This retrospective chart review was conducted in Union Hospital affiliated to Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China.

One hundred and thirty-seven patients with unilateral definite MD were enrolled between September 2012 and December 2019. For all patients, a thorough history inquiry, otoscopy, neurotological evaluations (audiometry, impedance, videonystagmography, caloric test, etc.) and imaging examination were conducted for differential diagnosis. The diagnosis of MD was established following the diagnostic guidelines of MD outlined by the American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS) in 1995 [17]. Sixty-nine subjects without audio-vestibular symptoms were enrolled as the control group.

The exclusion criteria were: (1) middle or inner ear anomaly; (2) middle or inner ear infections (otitis media, mastoiditis, labyrinthitis etc.); (3) retro-cochlear lesions (vestibular schwannoma, internal acoustic canal stenosis etc.); (4) having received previous ear surgery or intratympanic injections; (5) head trauma; (6) bilateral MD; (7) systemic diseases; (8) disorders of central nervous system (vestibular migraine, multiple sclerosis, cerebellar infarction, etc.).

This study was conducted according to the tenets of the Declaration of Helsinki. Informed consent was obtained from each patient and control subject. The project was approved by the ethical committee of Tongji Medical College of Huazhong University of Science and Technology.

For all MD patients included, pure tone audiogram was performed during the interictal period. Furthermore, some patients received additional audio-vestibular evaluations, including the electrocochleogram (EcochG) and caloric test. Within 48 h before testing, all subjects were instructed to refrain from alcohol, caffeine or medications that would affect the results of vestibular tests, for instance, sedatives or anti-depressants.

The clinical stage of MD was determined, based on hearing threshold average of the affected side, according to the AAO-HNS guidelines (1995) [17]. EcochG and caloric test were performed as described by previous literature [18]. For EcochG, summating potential (SP) and action potential (AP) were recorded, and the SP/AP ratio was calculated. The SP/AP ratio ≥ 0.4 was deemed as positive. During caloric test, the maximum slow phase velocity (SPVmax) of caloric nystagmus was measured following each air irrigation, and the canal paresis (CP) was calculated following the Jongkees’ formula. The interaural asymmetry of the caloric nystagmus ≥ 25% was considered abnormal. According to the published criteria [19], if the summated SPVmax of the induced nystagmus was < 20°/s after 4 air irrigations, the caloric response is believed to indicate bilateral vestibular hypofunction. In this case, ice water irrigation (4℃, 1.0 ml) would be used to confirm the caloric unresponsiveness.

All MRI examination were conducted using the Verio or Magnetom Trio 3 T scanners (Siemens, Erlangen, Germany) with a 12-element phased array coil. Among the routine IAC MR imaging sequence, three-dimensional sampling perfection with application optimized contrasts using different flip angle evolutions (3D-SPACE) was used, (1) to examine the anatomical configurations of AICA/PICA, (2) to exclude inner ear malformation, retro-cochlear pathology and lesions in the CPA. The parameters for the 3D-SPACE sequence were: repetition time (TR), 1000 ms; echo time (TE), 135 ms; slice thickness, 0.5 mm; field of view (FOV), 200 × 200 mm; matrix, 384 × 384; average,2; bandwidth, 289 Hz/Px.

All MRI data were transferred to the workstations and imaging analyses were performed on a picture archiving and communication system (PACS). Radiological data were intermixed and reviewed by two senior neuroradiologists (L.P with an experience of over ten years and L.J over five years) who were blinded to the clinical data. In this study, the Kazawa classification systems were adopted: type IA: non-loop AICA/PICA in the CPA cistern; type IB: non-loop AICA/PICA entering the IAC; type IIA: loop type AICA/PICA in the CPA cistern; and type IIB: loop type AICA/PICA entering the IAC [6]. Supplementary Figs. 1–4 demonstrated typical examples of the branching patterns of AICA/PICA evaluated by Kazawa grading systems.

Statistical analyses were performed by using software SPSS (version 26.0.0.2). All continuous variables are presented as means ± standard deviations (SD) or median and interquartile range (IQR 25th–75th percentiles) after verification of normal distribution. Categorical variables are presented as counts and percentages. Data were tested for normal distribution using the Kolmogorov–Smirnov test. The McNemar-Bowker test was used for the categorical variable comparison between the MD-affected and non-affected side. The distribution of AICA/PICA anatomical variations in different symptoms and hearing outcomes were compared using Chi-square test, Fisher's precision probability test, One-Way ANOVA and Kruskal–Wallis H test. The interobserver agreement on radiological assessment was estimated using kappa value. The level of agreement was generally recognized as follows: poor, less than 0.20; fair, 0.21–0.40; moderate, 0.41–0.60; substantial, 0.61–0.80; and almost perfect, 0.81–1.0. The criterion for statistical significance was set at p < 0.05.