ASTHMA, a disorder of respiration characterized by severe paroxysms of difficult breathing (dyspnoea), usually followed by a period of complete relief, with recurrence of the attacks at more or less frequent intervals. The term is often loosely em ployed in reference to states of embarrassed respiration, which are plainly due to permanent organic disease of the respiratory organs (see RESPIRATORY SYSTEM : Pathology).
The attacks occur quite suddenly ; in some patients at regular, in others at irregular, intervals. They are characterized by ex treme difficulty both in inspiration and expiration, but especially in the latter, the chest becoming distended and the diaphragm immobile. In the case of "pure," "idiopathic" or "nervous" asthma there is no fever or other sign of inflammation. But where the asthma is secondary to disease of some organ of the body, the symptoms will depend largely on that organ and the disease present. Such secondary forms may be bronchitis, car diac, renal, peptic or thymic.
The mode of onset differs greatly in different cases. In some the attack begins suddenly and without warning, but in others various sensations well known to the patient announce that an attack is imminent. The commonest warning is that of an intense desire for sleep, so overpowering that though the patient knows his only chance of warding off the attack is to keep awake he is utterly unable to fight against his drowsiness. Among other patients a condition of unwonted mental excitement presages the attack. The secondary forms of the disease may be ushered in by flatulence, constipation and loss of appetite, and a common symptom is a profuse diuresis, the urine being watery and nearly colourless. In the majority of instances the attack begins during the night, sometimes abruptly but often by degrees. A few hours after midnight he is aroused from sleep by a sense of diffi cult breathing. In some cases this is a slowly increasing con dition, not becoming acute for an hour or more. But in others the attack is so sudden, so severe, that the patient springs from his bed and makes his way at once to an open window, struggling for breath. The face is pale, anxious, and it may be livid. The veins of the forehead stand out, the eyes bulge and perspiration bedews the face. The head is fixed in position, and likewise the powerful muscles of the back to aid the attempt at respiration. The breath is whistling and wheezing, and if it becomes necessary for the patient to speak, the words are uttered with great diffi culty. The chest is almost motionless, and the respirations may become extraordinarily slowed. Inspiration is difficult as the chest is already over-distended, but expiration is an even greater struggle. The attack may last from an hour to several days, and between the attacks the patient is usually quite at ease. But notwithstanding the intensely distressing character of the attacks, asthma is not one of the diseases that shorten life, except by way of secondary changes it induces in the heart.
Though the causes of asthma are known to be diverse, recent investigation has shown that it depends essentially upon swelling of the mucous lining of the bronchioles and local outpouring of a mucous secretion. It is possible, too, that the muscular coats of the air-tubes are in a state of spastic contraction during an attack. Asthma is a special example of anaphylaxis (q.v.); the patient being hypersensitive to some protein or derived substance, exposure thereto by way of inspired air leads to anaphylactic phenomena predominating in the lungs. On subcutaneous inocu lation with minute quantities of infusions of various substances the occurrence with one of them of a local oedema will indicate the substance to which the asthmatic patient is hypersensitive and should avoid in order to be free from attacks.
See F. M. Rackemann, "Clin. Study of i so cases of bronchial Asthma," Arch. Int. Med. (1918) , xxii., p. 517 ; F. Coke, Asthma (1923) ; W. M. Duke, Allergy, Asthma, Hay Fever, etc. (1925) ; J. Adam, Asthma and its radical treatment (1926).