Neuralgia is pain in one or more nerves that occurs without stimulation of pain receptor (nociceptor) cells. Neuralgia pain is produced by a change in neurological structure or function rather than by the excitation of pain receptors that causes nociceptive pain. Neuralgia falls into two categories: central neuralgia and peripheral neuralgia. This unusual pain is thought to be linked to four possible mechanisms: ion gate malfunctions; the nerve becomes mechanically sensitive and creates an ectopic signal; cross signals between large and small fibers; and malfunction due to damage in the central processor.

Neuralgia was first recognized by Silas Weir Mitchell, a neurologist in the American Civil War, who noticed hyperalgesia and chronic pain in patients who had nerve lesions in the extremities and also some cases where no lesion was observed: These causalgias were certainly major by the importance of the symptoms, but stemmed from minor neurological lesions". Mitchell termed the condition “causalgia” which has since become known as “Complex Regional Pain Syndrome Type 1 and Type 2” (CRPS). CRPS Type I is a syndrome that develops after an initiating noxious event ., and Type 2 describes a case when nerve damage is clear.

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Neuralgia is often difficult to diagnose, and most treatments show little or no effectiveness. Diagnosis typically involves locating the damaged nerve by identifying missing sensory or motor function. This may involve tests such as an EMG test or a nerve conduction test. Neuralgia is more difficult to treat than other types of pain because it does not respond well to normal pain medications. Special medications have become more specific to neuralgia and typically fall under the category of membrane stabilizing drugs or antidepressants such as Cymbalta. The antiepileptic medication(AED) Lyrica was developed specifically for neuralgia and other neuropathic pain as a successor to Neurontin (gabapentin).

Under the general heading of neuralgia are trigeminal neuralgia (TN), atypical trigeminal neuralgia (ATN), and postherpetic neuralgia (caused by shingles or herpes). Neuralgia is also involved in disorders such as sciatica and brachial plexopathy with neuropathia. Neuralgias that do not involve the trigeminal nerve are occipital neuralgia and glossopharyngeal neuralgia.

In the case of trigeminal neuralgia the affected nerves are responsible for sensing touch, temperature sensation and pressure sensation in the facial area from the jaw to the forehead. The disorder generally causes short episodes of excruciating pain, usually for less than two minutes and usually only one side of the face. The pain can be described in a variety of ways such as "stabbing," "sharp," "like lightning," "burning," and even "itchy". In the atypical form of TN, the pain presents itself as severe constant aching along the nerve. The pain associated with TN is recognized as one of the most excruciating pains that can be experienced.

Simple stimuli such as eating, talking, making facial expressions, washing the face, or any light touch or sensation can trigger an attack (even the sensation of a cool breeze). The attacks can occur in clusters, as an isolated attack, or be completely constant. Some patients will have a muscle spasm which led to the original term for TN of "tic douloureux" ("tic", meaning 'spasm', and "douloureux", meaning 'painful', in French).

Neuralgia is a form of chronic pain and can be extremely difficult to diagnose. Postherpetic neuralgia is the easiest to diagnose because it follows an obvious cause (shingles). Neuralgia is a rare disease. Women are more likely to be affected than men, and those over 50 are at the greatest risk. In some cases, multiple sclerosis is related to nerve damage, causing the pain, so doctors will likely ask about family history to help diagnose. Nothing unusual can be seen in brain scans, so diagnosis is usually based on the description of the symptoms and the response to the medication or procedures.

Mechanisms

By understanding the neuroplastic changes following nerve damage, researchers may be able to better understand the mechanism of hyperexcitability in the nervous system that is believed to cause neuropathic pain.

Peripheral nerve injury

A neuron’s response to trauma can often be determined by the severity of the injury, classified by Seddon's classification. In Seddon’s Classification, nerve injury is described as either neurapraxia, axonotmesis, or neurotmesis. Following trauma to the nerve, a short onset of afferent impulses, termed “injury discharge”, occurs. While lasting only minutes, this occurrence has been linked to the onset of neuropathic pain.

