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Breathing in CMD


One of the primary complications that inhibits the quality of life in people with CMD is the compromise of breathing function, the inability to perform an effective cough, and the retention of carbon dioxide without appropriate breathing support. Signs of respiratory decline often occur first during sleep, in the form of hypoventilation (slower, shallower breathing), when respiratory effort is at its lowest, gradually spilling over into wakefulness with greater frequency of symptoms while awake.

Breathing? But this is a muscle condition...

People with CMD have problems with ventilation because:

  1. Expansion of the chest is limited by weakness in the diaphragm and chest wall muscles, contractures (tightness) in the joints between the ribs and vertebral bodies, and scoliosis (abnormal curvature of the spine);

  2. These limitations reduce the amount of air that can be inhaled in each breath;

  3. The decreased amount of air inhaled and exhaled causes carbon dioxide levels to buildup;

  4. A weak, ineffective cough can result in mucus build up, leading to frequent respiratory infections.

The Basics of Breathing

When we inhale, we oxygenate; the oxygen moves into the lungs and is absorbed into the circulatory system. When we exhale, we ventilate; carbon dioxide is transported out of the blood, into the lungs, and out the body. Proper ventilation requires enough muscle strength to expand the lungs during inhalation. Depending on how weak the affected individual is, they may or may not be able to take in a large enough breath, which means they have a smaller amount of air to breathe out. These smaller breaths are not enough to expel the carbon dioxide, causing CO2 to slowly build up over time.

Another often overlooked issue is that adequate airflow is extremely important in driving airway mucus up to the back of the throat to be expelled through a cough. However, with shallow breathing, there is not enough expiratory airflow, therefore retaining airway mucus.  Finally, shallower breathing leads to reduction of the size of alveolar units – the smallest units of the lung where gases (oxygen and carbon dioxide) cross over into and out of circulation, respectively. Collectively, across many lung regions, this will lead to reduced total lung volumes. For a variety of reasons, this makes breathing less efficient, and individuals begin to breathe shallower and faster, making for undue calorie expenditure. When lung volumes reach certain lower thresholds, lung units may begin to collapse (called atelectasis).

Key Point: People with CMD do not usually have a problem with oxygenation; they are more likely to experience hypercapnia (elevated carbon dioxide levels) and retained mucus, leading to an increased frequency of respiratory illness and infection.

Hypoxemia & Hypercapnia


Hypo = low
Hyper = high


The act of breathing allows 2 processes to occur. Breathing in facilitates oxygenation, while breathing out facilitates ventilation (getting rid of stale air). Imbalances in these two major functions lead to the development of low oxygen levels (hypoxemia) and/or elevated carbon dioxide levels (hypercapnia).


Hypoxemia is usually due to problems with the lung tissue, such as pneumonia or asthma, or as a result of mucous plugging where lung secretions cannot be cleared (atelectasis). These issues affect the actual lung airways or lung tissue and prevent inhaled oxygen from getting through the lung tissue into the blood stream. People with CMD do not usually have a problem with oxygenation unless the issues described here are also present. However, they are more likely to experience hypercapnia, which, if severe enough, can lead to associated hypoxemia.


Night Time Ventilation


People with CMD often first develop problems with ventilation during sleep. We all have slightly shallower breaths and slower breathing rates while sleeping, called hypoventilation. Our oxygen levels also slightly decrease, in part, because effort reduces, lung volumes reduce in the recumbent position, and tissues in the back of the throat relax and may somewhat obstruct breathing. In an unaffected individual, these changes go largely unnoticed, and are well-tolerated. However, in people with CMD, these changes can significantly impact ventilation during sleep.

In the supine position (lying down), chest wall expansion and diaphragmatic movement is more difficult as compared to sitting or standing upright. As a result, shallower breathing occurs, which if left untreated, causes carbon dioxide levels to rise. Eventually, oxygen levels may also fall.

Key Point: Hypoventilation is treated not with oxygen, but with ventilatory support (ventilator or BiPAP), to increase the amount of air volume exchanged during breathing.


Signs of Hypoventilation


There are a number of symptoms associated with the onset of breathing problems in people with CMD, including:


  • Difficulty sleeping – feeling “breathless” while lying down

  • Non-refreshing (non-restorative) overnight sleep

  • Morning headaches

  • Daytime Fatigue

  • Difficulty Concentrating

  • Loss of Appetite

  • Altered mental status


Any of these symptoms in a person with CMD may indicate the immediate need for assisted ventilation at night and even during the day. However, with proactive pulmonary care, an affected individual should never reach the point where they are experiencing these symptoms as a result of insufficient respiratory function.


A vital function associated with the loss of respiratory function is the ability to cough and effectively clear airway secretions.

The lining of our airways produces small amounts of mucous that trap dirt and bacteria – all of these are normally expelled during a cough. A cough begins with a deep breath in; the glottis closes (i.e. the vocal cords completely come together to create a seal), allowing pressure to build up in the lungs as the muscle of the chest and abdomen begin to contract. The respiratory muscles continue to contract and the glottis opens suddenly, forcing air out of the lungs/airways in a quick burst.

