The form of neuropathy may be further broken down by cause, or the size of predominant fiber involvement, i.e. large fiber or small fiber peripheral neuropathy. Frequently the cause of a neuropathy cannot be identified and it is designated idiopathic.
Neuropathy may be associated with varying combinations of weakness, autonomic changes and sensory changes. Loss of muscle bulk or fasciculations, a particular fine twitching of muscle may be seen. Sensory symptoms encompass loss of sensation and "positive" phenomena including pain (for a more detailed discussion, see peripheral neuropathy).
Central neuropathic pain is found in spinal cord injury, multiple sclerosis, and some strokes. Fibromyalgia, a disorder of chronic widespread pain, is potentially a central pain disorder and is responsive to medications effective for neuropathic pain.
Aside from diabetes (see Diabetic neuropathy) and other metabolic conditions, the common causes of painful peripheral neuropathies are herpes zoster infection, HIV-related neuropathies, nutritional deficiencies, toxins, remote manifestations of malignancies, genetic and immune mediated disorders.
Neuropathic pain is common in cancer as a direct result of cancer on peripheral nerves (e.g., compression by a tumor), as a side effect of some chemotherapy drugs, and as a result of radiation injury.
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.
Neuropathic pain may have continuous and/or episodic (paroxysmal) components. The latter are likened to an electric shock. Common qualities of the pain include burning or coldness, "pins and needles" sensations, numbness and itching. "Ordinary" pain results from exclusive stimulation of pain fibers, while neuropathic pain often results from the firing of both pain and non-pain (touch, warm, cool) sensory nerve fibers serving the same area. The result is signals that the spinal cord and brain do not normally receive.
The starting point for neuropathic pain is a lesion within the somatosensory system. Injured peripheral nerve fibers give rise to an intense and persistent input to the central nervous system, which, in some cases, induces secondary changes in the excitability of spinal cord, dorsal horn neurons. Current knowledge regarding the mechanisms of neuropathic pain is incomplete and is biased by a focus on animal models of peripheral nerve injury.
After a peripheral nerve injury, a neuroma develops at the nerve stump. Regenerating C-fibres of damaged axons develop ongoing spontaneous activity, abnormal excitability and a heightened sensitivity to chemical, thermal and mechanical stimuli. This phenomenon is called "peripheral sensitization."
As a consequence of the ongoing spontaneous activity following activation of primary nociceptors, STT neurons develop an increased background activity, enlarged receptive field and increased responses to afferent impulses, including normally innocuous tactile stimuli. This phenomenon is called central sensitization. Central sensitization has been proposed as an important mechanism of persistent neuropathic pain.
Other mechanisms, however, may take place at the central level after peripheral nerve damage. The loss of afferent signals induces functional changes in dorsal horn neurons. A decrease in the large fiber input decreases activity of interneurons inhibiting nociceptive neurons i.e loss of afferent inhibition. Hypoactivity of the descending antinociceptive systems or loss of descending inhibition may be another factor. With loss of neuronal input (deafferentation) the STT neurons begin to fire spontaneously, a phenomenon designated "deafferentation hypersensitivity.”
The phenomenon described above are dependent on changes at light-microscopic and submicroscopic levels. Aberrant regeneration, altered expression of ion channels, changes in neurotransmitters and their receptors as well as altered gene expression in response to neural input are at play.
Deciding on the best treatment for individual patients challenges both the art and science of medicine. Attempts to synthesize scientific studies into best practices are limited by such factors as differences in reference populations and a lack of head-to-head studies. Furthermore, there are few studies evaluating treatment combinations or the special needs of children.
It is common practice in medicine to designate classes of medication according to their most common or familiar use e.g. as "antidepressants" and "anti-epileptic drugs" (AED's). These drugs have alternate uses to treat pain because the human nervous system employs common mechanisms for different functions, for example ion channels for impulse generation and neurotransmitters for cell-to-cell signaling.
In addition to the work of Dworkin, O'Connor and Backonja et al., cited above, there have been several recent attempts to derive guidelines for pharmacological therapy. These have combined evidence from randomized controlled trials with expert opinion.
Favored treatments are certain antidepressants e.g tricyclics and selective serotonin-norepinephrine re-uptake inhibitors (SNRI's), anticonvulsants, especially pregabalin (Lyrica) and gabapentin (Neurontin), and topical lidocaine. Opioid analgesics and tramadol are recognized as useful agents but are not recommended as first line treatments.
In animal models of neuropathic pain it has been found that compounds which only block serotonin reuptake do not improve neuropathic pain. 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.
Tricyclic antidepressants may also work on sodium channels in peripheral nerves.
