The relation between flocculus volume and tinnitus after cerebellopontine angle tumor surgery

Chronic tinnitus is a common symptom after cerebellopontine angle (CPA) tumor removal. Sometimes, the tinnitus is gaze-modulated. In that case, patients can change the loudness or pitch of their tinnitus by ocular movements. During tumor removal by a retrosigmoid craniotomy, the cerebellar flocculus is manipulated by the surgical approach to access the tumor. The flocculus has been associated with tinnitus in rats, and is involved in eye-gaze control. This suggests that the flocculus may have a role in gaze-modulated tinnitus after CPA tumor removal. In order to investigate this hypothesis, the relation between the flocculus volume and the characteristics of postoperative tinnitus was studied.

A single-center cohort of 51 patients completed a questionnaire after CPA tumor removal. The questionnaire asked for the effect of eye movements on tinnitus and included the Tinnitus Functional Index (TFI). Tinnitus was present in 36 patients (71% of 51), of which 29 (81% of 36) described gaze-modulation. The median TFI was 22 (range 0-85). A postoperative MRI-scan of sufficient quality was available in 34 cases. The volumes of the (para)flocculi ipsilateral and contralateral to the surgery, and the ratio of these volumes were similar between patients with and without tinnitus. The TFI correlated with the volume of both ipsi- and contralateral (para)flocculus (r_s(23) = .516, p = .008 and r_s(23) = .430, p = .032). The ipsilateral-to-contralateral volume ratio of the (para)flocculi volumes was significantly lower in patients that could modulate the loudness of their tinnitus by eye gaze, compared to patients that could not (t(23) = 3.337, p = .003).

The lack of a relation between flocculus volumes and the presence of tinnitus, combined with the significant correlation between tinnitus severity and flocculus volumes, suggests that the flocculus may not be the primary source of tinnitus, but is likely to mediate tinnitus severity. The reduced ipsi-to-contralateral volume ratio in patients with gaze-modulated tinnitus suggests that atrophy of the flocculus on the surgery side triggers cross-modal interactions leading to modulation of tinnitus.