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CMT - A Disease of the Peripheral Nervous System

Linda here - We talk so much about the peripheral nervous system that I thought it might be good to know more about these often discussed but vastly misunderstood pathways of Schwann cells and myelin that keep our body vital. I asked occupational therapist, Susan Salzberg, OTR/L in North Carolina to answer some questions for us and thank her for the many hours she put into her beautifully written replies. Also, thanks to physiatrist, Dr. Greg Carter of Washington, for reviewing this.

Linda: Would you briefly explain the various nervous systems of the body and what they do.
Susan: When I think of CMT as a PERIPHERAL NEUROPATHY, I like to think of the nervous system as being divided into two parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). In its simplest form, this division is based on anatomy: the CNS is what is inside the brain and spinal cord. The PNS is everything else. Thus, the nerves that give us movement and feeling in our limbs are all in the PNS. In reality, it is not this simple, but this is a basis to start with.

From your high school biology class, you may remember that the nervous system has several other parts. These are not mutually exclusive. The Autonomic Nervous System (ANS) helps keep the body going on an even keel. It is the regulator of body functions, including digestion, body temperature and blood pressure. The fight or flight response, a reaction to stress, is controlled by the Autonomic Nervous System. Blushing, heart palpitations, clammy hands and dry mouth are possible emotional reactions related to this system. The Central Nervous System controls parts of the ANS and the Peripheral Nervous System controls parts.

Neurology books will often talk about Sensory Systems. These include the "five senses" plus balance: visual, olfactory (smell), gustatory (taste), auditory (hearing), tactile (sense of touch) and vestibular (balance). Again, parts of these systems are controlled centrally and parts are controlled peripherally. Nerves may also be referred to as sensory nerves or motor nerves. Sensory nerves are peripheral nerves that carry information from the outside world into your brain. Motor nerves activate muscles. For example, if we touch something hot, a sensory nerve will run up our arm to our brain. The brain will say "ouch, get me out of here" and will send a message down a motor nerve to the muscle that bends the arm... all in a matter of milliseconds!

There are other ways of dividing up the nervous system based on development and physiology, but I think these are the most familiar and most helpful in thinking about CMT.

Linda: Most of us know that CMT first affects the peripheral nervous system. Can you tell us exactly what the PNS covers.
Susan: The simplest way to think of the Peripheral Nervous System (PNS) is that it is whatever is outside the brain and spinal cord. The "best known" and most common symptoms of CMT are probably loss of sensation and weakness in the legs. If you think of the spine as the geographic center of the body, the first and worst symptoms are in the feet... the most peripheral (distant) part of the body if measured from the spine. In my experience this is true for all types of CMT. If our only symptoms were weak hands and legs with perhaps some resulting deformities, CMT would be very straight forward and much easier to manage.

But the nervous system and the body are not simple and straight forward. As an example, the cranial nerves can be called the peripheral nerves of the brain. They are numbered by Roman Numerals I-XII and have very specialized functions which I will list briefly:
I. Olfactory - smell. This is an unmyelinated nerve.
II. Optic - vision. The retina is the peripheral portion of this nerve.
III. Occulomotor - raises the eyelid, controls 4 of 6 muscles that move the eyeball, constricts the pupil, focuses the lens.
IV. Trochlear - moves eyeball down and out.
V. Trigemina - sensation to face, nose, mouth, forehead and top of head - chewing muscles.
VI. Abducens - moves eyeball to side.
VII. Facial - taste (front 2/3 of tongue), tears, facial muscles of expression.
VIII. Auditory/Vestibular - hearing and equilibrium.
IX. Glossopharyngeal - taste(back 1/3 of tongue), throat sensations, swallowing, salivation and carotid sinus (monitors blood pressure and pH of the blood and connects with cardiovascular and respiratory center in the brain stem).
X. Vagus - sensations from throat, larynx, heart, trachea, lungs and stomach, vocal cord muscles, some swallowing muscles.
XI. Spinal Accessory Nerve - a motor nerve with some fibers that join the Vagus - Jaw muscles and trapezius muscle.
XII. Hypoglossal Nerve - a motor nerve to the tongue muscles.

Linda: Is there any evidence that the central nervous system can be affected?
Susan: My response to this would be "No" although there is so much more we need to learn about CMT and the workings of our nervous systems.

CMT I is caused by deterioration of the myelin sheath. This is the coating or "insulation" on the peripheral nerves and is formed by Schwann cells. The nerves in the central nervous system also have a myelin coating on them, but it is formed by oligodendrocytes. Multiple sclerosis (a completely different problem from CMT) is caused by demyelination in the central nervous system. The myelin sheath on the cranial nerves is made by Schwann cells. Therefore, even though the cranial nerves are in the brain, they are really peripheral nerves. How sneaky!

