Nasal adenocarcinoma with diffuse metastases involving the orbit, cerebrum, and multiple cranial nerves in a horse.

A 9-year-old Trakehner gelding was examined because of right exophthalmus. Clinical findings included a lack of menace response in the right eye, reduced direct and consensual right pupillary light reflexes, ventrolateral strabismus of the right eye, mild right-sided facial asymmetry, a head tilt to the left, and increased extensor tone in the right limbs. Findings were suggestive of a multifocal lesion affecting the right forebrain; right optic, oculomotor, and facial nerves; and left vestibulocochlear nerve. Ultrasonographic examination of the right eye revealed a vascular retrobulbar mass. Computed tomographic imaging revealed a mass that filled the nasal cavity and invaded the forebrain. Necropsy revealed an undifferentiated nasal adenocarcinoma affecting the orbit with metastases to the right parotid gland, cranial cervical lymph nodes, fascial planes of the neck, and lungs. No evidence of direct involvement of the right facial and left vestibulocochlear nerves was found, suggesting the possibility of paraneoplastic peripheral neuropathy.