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-Barr