In the broad sense, a paraneoplastic syndrome can be defined as a disorder of an organ or tissue occurring at a site remote from a primary cancer or its metastases.
Paraneoplastic syndromes can affect virtually any organ or tissue, including the nervous system.
When used in the broad sense, nervous system paraneoplastic syndromes are quite common. They include nonmetastatic complications of cancer that secondarily affect the brain, such as strokes due to cancer-induced hypercoagulability, infections due to immune suppression from cancer, side effects of radiation or chemotherapy, and a variety of metabolic and nutritional disorders resulting from the cancer. In a more narrow sense, neurologic paraneoplastic syndromes, also known as remote effects of cancer on the nervous system, can be defined as disorders of nervous system function that are pathogenetically related to a cancer arising outside the nervous system but that are not ascribable to metastases or to the destruction of vital systemic organs by the tumor or its treatment. It is important to recognize that disorders that are clinically and pathologically identical to paraneoplastic syndromes also occur in patients without cancer, but at a lesser incidence. Paraneoplastic syndromes, defined in the narrow sense, can affect any portion of the nervous system, from the cerebral cortex to the neuromuscular junction to the muscles themselves.