Pulmonary interstitial glycogenosis (PIG) is a form of interstitial widening in infant lungs, typically seen below the age of 6 months. This entity was described in 2002 with the recognition of increased periodic acid-Schiff (PAS)-positive diastase-labile material in the interstitium in some infant lung biopsies, a finding confirmed by electron microscopy to represent glycogen within interstitial cells. This disorder is now considered to represent the same entity earlier described in 1992 as “cellular interstitial pneumonitis,” also called infantile cellular interstitial pneumonia (ICIP), descriptions that emphasize the cellular component of the interstitium rather than the glycogen-rich cytoplasm and matrix. Once considered rare, PIG/ICIP is a histologic pattern now recognized with some frequency in infant lung biopsies. Although this finding may be isolated, it is more often associated with other underlying disease affecting the lungs, such as congenital heart disease, pulmonary hypertension, or chronic neonatal lung disease due to hypoplasia or prematurity. It may be a diffuse or patchy finding in individual biopsies, and is a feature easily overlooked in the setting of superimposed injury.

The etiology of pulmonary interstitial glycogenosis remains unclear. Although it was initially suggested to represent a developmental abnormality, more recent theories suggest a nonspecific reactive and/or reparative process of the growing infant lung, particularly in light of its wider recognition, numerous associated disease states, and resolution in rare cases with follow-up biopsy. Notably, it has not been associated with systemic glycogen storage disease. It has been described in monozygotic twins, but in the setting of prematurity, and a genetic component remains unproven.