Paraneoplastic neurological syndromes

Paraneoplastic neurological syndromes (PNS) can be defined as remote effects of cancer that are not caused by the tumor and its metastasis, or by infection, ischemia or metabolic disruptions. PNS are rare, affecting less than 1/10,000 patients with cancer. Only the Lambert-Eaton myasthenic syndrome is relatively frequent, occurring in about 1% of patients with small cell lung cancer. PNS can affect any part of the central and peripheral nervous system, the neuromuscular junction, and muscle. They can be isolated or occur in association. In most patients, the neurological disorder develops before the cancer becomes clinically overt and the patient is referred to the neurologist who has the charge of identifying a neurological disorder as paraneoplastic. PNS are usually severely disabling. The most common PNS are Lambert-Eaton myasthenic syndrome (LEMS), subacute cerebellar ataxia, limbic encephalitis (LE), opsoclonus-myoclonus (OM), retinopathies (cancer-associated retinopathy (CAR) and melanoma-associated retinopathy (MAR), Stiff-Person syndrome (SPS), chronic gastrointestinal pseudoobstruction (CGP), sensory neuronopathy (SSN), encephalomyelitis (EM) and dermatomyositis. PNS are caused by autoimmune processes triggered by the cancer and directed against antigens common to both the cancer and the nervous system, designated as onconeural antigens. Due to their high specificity (> 90%), the best way to diagnose a neurological disorder as paraneoplastic is to identify one of the well-characterized anti-onconeural protein antibodies in the patient's serum. In addition, as these antibodies are associated with a restricted range of cancers, they can guide the search for the underlying tumor at a stage when it is frequently not clinically overt. This is a critical point as, to date, the best way to stabilize PNS is to treat the cancer as soon as possible. Unfortunately, about one-third of patients do not have detectable antibodies and 5% to 10% have an atypical antibody that is not well-characterized. As PNS are believed to be immune-mediated, suppression of the immune response represents another treatment approach.