As a medical student I was taught that beta-agonists such as salbutamol don't tend to work under the age of one. That is only partly true. Beta-agonists don't work for bronchiolitis and most children under 12 months with wheeze have bronchiolitis. However when the problem is a viral induced wheeze the inhaled Beta agonist is exactly what is needed even if they child is below a year old.
Bronchiolitis and viral induced wheeze have a similar presentation but very different mechanisms which is why one responds and the other doesn't. Bronchiolitis is essentially a problem of wetness. The viral lower respiratory tract infection causes the airways to be constricted by the accumulation of secretions. With viral induced wheeze, the infection has induced bronchospasm.
In both cases there will be a cough, coryza, wheeze and possibly some respiratory distress. Only one needs inhalers or nebulisers to be given. So how do I tell them apart?
The easiest way is to look at the child's age. There seems to be a fairly good split between the typical age groups of the two conditions. Bronchiolitis tends to affect those under 12 months old while viral induced wheeze tend to be seen in the over 12 month olds. Using that as a cut off will leave you being correct a lot of the time. Of course there are exceptions and they will usually be those children a few weeks or a couple of months either side of that cut off.
So for those who are well before their first birthday you can assume they have bronchiolitis. If the child has already started to outgrow the clothes that they were given for their first birthday, you can assume they have a viral induced wheeze. But what about the ones who are too close to call?
One thing that helps is the prodrome. Typically, children with bronchiolitis have a few days of being snotty before the cough develops. Then there is a daily worsening of cough followed by feeding difficulties. By day 3 of the cough there may be fast breathing and an audible wheeze.
With viral induced wheeze there is a variable length of coryzal illness from a day to a week. What is noticeably different is the onset of the wheeze and respiratory distress. This will usually happen over the space of hours, not days.
The other factor that helps is the severity of the symptoms. Consider our 11 month old with wheeze: If bronchiolitis affects children from birth to about 15 months of age, the most severely affected will be the littlest babies. Wet lungs when you are a few weeks old is no walk in the park. So by the time you are 11 months old, bronchiolitis is less likely to be severe.
Why not just try inhalers with all wheezers? The answer is that it is possible that this might make the child worse.
If an infant has bronchiolitis, they fight the good fight against wet lungs. They succeed against the odds since they are less able too feed and use more energy in the effort of breathing. What we must do as clinicians is avoid making this worse. Don't give unnecessary antibiotics that will fill and irritate their stomachs. Don't send them home with ineffective inhalers which will result in a routine of upsetting and tiring out the child.
Conversely, make sure that children with viral induced wheeze get enough beta-agonist. This might be quite a lot (the salbutamol paradox).
So, because it is an important distinction, use age, then prodrome and then severity in that order to decide if it is bronchiolitis or viral induced wheeze.