This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
The attacks of dyspnoea which occur in spasmodic asthma in all probability depend upon spasmodic contraction of the unstriped muscular fibres in the bronchi. In some cases no definite cause can be assigned for the occurrence of these attacks, though a gouty tendency in the patient, or the imperfect elimination of waste products, as in renal diseases, increases the tendency to their occurrence. In other cases they appear to be occasioned by irritation, either in the mucous membrane of the respiratory tract or irritation of some other part of the body. Thus they appear sometimes to be brought on reflexly, by irritation of the nose by polypi, by certain odours, or the inhalation of irritating dust, especially pollen of grass, or by congestion of the mucous membrane in ordinary coryza. Sometimes irritation of the pharynx by enlarged tonsils appears to bring them on, and they frequently arise from bronchial catarrh. At other times they may occur in consequence of indigestion, constipation, of worms in the intestine, of disease of the uterus or ovaries, or of pregnancy.
1 Roy and Graham Brown, Journ of Phys. vol. vi.