The concept of a hyperactive neurovascular compression syndrome such as HFS and trigeminal neuralgia has been accepted since Dandy’s first report, and microvascular decompression for these syndromes has acceptable surgical outcomes [11–14]. Due to the anatomically close relationship between the cochlear nerve and the facial nerve, some attempts have been made to perform microvascular decompression to treat tinnitus, based on the assumption that tinnitus is a type of hyperactive neurovascular compression syndrome. However, the response rate of microvascular decompression for tinnitus varies from 40 to 77%, and the tinnitus-free rate is lower than the response rate [2–5]. In contrast, an interesting report about tinnitus accompanied by HFS has been published. Ryu et al. investigated the results of microvascular decompression for tinnitus accompanied by HFS. Tinnitus in 7 of 10 (70%) patients was completely resolved after surgery, which is comparable to the surgical result for HFS and trigeminal neuralgia . This unexpectedly high success rate may be related to the differences in clinical characteristics of patients with HFS in whom offending vessels run very close to the cochlear nerve. However, neurovascular compression has been evident on MRI in tinnitus patients without HFS in other reports. Therefore, the pathophysiologic mechanism of tinnitus accompanied by HFS may be different from the pathophysiological mechanism of tinnitus alone.
Initially, we assumed that the pathophysiological mechanism of tinnitus accompanied by HFS might be quite similar to the two suggested pathophysiological mechanisms of hyperactive neurovascular compression syndrome, ephaptic transmission and hyperactive cranial nerve nucleus and/or higher brain structure. If tinnitus accompanied by HFS is caused by ephaptic transmission of the cochlear nerve, then MEG for these patients should have shown decreased auditory cortical activity and a delayed latency from the auditory stimulus and cortical evoked field. However, MEG results of the AEF waveform for our patients with tinnitus accompanied by HFS showed simultaneously increased auditory cortical activity and decreased N100m latency on the HFS side compared with patients with HFS without tinnitus (Tables 2 and 3). This result suggests that tinnitus accompanied by HFS is unlikely to be caused by ephaptic transmission of the cochlear nerve by neurovascular compression. Therefore, hyperactivity and hyper-conductivity of the central auditory nervous system may play a key role in the pathophysiological mechanism .
A recent study was published about normal-hearing patients with tinnitus. In this study, the authors observed a shortening of the I-V latency and enlarged Na and Pa amplitudes, and concluded that the cause of tinnitus in these patients seemed to have originated from the central nervous system . Although the patients in that study did not have HFS, the tinnitus conditions were similar to our patients. The results of our study suggest that tinnitus may originate in the central nervous system, rather than the cranial nerve or the root entry zone.
Although the pathophysiology of tinnitus is still controversial, an interesting theory regarding the pathophysiological mechanism of tinnitus has been introduced recently. According to the theory, the dorsal cochlear nucleus in the pons, which is modulated by multi-sensory input, is a strong candidate for the origin of tinnitus [16–18]. Multi-sensory input can affect dorsal cochlear nucleus granule cells, leading to changes in dorsal cochlear nucleus principal cells . Thus, sensory stimuli can modulate cochlear function and may be the cause of tinnitus. Further support for the relationship between sensory stimuli and tinnitus is found in patients with tinnitus who are treated with botulinum toxin. Abnormal movement in the head and neck area can be associated with tinnitus, and this type of tinnitus is successfully cured with botulinum toxin injections into the affected muscle [20, 21].
According to our MEG results and recent theories, hyperactivity of the dorsal cochlear nucleus may be one of the major pathophysiological mechanisms of tinnitus accompanied by HFS. In our series, one of eight tinnitus patients, who had high-pitch continuous tinnitus accompanied by HFS, continued to suffer from tinnitus even though there was no evidence of facial spasm after microvascular decompression. These results are comparable with previous outcome reports in which 2 out of 10 patients continued to experience tinnitus after successful microvascular decompression for HFS, although some of these patients experienced a decrease in hearing ability prior to surgery . An irreversible change in the central auditory nervous system may be attributable to the symptoms of these patients, and further study might be helpful in understanding this change.
Hyperactivity of the central auditory nervous system should also be considered in the pathophysiological mechanism of tinnitus accompanied by HFS. Hyperactivity of the dorsal cochlear nucleus may be induced by both neurovascular compression of the cochlear nerve and multi-sensory input from facial sensory stimuli. In our study, we could not conclude which factor had more influence on this hyperactivity. A further clinical trial, which involves controlling facial sensory input with botulinum toxic injections instead of microvascular decompression, will be used to determine the cause of central auditory hyperactivity.