Why Respiratory Muscles Matter
As DMD progresses, the muscles that allow us to breathe (respiratory muscles) also begin to weaken. This results in Respiratory Function Decline, which leads to difficulties with breathing and coughing.
As time goes on, respiratory muscle weakness can result in serious health problems. However, there are ways to reduce the impact of Respiratory Function Decline in those with DMD, including breathing support and airway clearance.
Stages of DMD and Respiratory Health
DMD can affect respiratory health in various ways. At diagnosis (usually around age 5) most patients with DMD breathe normally and have normal respiratory function. In the early stages of worsening respiratory function, the effects can be subtle or even symptomless, but with time they become more pronounced.
Ambulatory (Able to walk)
As long as patients with DMD are able to walk, they are unlikely to have serious respiratory problems. Even so, at this stage, gaining a good understanding of respiratory health and getting their lung function measured is wise. This can then be used to better understand other tests you may need to take down the road. Read more about respiratory monitoring.
Non-ambulatory (Wheelchair users)
Once using a wheelchair full-time, the risk of respiratory issues increases and becomes a priority for patients and their carers. Early signs that the respiratory muscles are weakening – like headaches, fatigue, and restless sleep – aren’t always obvious. To learn more about those early signs, see respiratory symptoms.
With increasing age, the progressive weakening of the respiratory muscles affects the ability to cough effectively enough to clear the lungs and airway, leaving patients susceptible to infections. Read more about later respiratory symptoms.
About Respiratory Muscles
Muscles make breathing possible
It’s important to understand the role respiratory muscles play and how these are affected in patients with DMD.
The ability to breathe normally requires the coordinated movement of a collection of muscles that expand and contract the rib cage and chest wall, moving air in and out of the lungs.
The abdominal muscles
These muscles are also very important during heavy breathing and coughing. When the abdominal muscles contract, they push the diaphragm upwards against the lungs, which forces more air out.
The diaphragm plays an important role in breathing. It sits at the bottom of the rib cage and moves up and down. When the diaphragm moves down, it creates suction in the chest, which pulls air into the lungs. During relaxed breathing, the elasticity of the chest wall and lungs pushes air back out. This is aided by the diaphragm as it relaxes and moves back up, further compressing the lungs.
The intercostal muscles (internal and external)
These are muscles between the ribs. The external intercostal muscles further expand the chest cavity when breathing in, helping air into the lungs. In a healthy person, the internal intercostal muscles are only needed during heavy breathing, by providing extra force to push air out of the lungs.
Sitting in a wheelchair and abnormal curvature of the spine (also called scoliosis) can affect posture and shape of the chest cavity, which can make it harder to breathe. Read about the importance of respiratory monitoring and respiratory devices to improve breathing.
Complications and symptoms of Respiratory Function Decline
While Respiratory Function Decline can start earlier, once patients become wheelchair users or non-ambulatory, they are more likely to start to experience subtle respiratory symptoms.
As long as patients with DMD can walk, most of them won’t have serious respiratory concerns. Once a person with DMD becomes non-ambulatory, the signs of Respiratory Function Decline may start to become more obvious. It’s important to understand how even early respiratory concerns can lead to more serious respiratory problems in the future.
Early complications and symptoms
Symptoms of Respiratory Function Decline are often not apparent until several years after patients stop walking; however, the underlying decline in respiratory muscle function begins much earlier and gets progressively worse with age. Even when they do appear, the signs of Respiratory Function Decline may initially appear subtle and unconnected to the lungs.
Not feeling rested
Difficulty staying awake
Learn more about how respiratory function is measured.
Sleep disordered breathing
Disrupted sleep impacts the ability to think, how the body grows, and quality of life. Due to their weakened respiratory muscles, patients with DMD might be more susceptible to sleep problems. With awareness of the signs and symptoms of sleep disordered breathing, you can be on the lookout for night-time breathing difficulties.
The video below will provide more information about some problems with breathing at night patients with DMD might have.
This video has been developed in the USA by PPMD, and it is being shared with their permission.
Obstructive Sleep Apnoea (OSA)
OSA is a partial or complete blockage of the upper airway that occurs while a person sleeps. It’s usually caused by weakness of the upper airway muscles. OSA not only interrupts sleep, it can also cause a drop in oxygen in the blood (hypoxemia) or an elevation of carbon dioxide levels (hypercapnia). Why does this matter? Well, the lungs are tasked with providing oxygen to the body and getting rid of carbon dioxide, and want to keep oxygen high and carbon dioxide low because that helps all organs to function as well as possible.
Shallow breathing at night
Normally, when a person goes to sleep, the muscles used for breathing during the day relax and they begin to take shallower breaths. However, for those with DMD, this is complicated by having weakened breathing muscles, causing them to breathe even less effectively. They may not be able to breathe deeply enough to bring in all the oxygen needed or to remove all the carbon dioxide that the body produces. This is called hypoventilation. Hypoventilation causes patients with DMD to wake up during the night, preventing them from getting sustained, restful sleep. It also causes daytime fatigue and sleepiness.
How do you know if someone has sleep disordered breathing?
There are tests that can gauge the level of disordered breathing.
Read about the tests and what they measure in respiratory monitoring.
Later complications and symptoms
Eventually respiratory-related symptoms begin to appear. These symptoms can also be subtle – like not being able to cough hard enough – but may result in repeated episodes of serious chest infections that require antibiotics or hospitalisation. Find out more about respiratory complications.
Ineffective cough and respiratory infections
Coughing is the body’s way of clearing the airways of the lungs. When mucus and other secretions build up, coughing helps remove them. The stronger and more forceful the cough is, the more effective it is at keeping the airway clear of secretions. When the muscles weaken, the lungs can’t take in as much air as they should. Weakened muscles also can’t generate enough force to clear the airway. This can lead to blockages and repeated infections that can result in hospitalisation in patients with DMD because of their weakened muscles.
These infections, including mild upper-respiratory tract infections, may in turn worsen respiratory muscle fatigue leading to a vicious cycle of recurrent, ever-worsening chest infection and inflammation, ineffective cough and mucus retention, and, potentially, collapsed airways (atelectasis), low blood oxygen levels and, eventually, respiratory failure.
The vicious cycle of respiratory decline
As Respiratory Function Decline progresses, there is a resulting decrease in the ability to cough. Decreased ability to cough can lead to more infections, which – in turn – can cause a further decline in overall respiratory function (including worsening hypoventilation).
But there are ways you can manage a weak cough, like using cough assist devices or manual cough techniques. Read about cough assistance devices.
Daytime shallow breathing (hypoventilation)
Weakened breathing muscles mean the levels of oxygen and carbon dioxide in your blood start to shift in ways that aren’t good for the body. Initially this shift in levels happens at night. However, as the respiratory muscles continue to weaken, reduced levels of oxygen (caused by shallow breathing) and high levels of carbon dioxide (known as hypercapnia) show up during the day too. As hypoventilation progresses to a 24 hour burden, patients usually require increasing levels of assistance to help them breathe.