The goal of this retrospective, cross-sectional study was to describe the different etiologies of trigeminal neuropathy based on clinical and MRI findings and to evaluate the significance of associated concomitant disorders. MRI studies of 49 dogs with trigeminal neuropathy were blindly reviewed and were classified into the following three groups: neoplasia, neuritis, or idiopathic trigeminal neuropathy (ITN). Thirty-one percent were suspected to have neoplasia (all unilateral), 16% to have neuritis (1 bilateral and 7 unilateral), and 53% to have ITN (4 unilateral and 22 bilateral). Dogs with clinical bilateral trigeminal dysfunction were most likely to have a diagnosis of ITN (predicted probability 95.7%). Unilateral clinical signs were significantly associated with neoplasia or neuritis compared with ITN (P < .001 and P = .002, respectively). Even with marked brainstem neoplastic involvement, central neurological deficits may be absent. Sensory impairment was significantly associated with either neoplasia or neuritis compared with ITN (P = .007 and P = .03, respectively). Ipsilateral noninfectious middle ear effusion was only seen in dogs with neoplasia (33%). Horner’s syndrome was present in 12% of all dogs (2 dogs in each group). Dogs with neoplasia were significantly older than dogs with neuritis (P = .02) and ITN (P = .002). JAAHA-MS-6997

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