But life isn't simple. In the axonal form of CMT the myelin is OK, there are just fewer nerve fibers. I don't think we know why there are fewer fibers. Could there also be fewer nerve fibers in the CNS? The other possibility is that a person with CMT may also have a separate central nervous system problem. It is an equal opportunity world for stroke, brain tumor or brain injury!

Linda: Some of our readers experience hearing and eyesight problems. What system is that?
Susan: These both may be cranial nerve problems.

Hearing loss is well documented in CMT. This is due to involvement of the auditory division of cranial nerve VIII. The nerve impulse is impaired when going from the hair cells of the inner ear to the portion of the brain that interprets sound, thus reducing our ability to hear. Not everyone with CMT has this problem.

In my experience, problems involving the visual system are not as well documented in CMT, but I have no doubt they exist. Cranial nerves II, III, IV and VI have to do with vision and eye muscles. During pregnancy, when my CMT got worse, I experienced cross-eyes. This cleared up after my baby was born. I think that cranial nerve VI was involved.

Linda: What about facial atrophy and facial tics. What's happening there?
Susan: This may also be a cranial nerve problem. The likely culprits would be nerves VII or V. With VII, there may also be loss of taste in the anterior 2/3s of the tongue (sweet, salt). Nerve V also provides sensation to the face, sinuses and jaw area.

I am not an expert on tics but would assume with CMT that they are really fasciculations caused by muscle weakness.

Linda: What are fasciculations and what causes them?
Susan: Muscles that work well, and work as they should, have a good strong contraction when we use them.

If the muscle doesn't get a steady message from the nerve that supplies it, the muscle will not contract normally.

If the nerves which supply a specific muscle fire at different times rather than together, the muscle will give a weak, rhythmic twitch rather than a strong contraction. This is thought of as a sign of irritation or fatigue and can occasionally happens in healthy people.

So a fasciculation is an involuntary muscle twitch which can actually be seen under the skin. When I think of fasciculations, I think of a defective electrical wire sputtering and causing a lamp to flicker rather than glow with a steady light.

Linda: Some of us have burning feet and other parts of our body burn. What causes that?
Susan: In my experience, people with neuronal CMT are more likely to have uncomfortable sensations and people with damaged myelin are more likely to have numbness. Both problems interfere with our body's "warning system."

If we were walking on hot sand, we would expect our feet to burn. Irritation and discomfort are our body's warning system to make an avoidance reaction. Discomfort from false sensations can range from annoying to impossible to live with. These are called "paresthesias" or
"hyperesthesias" by the medical folks and are a very hot topic for people who deal with pain management. I am not sure what causes them. Controlling the pain seems to be a matter of trial and error.

Linda: We've heard of fainting spells and even little seizures. Can that be our CMT?
Susan: We all know that passing out cold or just feeling faint is caused by the brain not getting enough blood, usually because of insufficient blood pressure. What factors in people with CMT could contribute to this? Weak muscles in the limbs may not push blood back to the brain quickly enough in some instances. If cranial nerve IX is affected, the mechanisms that regulate blood pressure may not work well. People who experience episodes of faintness need to see their physician to make sure that there are no pressing medical problems that need to be addressed. If the fainting spells are related to CMT issues, there may be ways of managing them.

Seizures are more complicated. They last longer, are not relieved by lying down and usually involve incontinence. Seizures are thought of as short-circuits within the central nervous system.

Linda: Some of us also have problems swallowing, choking on food and water, even our own saliva.
Susan: Eating and drinking involve several cranial nerves (V, VII, IX & XII) - both sensory and motor components. It is obvious that we need our muscles to chew, move food around in our mouth with our tongue and swallow. We also need excellent sensation in our mouth and throat to make sure we swallow safely. Swallowing is a complex process that directs food from our mouth to our stomach while avoiding our lungs. If we have a subtle impairment and "swallow down the wrong tube," we will choke and gag. This protects our airway. Our lungs do not want to receive the food! If our impairment is more severe, we may aspirate food into our lungs. If this happens enough, we may develop aspiration pneumonia. In aspiration, pneumonia it is common for the lower lobes to be involved because the food sinks to the bottom of the lungs.

"Silent aspiration" can occur if sensation is impaired to the point that the trachea does not feel the food passing through. This is an important concept because the body's defense system (irritation) isn't working. There is no choking or coughing, but the food or liquid is sliding right down into the lungs.

Speech and Language Pathologists can often teach "safe swallow techniques" to people who are at risk for aspiration.

Linda here - A huge thanks to Susan. Now we all know more.