Decoding the Myth and Laying it to Rest
The differences between what the two words represent help us to know which side of the argument is the accurate side.
The looming question inherent in the argument is exceedingly easy to answer. Is CMT a type or form of Muscular Dystrophy, does CMT fall under the umbrella of MD, and is CMT classified as MD? The clear-cut easy answer is no, CMT is not any of these things, categorically. Well, then, what is CMT?
CMT is many things. CMT is an inheritable peripheral neuropathy. CMT is a genetic neuromuscular disease. CMT is a peripheral polyneuropathy. CMT is a disease of the peripheral nervous system that exerts its effects on the muscles those nerves control. There are many different descriptors and many different acronyms that explain what CMT is. For everything that CMT is, CMT is not a muscle disease. How do we know this though?
CMT is a disease of the peripheral nerves. Every cause of CMT is a mutation in a gene that codes a molecular process in the peripheral nerves. These mutations cause a disruption in the molecular process that the host gene controls in the peripheral nerves. This is the oversimplified way that CMT directly affects the nerves that control the muscles but does not directly affect the muscles themselves.
For all the plethora of causes of CMT, none are a genetic mutation that disrupts a molecular process in muscle tissue. The muscles suffer because the nerves that control them are diseased, but not because the muscles are directly diseased. It’s quite an easy premise, yet so inherently complex. It doesn’t have to be quite so complex though.
The word use that explains a component of the disease process of CMT helps to sort out how CMT is not a muscular dystrophy disease. Likewise, word use that describes a component of muscular dystrophy diseases helps to sort out how and why CMT is not any type of muscular dystrophy disease.
The Root of The Matter
CMT causes muscle atrophy. It’s a hallmark of CMT in especially the feet, lower legs, and hands. What is atrophy? Atrophy is muscle wasting, right? That’s easy. Atrophy has a specific medical definition though, just as does dystrophy. Atrophy and dystrophy both describe wasting, but each have very distinct medical definitions, and they are not interchangeable with one another.
Atrophy describes tissue wasting that is the result of a process that is separate from that tissue. The muscle wasting in CMT, by definition, is atrophy because the wasting occurs as a result of a disease process in something that is not muscle tissue. Specifically, in CMT, that disease process is in the peripheral nerves, but the atrophy is in a different tissue than the peripheral nerves—skeletal muscle. Hence, the wasting is atrophy. There are three types of muscle atrophy, each with a different root cause.
Physiologic atrophy is atrophy caused by not using the muscles enough. This can usually be reversed by proper diet and exercise. Pathologic atrophy is usually seen with aging and malnutrition/starvation. Neurogenic atrophy is muscle atrophy that occurs with injury to, or disease of, the nerve that controls the atrophied muscle.
CMT causes neurogenic atrophy. The neurogenic atrophy is the result of the disease process within the peripheral nerves, but not because of a disease process within the muscle tissue. Neurogenic atrophy is the most severe of the three types of atrophy because muscle tissue lost to neurogenic atrophy is not recoverable. Often, CMTers can become sedentary as a result of disease progression. This brings about physiologic atrophy on top of the neurogenic atrophy that CMT already causes, thereby making matters worse for the CMTer. Physiologic atrophy can be overcome though, and even prevented via healthy diet and adequate proper exercise, even when neurogenic atrophy cannot be undone. Dystrophy, however, is different than atrophy.
Like atrophy, dystrophy has a specific medical definition. Dystrophy describes tissue wasting that is the direct result of a disease process within the tissue that is wasting. With muscle dystrophy, or muscular dystrophy as it is often called, as opposed to muscle atrophy, the muscles themselves are directly diseased, and the muscle wasting is the direct result of the muscle tissue being diseased. In muscular dystrophy diseases, by definition, the muscle wasting is dystrophy because the muscle tissue itself is diseased, and that results in the muscle tissue wasting. Hence, the origin of the name itself, muscular dystrophy: skeletal muscle tissue wasting as a result of a disease process within the muscle tissue, and not from a disease process in a tissue that is outside of or separate from the muscle tissue.
The Etymology Made Easy
The root word of atrophy and dystrophy is trophy. The word “trophy” originates from the Greek word for food, “trophe.” Adding the negative prefix, “a,” gives us “atrophe,” meaning “lack of food,” thus giving rise to “atrophy:” wasting away due to lack of food. In CMT, skeletal muscle wastes away because the disease process in the peripheral nerves prevents the skeletal muscles from receiving proper signals that are required for the muscles to maintain adequate nutritional balance. In this context, the nerve signals are the food, and the nutritional balance is the amount of adequate healthy nerve signals the muscles need in order to properly function. The disease process in the nerves cause the muscles to waste. The wasting, by medical definition, is therefore atrophy.
Building off of trophe, the prefix “dys” is the medical prefix for bad or difficult, as in “dysfunction,” meaning bad function, or as in “dyspnea,” meaning difficult breathing. Added to trophe, “dystrophe” means bad food or difficult nutrition, thereby giving rise to “dystrophy:” wasting away due to a dysfunction in the nutritional process. In muscular dystrophy, the disease process within the muscle causes the muscle tissue to waste from a dysfunction in the ability to process what the muscle tissue needs to maintain nutritional balance. In this context, the nerve signals are the food that need to be processed, and the mechanisms by which the muscle tissue processes this food in order to maintain its nutritional balance of adequate and healthy nerve signals is dysfunctional. Due to this contextual nutritional dysfunction within the muscle tissue, the muscle tissue wastes away. This type of wasting, by medical definition, is therefore dystrophy.
