Brain Tumors

Mary O. Smith • Lisle W. George

Nervous system tumors of ruminants include medulloblastoma, ependymoblastoma, neurofibrosarcoma, angioblastoma, meningioma, meningeal hemangioma, neurofibroma, schwan­noma, choroid plexus papilloma, pituitary adenocarcinoma, primitive neurectodermal tumor, and reticulosis.^1-5 CNS tumors of horses include pituitary adenomas, microgliomas, medullo- epithelioma, choroid plexus papilloma, ependymoma, neuro­fibroma, meningioma, meningeal carcinoma, and reticulosis.^1,6-10 Secondary tumors that invade the CNS include melanoma, lymphosarcoma, adenocarcinoma, squamous cell carcinoma, hemangiosarcoma, and osteoma.^1,10-15 Of these, lymphosarcoma is most common.^16,17 Metastatic invasion to the CNS occurs either by vascular routes or by extension along the peripheral nerve rootlets.¹⁸ Local extension from adjacent tissue, such as the paranasal sinuses, can also occur.¹¹

Clinical signs of brain tumors vary with the location and include abnormalities of gait (ataxia, paresis, hypometria/ hypermetria); seizures; altered mentation (especially dull­ness); facial paresis or paralysis; facial anesthesia or analgesia; dysphagia; head tilt; strabismus; nystagmus; and loss of the menace reflex.^11,18-20 Migration of facial tumors (squamous cell carcinomas) into the cranial vault through the cranial nerve foramina may also result in facial swelling, exophthalmos, Horner syndrome, or asymmetric airflow through the nares.¹⁷ Pituitary adenomas of aged horses (see Chapter 41) rarely cause neurologic disease, but they secrete melanocyte-stimulating hormone, which stimulates the adrenal cortex and causes Cushing disease. Some tumors are discovered incidentally at necropsy.

Antemortem diagnostic tests include radiographs of the skull (for tumors that spread locally and some metastatic tumors), and electroencephalography to elucidate brain dysfunc- tion.¹¹ Where available, CT or MRI can greatly facilitate diagnosis, but limited availability and considerations of cost 18 20 22

restrict their use in most cases.¹⁸,

Treatment of brain tumors in horses and livestock is generally not feasible because of limitations of cost, nursing care chal­lenges after craniotomy, lack of access to radiation therapy for large animals, and considerations of safety for personnel who handle animals with significant neurologic deficits. Palliative treatment such as corticosteroids may reduce clinical signs temporarily in some animals. Euthanasia is the choice for most large animals with brain tumors.