When an axon is severed, the segment of the axon distal to the cut degenerates and is absorbed by Schwann cells. The proximal segment fuses, retracts, and swells, forming a “retraction bulb.” The synaptic terminal function is lost, as axoplasmic transport ceases and no neurotransmitters are created. The nucleus of the damaged axon undergoes chromatolysis in preparation for axon regeneration. Schwann cells in the distal stump of the nerve and basal lamina components secreted by Schwann cells guide and help stimulate regeneration. The regenerating axon must make connections with the appropriate receptors in order to make an effective regeneration. If proper connections to the appropriate receptors are not established, aberrant reinnervation may occur. If the regenerating axon is halted by damaged tissue, neurofibrils may create a mass known as a neuroma.

In the event that an injured neuron degenerates or does not regenerate properly, the neuron loses its function or may not function properly. Neuron trauma is not an isolated event and may cause degenerative changes in surrounding neurons. When one or more neurons lose their function or begin to malfunction, abnormal signals sent to the brain may be translated as painful signals.

Central neuronal injury

Neuronal injury in the central nervous system (CNS) typically leads to local degeneration of the nerve axon and myelin sheath. Axonal debris in the CNS is eliminated by macrophages. Trauma to neurons in the CNS also causes a proliferation of glial cells that form a glial scar. Development of the glial scar is thought to inhibit regeneration of central neural connectionscitation needed. The damaged nerve terminal begins to swell and glial cells push the defective terminal away from connections to other neurons. Often, aberrant sprouting of damaged CNS neurons, specifically sensory neurons, results in neuralgia.

Diagnosis

Diagnosis of neuralgia is difficult, and misdiagnosis is common. Diagnosis typically involves locating the damaged nerve by stimulation of the specific damaged pathway or by identifying missing sensory function. The most common test for neuralgia is a nerve conduction study, such as using microneurography in which a peripheral nerve is stimulated and recordings are taken from a purely-sensory portion of the nerve.

When assessing neuralgia to find the underlying mechanism, a history of the pain, description of pain, clinical examination, and experimental examination are required. Since pain is subjective to the patient, it is important to use a pain assessment scale, such as the McGill Pain Questionnaire. Qualifying the severity of the pain is essential in diagnosis and in evaluating the effectiveness of the treatment. Clinical examinations usually involve testing responses to stimuli such as touch, temperature, and vibration. Neuralgia can be further classified by the type of stimuli that elicits a response: mechanical, thermal, or chemical. Response to the course of treatment is the final tool used to determine the mechanism of the pain. Future research must focus on the relationships between all of these categories.

Laser evoked potentials

Neuropathic pain is often the result of a lesion in spinothalamic pathways. Laser evoked potentials (LEPs) are measurements of cortical responses using lasers to selectively stimulate thermonociceptors in the skin. Lasers can emit a radiant-heat pulse stimulus to selectively activate A-delta and C free nerve endings. By specifically targeting pain and temperature pathways and measuring cortical responses, clinicians can identify even minute lesions in the spinothalamic pathways. LEP abnormalities are strongly indicative of neuropathic pain, while a normal LEP is often more ambiguous. LEPs have high sensitivity and are very reliable in assessing damage to both central and peripheral nervous systems.

Quantitative sensory testing

Another method for testing the proper function of a nerve is Quantitative sensory testing (QST). QST relies on analysis of a patient’s response to external stimuli of controlled intensity. A stimulus is applied to the skin of the nerve area being tested in ascending and descending orders of magnitude. Clinicians can quantify the mechanical sensitivity of the tactile stimulus using von Frey hairs or Semmes-Weinstein monofilaments. Also, weighted needles can be used to measure pin-prick sensation, and an electronic vibrameter is used to measure vibration sensitivity. Thermal stimuli are quantified by using a probe that operates on the Peltier principle.

One problem with QST is that abnormalities may be observed in non-neuralgia pains, often making it inconclusive in diagnosis. Also, QST is very time consuming and relies on expensive equipment.