From Where Does the CMT is MD Arise?
The root of the misconception that CMT is a type of MD stems from the MDA, an organization, including CMT as a disease that it provides research funding and patient services for. The notion that this then qualifies or classifies CMT as muscular dystrophy is incorrect. The MDA clarifies this within the structure of how they categorize and classify the many diseases they “cover.” It may come as a surprise, too, that muscular dystrophy diseases represent only about 1/6th of the diseases that the MDA provides research funding and patient services for.
Muscular Dystrophy as a disease is a group of eight diseases according to the Muscular Dystrophy Association. The MDA’s description of the Muscular Dystrophy disease classification reads, “The muscular dystrophies are a group of diseases that cause weakness and degeneration of the skeletal muscles.”
The MDA classification in its entirety lists eight diseases as muscular dystrophies. These are Becker MD, the Congenital Muscular Dystrophies (CMD) as a single entry (Bethlem CMD, Fukuyama CMD, Muscle-Eye-Brain disease (MEBs), Rigid Spine Syndromes, Ullrich CMD, and Walker-Warburg syndromes (WWS)), Duchenne MD, Emery Dreifuss MD [not to be confused with Emery Dreifuss Syndrome – a connective tissue disorder], Fascioscapulohumeral MD, Limb-Girdle MD, Myotonic dystrophy (DM), and Oculopharyngeal MD. This is the exhaustive list of every muscular dystrophy the MDA includes. In an email, the MDA clarified for this discussion that there are only these eight muscular dystrophy diseases, and CMT is not one of the eight (emails provided in the Appendix).
Beyond these eight muscular dystrophies in the MDA’s muscular dystrophy disease classification, are forty-three non-muscular dystrophy diseases that are organized into six separate disease type categories. I’m not going to list all forty-three diseases that are not muscular dystrophy diseases “covered” by the MDA. The six non-muscular dystrophy disease categories, however, are Motor Neuron Diseases, Ion Channel Diseases, Mitochondrial Diseases, Myopathies, Neuromuscular Junction Diseases, and finally Peripheral Nerve Diseases where we find CMT and GAN [GAN is a type of axonal CMT].
The MDA description of their classification of peripheral nerve diseases reads, “In peripheral nerve diseases, the motor and sensory nerves that connect the brain and spinal cord to the rest of the body are affected, causing impaired sensations, movement or other functions.” This is the MDA’s description of the category they place CMT into – a disease that affects the peripheral nerves. The MDA’s CMT webpage explains that CMT is a spectrum of nerve disorders, and the page makes no mention of CMT being a muscle disease nor a type of muscular dystrophy disease. The MDA themselves explain that CMT is not a muscular dystrophy disease.
It is said that CMT “falls under the umbrella of MD.” I prefer not to use the term, “umbrella.” The use not only fosters confusion, but the statement itself has a fundamental inaccuracy. MD is an acronym that represents a group of diseases, and nothing more. This group of diseases is not a group that CMT belongs to. Therefore, the statement and all that it implies is inaccurate. It would be a more accurate statement to say that CMT “falls under the umbrella of the MDA.” It would be more accurate because the MDA, as a non-profit organization, includes CMT as one of the forty-three non-muscular dystrophy diseases that it provides research funding and patient services for, or “covers under its umbrella.” There is, however, a distinct and fundamental difference between MD—a group of muscle tissue diseases, and MDA—a non-profit organization.
CMT causes muscle atrophy, but CMT does not cause muscle dystrophy. At its very core, the muscular dystrophy diseases are so called because the disease causes muscle dystrophy. Becker Muscular Dystrophy, for example, is a disease of muscle tissue, and the disease causes muscle dystrophy. The same holds true for each of the other seven muscular dystrophy diseases. The muscle wasting is medically defined as dystrophy because the tissue that is wasting is diseased. In contrast, CMT is a disease of the peripheral nerves that control the muscles. Because the muscle wasting in CMT is not caused by the muscle tissue itself being diseased, the wasting is medically defined as atrophy. By this very premise and definition, CMT does not meet the medical criteria to be a muscular dystrophy disease, and is therefore not a type of MD. If CMT is not MD, why does the MDA include it?
The MDA was founded in 1950 by Paul Cohen, who had muscular dystrophy disease. The MDA explains that the organization was originally founded to focus on the eight types of muscular dystrophy diseases. Over the years, however, their mission has expanded to cover an additional forty-three non-muscular dystrophy neuromuscular diseases. CMT is one of these forty-three. CMT was included because CMT is a neuromuscular disease – neuro meaning nerve, and muscular meaning muscle, with the two combined defining a disease of the nerves that affects muscles—but not because CMT is a muscular dystrophy disease. I contacted the MDA for comment on whether CMT was a form of MD, and their public relations office responded with, “No Sir, CMT is not a form of muscular dystrophy,” and they explained that, “CMT is a peripheral neuropathy, which is a different disease than muscular dystrophy, both muscular dystrophy and CMT are types of neuromuscular disease, but they are not the same thing.”
In Closing
In the end, not only because of everything discussed here, but also because of many more criteria, CMT is not a type or form of any muscular dystrophy disease. Yes, the MDA is an organization who provides research funding and patient services for CMT. No, CMT does not fall “under the umbrella of MD.” MD and MDA are two completely different acronyms that are not interchangeable with one another. The MDA including diseases within their funding program does not make those diseases a muscular dystrophy disease, and this is especially true for CMT.
