Of all the muscles in the body, the breathing muscles are the only ones (other than the heart) that are absolutely necessary for survival. Neuromuscular diseases such as muscular dystrophy, ALS, spinal cord injury, and many others affect the respiratory system. In fact, one of the most common reasons for illness and hospitalization for people with neuromuscular disease is pneumonia and breathing problems. The reason for this is that the breathing muscles – including the diaphragm, chest muscles, and abdominal muscles – can be affected in patients with neuromuscular disease to the point where breathing, coughing, and swallowing safely without aspiration may become a problem. Because the stakes are high, it is important for you to be prepared to get the best care possible by asking good questions and expecting your health care team to monitor your breathing system at each visit.
There are three functions of the breathing system that need to be discussed and monitored at each of your clinic visits with your pulmonary, neurology, rehabilitation medicine, or other care providers. The three functions are: 1) ventilating or getting air into and out of the lungs, 2) coughing and clearing the lungs of secretions, and 3) protecting the lungs from aspiration during swallowing. I will describe each of the three parts of the breathing system in detail and provide you with questions and tests that you should ask about.
Ventilating means moving air into and out of the lungs and getting oxygen from the air into your body and carbon dioxide, a waste product of your body, out and into the air. The muscles that are required for these functions include mainly the diaphragm but also some of the chest muscles. When these muscles are not functioning adequately, carbon dioxide can build up in your body and later, as weakness progresses, oxygen levels can drop. The first signs of not ventilating well (“hypoventilation”) often occur during sleep. As the rest of the body relaxes during sleep, the weakened diaphragm has to take on a bigger breathing workload. If the body cannot maintain the workload during sleep, carbon dioxide levels will build in the body. This can lead to symptoms such as waking up with headaches in the morning, more frequent awakenings at night, feeling “groggy” or sleepy during the day, or even finding it difficult to concentrate. Your healthcare team should ask you about these symptoms. If hypoventilation progresses to the daytime, you may notice shortness of breath while you are awake. Tests that measure your ability to ventilate include a vital capacity measurement (spirometry), maximal inspiratory pressure measurement, carbon dioxide measurement (arterial blood gas, end-tidal or transcutaneous), and possibly a sleep study if symptoms are present.
Cough is one of the most important protective mechanisms to keep bacteria and foreign materials out of the lung. In the case of lung infection, this can be a lifesaving response by your body to get bacteria and secretions up and out of the lung. Having an adequate cough depends on proper function of both the throat muscles and the abdominal muscles (and to a minor degree the chest muscles). Symptoms of having an inadequate cough include being unable to clear secretions (the inability to get the secretion up and out of the lungs) as well as a weak sounding cough. The best measure of cough is to measure a peak cough flow which can be measured in clinic with an asthma peak flow meter. A low value (less than 270 liters per minute) may be an indication for cough support which can be done manually or with machines that can assist cough.
Proper swallowing is also critical to maintaining health of the lungs. Swallowing dysfunction from weak throat muscles can put you at risk of aspiration because the food tube (esophagus) and the windpipe (trachea) are so close together. With a weak swallow, liquid or food particles can enter the lung and increase the likelihood of pneumonia, especially if the cough muscles are also weak. Your clinician should ask about choking episodes, coughing episodes with eating and drinking, and your ability to swallow the saliva that your body normally produces. Testing for swallowing dysfunction (which is often specialized) is best done by a speech and swallowing specialist.