• Kenneth Raymond

CMT Respiratory Involvement: What It Is and What It Is Not

Updated: Oct 29

Clarifying the Misconceptions of CMT-Related Respiratory Impairment

Kenneth Raymond and Ashraf Elsayegh, MD, FCCP, FAASM






When the respiratory muscles are affected by CMT, the result is a very specific kind of respiratory impairment. This respiratory impairment, however, is shrouded in misconceptions and misunderstandings that often lead to poor treatment choices and therapeutic outcomes. Charcot-Marie-Tooth Association (CMTA) Advisory Board member and CMT pulmonology expert Ashraf Elsayegh, MD, FCCP, FAASM, Division of Pulmonary/Critical Care, Cedars Sinai Medical Center, Associate Clinical Professor of Medicine UCLA School of Medicine, explains that respiratory impairment is grouped into two basic categories: diseases of lung tissue (lung disease), and diseases affecting the chest cavity (thoracic cavity respiratory disease). CMT-related respiratory impairment is a respiratory disease of the thoracic cavity, whereas diseases such as COPD are diseases of lung tissue, the two are not related nor connected, and one does not cause the other. Understanding the fundamental differences between lung disease and thoracic cavity disease is key to achieving successful therapeutic outcomes.


Charcot Marie Tooth disease, or CMT for short, is an expansive and complex inheritable neuromuscular disease that can affect motor nerves, sensory nerves, and/or autonomic nerves, or any combination of these. The motor nerves are the peripheral nerves that control skeletal muscle function. The sensory nerves are the peripheral nerves that carry sensory signals (touch, temperature, etc.) from all parts of the body to the spinal cord. The autonomic nerves are the peripheral nerves that control automatic processes, such as heart rate, organ function, etc. Because CMT affects the nerves that control skeletal muscle function, CMT has the potential to affect every skeletal muscle group, including the respiratory muscles. And when CMT does, it causes muscle weakness.


The Basics


When CMT causes respiratory impairment, it is called CMT-Induced Neuromuscular Respiratory Muscle Weakness. This is a very specific type of respiratory impairment caused by weakened respiratory muscles. Respiratory muscles are used to expand and contract the chest cavity (thoracic cavity), which, in turn, facilitates breathing. When a CMTer’s respiratory muscles become weakened due to the neuromuscular disease affects of CMT, the ability to fully expand the chest cavity so that the lungs can completely inflate and fully fill with air becomes impaired. The result is an impairment of the ability to draw a full breath, and this leads to shortness-of-breath, or SOB for short. The medical term for this scenario is hypoinflation.


Hypoinflation is a condition in which the lungs don’t fully inflate when drawing a breath. CMTer’s who have CMT-induced neuromuscular respiratory muscle weakness have an impaired ability to fully inflate their lungs as the result of a weakening of the muscles whose job it is to fully expand the chest cavity when taking a breath. Dr. Elsayegh explains that this type of hypoinflation is the result of weakened respiratory muscles and is not the result of lung disease. This is an important distinction. When CMT causes respiratory symptoms, the root cause is weakened respiratory muscles. The cause is not with the lungs or with the airways. CMT is not a disease of the lungs and nor is CMT a disease of the airways.


Restrictive vs. Obstructive vs. Neuromuscular


A widely held misconception is that CMT can cause both restrictive lung disease and obstructive lung disease, and CMT-related respiratory impairment is itself a restrictive lung disease. CMTers have even been diagnosed by their doctor with “CMT-Related Restrictive Lung Disease.” Full disclosure: I have until authoring this article understood and described CMT-related respiratory impairment to be a restrictive lung disease. Full stop. CMT does not cause restrictive lung disease, per se (restrictive lung disease can occur as a consequence of the effects of neuromuscular respiratory muscle weakness), and CMT-induced neuromuscular respiratory muscle weakness is not a restrictive lung disease. As Dr. Elsayegh explains, CMT-induced neuromuscular respiratory muscle weakness is not a restrictive lung disease or an obstructive lung disease, and it is not any kind of lung disease. To fully understand this, we have to dive into the fundamental differences between each of these.


Restrictive lung disease is a disease of lung tissue in which the lungs cannot fully expand due to a stiffening or hardening of lung tissue. Examples of restrictive lung disease include sarcoidosis, pulmonary fibrosis, and lung disease that can occur as a consequence of scoliosis. Each of these restrict the lungs from fully inflating—a hardening of the lungs.


Obstructive lung disease, commonly referred to as Chronic Obstructive Pulmonary Disease, or COPD for short, is a disease of lung tissue and airways in which the lungs can’t fully empty on exhale. COPD is a blanket term for several lung diseases that are each an obstructive lung disease. Examples of obstructive lung disease are emphysema, chronic bronchitis, and asthma. Each of these either obstruct the airways inside the lungs and slow down the movement of air within the lungs or destroy the alveoli resulting in an inability to fully empty. This results in air becoming trapped in the lungs before taking the next breath—a condition called air trapping. COPD also results in hyperinflation, a condition in which the lungs become much larger in size.


Neuromuscular respiratory muscle weakness causes a type of respiratory impairment that is the result of weakened respiratory muscles due to the effects of a neuromuscular disease. The muscle weakness can progress enough to cause an impairment of the ability to fully expand the thoracic cavity causing the lungs to not fully inflate with each breath. This type of respiratory impairment is not caused by diseased or damaged lung tissue but is caused by weakened muscles. Examples of neuromuscular diseases that can cause neuromuscular respiratory muscle weakness are CMT, ALS (Lou Gehrig’s disease), and Myasthenia Gravis.


Is It CMT, or…?


Restrictive lung disease, obstructive lung disease, and CMT-induced neuromuscular respiratory muscle weakness each cause SOB. How each cause SOB is different from one another. Restrictive lung disease causes SOB by impairing the ability to fully inflate the lungs due to lung tissue losing its elasticity and expandability. Obstructive lung disease causes SOB by impairing the ability to fully empty the lungs on exhale due to the airways inside the lungs becoming obstructed (usually by mucus), due to inflammation and constriction of the airways, or due to loss of lung alveoli. CMT-induced neuromuscular respiratory muscle weakness causes SOB by impairing the ability of the chest cavity to fully expand, which then limits how much the lungs inflate with each breath. When a CMTer has respiratory symptoms, such as SOB, how can a pulmonologist know if it’s due to CMT or something else?


A Pulmonary Function Test, or PFT for short, is a test that pulmonologists use for measuring pulmonary function, just as the name suggests. Specifically, a PFT is used to measure how well the respiratory system is working. A PFT measures several different parameters. The data garnered from these parameters tell the pulmonologist the overall condition of the respiratory system. The data can show if there is respiratory impairment, the data can show the severity of any present respiratory impairment, and the data can show if any present respiratory impairment is restrictive or obstructive, The data can also indicate the presence of respiratory muscle weakness. In an oversimplification, on PFT’s, restrictive lung disease will show hypoinflation (the lungs not fully inflating) and obstructive lung disease will show air trapping (the lungs not fully emptying). Because of its associated hypoinflation, explains Dr. Elsayegh, CMT-induced neuromuscular respiratory muscle weakness will exhibit a restrictive lung disease pattern on PFT's. This likely is from where the confusion and misconceptions arise regarding the type of respiratory impairment CMT can cause.


CMT-induced neuromuscular respiratory muscle weakness being considered a restrictive lung disease likely has its roots in the restrictive lung disease pattern exhibited on PFT’s. This type of respiratory impairment exhibits a restrictive lung disease pattern, Dr. Elsayegh explains, because CMT-induced neuromuscular respiratory muscle weakness i