Typewriter Tinnitus: Value of ABR as a Diagnostic and Prognostic Indicator

INTRODUCTION: Typewriter tinnitus refers to a special kind of staccato tinnitus, which is mostly described by patients as Morse code, popcorn, or machine-gun. It has been accepted that the mechanism of typewriter tinnitus is caused by the neurovascular compression of the cochleovestibular nerve. Patients who suffered from typewriter tinnitus have exhibited a good response to carbamazepine or oxcarbazepine, but there is a risk of recurrence after treatment cessation. The present study aims to determine the value of auditory brainstem response (ABR) in diagnosing typewriter tinnitus and predicting relapse after drug withdrawal. METHODS: Patients who presented with typewriter tinnitus from March 2019 to March 2022 were included for the present retrospective study. The auditory and vestibular test results and drug treatment effects were collected and analyzed. Patients with idiopathic unilateral subjective tinnitus, who were matched by age to patients with typewriter tinnitus at a ratio of 2:1, were consecutively recruited for the control group. RESULTS: Eighteen patients with typewriter tinnitus and 38 controls were included. Ears with typewriter tinnitus had longer interpeak latency (IPL) I-III, and wave III and V latencies, and a higher ratio of IPL I-III ≥2.3 ms based on ABR, when compared to the unaffected side and controls (p<0.05). Seventeen patients with typewriter tinnitus responded positively to medication. Among these patients, seven patients had a relapse after drug cessation, while 10 patients did not have a relapse. The relapse group had significantly longer IPL I-III and wave V latency, older age, and poorer hearing, when compared to the nonrelapse group (p < 0.05). Furthermore, IPL I-III had the largest area under the receiver operating characteristic curve, and the optimal cutoff was 2.4 ms (sensitivity, 100.0%; specificity, 66.7%). There were no significant differences in other demography or other clinical test results between the relapse and nonrelapse groups (p > 0.05). Ramsay Hunt syndrome and neuromyelitis optica spectrum disorders were identified in two cases. CONCLUSION: Prolonged IPL I-III based on ABR can help in the diagnosis of typewriter tinnitus and its prognosis after treatment cessation. Patients with IPL I-III greater than 2.4 ms, older age and poorer hearing are more likely to relapse. In addition to the neurovascular conflict of the cochleovestibular nerve, the etiologies of neuroinflammation and demyelinating diseases are also possible for typewriter tinnitus.