It's tricky: Rating alleviating maneuvers in cervical dystonia

Abstract

Objectives: To investigate hypothesized sources of error when quantifying the effect of the sensory trick in cervical dystonia (CD) with the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS-2), test strategies to mitigate them, and provide guidance for future research on the sensory trick.

Methods: Previous analyses suggested the sensory trick (or "alleviating maneuver", AM) item be removed from the TWSTRS-2 because of its poor clinimetric properties. We hypothesized three sources of clinimetric weakness for rating the AM: 1) whether patients were given sufficient time to demonstrate their AM; 2) whether patients' CD was sufficiently severe for detecting AM efficacy; and 3) whether raters were inadvertently rating the item in reverse of scale instructions. We tested these hypotheses with video recordings and TWSTRS-2 ratings by one "site rater" and a panel of five "video raters" for each of 185 Dystonia Coalition patients with isolated CD.

Results: Of 185 patients, 23 (12%) were not permitted sufficient testing time to exhibit an AM, 23 (12%) had baseline CD too mild to allow confident rating of AM effect, and 1 site- and 1 video-rater each rated the AM item with a reverse scoring convention. When these confounds were eliminated in step-wise fashion, the item's clinimetric properties improved.

Conclusions: The AM's efficacy can contribute to measuring CD motor severity by addressing identified sources of error during its assessment and rating. Given the AM's sensitive diagnostic and potential pathophysiologic significance, we also provide guidance on modifications to how AMs can be assessed in future CD research.

Keywords: Cervical dystonia; Clinimetrics; Sensorimotor integration; Sensory trick; Spasmodic torticollis.

Fig 1.

Proportions of patients given sufficient time to demonstrate AM. Number of patients per site, and number not permitted to demonstrate AM with sufficient duration.

Fig 2.

AM rating variability is higher for very mild CD Inter-rater variability in scoring the AM for insufficiently vs. sufficiently severe patients. (SD = standard deviation among video raters).

Figure 3.

One of each of the site- and video-raters rated AM in reverse of the rating scale. Test Information Functions for Site Ratings (left) and Video Ratings (right). The x-axis (Theta) represents the severity of the latent trait of CD severity and the y-axis represent the scaling for the AM item. Accelerating curves correspond to the expected direction (Correct Scoring, solid lines) and decelerating curves correspond to the unexpected direction (Reverse Scoring, dashed lines).