Bronchial Asthma

Bronchial asthma is an acute irritation of the airways that produces bronchial smooth muscle contraction, giving airway narrowing with some thickening of the walls by oedema and an inflammatory response. Mucus produced in the bronchi is more viscid than usual. Drying mucus may plug bronchi and cause small areas of atelectasis, presumably with neighbouring overinflation. Occasionally segmental collapse or larger may occur. On breathing out the airway has to remain open if air is not to be trapped in the retracting lung. Asthma increases the probability that the airway will be closed before the gas, potentially for exchange to atmosphere by breathing, has been evacuated. The disorder is initiated by inspiration of allergens and is mediated by immunoglobulin IGE in a type I inflammatory response, which releases histamine from neighbouring mast cells and leucotrienes.

The allergen has the theoretical potential to produce a later 'Type III' immune response, but this is more usually a feature of extrinsic allergic alveolitis than asthma, unless complicated by aspergillosis. Patients with allergies and asthma can be more vulnerable to opportunist organisms.

Clinicians don't need subtle radiology for diagnosis. The history and clinical findings are usually unambiguous, if the clinican is systematic. The radiologist is there to look for complications and to consider alternatives. In these days of retroviruses, odd infections will turn up at the most inconvenient times.

The air in the overinflated lungs can leak into the lung interstitium and is thought to leak near the junction of terminal bronchiole with the basic pulmonary unit and track back. The dilated long hilar lymphatics, when filled with air, give the subtle "tramline shadows" so beloved of Radiologists. This way the gas tracks to the hila and mediastinum. Gas in the soft-tissues of the neck is typical of this process. It is not found in the cases of pneumothorax unless the trauma, associated with the pneumothorax, has breached the parietal pleura. In one case, there is gas above the left lung and this is probably pleural, perhaps gas tracking in the outermost lymphatics to the pleural space. Such pleural accumulations of gas are uncommon.