In most series, a mixed sensorimotor polyneuropathy has been 4 to 5 times more frequent than a purely sensory one. However, the latter is a more specific syndrome identified with lung cancer (described originally by Denny-Brown); it is characterized by a loss of all modalities of sensation spreading from the distal to the proximal segments of the limbs and eventually to the trunk and face. There is loss of tendon reflexes, but motor power may be retained. It has also been appreciated that the sensory loss in the beginning may have a multifocal distribution. Another variety is characterized by initial sensory ataxia, similar to that discussed in the earlier section “Acute
Sensory Neuronopathy (Sensory Ganglionopathy).” The illness reaches its peak in a few weeks or months and in a very few instances the development has been as rapid as that of GBS.
The pathologic changes are those of an inflammatory and destructive sensory neuropathy and neuronopathy (ganglionitis) and are sometimes part of a more widespread disorder of the nervous system related to the anti-Hu antibody (also termed antinuclear neuronal antibody type 1 (ANNA-1); see Chap. 30). This polyneuropathy has proven to be most typical of small cell cancer of the lung. In a series of 71 patients with paraneoplastic sensory neuronopathy reported by Dalmau and colleagues, more than half were associated with symptomatic inflammatory lesions in the temporal lobes (limbic encephalitis), the brainstem, and, rarely, the anterior horn neurons of the spinal cord. Other distinctive paraneoplastic syndromes such as cerebellar degeneration and Lambert-Eaton myasthenic syndrome were combined with polyneuropathy in isolated cases and there were signs of dysautonomia in 28 percent. In other words, there are many cases of sensory neuropathy and neuronopathy that exist in parallel with various neoplasms, including some of our patients with lymphomas.
The CSF protein is mildly elevated but usually acellular. Sensory potentials are usually absent in all nerves after a few weeks, but may be spared early on. The localization of anti-Hu antibody to the several affected regions of the nervous system and to the tumor itself has led to speculation that the lung tumors are typically small or inevident because the antibody suppresses tumor growth. Almost all cases of paraneoplastic sensory neuropathy and a proportion of the more nondescript sensory predominant or sensorimotor paraneoplastic polyneuropathies also demonstrate anti-Hu antibodies, making this testing useful in distinguishing paraneoplastic varieties of sensory neuropathy and neuronopathy from those caused by postinfectious or immune disorders such as Sjögren syndrome and HIV infection. The finding of high antibody titers should lead to chest imaging and, in appropriate cases, bronchoscopic or positron emission tomography (PET) examinations to detect an underlying cancer. A rare vasculitic mononeuropathy multiplex that occurs with cancer is discussed further on.