For affected individuals without the ability to effectively cough, there is an increased chance of respiratory infection or mucus plugging and collapse of lung segments. People with CMD often have a weak cough due to the same issues that cause breathing problems (weak muscles in the chest wall and diaphragm, contractures between the ribs, scoliosis).

Surveillance and Intervention


Surveillance of pulmonary function is critically important to catch early signs of breathing issues, and includes:

Pulmonary Function Testing (PFT)

Pulmonary Function Tests measure the efficiency of the respiratory system. Key measures in CMD-affected individuals are Forced Vital Capacity (FVC) and Peak Cough Flow (PCF/CPF), and respiratory muscle strength tests. FVC measures the amount of air expelled during a forceful exhalation after filling the lungs with a much air as possible. CPF measures the strength of an individual’s cough. Respiratory muscle strength tests measure the maximal inspiratory or expiratory pressures that an individual can generate. These measures provide useful information, particularly when initiated early in life, and tracked over time, to determine the course of progression and anticipating future complications.

Pulmonary Function Testing should be performed in both the sitting and supine positions. Diaphragmatic involvement will be indicated in the change in FVC between the two positions. This test should be repeated at least annually to evaluate the rate of respiratory decline and intervene with additional testing and the appropriate treatments. Pulmonary Function Testing should begin by age 5, even if no signs of breathing dysfunction are obvious.

Based on PFT results and overall decline in function, a polysomnogram (sleep study) may be ordered to evaluate breathing function while sleeping, and whether there is a need for non-invasive ventilation.


Polysomnography (Sleep Study)


Because most symptoms of breathing dysfunction occur initially while sleeping, it is important to assess for signs of hypoventilation. Sleep studies measure oxygen and carbon dioxide levels during sleep, and evaluate if there are any periods of apnea (pauses in breathing). These may indicate the need for ventilatory support through the use of BiPAP or a ventilator while sleeping. This test should be repeated annually to evaluate the rate of respiratory decline and to adjust the settings of the BiPAP or ventilator.


If Pulmonary Function Testing and Polysomnography indicate there may have been long term untreated breathing issues, an Echocardiogram should be ordered to ensure there is no evidence of right-side heart strain as a result of the prolonged respiratory insufficiency.

​Cough Assist Machine

Regular use of a Cough Assist Machine or mechanical insufflation-exsufflation device will help expel mucus when a CMD-affected individual cannot effectively do soon their own. As the name suggests, a Cough Assist machine enhances or augments a natural cough. The machine delivers a large volume of air (positive pressure) to inflate the lungs. After achieving this, the machine reverses flow rapidly and briefly to  simulate the expiratory flow of a cough. While machine settings can be finely adjusted for comfort, they should be determined by a skilled respiratory therapist or physician. Daily use of Cough Assist can help maintain clear airways and reduce the chance of respiratory infection, and can also offer a way to stretch chest muscles to help maintain compliance and breathing function. Many CMD-affected children and adults who use Cough Assist daily, report that breathing feels easier following treatment, and studies have shown that regular use may maintain and even sometimes improve breathing function.


When someone with CMD develops a respiratory illness or infection, their ability to expel mucus may be compromised, and their chances for developing atelectasis (collapsed lung tissue) or pneumonia are increased. If one's oxygen saturation level is below 95%, it is important in these instances to deliver positive air pressure via a cough assist machine or a ventilator/BiPAP machine to increase the volume of air delivered to the lungs. The instinct is often to deliver oxygen, but oxygen alone cannot overcome this issue. The goal is to enhance the depth of breathing, (the amount of air inhaled with each breath), and while oxygen saturation is a good indicator of respiratory distress, simply delivering oxygen is not the answer. A small device called a pulse oximeter can be purchased at your local pharmacy or online store without a prescription for under $25. It is important for everyone affected with CMD to own a pulse oximeter so that oxygen saturation, particularly when ill with a respiratory virus or infection, can be monitored, and steps taken to increase ventilatory support and airway clearance.

Video: How to Get Through Flu Season

It is imperative that individuals affected with any neuromuscular disorder, as well as those they come into frequent contact with, get an annual flu shot. The injection version that contains a dead virus rather than the mist version that contains a live virus is recommended for both the affected individual and their families/care givers. The flu shot can take several weeks to take effect so it is important to obtain your flu shot as early in the season as possible.

Cure CMD would like to thank Dr. Oscar H. Mayer from Children's Hospital Philadelphia and Dr. Hemant Sawnani from Cincinnati Children’s Hospital Medical Center for their guidance and tireless support of the CMD community.

Additional References

For a full list of recent peer-reviewed publications on breathing in neuromuscular conditions,
please also visit our publication library on this topic.

Respiratory Function in Neuromuscular Conditions:


Daytime Ventilation:


Cough Assist & Hyperinflation Therapy


Surgeries and Hospitalizations

Basics of Breathing
Night Time Ventilation
Signs of Hypoventilation
Pulm Guide
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