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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