Longitudinal patient-reported mobility assessment in fibrodysplasia ossificans progressiva (FOP)

Background: Fibrodysplasia ossificans progressiva (FOP) is a rare, disabling genetic disorder characterized by episodic soft tissue swelling (flare-ups) that leads to progressive heterotopic ossification and restricted joint mobility. Methods: Here we present the first longitudinal patient-reported mobility assessment (PRMA) in FOP based on a simple evaluation tool. At initial presentation and follow-up (1–11 year span; median: 6 year span), 64 patients (36 females; 28 males) with classic FOP completed a questionnaire designed to rapidly assess mobility at 15 sites (three axial; six upper limb, and six lower limb). In order to validate this instrument, twenty-one of 64 patients (33%) underwent a cumulative analogue joint involvement scale (CAJIS) evaluation by two physicians within six months of their second self-assessment. Results: We found that: 1) mobility changes were episodic and regional, occurring first in the neck and trunk, followed by the upper limbs and finally the lower limbs; 2) interval improvements in mobility did occur, most notably in the lower limbs (18%), and less so in the upper limbs (12%) and trunk (3%), and 3) patient-reported mobility assessments correlate highly (R² = 0.81) with physician-reported CAJIS evaluations. Conclusion: This is the first longitudinal PRMA in FOP and provides a simple and valid tool that can be used in the design and evaluation of clinical trials in this progressively disabling disease.