Spinal muscular atrophy (SMA) is characterized by its hallmark progressive motor function decline. But breathing difficulties also are common in the more severe types of the disease.
The leading genetic cause of death in infants and toddlers, SMA is divided into five main types: 0, 1, 2, 3, and 4. These types are based on the age of symptom onset and the highest motor milestone achieved, with type 0 as the disease’s most severe form, with onset before birth. Type 4 is its mildest form, with adult-onset.
SMA is caused by the progressive loss of the specialized nerve cells that control voluntary movement, which leads to muscle weakness and atrophy.
But progressive weakness of the muscles supporting the chest wall — called the intercostal muscles — also can cause severe breathing and coughing difficulties. These issues increase the risk of a carbon dioxide buildup in the lugs, and of lung infections.
In children with SMA types 0, 1, and 2, breathing problems can be especially severe.
Indeed, respiratory problems are the main cause of illness and the most common cause of death among children with these three disease types.
Breathing issues in these patients are due to the severe weakness of the intercostal muscles, which leave the diaphragm as the main breathing muscle. The diaphragm is the thin muscle that sits at the base of the chest and separates it from the abdomen.
The impairments in SMA may lead to lung underdevelopment, reduced lung function, and difficulties in coughing and clearing lung secretions, which increase the risk of hypoventilation and infection. Hypoventilation is characterized by low oxygen and a carbon dioxide buildup due to too shallow or too slow breathing.
Aspiration pneumonia, a lung infection that develops due to food or liquid getting into the airway, is particularly common among these patients due to throat muscle weakness and subsequent poor coordination of the swallow with airway closure.
Breathing issues and hypoventilation are usually worse at night, clinicians note. In a lying or supine position, most muscles relax and the abdominal contents push up against the diaphragm, making it harder for patients to take deep breaths. This may lead to sleep problems, such as obstructive sleep apnea, and to daytime sleepiness and headaches.
Most patients with the milder forms of SMA — types 3 and 4 — may never experience breathing problems.
Patients with breathing difficulties may need temporary or permanent ventilatory support. In more severe cases, this can include tracheostomy, a surgical procedure in which an opening is created in the windpipe for mechanical ventilation.
It is important that patients and their families learn and practice techniques that help with coughing and airway clearance, so as to prevent breathing problems and minimize the impact of lung infections.
Last updated: Nov. 3, 2021
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