Fractal geometry of airway remodeling in human asthma

Rationale: Airway wall remodeling is an important aspect of asthma. It has proven difficult to assess quantitatively as it involves changes in several components of the airway wall. Objective: To develop a simple method for quantifying the overall severity of airway wall remodeling in asthmatic airways using fractal geometry. Methods: Negative-pressure silicone rubber casts of lungs were made using autopsy material from three groups: fatal asthma, non-fatal asthma, and nonasthma control. All subjects were lifelong nonsmokers. A fractal dimension was calculated on two-dimensional digital images of each cast. Results: Nonasthma control casts had smooth walls and dichotomous branching patterns with nontapering segments. Asthmatic casts showed many abnormalities, including airway truncation from mucous plugs, longitudinal ridges, and horizontal corrugations corresponding to elastic bundles and smooth muscle hypertrophy, respectively, and surface projections associated with ectatic mucous gland ducts. Fractal dimensions were calculated from digitized images using an information method. The average fractal dimensions of the airways of both the fatal asthma (1.72) and nonfatal asthma (1.76) groups were significantly (p < 0.01 and p = 0.032, respectively) lower than that of the nonasthma control group (1.83). The lower fractal dimension of asthmatic airways correlated with a decreased overall structural complexity and pathologic severity of disease. Conclusion: Fractal analysis is a simple and useful technique for quantifying the chronic structural changes of airway remodeling in asthma.