Central Nervous System Hemangiosarcoma in a Horse

An 18-year-old 600-kg Percheron cross gelding presented with neurologic signs of 24-hour duration. The gelding had no history of illness. Initial physical examination showed abnormalities that included a small corneal ulcer in the left eye and periocular skin abrasions on the right side. The gelding had depressed mentation and right head turn. The horse circled to the right, had a tendency to lean and fall to the right, and was ataxic in all limbs. Cranial nerve dysfunction included bilateral facial paralysis, spontaneous horizontal nystagmus (fast phase to the left), diminished tongue tone (without appreciable atrophy), right nasofacial hypalgesia, and pharyngeal dysphagia. General proprioceptive deficits were present in the right thoracic and pelvic limbs, while extensor tone was present in the left thoracic and pelvic limb muscles. Neurologic examination findings were consistent with a diffuse brainstem lesion, mainly on the right side and extending from the rostral to the caudal medulla. A right telencephalic or diencephalic lesion could not be ruled out to explain the head turn and circling to the right. Based on the presence of a diffuse or multifocal lesion and neuroanatomic lesion localization, the primary differential diagnosis included infectious causes of meningoencephalitis (equine protozoal meningoencephalomyelitis [EPM], atypical presentation of equine herpes virus I, rabies, or West Nile virus), trauma, and neoplasia with associated mass effect. Although the clinical signs were acute in onset, trauma was not likely, given the multifocal nature and region of the brain involved. Neoplasia was considered possible due to brainstem involvement; however, acute onset is unlikely with brain tumors. Neoplasia remained on the list of possible causes, as some of the clinical signs may have been missed before presentation or if hemorrhage was associated with the tumor. The left facial nerve paralysis was attributed to trauma that occurred during transportation, as the referring veterinarian reported that the horse did not have evidence of the left corneal ulceration immediately before referral. Other than a mild lymphopenia (1,863 3 10 3 /mL; refer