Lamotrigine may have a special role in treating two conditions for which there are few alternatives, namely post stroke pain and HIV/AIDS-related neuropathy in that subgroup on antiretroviral therapy.
Several opioids, particularly methadone have NMDA antagonist activity in addition to their µ-opioid agonist properties.
There is little evidence to indicate that one strong opioid is more effective than another. Expert opinion leans toward the use of methadone for neuropathic pain, in part because of NMDA antagonism. It is reasonable to base the choice of opioid on other factors.
Repeated topical applications of capsaicin, are followed by a prolonged period of reduced skin sensibility referred to as desensitization, or nociceptor inactivation. Capsaicin not only depletes substance P but also results in a reversible degeneration of epidermal nerve fibers. Nevertheless, benefits appear to be modest.
Marijuana and its active ingredients are called cannabinoids. Unfortunately, strongly held beliefs make discussion of the appropriate use of these substances, in a medical context, difficult. Similar considerations apply to opioids.
A recent study showed smoked marijuana is beneficial in treating symptoms of HIV-associated peripheral neuropathy. Nabilone is an artificial cannabinoid which is significantly more potent than delta-9-tetrahydrocannabinol (THC). Nabilone produces less relief of chronic neuropathic pain and had slightly more side effects than dihydrocodeine.
The predominant adverse effects are CNS depression and cardiovascular effects which are mild and well tolerated but, psychoactive side effects limit their use. A complicating issue may be a narrow therapeutic window; lower doses decrease pain but higher doses have the opposite effect.
Sativex, a fixed dose combination of delta-9-tetrahydrocannabinol (THC) and cannabidiol, is sold as an oromucosal spray. The product is approved in Canada as adjunctive treatment for the symptomatic relief of neuropathic pain in multiple sclerosis, and for cancer related pain.
Long-term studies are needed to assess the probability of weight gain, unwanted psychological influences and other adverse effects.
Dextromethorphan is an NMDA antagonist at high doses.
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 limited by unacceptable side effects.
Lesioning operations on the sympathetic branch of the autonomic nervous system are sometimes carried out.
A 2007 review of studies found that injected (parenteral) administration of alpha lipoic acid (ALA) was found to reduce the various symptoms of peripheral diabetic neuropathy. While some studies on orally administered ALA had suggested a reduction in both the positive symptoms of diabetic neuropathy (including stabbing and burning pain) as well as neuropathic deficits (paresthesia), the metanalysis showed "more conflicting data whether it improves sensory symptoms or just neuropathic deficits alone". There is some limited evidence that ALA is also helpful in some other non-diabetic neuropathies.
Benfotiamine is a lipid soluble form of thiamine that has several placebo controlled double blind trials proving efficacy in treating neuropathy and various other diabetic comorbidities.
Transcutaneous electrical nerve stimulation (TENS) may be worth considering in chronic neurogenic pain. TENS, with certain electrical waveforms, appears to have an acupuncture-like function.
Infrared photo therapy has been used to treat neuropathic symptoms. However, recent work has cast doubt on the value of this approach.
Neuromodulation is a field of science, medicine and bioengineering that encompasses both implantable and non-implantable technologies (electrical and chemical) for treatment purposes.
Implanted devices are expensive and carry the risk of complications. Available studies have focused on conditions having a different prevalence than neuropathic pain patients in general. More research is needed to define the range of conditions for which they might be beneficial.
Spinal cord stimulators, use electrodes placed adjacent to, but outside the spinal cord. The overall complication rate is one-third, most commonly due to lead migration or breakage. Lack of pain relief sometimes prompts device removal.
Infusion pumps deliver medication directly to the fluid filled (subarachnoid) space surrounding the spinal cord. Opioids alone or opioids with adjunctive medication (either a local anesthetic or clonidine) or more recently ziconotide are infused. Complications such as, serious infection (meningitis), urinary retention, hormonal disturbance and intrathecal granuloma formation have been noted.
There are no randomized studies of infusion pumps. For selected patients 50% or greater pain relief is achieved in 38% to 56% at six months but declines with the passage of time. These results must be viewed skeptically since placebo effects cannot be evaluated.
Stimulation of the primary motor cortex through electrodes placed within the skull but outside the thick meningeal membrane (dura) has been used to treat pain. The level of stimulation is below that for motor stimulation. As compared with spinal stimulation, which requires a noticeable tingling (paresthesia) for benefit, the only palpable effect is pain relief.
The best long-term results with deep brain stimulation have been reported with targets in the periventricular/periaqueductal grey matter (79%), or the periventricular/periaqueductal grey matter plus thalamus and/or internal capsule (87%). There is a significant complication rate which increase over time.