Malignant disorders may produce neuromuscular syndromes in a variety of ways, for some of which it is still difficult to determine the exact pathophysiology. In the myopathic and neuropathic disorders, one possible explanation is that they are due to a virus such as is found in the rare "
progressive multifocal leukoencephalopathy". This is seen in association with the
malignant lymphomas and with other conditions such as
sarcoidosis where immune responses may be altered by either the disease or the treatment. No viral material has been found in the nonmetastatic
neurological disorders apart from
progressive multifocal leukoencephalopathy. An alternative theory is that there may be an autoimmune process, the nervous system sharing some
antigenic determinant with the
neoplasm (Urich, 1967). The prognosis in the paraneoplastic
neurological disorders is usually poor. As well as the direct threat to life posed by the malignant disease, when the
neurological disorder is due to destruction of neurones (for instance cerebellar degeneration or sensory neuronopathy) recovery of function is impossible. Spontaneous remissions have been recorded in cases of proximal
muscle weakness and sensorimotor neuropathy, but it is difficult to know whether the remissions have been truly spontaneous or related to treatment (excision of the
neoplasm or administration of
steroids). Further immunological and virological studies will probably reveal the answers to some of the outstanding problems. In the meantime the clinician must continue to investigate patients with
muscular weakness for evidence of an occult
neoplasm, and to repeat investigations if no other cause for the
neurological disorder is found. Also, in patients with known malignant disease, apart from trying to differentiate forms of neuromyopathy from the effects of
metastases the various metabolic disorders must be considered because the therapeutic possibilities are a little more promising in the paraneoplastic endocrine disorders. Ross (1975) wisely said that "
cancer has replaced
syphilis as the great imitator".