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Polyneuropathy: A Disease of the Longest Nerve-Fibers

The peripheral nerves are bundles containing many individual nerve-fibers, and are similar to telephone cables carrying many individual wires. There are two basic types of nerve-fibers–motor and sensory. The motor fibers carry electrical impulses outward from the spinal cord to the muscles, causing them to contract. The sensory fibers carry electrical impulses inward from the skin, joints and other structures to the spinal cord, providing the nervous system with inputs, among others, concerning the senses of touch, pain and temperature.

Peripheral nerves can be pinched or injured in specific locations. When this occurs, the problem is called a “mononeuropathy,” meaning that a single peripheral nerve is affected. Examples of mononeuropathy include carpal tunnel syndrome in which the median nerve is pinched at the wrist, and peroneal neuropathy in which the peroneal nerve is injured near the knee. Because the median and peroneal nerves contain both motor and sensory fibers, people with these conditions can experience both weakness and numbness.

In carpal tunnel syndrome, certain muscles of the thumb can become weak, while numbness affects the thumb, index finger, middle finger and part of the ring finger–but not the little finger. In peroneal neuropathy muscles that lift the front and outer edges of the foot can become weak, while numbness affects the outer surface of the calf and the top of the foot–but not its bottom. In cases of mononeuropathy only the structures connected to that one nerve’s fibers are affected.

In contrast, “polyneuropathy” produces a pattern of weakness and numbness completely different from that seen in mononeuropathies. Instead of affecting the fibers of just a single peripheral nerve, polyneuropathy simultaneously impacts fibers traveling in numerous peripheral nerves.

In usual cases of polyneuropathy it is the longest nerve-fibers that are most at risk, while the shorter nerve-fibers are less affected. In brief, polyneuropathy is a “length-dependent” neuropathy. Because the longest nerve-fibers in the body are those that run from the lower back to the feet, in typical cases of polyneuropathy the first part of the body to become weak or numb is the feet.

In polyneuropathy muscles ordinarily served by more than one peripheral nerve can become weak, and the numbness extends beyond the territory of any single nerve. If a person with polyneuropathy pulled on stockings, he or she could cover the parts of the legs affected by weakness and numbness. Thus, the weakness and numbness affecting the legs are described as showing a “stocking” pattern of loss.

When the medical condition responsible for the polyneuropathy causes worsening damage to the peripheral nerves, the stockings climb ever higher as the next-longest nerve-fibers become involved. By the time a person’s stockings climb as high the knees, he or she might also notice symptoms in the fingers. This is because the nerve-fibers running from the neck to the fingers are about as long as those running from the lower back to the knees.

If a person with polyneuropathy affecting the hands and arms pulled on gloves, he or she could cover the parts of the arms affected by weakness and numbness. Thus, the weakness and numbness affecting the arms are described as showing a “glove” pattern of loss, and when legs and arms are simultaneously impacted, it is called a “stocking-glove” pattern.

Medical doctors are usually able to detect polyneuropathy from patients’ histories of symptoms and their physical examinations, but tests of muscle and nerve electricity–called electromyography and nerve conduction studies–are often helpful in characterizing the extent and pattern of nerve impairment.

Polyneuropathy is more of a category of nerve impairment than a final diagnosis, and numerous diseases can produce the same end-result of stocking-glove loss.

Diabetes is the most common cause of polyneuropathy in both the U.S. and the rest of the world. Blood sugars are elevated in people with diabetes, but the extent of polyneuropathy is not strictly related to how bad the blood sugars are, or for how long they have been elevated. For example, one person with severe, long-term elevations of blood sugars might have very little polyneuropathy, while another person might have polyneuropathy as the very first symptom of their diabetes. At present there is no good treatment for the polyneuropathy of diabetes apart from best-achievable control of blood sugars, but when annoying sensations like burning or tingling are present, these can be managed with topical or oral medications.

Ingestion of toxic chemicals can also produce polyneuropathy, and alcohol is the chemical most frequently involved. And while people with heavy and prolonged use of alcohol are more likely than light drinkers to develop this complication, here, too, some people seem more susceptible to this problem than others. Abstinence can keep the polyneuropathy from worsening, but the already damaged nerve-fibers might not fully recover. Because people with alcoholic polyneuropathy often lack sufficient quantities of thiamine, a vitamin important to the nerves, supplementing well-rounded, nutritious meals with this vitamin is usually helpful.

Inherited polyneuropathy can be transmitted in families in either a dominant or recessive form. In families with dominant transmission a bad gene from just one parent is sufficient to produce the disease in a child. In families with recessive transmission defective genes from both parents are required in order to produce the disease.

As a final illustration of the range of disease processes that can cause polyneuropathy let’s consider Guillain-Barré (pronounced GEE-on bah-RAY) syndrome, also known by the more cumbersome term of acute inflammatory demyelinating polyradiculoneuropathy. In contrast to the diabetic, alcoholic and genetic forms of polyneuropathy that typically worsen at a pace measured in months or years, Guillain-Barré develops in a matter of days. The affected patient usually notices weakness about the ankles, followed rapidly by weakness about the knees, hips, arms and even of the muscles controlling breathing. Symptoms usually peak within two weeks during which time a patient should be monitored in a hospital in case a ventilator is needed to support breathing. Subsequent recovery of strength occurs over a course of weeks to months.

Guillain-Barré syndrome involves inflammation of nerves and nerve-roots (spinal cord connections) caused by an overactive immune system. This is a so-called auto-immune disease in which a person’s immune system attacks a tissue within their own bodies, in this case the nerves. Certain treatments that temporarily suppress the action of the immune system have been shown by randomized, controlled trials–the gold-standard of medical proof–to improve outcome in this condition.

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