Evolution of DL_CO in LAM: Historical Perspectives and the Role of Advanced Imaging

Lymphangioleiomyomatosis (LAM) is a rare cystic lung disease affecting women and is characterized by the proliferation of abnormal smooth muscle-like cells within the lungs, kidneys, and lymphatic system. Forced expiratory volume in 1 second (FEV₁) and diffusing capacity of the lungs for carbon monoxide (DL_CO) are two commonly used markers for evaluating the status of LAM, although the disease may predominantly be associated with changes in only one of these parameters. In this special feature, we trace the historical evolution of DL_CO and FEV₁ in LAM up to their current uses, beginning with their relationship in early studies with histopathology and imaging. We transition to the use of DL_CO and FEV₁ in the context of sirolimus therapy and monitoring rates of change in lung function. Finally, we examine modern imaging modalities and how these techniques have contributed to our understanding of LAM progression, with a focus on the unique and perhaps undervalued role of DL_CO. The LAM Histology Score (LHS), which measures the involvement of cysts and LAM cells in the lung via biopsy, relates to disease stages and aligns more with DL_CO than FEV₁. The Cyst Score, calculated from high-resolution computed tomography (HRCT) scans, is a measure of the lung parenchyma occupied by cysts and correlates with disease progression. Large cysts as visualized by HRCT predominantly influence FEV₁, whereas smaller cysts, which impact a greater surface area of the lung and may be underestimated, tend to affect DL_CO.