Neuropathy

Neuropathy is usually short for peripheral neuropathy, meaning a disease of the peripheral nervous system. Strictly speaking, however, neuropathy is any disease that affects any part of the nervous system.

The four major forms of nerve damage are polyneuropathy, autonomic neuropathy, mononeuropathy, and mononeuritis multiplex. The most common form is peripheral polyneuropathy, which mainly affects the feet and legs.

Often the form of neuropathy is further broken down as to cause (see below), or other type, such as small fiber peripheral neuropathy, which is idiopathic.

Aside from diabetes (see Diabetic neuropathy), the common causes of neuropathy are herpes zoster infection, HIV-AIDS, toxins, alcoholism, chronic trauma (such as repetitive motion disorders) or acute trauma (including surgery), various neurotoxins and autoimmune conditions such as celiac disease, which can account for approximately 16% of small fiber neuropathy cases. Neuropathic pain is common in cancer as a direct result of the cancer on peripheral nerves (e.g., compression by a tumor), as a side effect of many chemotherapy drugs, and as a result of electrical injury. In many cases no apparent causes can be found, in this case the neuropathy is "idiopathic" meaning no cause is found.

Neuropathy often results in numbness, abnormal sensations called dysesthesias and allodynias that occur either spontaneously or in reaction to external stimuli, and a characteristic form of pain, called neuropathic pain or neuralgia, that is qualitatively different from the ordinary nociceptive pain one might experience from stubbing a toe or hitting a finger with a hammer.

Neuropathic pain is usually perceived as a steady burning and/or "pins and needles" and/or "electric shock" sensations. The difference is due to the fact that "ordinary" pain stimulates only pain nerves, while a neuropathy often results in the firing of both pain and non-pain (touch, warm, cool) sensory nerves in the same area, producing signals that the spinal cord and brain do not normally expect to receive.

Neuropathic pain can be very difficult to treat. Sometimes strong opioid analgesics may provide only partial relief. Opioid analgesics are to be considered only as a tertiary treatment. Several classes of medications not normally thought of as analgesics are often effective, alone or in combination with opioids and other treatments. These include tricyclic antidepressants such as amitriptyline (Elavil®), anticonvulsants such as gabapentin (Neurontin®) and pregabalin (Lyrica®).

In animal models of neuropathic pain it has been found that compounds which only block serotonin reuptake do not improve neuropathic pain.^{[1][2][3][4][5][6][7][8]} Similarly, compounds that only block norepinephrine reuptake also do not improve neuropathic pain. Compounds such as duloxetine, venlafaxine, and milnacipran that block both serotonin reuptake and norepinephrine reuptake do improve neuropathic pain. Antidepressants usually reduce neuropathic pain more quickly and with smaller doses than they relieve depression. Antidepressants therefore seem to work differently on neuropathic pain than on depression, perhaps by activating descending norepinephrinergic and serotonergic pathways in the spinal cord that block pain signals from ascending to the brain.

The newer anticonvulsants gabapentin and pregabalin appear to work by blocking calcium channels in damaged peripheral neurons. Tricyclic antidepressants may also work on sodium channels in peripheral nerves. The anticonvulsants carbamazepine (Tegretol®) and oxcarbazepine (Trileptal®), especially effective on trigeminal neuralgia, are thought to work principally on sodium channels.

In general, the antidepressants seem to be most effective on continuous burning pain, while the anticonvulsants seem to work best on sudden, lancinating, "shock-like" pains that appear to involve large numbers of peripheral nerves improperly firing together.

In some forms of neuropathy, especially post-herpes neuralgia, the topical application of local anesthetics such as lidocaine can provide relief. A transdermal patch containing 5% lidocaine is available. Ketamine in a transdermal gel is also frequently effective when the neuropathy is localized. Neurontin 100mg/g PLO gel is also effective for treating peripheral neuropathy, including Carpal Tunnel Syndrome.

In some neuropathic pain syndromes, "crosstalk" occurs between descending sympathetic nerves and ascending sensory nerves. Increases in sympathetic nervous system activity result in an increase of pain; this is known as sympathetically-mediated pain. Reducing the sympathetic nerve activity in the painful region with local nerve blocks or systemic medications such as clonidine may provide relief.

The NMDA receptor seems to play a major role in neuropathic pain and in the development of opioid tolerance, and many experiments in both animals and humans have established that NMDA antagonists such as ketamine and dextromethorphan can alleviate neuropathic pain and reverse opioid tolerance. Unfortunately, only a few NMDA antagonists are clinically available and their use is usually associated with unacceptable side effects.

Several opioids, particularly methadone, have NMDA antagonist activity in addition to their μ-opioid agonist properties that seems to make them effective against neuropathic pain, although this is still the subject of intensive research and clinical study. Methadone has this property because it is a racemic mixture; one stereo-isomer is a μ-opioid agonist; the other is a NMDA antagonist.

A recent study showed smoked marijuana is benefitial in treating HIV-associated periphial neuropathy.^[9]

In addition to pharmacological treatment there are several other modalities that help some cases. While lacking double blind trials, these have shown to reduce pain and improve patient quality of life particularly for chronic neuropathic pain: Interferential Stimulation; Acupuncture; Meditation; Cognitive Therapy; and prescribed exercise.

In more recent years, Photo Energy Therapy devices are becoming more widely used to treat neuropathic symptoms. Photo Energy Therapy devices emit near infrared light typically at a wavelength of 890nm. This wavelegnth is is believed to stimulate the release of Nitric Oxide, a Endothelium-derived relaxing factor into the bloodstream, thus vasodilating the capilaries and venuoles in the microcirculatory system. This increase in circulation has been shown effective in various clinical studies, to decrease pain and improve sensation in diabetic and non-diabetic patients.^{[citation needed]}

• Nerve
• Peripheral nervous system
• Neuritis
• Neuralgia
• Small fiber peripheral neuropathy
• Phantom limb
• Phantom pain

• Special Interest Group on Neuropathic Painof the International Association for the Study of Pain (IASP)
• The Neuropathy Association
• Join a Neuropathy Support Group on Yahoo!

• Nep Know More Provides Additional Help and Information on Neuropathic Pain
• Easy to Read Page On Neuropathy
• A neuropathic series of articles from a neurologist who researches neuropathic pain
• Up to 16% of Patients with Small Fiber Neuropathy May Have Celiac Disease
• National Diabetes Information Clearinghouse
• Information about Neurology Article on marijuana's effect on neuropathic pain
• Nitric Oxide and its Role in Diabetes, Wound Healing and Peripheral Neuropathy