All of the following will cause the mediastinum to shift away from the pathology EXCEPT

• A. Diaphragmatic hernia
• B. Atelectasis
• C. Pneumothorax
• D. Pleural effusion

Answer: (B)

Mediastinal shift reflects the balance of volume loss versus pressure on the affected side. When a side loses volume or collapses, the mediastinum tends to be drawn toward that side. If a pathology causes increased pressure or a space-occupying effect on one side, the mediastinum is pushed away from that side.

A diaphragmatic hernia can occupy space in the chest and reduce ipsilateral lung volume, producing a shift away from the side with the herniation. A pneumothorax, particularly tension pneumothorax, raises intrathoracic pressure on the affected side and pushes the mediastinum away. A large pleural effusion adds fluid mass and pressure on that side, also shifting the mediastinum away. Atelectasis, however, causes loss of volume on the affected side, pulling the mediastinum toward the collapsed lung. Therefore, atelectasis is the exception to mediastinal shift away from the pathology.