Internal auditory canal metastasis as the only brain lesion in progressive lung adenocarcinoma: Case report and literature review.

Background: Metastases to the internal auditory canal (IAC) are exceptionally rare, especially as a solitary lesion without associated parenchymal brain or leptomeningeal involvement. Lung adenocarcinomas with a signet-ring cell component are aggressive and prone to early dissemination. Case presentation: We report a 64-year-old man with stage IV lung adenocarcinoma with a signet-ring cell component who developed rapidly progressive left-sided facial palsy, vertigo, and hearing loss. Brain MRI revealed a solitary enhancing lesion in the left IAC. Considering the clinical context and rapid symptom progression, a metastatic lesion was suspected. The patient was treated with corticosteroids and radiotherapy. At three-week follow-up, he showed marked improvement in vertigo and hearing, with partial recovery of facial nerve function (House-Brackmann grade III). Conclusion: This case highlights the importance of recognizing rare sites of metastasis in patients with known malignancy. Early diagnosis and prompt treatment with a multidisciplinary approach can lead to meaningful neurological improvement and symptom relief, even in the context of ongoing systemic disease. Clinicians should be alert for IAC metastases when acute cochleovestibular or facial nerve symptoms occur.