Brain tumors develop with a frequency of 14.5 in 100,000 dogs ¹ and 2.2 to 3.5 in 100,000 cats. ^1,2 The tumor classification system most widely used is from the World Health Organization Classification System and can be found in Table 19-1 . Meningioma is the most common brain tumor in dogs and cats ^3-5 and accounts for approximately 45% of primary intracranial neoplasms in dogs ^4,5 and 59% in cats. ³ Glial cell tumors are the second most common primary brain tumor in dogs ^3,5 and the fourth most common in cats. ³ Brain tumors typically affect older dogs with a median age of 9 years (range, 4–13 years) and 95% are older than 5 years of age. ⁴ Golden Retrievers and Boxer dogs are at increased risk for intracranial neoplasia. ⁵ Dolichocephalic breeds may be at increased risk for meningiomas, whereas brachycephalic breeds may have increased risk for gliomas. ⁵ Cats with brain tumors are older with a mean age of 11.3 ± 3.8 years (range, 0.5–21.5 years). ³ Male cats are affected slightly more than female cats (male:female ratio of 1.5:1). ³ The domestic shorthair cat is the most common breed identified. ³

Definitive risk factors for development of brain tumors are unknown for dogs and cats. Inheritance may have a role in some breeds of dogs. Hormones such as estrogen and progesterone can influence tumor genesis. ^7,8 Young cats with mucopolysaccharidosis type I have a high incidence of meningiomas, providing suspicion for a genetic basis. ⁹

Clinical signs of brain tumors develop as a result of damage to surrounding normal neural tissue from tumor expansion and associated edema. ¹⁰ Brain edema is a prominent feature of intracranial neoplasms. Brain edema may initially cause few or no clinical signs. As the edema worsens, a mass effect occurs with distortion and displacement of brain tissue, subsequently causing a rise in intracranial pressure ¹¹ and shift of brain tissue to areas of lower pressure. Neurologic deficits produced by these displacements are additive to the clinical signs caused by the tumor itself. ¹² Table 19-2 summarizes clinical signs associated with herniation. Hydrocephalus may also occur, either as a result of altered cerebrospinal fluid (CSF) resorption or ventricular obstruction. ^13,14

Common neurologic signs observed in animals with brain tumors include altered mentation (obtundation, stupor, coma), seizures, ataxia, circling, and behavioral changes. Seizures (45%) are the most common clinical sign in dogs, but circling (23%), ataxia (21%), and head tilt (13%) also occur. ¹⁵ Brain neoplasia should be considered as a differential diagnosis when a dog has its first seizure after 4 years of age. ⁴ Clinical signs in cats are similar but often are vague or nonspecific, with anorexia and lethargy being most common. Reported neurologic signs in cats include altered mentation (26%), circling (23%), seizures (23%), ataxia (16.9%), and behavioral changes (15.6%). ³ A sudden onset of aggression is the most common behavioral change reported in cats. ³

A minimum database including a complete blood count, serum biochemical analysis, and urinalysis should be performed to evaluate for metabolic abnormalities in animals suspected of having a brain tumor. Radiographs of the thorax and abdomen as well as abdominal ultrasound are recommended to rule out metastatic and concurrent disease prior to advanced diagnostics and surgery. ⁵ Neuroanatomic localization of the central nervous system (CNS) lesion will vary based on location of the tumor and amount of mass effect caused by the edema and tumor. ^16,17 Table 19-3 summarizes clinical signs associated with specific lesion localization within the brain.