Interstitial Pneumonias

Lymphangioleiomyomatosis (LAM)

A peribronchiolar cystic space has alveolar walls and nodules of smooth muscle (arrows) that contain narrowed vascular channels-- veins and lymphatics.

Here, the abnormal bundles of smooth muscle constrict a small vein. The smooth muscle reacts with antibodies to muscle cells: actin and desmin. It differs from smooth muscle in reacting with HMB45, an antibody to melanoma cells.

A trichrome stain shows the smooth muscle (red) and connective tissue (blue) that constrict the vessels.

Summary of histologic features of LAM

Pathogenesis of LAM--hypothesis: The slowly progressive smooth muscle proliferation of LAM produces emphysematous change that is similar to that seen in interstitial emphysema. In LAM, venous obstruction increases lymphatic flow. Lymphatics, also obstructed, in interlobular septa and around bronchovascular bundles dilate and constrict bronchioles. Resulting acinar overdistension causes alveolar rupture into the interstitium. The interstitial emphysema, in turn, causes hemorrhage (and hemosiderosis) as it dissects along tissue planes to the pleura.

The diagram shows the peribronchiolar and interlobular septal obstructed lymphatics. Rupture of air into the interstitium and lymphatics causes more compression of airways and can dissect to the pleura to cause pneumothorax.