All peripheral nerves are myelinated, except C fibres – which carry information about pain. These C fibres are sometimes called ‘slow fibres’ because the signal travels slowly due to the lack of myelination.
Specific causes of peripheral neuropathy
The mnemonic DAVID can be used as a method to remember specific causes of peripheral neuropathies
- D -Diabetes
- A -Alcoholism
- V -Vitamin Deficiency – B12
- I -Infective / inherited - Guillain-Barre / Charcot-Marie-Tooth
- D -Drugs – e.g. isoniazid
There are 6 main mechanisms of peripheral nerve degeneration:
- Demyelination – e.g. Guillain-Barre syndrome, post-diphtheric neuropathy, hereditary sensory-motor neuropathies - usually occurs as a result of schwann cell damage.
- Axonal degeneration – usually occurs in toxic neuropathies – conduction speed remains normal, and the axon tends to dies of peripherally first, then more proximally. Surviving axons often strive to re-innervate affected areas
- Wallerian degeneration – this describes fibre degeneration when the fibre is cut or crushed. Both the axon and the myelin sheath will degenerate over several weeks after the incident.
- Compression – e.g. carpal tunnel syndrome – there is local demyelination at the site of compression. Occurs when the compression is not as severe as in wallerian degeneration.
- Infarction – e.g. polyarteritis nodosa, Churg-Strauss syndrome, diabetes – there will also be wallerian degeneration distal to the infarct.
- Infiltration – e.g. leprosy, malignancy, inflammation, sarcoidosis
Regeneration can and does occur, although it is limited. There can either be:
- Remyelination – more likely to occur than axonal regrowth
- Axonal regrowth – this can occur at a rate of up to 1mm per day
Neuropathy – a pathological process affecting (a) peripheral nerve(s)
Mononeuropathy – only one nerve involved
Mononeuritis Multiplex – usually describes a condition that affects several nerves, but in no discernable pattern, and no underlying unifying condition. i.e. the nerves have been individually damadged.
Polyneuropathy – many nerves involved. Usually describes a symmetrical disease, and it usually begins distally. Can be sensory, motor or mixed. Classified into demyelinating, or axonal types. Loads of different types and different classifications. Often there is loss of reflexes.