Vestibular schwannomas (VS) are benign tumors that frequently occur at the cerebellopontine angle (CPA), usually arising from the internal acoustic canal (IAC). Intralabyrinthine schwannomas, on the other hand, are far less common (1% of all VS) and they tend to extend toward the CPA due to the absence of a bony barrier [1]. And VS have a slow growth rate, as previously reported in the literature, with overall mean growth rates ranging from 1.15 to 2.4 mm/year [2]. So the lateral extension of a VS involving the middle ear is very rarely reported. Furthermore, it is even more rarely to be found in the form of an external auditory canal (EAC) mass and only a few cases have been reported in the literature to date. To the best of our knowledge, only five cases have been reported from 1950 to 2021. Here, we present two cases of VS presenting as tumor of the EAC.

Vestibular schwannoma is a benign tumor, and in 80% of VS, it is found as a CPA tumor. It commonly occurs on the Schwann cell of the vestibular nerve sheath within the IAC and tends to extend to the CPA due to the absence of bony barrier. Intralabyrinthine schwannomas are much less common (1% of all VS), with a prevalence of 0.1%-0.4%; they occur in the peripheral intralabyrinthine branches of the vestibulocochlear nerve [3]. Tieleman, et al. [4] suggested that most of the inner ear schwannomas (80%) occur in the cochlea, particularly in the basal and second turn of the cochlea and more frequently in the scala tympani.

In a literature review, there were a few cases where VS has progressed and invaded the middle ear, but cases where it has invaded the EAC and diagnosed as EAC mass were very rare that only five cases have been reported since the first report by Trans, et al. in 1987 (Table 1) [5-9]. In these cases, it is difficult to accurately identify the origin of the tumor, and the tumor that originated in the IAC, where VS commonly occurs, can be considered to have progressed medially to the CPA and externally to the inner and middle ears. Alternatively, it is conceivable that the tumor that occurred within the labyrinth progressed medially to the IAC and CPA and externally to the middle ear. Intralabyrinthine schwannoma is a tumor originating from the cochlea, vestibule, or semicircular canals corresponding to the membranous labyrinth. Primary labyrinthine schwannoma was previously considered rare, but more than 150 cases have already been reported until recently [10]. In addition, case of multifocal origin tumors independently from the IAC and intralabyrinthine can be considered. However, multiple unilateral VS occurring without the features of neurofibromatosis type 2 are considered very rare as only three cases have been reported to date [11].

Regarding the case where VS has progressed to the EAC at the time of diagnosis, none of the studies clearly mentioned the origin of the tumor. Furthermore, in all these studies, the tumors have been removed through surgical treatment using the translabyrinthine approach. The patients presented here were middle-aged (43-57 years old) individuals, and the second patient is the first reported male case. Both of them did not have relevant clinical backgrounds, such as neurofibromatosis.

The difference between our first case and the other reported cases is that in the former, the tumor progressed into the middle ear through a round window, based on the CT and surgical findings, which is consistent with Venkatasamy, et al.’s [1] claims regarding the progression of the VS. In most cases of EAC mass, the first selected imaging test would be probably contrast enhanced temporal bone CT scan. The axial view shown in this case showed connectivity of mass through the round window and enlargement of IAC which is not frequent findings in an EAC mass. This suggests that when the EAC mass invades the middle ear, careful examination of the round window and confirmation of IAC enlargement can provide useful information for early differential diagnosis of extensive VS.

Vestibular schwannoma has a very slow growth rate, and there are different views regarding aggressive treatment of the tumor at the time of diagnosis. Our second case and the case reported by Carreño, et al. [8] had tumor recurrence after the initial treatment. Careful preoperative assessment of the tumor extension and selection of appropriate treatment are mandatory for extended VS.

Through further analysis and consideration of VS extending to the middle ear and EAC, which is rare, as in the present cases, it will be possible to determine the cases requiring early aggressive treatment. And since round window seems to be the main route of extension of VS to the middle ear, careful examination of theround window on MRI is mandatory before surgical treatment.