- Asthma is characterised by a triad of airway inflammation, reversible airway obstruction, and hyper-reactivity of the airways to a variety of stimuli.
- Functionally the disease is characterised by wide variations, over short periods of time, in resistance to air flow in intrapulmonary airways. This increase in airway resistance is reversible, particularly with bronchodilators.
- Intrinsic asthma – no definite precipitating cause. Young and middleage onset.
- Extrinsic or atopic asthma – immediate type 1 hypersensitivity reaction to specific antigen or allergen (e.g. pollens or chemicals). Removal of the stimulus produces a clinical improvement.
- Attacks of breathlessness, chest tightness and wheeze. Often severe and requiring ventilatory support.
- Occasionally, mild symptoms such as a persistent cough predominate.
- The patient is hypoxic with low pO2 and usually a low pCO2.A high pCO2 is a sinister sign heralding severe fatigue and respiratory failure.
- There is reduced forced vital capacity (FVC) and forced expiratoryvolume in 1 s (FEV1). These improve following the administration of bronchodilators.
- The residual lung volume and total lung capacity are increased due to air trapping.
Fig: Asthma. The lungs are hyperinflated; there is bilateral diaphragmatic flattening; 7 anterior and 11 posterior ribs are visible; lungs are otherwise clear.
- Early in the disease the radiology may be entirely normal. Chronic asthma is associated with a number of distinct radiographic changes.
- CXR – lungs are hyperinflated with flattened hemidiaphragms of limited excursion. There is bronchial wall thickening (>1 mm). This is a more marked finding in children and in adults with infection. There is also hilar enlargement due to a combination of lymphadenopathy and pulmonary hypertension.
- CT may confirm thickened bronchi, but also areas of mosaic perfusion on ‘lung windows’. These represent variable alternate areas of air trapping set against normally perfused and aerated lung.
- Always look for complications of asthma:
- Pneumothorax or rarely pneumomediastinum.
- Consolidation secondary to pulmonary infection.
- Mucus plugging and subsequent lobar or segmental lung collapse.
- In 2% allergic bronchopulmonary aspergillosis (ABPA) – dilated central bronchiectasis, with mucus plugging, associated with eosinophilia.
- The radiological features are similar to those of cystic fibrosis.
- However the clinical mimics of asthma are important to differentiate. They can often be diagnosed with imaging and should be considered, particularly in atypical cases.
- Inhaled foreign body (particularly in children). Look for compatible history and asymmetric air trapping. CT imaging or bronchoscopy should be considered.
- Left heart failure. Look carefully for early interstitial pulmonary oedema.
- Tracheobronchial stenosis – tracheal narrowing will be visible on the CXR.
- Hypersensitivity pneumonitis – look for ground glass change and centrilobular nodules on CT.
- Post infectious bronchiolitis – air trapping on HRCT.
- Bronchodilators – inhaled and nebulised (e.g. salbutamol).
- Inhaled steroids _ oral/IV steroids.
- Early consideration for ventilatory support.
- Maintenance inhalers and good technique to prevent attacks.
- Andrew P., Mangerira C. and Rakesh R.A-Z Chest Radiology, pp.42-45