Peripheral nerves may be enlarged and is usually most pronounced in limb girdle plexuses. Similar enlargements may also be present in nerve roots and spinal ganglia. This is supported by the fact that dietary treatment with reduction of phytanic acid levels results in a marked clinical improvement and a tendency to normalization of nerve conduction velocity. However, it is not known how phytanic acid induces the endoneurial, the nerve fibre and the Schwann cell changes. The pathogenesis, the chemical composition and the sites of deposits vary markedly between the different forms of amyloidsis.
Neurologic manifestations occur particularly in the heriditary and primary forms of amyloidosis.
The somatic and the autonomic peripheral nervous system may both be involved, causing a sensorimotor peripheral neuropathy and autonomic dysfunctions such as postural hypotension, impotence, inability to sweat and gastrointestinal disturbances. The clinical deficits are associated with lesions of the nerve fibres and marked deposits of amyloid in the afflicted nerves. There is thus a profound abnormality in the microenvironment of the nerve fascicles in amyloid neuropathy. The detailed pathomechanisms of FAP are still not clear. It also seems reasonable to assume that the abnormal plasma TTR over a very long period of time will pass the endoneurial vessels.