4. Discussion
AN is the most common posterior cranial fossa tumor in adults, accounting for 6%–8% of all intracranial tumors and more than 80% of tumors in the CPA [19]. The early symptoms in patients with AN are mainly unilateral sensorineural hearing loss, tinnitus and vertigo [20]. SSNHL occurs in 10% to 20% of AN patient at some point in their medical history, but the incidence of AN is somewhat lower in patients with SSNHL [4]. Sensorineural hearing loss is the major presentation of AN patients and is often accompanied with tinnitus. In the present study, 90 percent (9/10) of AN patient who present SSNHL as an initial symptom complained of tinnitus, which is consistent with the incidence of tinnitus in patients with AN (51%~ 92%) that reported in the previous studies [21-23]. Even though tinnitus is a nonspecific symptom, clinicians should still be alert to patients with unilateral SSNHL and tinnitus to avoid missing the diagnosis. Meanwhile, the absence of accompanying symptoms does not eliminate the possibility of a tumor.
Previous studies have shown that the audiogram configuration may be trough shaped or U-shape in AN patient who presenting with SSNHL as a primary symptom [7, 21]. Furthermore, a recent study has found that the trough shaped or U-shape audiogram was significantly more prevalent in patients with AN than in ordinary idiopathic SSNHL patients, and the incidence of AN in SSNHL patients with trough shaped or U-shape audiogram was significantly higher in SSNHL patients with other audiogram configurations [24, 25]. This study suggests that a trough shaped or U-shape audiogram in patients with SSNHL may indicate the presence of AN. In our study, the incidence of U-shape audiograms (20%, 2/10) in SSNHL patients with AN was not significantly specific compared to other configurations, which may be due to the small sample size.
As the gold standard for AN diagnosis, MRI is the preferred examination and can provide exquisite tumor characterization, surgical planning, and post-therapeutic assessment [19]. High-resolution MRI can detect tumors smaller than 1cm located in the IAC and differentiate AN from other masses such as facial nerve schwannoma, meningioma, epidermoid cyst, arachnoid cyst, aneurysm, and metastasis [26, 27]. According to previous studies, AN can be successfully diagnosed and largely differentiated from other lesions with 96% to 100% sensitivity and 88% to 93% specificity with the combination of T1 and T2- weighted MRI [28]. In the present study, all the enrolled SSNHL patients were undergone contrast-enhanced MRI and 10 were diagnosed as AN. In addition to MRI, ABRs have been used widely as a screening procedure for the diagnosis of AN, particularly when MRI is not available. In our study, abnormal ABR results were obtained in all patients, and the overall ABR sensitivity in diagnosing AN in SSNHL was 100%. Due to the small sample size, our results do not indicate the sensitivity of the ABR. We have known that ABR testing has limits. The reported sensitivity of ABR for the diagnosis of AN varies between 63 and 97%, however, for small AN, its sensitivity decreases significantly to 8%-42% and ABR is not possible when the hearing loss exceeds 80 dB in the 2000 to 4000Hz frequency [4, 29]. A recent modelling study has found that the cost-saving with ABR prior to MRI does not seem to outweigh the number of missed patients with AN and other important pathologies that would have been detected when using standalone MRI [30]. Therefore, for SSNHL patients, we recommend that ABR and MRI should be combined to improve the accuracy of detection and prevent misdiagnosis and missed diagnoses, especially for small AN.
Previous studies have pointed out that the pathogenesis of SSNHL in AN patient involves mechanical compression of the adjacent cochlear nerve, based upon the conjecture that the nerve fibers responsible for middle-frequency hearing are in a position more susceptible to tumor compression [14]. Although hearing loss due to nerve compression is theoretically progressive, a sudden enlargement of the tumor (e.g., hemorrhage or cystic degeneration) could compress the cochlear nerve enough to cause sudden hearing loss [31]. Nevertheless, it has been reported that tumor size is not to be correlated with the grade of hearing loss, and the correlation between tumor size and the incidence of SSNHL is also controversial [9, 16]. In this study, we found no significant correlation between tumor size and grade of hearing loss. In addition, the tumor size and configuration of audiograms were also unrelated. These results were consistent with other previous studies.
Many studies have observed that hearing recovery occurred in some AN patients presented with SSNHL after corticosteroid therapy and reported the recovery rate ranging from 16.7% to 44.4% [10, 31, 32]. It is well-known that SSNHL patients with different types of audiogram configurations have obvious differences in the hearing recovery [1]. Several studies have found that the recovery rate of SSNHL in AN patient was also significantly related to audiogram pattern [24, 25]. In 2021, Wasano et al. [16] revealed that the recovery of hearing in patients with U-shaped audiograms was significantly greater than in patients having the other audiogram forms, and the recovery rate decreased as the SSNHL episodes in patients increased. In 2017, Cho et al. [27] reported that non-tumorous lesions (intra-labyrinthine hemorrhage and labyrinthitis) showed a poorer treatment response than that of AN in patients with SSNHL. Hearing recovery may be due to the regression of tumor edema caused by corticosteroid treatment and/or the absorption of hemorrhage from the tumor itself or in the vicinity of the tumor [33]. In the present study, 25% (2/8) SSNHL patients diagnosed as AN showed hearing recovery after drug treatment. This rate was consistent with previous studies. These findings suggested that a therapeutic response to corticosteroid treatment for SSNHL does not exclude the presence of AN and all patients with SSNHL should undergo MRI to prevent misdiagnosis and delays in potential treatment.
5.
Conclusion
In conclusion, we have reported on a series of 10 AN patients presented SSNHL as a primary symptom and treated as SSNHL initially. MRI is the most effective examination for the diagnosis of small AN. This study demonstrated that the hearing loss of these patients may improve with corticosteroid treatment. Therefore, we recommend that all patients presenting with SSNHL, regardless of whether the hearing loss responds to drug treatment, should undergo MRI to rule out AN and avoid delayed treatment due to missed diagnosis. In addition, as an effective screening procedure, ABR is also important for the diagnosis of AN.