Psychometric Values of a New Scale: The Rett Syndrome Fear of Movement Scale (RSFMS)

(1) Background: One of the characteristics associated with Rett syndrome (RTT) is a fear of movement (FOM). Despite the grave consequences on health, function, and the caregiver’s burden associated with bradykinesia accompanying FOM, there is no specific FOM assessment tool for RTT. (2) Objective: To construct and assess the psychometric values of a scale evaluating FOM in RTT (Rett syndrome fear of movement scale—RSFMS). (3) Methods: Twenty-five girls aged 5–33, including a research group (N = 12 individuals with RTT) and control group (N = 13 typically developing girls at equivalent ages). The Pain and Discomfort Scale (PADS) and Facial Action Coding System (FACS) assessed the participants’ behavior and facial expressions in rest and movement situations. (4) Results: Significant behavioral differences were recorded in these rest and movement situations within the research groups using the RSFMS (p = 0.003), FACS (p = 0.002) and PADS (p = 0.002). No differences in reactions were found within the control group. The new scale, RSFMS, was found to show a high inter- and intra-rater reliability (r = 0.993, p < 0.001; r = 0.958, p < 0.001; respectively), good internal consistency (α = 0.77), and high accuracy (94.4%). (5) Conclusions: The new scale for measuring FOM in RTT, the RSFMS, was validated using the FACS and PADS. The RSFMS was found to be a tool that holds excellent psychometric values. The new scale can help clinicians working with individuals with RTT to plan appropriate management strategies for this population.