The cause is a partial obstruction of the bronchi and bronchioles, which is caused by contraction of their wall muscles. In contrast to bronchitis, in which there is constant wheeziness, attacks of asthma come and go, with wide variations in the degree of obstruction at different times. Although the condition cannot be cured, each attack can be relieved by treatment or, if untreated, usually ends naturally. Most asthma is triggered by allergic reactions to such things as pollen, cat or dog fur or house dust. More rarely, foodstuffs such as eggs or milk, or a flavouring, dye or preservative added to food may be the cause.
Some attacks occur for no apparent reason.
The main symptom is breathlessness accompanied by a feeling of painless tightness in the chest, along with wheezing, the amount of which varies greatly. Sometimes only a doctor's stethoscope can catch the sound of wheezing - or it may be loud enough to carry across a crowded room. Often, the sufferer finds that it helps to sit up straight, with the arms held stiff to support the chest. With increased breathlessness, breathing becomes increasingly shallow and fast, and wheezing sounds grow louder. In very severe attacks, the wheeze may lessen.