Neuropathic Pain

Neuropathic pain may result from efferent activity (sympathetically maintained pain) or from interruption of afferent activity (deafferentation pain).

Peripheral nerve injury or dysfunction can result in neuropathic pain. Examples are

• Mononeuropathies (involve a single nerve [eg, carpal tunnel syndrome, radiculopathy due to a herniated intervertebral disk])

• Plexopathies (involve multiple nerves within a particular neural plexus; typically caused by trauma, inflammation, or nerve compression, as by a tumor)

• Polyneuropathies (involve multiple nerves, often throughout the body; typically caused by various metabolic disorders, paraproteinemias, toxic exposures [eg, alcohol, chemotherapy]), hereditary predisposition, or, rarely, immune mediated mechanisms—see tables Causes of Peripheral Nervous System Disorders)

Mechanisms of neuropathic pain are complex and involve changes

• At the peripheral nociceptor and nerve level

• At the dorsal root ganglion

• In central nervous system (CNS) nociceptive pathways and terminal structures

At the peripheral nerve and nociceptor level, injury results in inflammation and in activation and over-representation of cation channels, particularly sodium channels. These changes reduce the threshold for activation and increase the response to noxious stimuli. In chronic states, the peripheral nerve continuously triggers nociceptive ectopic signals to the CNS. This bombardment of continuous peripheral nociceptive input leads to changes in receptive nociceptors (central sensitization); they are primed, interpret pain from minor stimuli (including nonpainful stimuli [allodynia]) as substantial pain, and interpret that pain as coming from a wider area than it is. These changes can be reversed, at least for a time, if the peripheral nociceptive input can be interrupted.

Central neuropathic pain syndromes (pain caused by dysfunction of somatosensory pathways in the CNS) can result from any CNS lesion, but these syndromes most commonly occur after stroke, result from spinal cord injury, or are associated with a multiple sclerosis demyelinating plaque. To be considered central neuropathic pain, the pain must occur in the area clinically affected by the CNS lesion; however, it does not need to involve the entire affected area. Central neuropathic pain develops only if the spinothalamic tract (pinprick, temperature sensation) malfunctions. If pinprick and temperature sensation are normal in the area of pain suspected to be central neuropathic pain, another pain source should be considered. The cause of pain in neurologically impaired patients is more commonly musculoskeletal (eg. shoulder pain related to arm paresis after a stroke or an upper extremity overuse syndrome in wheelchair-bound patients with a spinal cord injury).

Deafferentation pain is due to partial or complete interruption of peripheral or central afferent neural activity. Examples are

• Postherpetic neuralgia

• Central pain (pain after CNS injury)

• Phantom limb pain (pain felt in the region of an amputated body part)

Mechanisms are unknown but may involve sensitization of central neurons, with lower activation thresholds and expansion of receptive fields.

Neuropathic pain syndromes are sometimes associated with overactivity of the sympathetic nervous system. The sympathetic overactivity does not cause neuropathic pain, but it can contribute to its clinical features and severity. The pain that results is called sympathetically maintained pain, which depends on efferent sympathetic activity. Complex regional pain syndrome sometimes involves sympathetically maintained pain. Other types of neuropathic pain may have a sympathetically maintained component. What triggers sympathetic overactivity in some neuropathic pain states and not others is unknown. Mechanisms probably involve abnormal sympathetic-somatic nerve connections (ephapses), local inflammatory changes, and changes in the spinal cord.

Dysesthesias (spontaneous or evoked burning pain, often with a superimposed lancinating component) are typical, but pain may also be deep and aching. Other sensations—eg, hyperesthesia, hyperalgesia, allodynia (pain due to a nonnoxious stimulus), and hyperpathia (particularly unpleasant, exaggerated pain response)—may also occur.

Patients may be reluctant to move the painful part of their body, resulting in muscle atrophy, joint ankylosis, bone loss, and limited movement.

Symptoms are long-lasting, typically persisting after resolution of the primary cause (if one was present) because the CNS has been sensitized and remodeled.

• Clinical evaluation

Neuropathic pain is suggested by its typical symptoms when nerve injury is known or suspected. The cause (eg, amputation, diabetes, compression) may be readily apparent. If not, the diagnosis is often assumed based on the description of the symptoms; however, those descriptors (eg, burning) are neither sensitive nor specific for neuropathic pain. Thus, additional evaluation, including neurologic examination and electrophysiologic studies, are useful to confirm the diagnosis and to identify the injured nerve. Pain that is ameliorated by sympathetic nerve block is sympathetically maintained pain.

• Multimodal therapy (eg, physical methods, antidepressants, antiseizure drugs, psychotherapeutic methods, neuromodulation, sometimes surgery)

Successful neuropathic pain management starts with confirming the correct diagnosis and managing treatable causes (eg, herniated disk, carpal tunnel syndrome). In addition to drugs, mobilization and physical therapy are needed to desensitize areas of allodynia and prevent trophic changes, disuse atrophy, and joint ankylosis. Psychologic factors must be considered from the start of treatment. Anxiety and depression must be treated appropriately. If pain persists, neural blockade may help. When dysfunction does not respond to first-line treatments, patients may benefit from the comprehensive approach provided by a pain clinic.

Neuromodulation (spinal cord or peripheral nerve stimulation) is particularly effective for neuropathic pain.

Several classes of drugs are effective (see table Drugs for Neuropathic Pain), but complete relief is unlikely, and setting realistic expectations is important. The goal of pharmacologic management is to lessen neuropathic pain so that it is less debilitating.

Opioid analgesics can provide some relief but are generally less effective than for acute nociceptive pain and are associated with risk of dependence; adverse effects may prevent adequate analgesia.

Adjuvant analgesics, such as antidepressants and antiseizure drugs, are most commonly used to treat neuropathic pain, and their efficacy is supported by randomized trial data (1; see table Drugs for Neuropathic Pain).

is one of the most widely used drugs for such purposes. For effective analgesia, the dose should usually be > 600 mg orally 3 times a day, and many patients need a higher dose. Maximum dosage is usually considered to be 1200 mg orally 3 times a day.

For tricyclic antidepressantsserotonin and norepinephrine

is a mixed mechanism (serotonin and norepinephrine) reuptake inhibitor, which appears to be effective for diabetic neuropathic pain, fibromyalgia, chronic musculoskeletal pain (including low back pain), and chemotherapy-induced neuropathy. Doses that are efficacious for depression and anxiety and for pain management are similar.

Other potentially effective treatments include

• Spinal cord stimulation by an electrode placed epidurally for certain types of neuropathic pain (eg, chronic leg pain after spine surgery)

• Electrodes implanted along peripheral nerves and ganglia for certain chronic neuralgias (peripheral nerve stimulation)

• Sympathetic blockade, which is usually ineffective, except for some patients with complex regional pain syndrome

• Neural blockade or ablation (radiofrequency ablation, cryoablation, chemoneurolysis)

• Transcutaneous electrical nerve stimulation (TENS)