Pain Is a Warning
To understand neuropathic pain, it is necessary to understand “normal” pain, that is, what pain is and how it functions in normal circumstances. Pain is a warning. Its unpleasantness gets one’s attention in order for one to learn what can injure or damage the physical body. It is of significant evolutionary importance.
All pain is perceived in the brain; before that, it is just a signal from a point of injury. At the brain, this signal is used to locate where the pain is, how severe the injury is, and how painfully it should be perceived. Nociceptors are the nerve pain receptors that set off these signals when provoked by trauma, burns, inflammation, or infection.
From the nociceptors, the signal travels up that area’s nerve to the spinal cord, and where it enters is met by some altering, competitive signals. One of these is the brain’s inhibitory stimulus. (Once warned by pain, the brain makes an individual functional by dampening this pain so that he or she can get away from a source of injury.) Another alteration at the site is a connecting nerve called an “interneuron,” which also diminishes the pain as the signal crosses to the other side of the spine to rise up through spinal nerve tracts to the brain, the whole circuit giving feedback between the pain stimulation and its inhibition.
As an example, hitting one’s thumb results in an immediate pain signal (from an evolutionary standpoint, to stop hitting one’s thumb). Immediately after, the pain is not quite as severe. This is not because the thumb is any less injured; it is because of the dampening processes in action.
When Acute Pain Becomes Chronic Pain
If repeated signals continue to bombard the spinal areas where the pain is dampened, this system can become overwhelmed and recruit other previously inactive pain receptors in the spine. As this happens, the inhibitory interneuron changes into an “amplification” interneuron and pain signals continue to be generated and sent toward the brain at a more powerful level, even after the original source of the pain has long healed. Pain gets worse. Thus, the most common cause of chronic pain is inadequately treated acute pain.
When pain is due to nerve nociceptors firing off their pain signals by themselves, this is what is called neuropathic pain. Neuropathic pain will present as what the nociceptors involved in pain generation were designed to do–initiate sensations of itching, tingling, stabbing, aching, or devastating pain, but instead of initiating these sensations due to some injury, they have begun initiating them without provocation.
The Psychological Toll of Neuropathic Pain
Acute pain causes anger (the example of hitting the thumb with a hammer). Chronic pain is associated with depression. Overactive areas of the brain, as with neuropathic pain, “recruit” other areas from emotional centers, which is why a third of those with chronic pain have depression. Depression makes the perception of pain worse, and worse pain makes the depression worse. It becomes a vicious cycle that can only be stopped when both pain and its psychological effects are treated together.
How Is Neuropathic Pain Diagnosed?
It is impossible to prove someone is in pain, because pain is a subjective perception. The pain doctor is obligated to believe his or her patient, however, and to devise strategies to manage it. It is more helpful, however, for the doctor to understand what is going on in a patient’s pain syndrome than to label it with a diagnosis.
The Patient History
A history and physical exam are the initial diagnostic tools. The doctor will learn when the pain began, where it’s located now, and its progression, nature, and severity. It can be used to note changes due to pain, such as in skin, hair, nails, skin color, sweating, temperature changes, and muscle spasm. The pain itself requires a specific list of questions whose answers are helpful in understanding the pain:
- Frequency of pain: is it intermittent or continuous?
- Where is the pain located and does it radiate to other areas?
- What is the quality of the pain–burning, tingling, shooting, crushing, stabbing, etc.?
- What makes it better? What makes it worse?
- How has it changed life at home, in a marriage or relationship, in employment?
- Are there any medical conditions that make neuropathic pain more likely, such as diabetes? Herpetic outbreaks–simplex or zoster (shingles)? Previous stroke or closed head injury?
- How has it affected the patient besides its discomfort–depression, anger, or hopeless?
The Analog Pain Score
Pain is known as the 5th vital sign, after temperature, pulse rate, respiratory rate, and blood pressure. A patient will be asked to assign a number to pain severity, 0 being absent and 10 being the worst conceivable pain. This is helpful in determining the urgency of management and as an interval score in judging the success of any treatment.
The Physical Exam
The physical exam incorporates a neurological assessment that attempts to find a cause for the neuropathic pain. Since most neuropathic pain is of a mysterious origin, physical findings are often unhelpful. An evaluation of normal nerve function is limited to a check of reflexes and perception of pinpricks or other tactile testing devices. A previous stroke may only become known by the neurological findings sought when evaluating a patient for neurologic pain. Any damage to the brain can alter pain perception, the dampening process, or the psychological impact to pain’s perception.
Muscle spasm is a frequent condition associated with neuropathic pain, as patients will overtax muscles used to splint away from the painful area, overwhelming their capacity. This causes muscles to become acidotic which results in painful cramping. Occasionally the muscle spasm is so painful that the patient seeks help for it instead of the neuropathic pain that has caused it.
Occasionally, the path of a painful nerve tract can demonstrate small lumps of inflammatory nerve along its path, called trigger points. These are especially sensitive but can be used to advantage by injecting them with anesthetic or just in breaking them up by the needle alone.
Blood tests are indicated with neuropathic pain to identify any underlying conditions that make it more likely, such as diabetes (fasting glucose or hemoglobin A1c level) and rheumatic arthritis (RA factor–rheumatoid factor, C-reactive protein–CRP, erythrocyte sedimentation rate–ESR).
