Answers

Short answers

1 The chest radiograph shows a triangular shadow of soft tissue density behind the heart in the left lower zone. It also shows a loss of clarity in the adjacent descending thoracic aorta and the left hemidiaphragm in its medial portion, with depression of the left hilum. These features are consistent with left lower lobe collapse.

2 In a patient of this age with known asthma, the most likely cause is mucous plugging and bronchiole obstruction. In older patients or smokers, the most common cause of lobar collapse is a stenosing lesion secondary to bronchogenic carcinoma.

3 The patient should be admitted acutely and treated empirically with supplemental oxygen, bronchodilators, and steroids in accordance with national asthma guidelines. Physiotherapy should be used to dislodge the mucous plug. After treatment, repeat the radiograph to confirm reinflation. If the lung does not re-expand, the patient must be referred for urgent bronchoscopy.

Long answers
1 Radiological abnormality
Each lobe collapses in a characteristic way, and this depends on which airway is obstructed. This phenomenon was first described in 1973 by Benjamin Felson, a professor of radiology in the United States. In the case of proximal occlusion of the left lower lobe, the lobe collapses posteriorly and medially towards the spine. Lying behind the heart, it assumes a triangular shape with a straight lateral border, classically described as a "sail sign" on posteroanterior chest radiography (fig 2).

View larger version (160K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 2 Posteroanterior chest radiograph showing the straight lateral border of a triangular soft tissue mass behind the left heart (arrows). This forms the "sail sign" of left lower lobe collapse

 
It usually overlies the cardiac shadow and can easily be missed on poorly windowed or under-penetrated films. The collapsed lobe obscures the left hemidiaphragm, offering a subtle silhouette sign, and the horizontal fissure swings downwards, with the hilar displaced inferiorly.¹ The silhouette sign, described by Benjamin Felson, is based on the premise that an area of intrathoracic radiopacity that is in anatomical contact with a border of the heart, diaphragm, or aorta will obscure that border.² Ipsilateral rib crowding may be seen as a result of volume loss. Other features that help confirm the diagnosis include mediastinal and tracheal shift towards the side of the collapse, and possible herniation of the contralateral lung across the midline as a result of compensatory hyperinflation. The degree of hilar depression and compensatory hyperaeration is variable—it depends on the degree of collapse and the shift of the mediastinal structures within the thoracic cage.² Less commonly, a stenosing bronchogenic tumour may be seen as a soft tissue density overlying the left hilar point.

2 Most likely cause
Lobar collapse has many causes, and its incidence varies with age and clinical history. It is most commonly caused by a proximal stenosing bronchogenic carcinoma, and although most lung cancers are seen in men, the incidence in women is rising. Lung cancer is rarely diagnosed in people younger than 40, but the incidence rises steeply thereafter, with most cases (85%) occurring in people over the age of 60 with a history of smoking.³ In ventilated patients including neonates, malpositioning of the endotracheal tube can aerate one lung and occlude the contralateral side, whereas in infants, collapse related to an inhaled foreign body (such as a peanut) should always be considered. In older children and young adults, the most common cause of lobar collapse is as a complication of asthma.

Asthma is a chronic inflammatory disease characterised by reversible airflow limitation and airway hyper-responsiveness.⁴ Hypertrophy of the bronchiole smooth muscle is present, along with mucosal thickening as a result of associated inflammatory oedema. In response to immunological stimuli, goblet cell hyperplasia and consequent hypersecretion of mucus can cause both segmental and subsegmental airway plugging. Proximal occlusion of a bronchus causes loss of aeration, and as the residual air is gradually absorbed, lung volume reduces and the lung eventually collapses.⁵ The patient’s age and clinical history suggest that this is the most likely cause of her left lower lobe collapse.

3 Treatment
In acute collapse, removing the causative agent should allow lung re-expansion with no long term sequelae. At presentation, patients should be risk stratified and treated in accordance with British Thoracic Society guidelines for the management of asthma.⁶ Treatment includes supplemental oxygen, bronchodilators, and steroids, with the addition of empirical antibiotics if infection is suspected. Our patient had no signs of infection, so antibiotics are unlikely to be needed. Airway clearance is helpful in patients with hyersecretion states, including asthma, and associated atelectasis,⁷ and although not formally mentioned in the guidelines, chest physiotherapy is useful for treating asthma complicated by mucous plugging.⁸ All patients should have a repeat chest radiograph to check for re-inflation, and if lobar collapse persists, referral for early bronchoscopy is of paramount importance to exclude a more sinister cause.

Patient outcome
Our patient was treated in accordance with national guidelines and responded well to empirical management with the addition of daily chest physiotherapy. Her supplemental oxygen and nebulised bronchodilator therapy was slowly reduced, and daily chest radiographs showed a gradual re-expansion of her left lower lobe. Three days after admission, her chest radiograph returned to normal and her clinical parameters had improved enough to warrant discharge. She was converted back to inhaled bronchodilator therapy and discharged on a reducing course of oral steroids. Arrangements were made for appointments with an asthma nurse and follow-up at the chest clinic. Repeat chest radiography at six weeks remained normal.

Cite this as: BMJ 2009;339:b4008