The frequency of paraneoplastic syndromes depends on how one defines the disorder. Using the more restrictive definition, paraneoplastic syndromes affecting the nervous system are quite rare. There were no instances of sensory neuronopathy. In another series of almost 4,000 patients with lung cancer, only 14 were found to have an encephalomyelitis. The result is that an oncologist may go several years without seeing any patient with a paraneoplastic syndrome and may even come to doubt its existence.
Paraneoplastic syndromes are important to the neurologist for several reasons.
The disorders, as defined in the narrow sense, usually present themselves before the cancer is diagnosed. In many instances, the cancers are small and potentially curable. Thus, the neurologist has the opportunity to make the diagnosis not only of a paraneoplastic disorder, but also of an underlying cancer that may be curable. If the patient is already known to have cancer, a paraneoplastic syndrome may mimic or be mimicked by metastatic disease. Leptomeningeal metastases may mimic a paraneoplastic peripheral neuropathy.