Paraneoplastic encephalomyelitis (PEM) is an inflammatory disorder which occurs as an immune-mediated response to systemic cancer. The symptoms are not directly related to complications of the cancer or cancer-specific treatment. Both the central and peripheral nervous systems may be affected. Nearly all cancers have been associated with PEM, with small cell lung cancer (SCLC) being the most common (in 75% of cases). Clinical presentations vary based on the part of the nervous system affected and can be classified according to the following:

• Paraneoplastic myelitis with encephalomyelitis – Typical in patients with SCLC. Patients develop sensory neuronopathy with sensory loss that may involve the proximal areas such as the neck and trunk. Motor deficits may also occur.
• Limbic encephalitis – Patients present with acute or subacute mood and behavioral changes, short-term memory difficulties, cognitive dysfunction, and complex partial seizures with symptoms developing over days to weeks. Other common symptoms include hyperthermia, somnolence, and endocrine abnormalities associated with hypothalamic dysfunction. Two-thirds of patients will also have encephalomyelitis. Commonly associated cancers include SCLC, seminoma or other testicular tumors, breast cancer, Hodgkin lymphoma, and thymoma. Neurologic symptoms typically precede a cancer diagnosis by weeks to months.
• Brain stem encephalitis – Patients present with a wide spectrum of symptoms including opsoclonus, nystagmus, dysphagia, dysarthria, trigeminal sensory loss, sensorineural deafness, central hypoventilation, vertigo, and supranuclear, internuclear, and nuclear extraocular movement deficits. Malignant associations include SCLC, testicular tumors, breast cancer, hypernephroma, and prostate adenocarcinoma.