Drug testing is required when patients are on chronic pain medications. Drug diversion is a serious problem resulting in an overdose crisis, and pain doctors will screen their patients to assure the medication prescribed is being taken and that no unprescribed medications or illicit drugs (cocaine, methamphetamines) are being taken. This is not for strictly legal reasons, but for legitimate care of those in whom addiction and dependence may have influenced their behavior and put them in danger.
X-rays, ultrasound, CT, and MRI are used to focus on a limb with chronic neuropathic pain, but are also used to identify any structural conditions (nerve compression) or masses (benign and malignant tumors) that may be causing chronic pain.
- Nerve conduction studies can determine if the signal is traveling through the nerve normally, whether it is delayed, or whether the signal is weak or amplified.
- Electromyography is used to evaluate the electrical activity of muscle fibers innervated by nerves to determine whether the pain is primarily a nerve disease or a muscular disease. It is especially helpful in assessing muscle spasm.
How Can I Manage Neuropathic Pain?
Neuropathic pain is a complicated cross-traffic of nerve and muscle pathology, social dysfunction, mood alterations, and depression. When a patient fails to get relief, hopelessness amplifies the perception of pain. The treatment for neuropathic/chronic pain requires a multidisciplinary approach that includes medication, physical therapy, behavioral and psychological intervention, and surgical therapies that can include neurostimulators and implanted pumps for medication delivery into the spinal fluid.
Of all of these treatment modalities, the main three are medication, behavioral/psychological therapy, and physical therapy.
The line between chronic pain and neuropathic pain is often blurred, since neuropathic pain is a type of chronic pain. Opioid medication and other analgesics, a standard treatment for severe, unremitting pain, is often used in treating neuropathic pain, too. Anticonvulsants affect the nerve transmission of pain; anti-anxiety agents and antidepressants also have a role; and antispasmodics help when muscle spasm is involved. Since strong medication has an increasing potential for side effects, complications, and abuse, the usual protocol calls for a medication “pyramid” with multiple tiers of drugs to try, from the safest to the ones requiring the most scrutiny.
The types of drugs used in neuropathic pain include nonopioid analgesics (acetaminophen, NSAID), anticonvulsants (gabapentin, pregabalin), antidepressants, antispasmodics, and even medicinal marijuana.
Neuropathic pain, when initially treated with antidepressants, use the ones which inhibit the reuptake (keep levels higher) of neurotransmitters known to affect pain–serotonin and norepinephrine. (These are neurotransmitters found in the inhibitory pathway from the brain.)
Biofeedback and cognitive-behavioral therapy and psychological therapy are useful for both the physical pain and the psychological impact that augments it.
The goal of pain management is not to make one pain-free, as this is seldom accomplishable. Instead, the goal is to make a patient functional, either in spite of the pain or by mitigating its severity. One of the best approaches to restoring functionality is via physical therapy, which not only uses active and passive motion to reduce pain, but trains the patient how to alter lifestyle to keep the improvements in place.
Transcutaneous electrical nerve stimulators (TENS units) can send impulses to the spine to block the painful impulses. Spinal neurostimulation using implantable electrodes can do the same but target the painful area more specifically. Deep brain stimulation and other novel approaches are being investigated.
Besides implantable stimulators or pain pumps, interventional methods include injections of steroids and/or local anesthetic into the problem sites. Nerve ablation can effectively stop the pain transmission but can run the risk of affecting other nerve functions when the nerve has a mixed purpose of both sensory and motor roles.
Neuropathic Pain Prevention
Since the most common cause of chronic pain, including neuropathic pain, is primarily due to neglected, ignored, or inadequately treated acute pain, prevention is accomplished by treating acute pain before it can become chronic. Therefore, the prevention of neuropathic pain is by aggressively treating acute syndromes that result from trauma and injury, burns, chemical exposure, fractures, nerve compression, and other painful sources.
Acute nerve compression can be relieved surgically to prevent further pain. Medication can mitigate acute pain’s severity so that the spinal inhibitory pathways don’t become overwhelmed, setting up the dominoes that fall to result in chronic pain.
Any acute inflammation associated with injury, while serving as part of the healing process, can continue past its welcome and usefulness to result in neuropathic pain, so any inflammatory conditions that last beyond what is expected in an injury or infection should be cooled down with non-steroidal anti-inflammatories or corticosteroids.
Some complications of injury result in pain syndromes that are hard to explain or treat, such as complex regional pain syndrome (CRPS), which causes expanding areas of pain, motor dysfunction, and other manifestations. Early mobilization (physical therapy) after a limb injury may prevent CRPS. Nerve blocks may pause its progression, and behavioral therapy is useful to limit its impact or longevity.
Treatment of hopelessness is important in neuropathic pain, since a chronic pain syndrome can jeopardize relationships and threaten employment and income. The pain experience is consuming, but adding divorce and other estrangements and stopping the flow of income and support will severely impact anyone’s coping skills negatively, making the vicious cycle of pain-depression-pain spiral down more viciously. Because of the codependent nature of pain and the changes in the brain ic creates, treating neuropathic pain physically and the social and financial deterioration it causes, psychologically, at the same time, is the best way to prevent worsening of an already-challenging situation.