Is ALS always associated with shortness of breath?

A respiratory dysfunction has two fundamental effects: a reduction in oxygen supply (decreased inhalation) and an excessive buildup of carbon dioxide in the blood (decreased exhalation). The lack of oxygen is experienced as “shortness of breath” and can cause distressing symptoms.

In ALS, however, oxygen deficiency is not the primary concern. “Shortness of breath” and “air hunger” are therefore rarely present. Due to the different properties of oxygen and carbon dioxide in the blood, an ALS-related respiratory dysfunction first leads to an accumulation of carbon dioxide—even before a lack of oxygen develops. Excessive carbon dioxide buildup (hypercapnia) leads to fatigue and drowsiness, which—depending on the carbon dioxide concentration in the blood—can progress to a loss of consciousness (carbon dioxide narcosis).

The sequence of events—the buildup of carbon dioxide and the subsequent lack of oxygen in the blood—explains why people with ALS usually do not experience “shortness of breath.” Fatigue sets in first, which is caused by the buildup of carbon dioxide. Oxygen deprivation does not occur until there is severe respiratory dysfunction. Shortness of breath is not perceived, or is perceived only to a limited extent, because it is masked by the fatigue caused by carbon dioxide buildup.