Polyneuropathies are disorders of peripheral nerves, which can be affected by trauma, tumor, infection, nonspecific inflammation or immunological disorders. These disorders of peripheral nerves can result in loss of sensation and weakness of skeletal muscles leading to significant morbidity and crippling disability. Here is a short introductory account of these disorders of peripheral nerves.
1. Peripheral Nerves:
Peripheral nerves are bundles of large-diameter myelinated and small-diameter unmyelinated nerve fibers. Large-diameter myelinated fibers carry motor efferents as well as sensations of vibration and proprioception. On the other hand small-diameter carry sensations of pain and temperature along with mediation of autonomic nervous system.
2. Mechanism of Nerve Damage in Polyneuropathies:
There are 4 mechanisms by which nerve fiber damage occurs leading to development of polyneuropathy. These mechanisms include Axonal degeneration, demyelination, Wallerian degeneration and neuronopathy.
3. Axonal Degeneration:
This is most common type of underlying pathology of neuropathies. This type is associated with toxic, metabolic, nutritional and systemic disorders. 75% of these cases are distal symmetrical polyneuropathies. Longest fibers are affected first of all and this type of pathology mostly involves large-diameter nerve fibers causing sensory or sensory-motor polyneuropathy.
Demyelination is an immune-mediated disorder, which results in patchy demyelination of the nerve leaving axon intact. Demyelination can occur both distally or proximally and usually is asymmetric.
5. Wallerian Degeneration:
Wallerian degeneration occurs to the distal stump of the completely transected nerve or nerve having equivalent damage. Proximal segment heals at a variable rate and up to a variable extent.