Punch skin biopsy

Recently, skin biopsy has been used to investigate mechanoreceptors and their myelinated afferents. Though available in only a few research centers, skin punch biopsy is an easy procedure and is minimally invasive. Punch skin biopsy is used to quantify nerve fibers C fibers and A-delta nerve fibers through measurement of the density of intra-epidermal nerve fibers (IENF). Loss of IENF has been observed in several cases of neuropathic pain.

Atypical (trigeminal) neuralgia

Atypical trigeminal neuralgia (ATN) is a rare form of neuralgia and may also be the most misdiagnosed form. The symptoms can be mistaken for migraines, dental problems such as TMJ, musculoskeletal issues, and hypochondriasis. ATN can have a wide range of symptoms and the pain can fluctuate in intensity from mild aching to a crushing or burning sensation, and also to the extreme pain experienced with the more common trigeminal neuralgia. ATN pain can be described as heavy, aching, and burning. Sufferers have a constant migraine-like headache and experience pain in all three trigeminal nerve branches. This includes aching teeth, ear aches, feeling of fullness in sinuses, cheek pain, pain in forehead and temples, jaw pain, pain around eyes, and occasional electric shock-like stabs. Unlike typical neuralgia, this form can also cause pain in the back of the scalp and neck. Pain tends to worsen with talking, facial expressions, chewing, and certain sensations such as a cool breeze. Vascular compression of the trigeminal nerve, infections of the teeth or sinuses, physical trauma, or past viral infections are possible causes of ATN.

Glossopharyngeal neuralgia

Glossopharyngeal neuralgia consists of recurring attacks of severe pain in the back of the throat, the area near the tonsils, the back of the tongue, and part of the ear. The pain is due to malfunction of the 9th cranial nerve (glossopharyngeal nerve), which moves the muscles of the throat and carries information from the throat, tonsils, and tongue to the brain.

Glossopharyngeal neuralgia, a rare disorder, usually begins after age 40 and occurs more often in men. Often, its cause is unknown. But sometimes glossopharyngeal neuralgia results from an abnormally positioned artery that compresses the glossopharyngeal nerve near where it exits the brain stem. Rarely, the cause is a tumor in the brain or neck.

Occipital neuralgia

Occipital neuralgia, also known as C2 neuralgia, or Arnold's neuralgia, is a medical condition characterized by chronic pain in the upper neck, back of the head and behind the eyes.

Treatment

Treatment options include medicines, surgery, and complementary approaches.

High doses of anticonvulsant medicines—used to block nerve firing— and tricyclic antidepressants are generally effective in treating neuralgia. If medication fails to relieve pain or produces intolerable side effects, surgical treatment may be recommended.

Neural augmentative surgeries are used to stimulate the affected nerve. By stimulating the nerve the brain can be “fooled” into thinking it is receiving normal input. Electrodes are carefully placed in the dorsal root and subcutaneous nerve stimulation is used to stimulate the targeted nerve pathway. A technician can create different electrical distributions in the nerve to optimize the efficiency, and a patient controls the stimulation by passing a magnet over the unit.

Some degree of facial numbness is expected after most of these surgical procedures, and neuralgia might return despite the procedure’s initial success. Depending on the procedure, other surgical risks include hearing loss, balance problems, infection, and stroke. These surgeries include rhizotomy (where select nerve fibers are destroyed to block pain) and Microvascular decompression (where the surgeon moves the vessels that are compressing the nerve away from it and places a soft cushion between the nerve and the vessels).

Some patients choose to manage neuralgia using complementary techniques, usually in combination with drug treatment. These therapies offer varying degrees of success. Options include chiropractic, acupuncture, biofeedback, vitamin therapy, nutritional therapy, hot-cold compress, and electrical stimulation of the nerves.

Risks

Sleep deprivation and malnutrition have also been reported as byproducts of the pain. It is possible that there are other triggers or aggravating factors that patients need to learn to recognize to help manage their health. Bright lights, sounds, stress, and poor diet are examples of additional stimuli that can contribute to the condition. The pain can cause nausea, so beyond the obvious need to treat the pain, it is important to be sure to try to get adequate rest and nutrition.

See also

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