A tumor is an abnormal growth, whether noncancerous (benign) or cancerous (malignant). In many parts of the body, a noncancerous tumor causes few or no problems. However, any abnormal growth or mass in the brain or spinal cord can cause considerable damage.
Some cancers elsewhere in the body cause symptoms of nervous system dysfunction even though there is no evidence that nerve tissue has been invaded. These disorders are called paraneoplastic syndromes (see Paraneoplastic Syndromes). They can cause dementia, mood swings, seizures, incoordination, dizziness, double vision, and abnormal eye movements. The most common effect is dysfunction of peripheral nerves (polyneuropathy—see Polyneuropathy), resulting in muscle weakness, numbness, and tingling.