2.3 Audiological assessment
All patients received the essential audiological examinations, including pure-tone audiometry and ABR. The severity of hearing loss was classified based on criteria of the World Report on Hearing published by World Health Organization (WHO) in 2021 [15]. According to this criterion, severity of hearing loss is categorized into 7 grades based on hearing thresholds measured with PTA at 0.5, 1, 2, and 4 kHz:
1. Normal hearing, mean hearing thresholds less than 20 dB;
2. Mild hearing loss, mean hearing thresholds is 20 to < 35 dB;
3. Moderate hearing loss, mean hearing thresholds is 35 to < 50 dB;
4. Moderately severe hearing loss, mean hearing thresholds is 50 to < 65 dB;
5. Severe hearing loss, mean hearing thresholds is 65 to < 80 dB;
6. Profound hearing loss, mean hearing thresholds is 80 to < 95 dB;
7. Complete or total hearing loss/deafness, mean hearing thresholds is 95 dB or greater.
We also analyzed the patterns of hearing loss and the configuration of audiogram was categorized into 7 forms: low-frequency ascending form, U-shaped form, high-frequency descending form, flat form, profound form, dip form and other form [16] (see Table S1).
Results of the ABR were considered abnormal when they met at least one of the following criteria: (1) absent evoked response upon the compatible auditory threshold; (2) desynchronization of waves other than wave I; (3) interpeak latency (IPL) between waves I and III > 2.5 ms; (4) IPL between waves I and V > 4.4 ms; (5) Wave V interaural latency difference (ILD) > 0.2 ms; (6) interaural difference of IPL between waves I and V > 0.2 ms [17].
2.4 Imaging examination
All enrolled patients underwent a targeted MRI scan of the brain. The scans included high-resolution T2 sequences and contrast-enhanced T1-weighted MRI directed to the IAC and cerebellopontine angle (CPA). The tumor size was measured on MRI images, and the type of tumor within the IAC (intracanalicular) and the maximum diameter of the tumor in CPA were used according to the recommendation of the Summary and consensus in 7th International Conference on acoustic neuroma [6]. According to Koos grading standard, the tumor size is classified as follows: grade I, tumor is confined to the IAC (intracanalicular) and the maximum diameter is ≤ 1 cm; grade II, small tumor protrusion into CPA without contact with the brain stem, diameter ranges from 1.1 to 2 cm; grade III, tumor occupying the CPA with no brainstem displacement, diameter ranges from 2.1 to 3 cm; grade IV, large tumor with brainstem and cranial nerve displacement, diameter is more than 3.0 cm [18].