Total functioning capacity scale in Huntington's disease: natural course over time

The total functioning capacity (TFC) assessment has been integral to Huntington's disease (HD) research and clinical trials, measuring disease stage and progression. This study investigates the natural progression of function in HD, focusing on changes in TFC scores related to age and CAG-repeat length, and evaluates TFC's strengths and weaknesses in longitudinal studies.

Using Enroll-HD platform's clinical dataset version 5, including Registry-3, we analysed data from 21,079 participants, with 16,083 having an expanded CAG repeat. Our final analysis encompassed 15,527 patients and 52,457 visits, with TFC scores ranging from 0 to 13.

Alluvial charts show that most individuals maintain maximum functional capacity over time. 3505 individuals experienced change in TFC scores, over the subsequent 4 years, 2224 (64.1%) experienced declining TFC scores, while 661 (18.6%) showed improvement within a year. The remaining 17.3% exhibited stable TFC scores. Age-related changes followed a specific sequence: occupation, household chores/finances, daily living, and care. Longer CAG-repeat lengths were linked to earlier functional decline, with some geographic regions showing earlier losses in specific domains. Reduced penetrance CAG-repeat groups exhibited different trajectories from full penetrance HD participants.

When we focus on those who experienced a change in TFC score, the number of HD patients with regained functional capacity is substantial, even considering interrater variability, which may influence outcome assessments in clinical trials. The TFC effectively reflects changes in functional domains as intended. Analysis of the reduced penetrance group suggests potential selection biases in seeking medical attention earlier and for reasons unrelated to HD.