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Dear Editor, The vestibulo-ocular reflex (VOR) ensures that the visual image on the fovea remains steady during head motion, while the vestibulospinal reflex maintains posture during locomotion. These functions indicate that patients with bilateral vestibulopathy (BV) experience unsteadiness and oscillopsia during walking. BV may arise from exposure to ototoxic agents, or in association with autoimmune, genetic, or degenerative diseases.1,2 Here we describe a patient who presented with gait unsteadiness as the first manifestation of a hidden lung cancer, and showed rapid progression of unilateral or asymmetric vestibulopathy into a bilateral vestibular failure. A 75-year-old man suffered from progressive dizziness and unsteadiness during locomotion for about 4 months. The patient showed leftward head tilt and left skew deviation, spontaneous nystagmus beating rightward and clockwise during visual fixation that decreased slightly in darkness, and horizontal direction-changing gaze-evoked nystagmus (GEN). Smooth pursuit was impaired in both horizontal directions, but his saccades were normal. The visually enhanced VOR was impaired bilaterally. The fundus showed leftward ocular torsion, and the subjective visual vertical (SVV) was also tilted to the left. He tended to fall leftward and backward when attempting to stand, and had mild ataxia of bilateral upper extremities. Bedside head impulse tests (HITs) revealed corrected catch-up saccades when stimulating the left horizontal canal (HC). Video HITs also revealed a decreased VOR gain and corrective saccades for left horizontal and both posterior canals (Fig. 1A). However, caloric irrigation elicited bilaterally reduced responses (Fig. 1B). He additionally developed leftside facial palsy 2 weeks later. Bedside HITs were followed-up throughout his admission, which detected catch-up saccades in both directions 1 week later. Follow-up video HITs also revealed decreased gains and corrective saccades for all six semicircular canals (Fig. 1C). A rotatory chair test showed reduced gains of the VOR. Brain MRIs with gadolinium enhancement disclosed nodular enhancements within both internal auditory canals (IACs) along with an increased signal of the cerebellar vermis (Fig. 1D). Whole-body positron-emission tomography revealed a hypermetabolic mass in the hilum of the right lung. A biopsy of the pulmonary lesion confirmed the presence of small-cell lung cancer. The patient refused chemotherapy. His gait unsteadiness worsened to a bedridden status over the following 2 months. Most IAC lesions are benign tumors, with vestibular schwannoma accounting for 90% of them.3 Metastases to the IAC are rare, constituting 0.3–0.7% of cerebellopontine-angle tumors.4 Although uncommon, metastatic carcinoma of the IAC should be considered in the differential diagnosis of cochleovestibular dysfunction due to its potentially devastating outcomes. Differentiating IAC metastases from benign schwannoma is often difficult due to the similarity of their MRI features and manifestations.3,4 Severe pain in the mastoid areas has been considered characteristic of IAC metastasis in a few studies, but other studies (including the present one) failed to find any mastoid pain. Instead, rapid progression of the vestibular Sung-Hee Kim Hyo Jung Kim Ji-Soo Kim

Rapid Progression of Bilateral Vestibulopathy Due to Metastatic Lung Cancer