Mediastinal Shift

Christopher M. Walker, MD

DIFFERENTIAL DIAGNOSIS

Common

Pleural Effusion
Lobar Atelectasis
Pneumothorax

Less Common

Pneumonectomy
Radiation Fibrosis
Tuberculosis

Rare but Important

Hemothorax
Fibrothorax
Malignancy
Diaphragmatic Hernia
Scimitar Syndrome

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Direction of shift
Acuity of problem

Helpful Clues for Common Diagnoses

Pleural Effusion
- Contralateral mediastinal shift
- Opaque hemithorax
- Pleural nodularity or thickening raises suspicion of malignancy
- Causes of large unilateral fluid collections include
  - Infection, primary or metastatic malignancy, chyle, blood
- Smoothly thickened and enhancing parietal pleura indicates exudative effusion
- Chylous effusions are indistinguishable from transudative effusions
  - Secondary to obstruction by tumor, iatrogenic from surgery or trauma
Lobar Atelectasis
- Ipsilateral mediastinal shift
- Lobar collapse patterns
  - Obstructing neoplasm is more likely in outpatients
  - Mucus plug is more likely in inpatients
Pneumothorax
- Convex pleural line paralleling chest wall
- No vascular markings lateral to pleural line
- Tension pneumothorax is clinical diagnosis with symptoms/signs including
  - Chest pain, hypoxia, circulatory collapse
  - Physical examination findings of pneumothorax
- Suggestive/concerning radiographic findings of tension pneumothorax
  - Contralateral mediastinal shift
  - Flattening of diaphragm
  - Widening of rib interspaces
  - Complete collapse of lung

Helpful Clues for Less Common Diagnoses

Pneumonectomy
- Small hemithorax with ipsilateral thoracotomy
- Ipsilateral mediastinal shift
- Opaque hemithorax secondary to fluid in pneumonectomy space
Radiation Fibrosis
- Important threshold doses
  - Seldom visible ≤ 30 Gy
  - Nearly always visible ≥ 40 Gy
- Incidence increases with 2nd course of therapy
- Occurs 6-12 months after radiotherapy
- No further progression 2 years after radiotherapy
- Radiographic and CT findings
  - Ipsilateral mediastinal shift
  - Traction bronchiectasis, pleural thickening, and volume loss
  - Fibrosis does not obey lobar or segmental boundaries
  - Sharp and straight demarcation corresponding to radiation portals
Tuberculosis
- Ipsilateral mediastinal volume loss with extensive post-primary tuberculosis
- Activity difficult to determine without comparison radiographs
- Common locations include
  - Upper lobes or superior segments of lower lobes
- Radiographic or CT findings
  - Fibrosis
  - Traction bronchiectasis
  - Cavities
  - Adjacent emphysema
- CT findings of active disease
  - Tree-in-bud opacities indicating endobronchial spread of infection
  - Cavitation
  - Consolidation
  - Rim-enhancing lymphadenopathy

Helpful Clues for Rare Diagnoses

Hemothorax
- Most common cause is penetrating or blunt trauma
- Less common associations include
  - Aortic dissection, rupture of aneurysm, or coagulopathy
- Contralateral mediastinal shift
- CT demonstrates high-density fluid (> 30 HU)
- May see fluid-fluid level representing hematocrit effect
- Organization may lead to fibrothorax
Fibrothorax
- Marked unilateral pleural thickening ± calcification
- Ipsilateral mediastinal shift
- Causes include resolved
  - Hemothorax
  - Empyema
  - Tuberculosis effusion
Malignancy
- Contralateral mediastinal shift often caused by tumors metastatic to mediastinum or lung
- Causes include
  - Primary or metastatic germ cell tumors
  - Thymoma or thymic carcinoma
  - Mesothelioma
  - Endobronchial or extrabronchial mass causing lung collapse
  - Pleural metastases with large pleural effusions
Diaphragmatic Hernia
- Diaphragmatic rupture with herniation of viscera into thorax
  - Secondary to high-energy blunt or penetrating trauma
  - Associated injuries can be many, including pneumo-/hemothorax, rib fractures, and pulmonary contusion
- Bochdalek hernia
  - Bochdalek hernias are located “back and to the left”
  - Majority are small, contain fat, and are incidental
  - If containing bowel or kidney, may cause contralateral mediastinal shift
  - Large congenital hernias will usually be diagnosed prenatally by fetal ultrasound or MR
- Morgagni hernia
  - Anterior and right-sided in location
  - Most contain omentum with small vessels
  - Most common viscera to herniate is colon
Scimitar Syndrome
- Right lung hypoplasia with ipsilateral mediastinal shift
- Anomalous pulmonary venous return of a portion or all of affected lung
  - Vein parallels right heart border
- Decreased size of right pulmonary artery
- Systemic to pulmonary collaterals to portions of right lung
- Associated congenital heart disease in 25%

Image Gallery

Frontal radiograph shows typical radiographic features of chylous effusion from thoracic duct obstruction. Note the left hemithorax opacification with rightward mediastinal shift

Axial CECT shows large pleural effusion. Pleura uniformly enhances

indicating an exudative effusion. The left lung is completely atelectatic

(Left) Frontal radiograph shows typical radiographic features of pleural effusion from mesothelioma. Note the complete opacification of the left hemithorax