OBJECTIVES: DMD is an X-linked recessive disorder that causes progressive loss of muscle function in affected patients. Given variation in ages at symptom onset and rates of motor function decline, we aimed to develop data visualizations characterizing typical patterns of disease progression amid heterogeneity.

METHODS: Data were obtained from the North Star Clinical Network database. Function was measured by the North Star Ambulatory Assessment (NSAA), a validated rating scale comprised of 17 items (e.g., walk, hop) scored from 0 (less able) to 2 (more able) for a total score of 0-34. Mean NSAA total scores were modeled as quadratic functions of age. Average performance on specific NSAA functional tasks were described using heatmaps. A subgroup of representative patients with NSAA total score below 10 and age 11-13 years, consistent with loss of ambulation at age 12 years (the population median), was contrasted with the overall population.

RESULTS: The study sample included 376 patients with DMD with an average of 5.1 NSAA assessments each. Mean age and NSAA total score at first assessment were 7.1 years and 22.2 units, respectively. Average NSAA total scores followed an arc pattern of early improvement followed by later decline. Mean NSAA total score declines were more rapid among representative patients than in the broader population, with the last 15 NSAA total score points lost in 2 vs. 4 years. Accordingly, performance on specific NSAA tasks exhibited more abrupt declines among representative patients than indicated by broad population averages.

CONCLUSIONS: When patterns of disease progression are heterogeneous, as in DMD, the population average trajectory obscures the more rapid pace of decline experienced by the most representative individuals. As the pace of progression can have profound impacts on patients and caregivers, visualizations characterizing representative individual experiences amid population heterogeneity are important to understanding and communicating disease burden.