increased Labyrinthine Postgadolinium 3D-FLAIR Signal Intensity and Association with Hearing Impairment

DISCUSSION

This study assessed whether measured intralabyrinthine signal abnormalities on postcontrast 3D-FLAIR images correlate with sensorineural hearing loss. The results indicate that the measured intralabyrinthine signal is correlated with audiometric findings. Specifically, increased signal is associated with greater degrees of sensorineural hearing loss.

There is a growing body of evidence demonstrating that labyrinthine signal changes may reflect at least 1 contributing pathoetiology of hearing loss in VS. Importantly, combined FLAIR and contrast-enhanced technique may possibly capture both vascular and inflammatory changes. Where prior work investigating delayed gadolinium enhancement in VS suggests increased BLB permeability alongside increased T2 FLAIR signal, potentially reflecting inflammatory milieu, herein we demonstrate correlation on standard non-delayed imaging.⁸ Where recent work explores peritumoral signal on postgadolinium FLAIR images, possibly representing increased VS permeability/extravasation, how this finding may relate to the adjacent labyrinth remains unexplored.¹⁷ It seems conceivable that at least 1 minor mechanism of hearing loss may include local permeation of inflammatory cytokines, and proteins themselves may contribute to hearing loss. Supporting such a theory is the recent finding that micro-RNA secreted by human VS tissue sensitizes a mouse model to hearing impairment.¹⁸ Further narrowing down on such pathologies remains difficult; although direct vestibular sampling is possible, such technique is not commonly performed.^6,19 As such, the partial correlations identified in the present study may represent just 1 component of hearing impairment. Correlation of potential inciting and aggravating factors toward hearing loss may assist our detection of future potential hearing loss in the age of advanced MR spectroscopy, photon-counting CT material decomposition, and newer contrast techniques.

One prevailing problem in the field of VS is identifying factors that inform management selection (ie, observation, microsurgery, or radiosurgery) and optimal timing thereof, with the goal of hearing preservation. If certain clinical characteristics or imaging features portended a better outcome with a certain intervention over the anticipated natural history of hearing loss, early intervention may be considered. While increased signal in the ipsilateral labyrinth has been shown to correlate with hearing impairment at relatively early-stage disease, the trajectory of such changes relative to impairment remains unknown.¹⁰ Specifically, it remains to be determined whether signal aberration reflects irreversible damage for which other interventions/patient counseling would be beneficial versus an opportunity for identifying those best served with surgical or other therapy.

In the present study, 44 patients had follow-up 3D-FLAIR postgadolinium imaging and paired audiometry. No statistically significant associations comparing audiometric measures including PTA and WRS and signal intensity were found over the analyzed interval. It is important to note, however, that these results should be interpreted carefully. There were relatively few patients who had follow-up imaging, and the select group may be biased by those who were already selected for conservative management. As compared with prior work, patients with available 3D-FLAIR postgadolinium imaging represent later observational follow-up and demonstrated worse hearing.¹⁰ Specifically, increased mean age (66 versus 59 years), PTA (37 versus 29 dB hearing loss), and decreased WRS (80% versus 95%) are noted in the present study. Further, the average hearing impairment change per year was very small. As such, it remains unknown if signal intensity changes reflect completed damage versus slowly progressing insult or longer-term indolent changes.

This study is not without limitations. Signal intensities were normalized to both the contralateral labyrinth and pons. While both techniques have previously been used as a method of accounting for and normalizing signal intensity across patients, time, and scanners, there may be further subtle variability in the contralateral labyrinth in addition to labyrinthine and pontine microvascular changes, which could also be characterized by postgadolinium 3D-FLAIR techniques. Relative intensities between these comparators may further change magnitude given differences in timing, and careful consideration as to which sequences may represent which mechanistic pathologies and their natural trajectory is needed to improve sensitive assessment of signal changes. Future work using quantitative MR techniques and assessing normal trajectory may be useful toward mitigating these effects. The present patient cohort additionally is subject to increased chance of referral bias.²⁰ The overall cohort is additionally limited in that few patients with available 3D-FLAIR postgadolinium imaging had available follow-up to assess hearing decline and signal changes over time. Further, given the often-indolent growth of VS, longer-term observations are needed. Importantly, the present study hypothesized imaging correlates with hearing loss as a manner by which to explore mechanisms; measurement of SIRs and correlation with hearing is not indicated in routine practice outside of normal diagnostic considerations. Given the partial correlation coefficients observed herein, additional mechanisms and their imaging representations contributing to hearing loss remain.