Neuropathic pain is pain caused by nerves that aren't working as they should. Put simply, nerves are long thin cells that carry messages of body sensations to the brain, and commands for movement back to the body. The brain is made up of a complex network of nerves that interpret messages and communicate with each other, allowing us to think and feel. When nerves become damaged, messages can become disrupted or nerves can simply start firing on their own, creating false messages such as pain when there isn’t a problem. We call this neuropathic pain, pain that is caused by the nerves themselves malfunctioning.
In contrast, “nociceptive” pain is caused by an injury, physical pressure or inflammation in some part of the body. The nerves are working properly when they send pain messages to the brain telling you there is something wrong. Your nerves can also trigger a sub-conscious reflex movement that reduces damage, like pulling your hand back from a hot surface even before you realize you’ve burned your finger.
What does it feel like?
Neuropathic pain, pain caused by the nerves themselves, can be mild or severe, permanent or temporary. It can feel like burning, throbbing, electrical-like sensations, tingling “pins and needles”, increased sensitivity, itching or just numbness. Basically, whatever the nerve is supposed to do can either be blocked or “overdone”, depending on whether the damaged nerve stops transmitting signals or starts firing randomly because of damage or pressure. Sensations can be changed when sensory nerves are damaged or squeezed and, when motor nerves are affected, the ability to move can be blocked (for example, paralysis with a stroke) or uncontrolled (as when muscles twitch).
What causes it?
Neuropathies can often be hard to diagnose and treat, and there is a wide range of causes for damage to nerves. Some of these include:
Diabetes—High sugar in the blood causes damage to tiny nerves in the hand, feet and eyes, leading to numbness and loss of vision over time, especially when blood sugar levels are not well controlled.
Cancer and cancer treatments—A growing tumour can press on nearby nerves inhibiting their function or causing pain. Cancer drugs, with their toxicity spilling over onto our own cells, can cause damage to nerve cells too. Numb fingers are common with some chemo treatments, but these nerves usually repair themselves after treatments are finished when dosages are carefully monitored.
Neurological diseases—Some diseases cause symptoms by having an effect on nerves, like multiple sclerosis, a disease where the myelin “insulation” around nerves becomes damaged, resulting in nerves firing randomly or signals being unable to travel through the nerve.
Stroke—A stroke results from blood not reaching part of the brain, either because of a blood clot or because a blood vessel has burst. Without blood and the oxygen and nutrients it carries, nerve cells in the brain die and cease their function. Interestingly, sometimes other nerves can take over the function, allowing a significant amount of recovery. This is called “plasticity”… the ability of tissues to grow and change (somewhat like molding plastic!)
Shingles—After you have chicken pox, the herpes zoster virus doesn’t totally disappear. It stays dormant in nerve cells, waiting for its chance to come back as shingles, a nasty painful/itchy rash with fluid-filled bubbles on the surface of the skin. While it’s reactivated, the virus can cause damage to the nerve it’s growing in, sometimes resulting in ongoing pain signals being generated even after the virus infection is subdued and the rash is healed. This is known as “post herpetic neuralgia”, and can last weeks, months or even years. Treating the virus infection quickly with an anti-viral medication will reduce the nerve damage and chance of developing long-term problems. It is recommended to start treatment within 72 hours but the sooner, the better.
Phantom limb pain—It’s hard to imagine, but almost 80% of people with amputations can have pain or itching that their brain is telling them is occurring in the limb that is no longer there. These sensations are likely initiated in a nerve that has been cut or damaged in the surgery sending signals that the brain interprets as coming from the absent limb. It must be so frustrating to have an itch that you can’t scratch!
Nutrient deficiencies—Lack of a nutrient essential for nerve health, for example vitamin B12, can also be a cause of malfunction of peripheral nerves, those in the extremities. Interestingly, metformin, a commonly used medication for diabetes, can reduce absorption of B12, leading to a deficiency in significant numbers of patients, and increasing risk of numbness in the hands and feet that is often blamed on the diabetes itself.
And there are many other diseases that can cause misfiring of nerves. I occasionally get a burning sensation on the outside of one foot—it took me quite a while to figure out it was a compression neuropathy, caused by a muscle spasm in my calf pressing on the nerve to my foot. I get quick relief from this foot pain by massaging my calf. Who would have thought! Unfortunately, in some people the source and exact location of the problem are never identified.
Focal neuropathies are those focused in one spot of a single nerve, making them easier to treat once diagnosed. Peripheral neuropathies, for example diabetic neuropathy, can be widespread since the cause (high sugar in the blood) originates in the blood that circulates throughout the body.
Treatment
Of course, addressing the cause (like taking an anti-viral for shingles, keeping diabetes under control, or using massage as I do for my foot pain) is an ideal treatment if this is possible. Otherwise, using medication that modifies how nerves fire, like anticonvulsants, antidepressants or drugs that block certain pain receptors within the body, can be helpful to reduce discomfort. There are also drugs that can activate calming pain-blocking pathways, like the GABA pathway and alpha adrenergic pathway in the body.
Commonly used neuropathic pain medications include anticonvulsants like gabapentin/pregabalin, and carbamazepine (Tegretol), and tricyclic antidepressants like amitriptyline or desipramine. Magnesium supplements (especially if the diet is lacking in magnesium) are sometimes helpful, as well as substance P blockers like capsaicin (a non-prescription cream) or loperamide (an antidiarrheal medication that can also be made into a cream by a pharmacist). In difficult to treat neuropathies, pain receptor blocking drugs like ketamine (yes, the date-rape drug!) may be tried.
As a compounding pharmacist, I have often made some of these into a penetrating gel when the location of the damage was known and this avoids side effects commonly noted when taking it by mouth. Because the medication is delivered directly to where the problem is, much less is needed and side effects like drowsiness and stomach upset are essentially eliminated. This allows us to combine several drugs that work to block pain in different ways into one product. One favourite that often works well is a combination of ketamine (an NMDA blocker), gabapentin (an anticonvulsant AMPA blocker) and clonidine (an alpha-adrenergic blocker).
Sometimes the pain and sensitivity were so great, the person couldn’t tolerate rubbing a pain cream onto the skin. In these cases, we would supply a lidocaine spray in a water base (avoiding alcohol that could cause painful coolness as it evaporated) to numb the skin before using the pain gel.
It has been very gratifying, as a compounding pharmacist, to be able to help people who were not getting pain relief from the standard medication. It was at a pain conference that I learned how muscle spasms, where muscles become thickened and shortened, could cause pain in a different part of the body by pressing on nerves or pulling on tendons that attach muscles to bone. This results in pain being sensed in a different location than the origin of the problem, making it more difficult to diagnose the source.
If you suffer from neuropathic pain in spite of treatment with standard medication, I highly recommend asking a local compounding pharmacist trained in pain management to work with your doctor. Together the 3 of you can work to find alternative solutions to calm your pain.
References:
Types and causes of neuropathic pain—Medical News Today
Algorithm for Chronic Neuropathy—Professional Compounding Centers of America (PCCA)
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