Each of your lungs has a main bronchus (large air passage); these combine to form the trachea or windpipe. In the lung, the bronchi divide repeatedly into smaller and smaller passages (ultimately being called "bronchioles"), eventually ending at the alveoli (the air sacs where oxygen is absorbed and carbon dioxide is given off by your blood). In bronchiolitis the bronchioles become inflamed, and the swelling of the mucous membranes that line them narrows the air space and makes it hard for air to move in and out of the lungs. Also, the viruses that usually cause bronchiolitis damage the mucous membranes, resulting in release of mucus (usually clear, thick, and very sticky); this mucus also gets in the way of air exchange.
Bronchiolitis is usually viral. The most common offender is respiratory syncytial virus (RSV). Other common viruses causing bronchiolitis include influenza (both type A and type B), the "parainfluenza" viruses (which are not actually related to influenza), and adenoviruses. Measles can also cause bronchiolitis, but measles bronchiolitis is very rare in the United States and other countries where most people have been vaccinated against rubeola (the measles virus).
The symptoms of bronchiolitis include wheezing, caused by narrowing of the bronchioles. This is similar to asthma, except that, since the narrowing is caused by swelling and mucus blockage, bronchodilators such as albuterol may not be as helpful as they are in an asthma exacerbation. In severe bronchiolitis we can also see retractions (sinking in of the skin between the ribs, above the breastbone or collarbones, or beneath the ribs); this reflects the partial vacuum formed in the chest when you try to breathe and air does not enter your lungs as quickly as usual. With really bad bronchiolitis you may not get quite as much oxygen as you should, and you may then need extra oxygen until the swelling subsides.
Some patients with bronchiolitis can be relieved with albuterol, but in the hospitaL we prefer using adrenaline mist (Vaponefrin®:), which not only dilates the bronchioles but also directly reduces the swelling and mucus leakage and is a more effective treatment. Unfortunately, adrenaline has more side effects (like rapid heartbeat) than albuterol, so we usually do not prescribe it for home use. In some cases we will prescribe albuterol for home use for a child with bronchiolitis, mainly because it has fewer side effects than adrenaline. Steroids are occasionally used to help relieve the inflammation as well, but are not as effective as they are with other airway problems like asthma. Humidity (like cool mists) and fluids can also help by making the mucus thinner and easier to cough up.
The inflammation of bronchiolitis can continue for days to weeks in some cases, with persistent wheezing that does not easily go away with Vaponefrin. However, as long as you do not need extra oxygen and are not working too hard to breathe, the wheezing is actually not that big a problem. I admit children with bronchiolitis to the hospital only if they are working very hard to breathe or if they need supplemental oxygen, and I usually discharge them once they have improved and do not need extra oxygen, even though they still are wheezing.