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The term subsegmental atelectasis includes any loss of lung volume so small that it does not cause indirect signs of volume loss (as might be seen with larger atelectases). A subtype of subsegmental atelectasis is linear atelectasis (also known as discoid or plate-like atelectasis, and historically as Fleischner lines on chest radiographs).
Subsegmental vs linear atelectasis
There is confusion about the use of the terms "subsegmental atelectasis" and "linear atelectasis" (and their synonyms). From an academic point of view, the term linear atelectasis is reserved for atelectasis which appears primarily in the lung bases and is secondary to hypoventilation. Conversely, subsegmental atelectasis includes both linear atelectases and all other forms of atelectasis that do not involve a whole bronchopulmonary segment. In other words, every linear atelectasis is subsegmental atelectasis, but not every subsegmental atelectasis is linear atelectasis.
Linear atelectasis is usually due to a lack of adequate inspiration, and not due to any underlying airway obstruction. They are most commonly seen in post-surgical patients or those with abdominal pain, and may also be observed in the morbidly obese or immobile patients.
By definition, subsegmental atelectasis (regardless of its etiology) does not produce volume loss and subsequent shifting of mobile thoracic structures, and in most cases lacks clinical relevance and does not need to be reported.
Linear atelectases may result in minor linear densities of varying thickness usually parallel to the diaphragm, most commonly at the lung bases or less mobile regions of the lungs (e.g. lingula).
Other subsegmental atelectases present as linear or wedge-shaped densities and can affect any lung lobe. Unlike linear atelectases, subsegmental atelectases due to obstructive causes usually adopt a radial distribution rather than a horizontal one, as they are secondary to airway obstruction rather than parenchymal compression (as occurs in hypoventilation). Therefore, they follow the distribution of the bronchial and bronchiolar tree.