Teaching Case

A 17-year-old man complains of a 24-hour history of sudden thoracic and cervical pain. He reports a self-limited viral process with strong coughing attacks during the past week. No record of fever, vomits, or anorexia. Physical exam showed a slightly erythematous oropharynx and crepitation in the supraclavicular region.

Initial chest x-ray on PA view (Figure 1a) shows linear lucencies along pulmonary hila and paratracheal regions with subcutaneous tissue involvement. These findings are suspicious for pneumomediastinum associated with subcutaneous emphysema. Lateral x-ray view (Figure 1b) shows areas of lucency anterior to the heart and ascending aorta.

A thoracic contrast-enhanced CT on venous phase was performed (Figure 2):

Axial views confirm the presence of air within the supraclavicular region (Figure 3a) and mediastinum (Figure 3b).

Coronal multiplanar reconstruction shows linear bands of air parallel and adjacent to the bronchovascular sheaths (Figure 3c).

Background

Pneumomediastinum is a radiological finding depicted by air in the mediastinal space. There are three mechanisms described [1]:

1] Mucosal barrier disruption [tracheobronchial tree or oesophagus] allows unrestrained air passage towards the mediastinum.

2] Gas produced by microorganisms in the mediastinum or adjacent areas.

3] Rupture of alveolar acini.

Aetiologies are varied, among which infection, trauma or iatrogenesis must be ruled out. When there is no apparent explanation, pneumomediastinum is considered spontaneous [Hamman syndrome].

In 1937, Charles Macklin first studied the physiopathology of spontaneous pneumomediastinum [SPM] through experimental works. The release of air into the interstitium secondary to acini rupture causes distension of the secondary pulmonary lobule. As this progresses, the air dissects the central arteriobronchial sheath until reaching the pulmonary hilum [Figure 4], entering the mediastinal compartment [2].

Clinical and Imaging Perspective

Although relatively rare, SPM can occur at any age and must be suspected, especially if there is diagnostic uncertainty. Patients usually consult with non-specific chest pain and dyspnea. During a physical examination, one may observe crepitus due to soft tissue gas and a pulse-synchronous rasping sound during cardiac auscultation [Hamman sign].

Due to its unspecific clinical presentation, radiological evaluation is paramount. Chest radiographs on PA and lateral projections are generally the first assessment based on availability and rapidness. Cardiac silhouette and mediastinal contours must be carefully assessed, looking for linear or curvilinear lucencies [3]. A lateral view sometimes shows a radiolucent delineation of the pulmonary artery [Ring-around-the-Artery Sign] [4]. Peripheral soft-tissue margins might show subcutaneous emphysema.

CT key findings include linear air bands along the bronchovascular sheats from the distal bronchi to the hilum [Macklin effect]. Pneumopericardium appears confined below the aortic arch as opposed to pneumomediastinum, which usually extends cranially towards subcutaneous tissues. Hence, pneumomediastinum displays slim and linear soft-tissue partitions within the air corresponding to connective tissue.

Outcome

Even though it may seem an ominous diagnosis, retrospective cohort studies support self-limitation of SPM, opting for conservative management when concomitant factors, injuries, or underlying diseases are not present [5,6], like it was done in our case with excellent clinical course.

Take Home Message

• Alveola rupture generates emphysematous distension within the secondary pulmonary lobule provoking dissection of the arteriobronchial sheat [Macklin effect]. Finally, air migrates cranially and accumulates within the mediastinum.

SPM displays poor specific symptoms. Chest x-ray is the first approach, and CT is mandatory to rule out treatable and ominous aetiologies.