Background
Pulmonary Interstitial Glycogenosis (PIG) is a children’s interstitial lung disease (chILD) and was first described in 2002. This disorder is relatively rare and only few cases have been reported in the medical literature. However, given its relatively recent description and the fact that it is only diagnosed through lung biopsy, PIG may be under-recognized and under-reported. PIG has only been reported in infants, usually diagnosed within the first few months of life.
What causes PIG?
The cells of the body use glucose, a sugar, for energy. Most cells of the body use glucose from the blood, but it can be stored in a larger molecule called glycogen. Glycogen is found in large amounts in skeletal muscle and the liver and is used to supply energy to the body when needed. It is not typically found in large amounts in other cells of the body.
Pulmonary Interstitial Glycogenosis (PIG) is caused by an abnormal accumulation of glycogen in specific cells of the lung. These cells are located in the interstitium, the space between the air sacs in the lungs. The excess glycogen leads to a thickening of this space, making it difficult for oxygen to get from the air sacs into the bloodstream.
The cause of PIG remains unclear. The accumulation of glycogen has also been seen in other lung conditions to different degrees, especially those associated with poor lung growth. Based on these observations and the lack of inflammation in the lungs, it is thought that PIG is a result of abnormal development of the lungs in the fetus and young infant. However, a report of PIG in a pair of identical twins also point to the likely genetic predisposition for this disorder. Further studies are needed to accurately define this lung disorder.
Symptoms
Infants with PIG present with rapid and laborious breathing and the need for oxygen supplementation in the first few weeks of life. These symptoms are similar to those of many other respiratory disorders of infancy, such as respiratory distress syndrome and surfactant protein deficiencies. However, a common pattern in the presentation of these infants is an initial stable period, followed by an unexplained deterioration in their respiratory status several days or weeks after birth.
Diagnosis
As in other forms of chILD, several tests can help with the diagnosis.
- Lab work to rule out other causes of these symptoms, such as cystic fibrosis or immunodeficiency, is often performed.
- A high-resolution computed tomography (CT) scan of the lungs may show findings consistent with an interstitial lung disease. However, the imaging appearance of PIG is highly variable and non-specific for PIG. With the few case reports of PIG, it is currently difficult to make the diagnosis solely based on radiographic imaging.
- A bronchoscopy with bronchoalveolar lavage (BAL) may be performed which can look for infection, inflammation, and signs of aspiration into the lungs. Currently, a definitive diagnosis of PIG can only be made through lung biopsy. The biopsy tissue typically shows little inflammation. The hallmark of PIG is the accumulation of glycogen in the lung interstitial cells.
Treatment
As for any child, optimizing nutrition for adequate growth and the prevention of respiratory infections are important in the overall health. In addition, oxygen supplementation may be required.
Most of the reported infants with PIG have received and responded favorably to therapy with intravenous or oral corticosteroids. However, given the potential side effects of corticosteroids and that the evidence of this treatment is based on few patients, careful consideration before initiating treatment is warranted for each patient.
Prognosis
The cases of PIG described in medical literature have been associated with a favorable prognosis. Clinical improvement is noted in most cases and only one death has been reported in a premature infant with PIG. Again, these statistics are based on few patients. Caution must be exerted before drawing conclusions about the prognosis of PIG, especially if PIG is present along with lung growth abnormalities, in which case the prognosis may be poorer.
Future Directions
There is still much to learn about Pulmonary Interstitial Glycogenosis (PIG). Along with the chILD Foundation and the Children’s Interstitial Lung Disease Research Network (CHILDRN), there is an ongoing multi-center collaboration on a Rare Pediatric Lung Disease Patient Registry. This database will enable doctors and researchers to collect more information and better understand rare pediatric lung disorders such as PIG.