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Peripheral Nerve
Disorders |
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Peripheral nerve damage can be divided into
axonal damage or demyelination.
In axonal degeneration nerve-cell bodies are unable to maintain long axonal
processes. The result is degeneration of axons, starting at the periphery and
progressing up towards the neuronal cell body. This process is often termed a
dying back neuropathy and is commonly due to toxic and metabolic damage to
nerve-cell bodies. Loss of axons can be inferred by reduced amplitude of nerve
impulses when measured electrically from affected nerves.
In segmental demyelination the disease process spares axons but damages Schwann
cells and myelin. Loss of myelin can be inferred by slowing of conduction
velocity when measured electrically. Loss of myelin is mainly seen in
immune-mediated disease, as well as with several toxins.
Provided that the cell body is intact, and that there is no scarring in the
nerve, axons and myelin may regenerate, allowing recovery from damage to
peripheral nerves.
Regeneration in the peripheral nervous system
The sequence of events that follows sectioning of axons in a nerve is termed
Wallerian degeneration and, under favourable circumstances, this is followed by
axonal regeneration.
1 When an axon is severed or damaged, the axon and myelin distal to the injury
degenerate. The degenerate myelin and axon are removed by macrophages and
Schwann cells. The target tissue, e.g. muscle, is denervated and atrophies.
2 Schwann cells in the distal nerve proliferate and enlarge within the
still-intact basement membrane tube enclosing them.
3 The nerve-cell body becomes swollen, the nucleus enlarges and there is an
increase in the quantity of cytoplasmic intermediate filaments, termed central
chromatolysis (a structural change reflecting regeneration). The stump of the
proximal axon swells
and several small axon sprouts grow out down the column of proliferated Schwann
cells, which acts as a guide for the regenerating axon. The axons grows 2-3 mm
per day, eventually re-innervating the denervated tissue.
4 The axon is re-myelinated, but new myelin segments between nodes of Ranvier
are shorter than in the original nerve.
Axons capacity for regeneration allows surgical repair of peripheral nerves by
nerve anastomosis after severance. Grafting of a portion of nerve is required
when there has been scarring, as axons can only grow down the intact basement
membrane tubes of Schwann cells, not through a collagenous scar.
Laceration, compression and entrapment are common mechanical causes of nerve
dysfunction
One of the most common causes of peripheral nerve dysfunction is mechanical
trauma. Laceration of nerves is seen after penetrating trauma and is associated
with some bone fractures. The nerve distal to the laceration undergoes Wallerian
degeneration. If surgical anastomosis is performed, axons may re-grow to
re-innervate the denervated tissues.
Entrapment and compression of nerves is commonly seen in several sites.
Nerve roots are compressed in intervertebral foramina by prolapsed
intervertebral discs or osteophytes due to osteoarthritis of the spine.
The median nerve is compressed by swellings in the carpal tunnel at the wrist.
The ulnar nerve can become compressed in the flexor carpal tunnel at the
medial epicondyle of the humerus.
The common peroneal nerve can be compressed at the neck of the fibula.
Compressed nerves develop segmental demyelination and abnormal conduction but,
with prolonged damage, there is additional axonal degeneration.
Acute immune-mediated demyelination is the most common cause of an acute
peripheral neuropathy
Guillain-Barrι syndrome, also known as acute post-infectious polyradiculoneuropathy, is the most common form of acute neuropathy. It is an
immune-mediated demyelination of peripheral nerves, usually seen 2-4 weeks after
a viral illness, but also triggered after a variety of infective processes.
Histologically, nerves show infiltration by lymphoid cells, with phagocytosis of
myelin by macrophages.
Widespread demyelination in peripheral nerves causes motor weakness, often
leading to respiratory failure, with less prominent sensory changes. Re-myelination
usually occurs over a period of 3-4 months and is associated with recovery in
most cases.
Chronic demyelinating polyradiculoneuropathy is a chronic form of
immune-mediated demyelination in which repeated episodes of demyelination and
re-myelination cause proliferation of Schwann cells, often leading to physical
swelling of peripheral nerves (hypertrophic neuropathy).
Peripheral neuropathy may occur as a paraneoplastic syndrome, several cases
being due to formation of autoantibodies to the tumour, which cross-react with
myelin.
Neuropathies may be caused by toxins, causing either demyelination or axonal
degeneration
Many toxins cause damage to peripheral nerves, either by damaging myelin or by
causing damage to neurons, leading to axonal degeneration.
Chronic alcohol abuse} is a common cause of peripheral neuropathy, which may
be due to the direct toxic effects of alcohol as well as the effects of vitamin
deficiency.
Drugs, e.g. isoniazid, sulphonamides, vinca alkaloids, dapsone and chloroquine,
are important causes of neuropathy.
Occupational exposure to chemicals such as lead, arsenic, mercury, and
acrylamide.
Most toxins produce a 'dying back' axonal neuropathy that results in symmetrical
sensory/motor neuropathy with a 'glove and stocking' distribution, reflecting
loss of the ends of the longer axons.
Diabetes is the most common metabolic disorder causing neuropathy
Several metabolic disorders are associated with the development of a peripheral
neuropathy, the most common being diabetes.
Diabetes mellitus is associated with four main patterns of neuropathy:
symmetrical predominantly sensory peripheral polyneuropathy, autonomic
neuropathy, proximal painful motor neuropathy, and cranial mononeuropathy
(mainly third, fourth and sixth nerves).
The sensory and autonomic neuropathy is mainly due to a combination of axonal
degeneration and segmental demyelination, whereas the motor neuropathy and
cranial mononeuropathy are caused by vascular disease in blood vessels supplying
nerves.
Vitamin deficiency is an important cause of peripheral neuropathy, particularly
B1 (thiamine) and B12.
Amyloid may infiltrate peripheral nerves and cause a neuropathy. In addition to
secondary amyloid associated with systemic inflammatory diseases or myeloma,
several types of familial amyloid cause a specific neuropathy due to a defect in
the gene coding for transthyretin (prealbumin).
The main tumours of peripheral nerve are benign nerve-sheath tumours,
neurofibromas and schwannomas
The main tumours of peripheral nerve arise from the nerve sheath and are termed
schwannomas and neurofibromas. Uncommonly, malignant peripheral nerve-sheath
tumours can develop (neurofibrosarcomas and malignant schwannomas).
Schwannomas, which are usually solitary tumours, can occur on any peripheral
nerve. They are rounded lesions, typically 1-2 cm in diameter when removed,
composed of spindle-shaped cells with features of Schwann cells. Schwannomas
affecting the eighth cranial nerve are termed acoustic neuromas and may be part
of the neurofibromatosis 2 syndrome (NF1).
Neurofibromas, which may be solitary or multiple, are nodular or plexiform in
type. Nodular neurofibromas are discrete tumours that form a defined mass,
whereas plexiform neurofibromas diffusely affect nerves over a wide area, making
surgical removal very difficult. Nodular neurofibromas are fusiform or rounded
tumours composed of spindle cells that resemble perineurial cells. Plexiform
neurofibromas often appear as boggy areas of ill-defined swelling and are
composed of nerves expanded by a proliferation of perineurial cells and
extracellular matrix material. Multiple neurofibromas are seen as part of the
neurofibromatosis 1 syndrome (NF2).
Malignant nerve-sheath tumours may arise de novo or may occur as malignant
change in an existing benign tumour, usually in cases of neurofibromatosis. They
behave as soft-tissue sarcomas. |
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