Content overlap of 91 dystonia symptoms among the seven most commonly used cervical dystonia scales

Dystonia is the third most common movement disorder [1, 2]. It is characterized by sustained or intermittent muscle contractions causing abnormal movements or postures [3]. Currently, dystonia is classified along two axes: clinical characteristics, including age at onset, body distribution, temporal pattern and associated features (additional movement disorders or neurological features); and etiology, which includes nervous system pathology and inheritance [3]. Focal dystonia, which affects only one body region is the most common. Cervical dystonia (CD) is the one with the highest prevalence (from 57/1mln to 4000/1mln) [4]. Over the years multiple scales assessing symptoms and severity of CD were developed. The most popular ones are Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) [5]; Tsui scale [6]; Burke-Fahn-Marsden Dystonia Rating Scale (BFMDRS) [7]; Cervical Dystonia Impact Profile 58 (CDIP-58) [8]; Craniocervical Dystonia Questionnaire 24 (CDQ-24) [9]; Cervical Dystonia Severity Rating Scale (CDSS) [10]; Dystonia Discomfort Scale (DDS) [11]; The Dystonia Non-Motor Symptoms Questionnaire (DNMSQuest) [12]; Unified Dystonia Rating Scale (UDRS) [13] etc.

However, different scales focus on different features which might result in a poor assessment of disease severity. This problem mainly applies to non-motor symptoms (NMS) of dystonia. For instance, not all of the scales evaluate NMS as some of them were developed before the subject emerged [14]. In recent decades NMS have been the subject of extensive research – the most popular symptoms are: sleep disorders, anxiety and tiredness [15, 16]. Until now, there is no universal tool to assess them. There is a possibility of overlooking the symptoms by using one specific questionnaire.

Eiko Fried (2017) proposed a methodology for evaluating different assessment tools in order to designate the level of overlap of symptoms across the questionnaires [17]. Such analysis might give valuable information about the reproducibility and generalization of findings made by one tool. When the overlap is poor, it might indicate that the symptoms assessed by one tool are idiosyncratic and cannot be found by another tool – putting in doubt the comparability of different studies. Moreover, this methodology has been already used in several different studies [18‐21]

The aim of our study was to evaluate the degree of overlap of most often used CD rating scales which were identified by the literature review. Furthermore, we gave recommendations on which scales should be combined to give more specific information about dystonia severity in research.

From the included articles most commonly used scales were TWSTRS (in 162 articles), Tsui scale (in 40 articles), BFMDRS (in 14 articles), CDIP-58 (in 12 articles), CDQ-24 (in 9 articles), CDSS (in 3 articles), DDS (in 3 articles) and DNMSQuest (in 2 articles). Detailed information about the number of articles in which each scale was used can be found in Fig. 2.

The analysis extracted 91 distinct CD symptoms (Fig. 3). 52 (62%) symptoms are examined by more than one scale. Whereas none of the symptoms appear in six or seven instruments simultaneously (Table 2). Furthermore, each symptom is present in two instruments on average.

In Table 3 the analysis of specific, compound and idiosyncratic symptoms can be found. CDIP-58 consists of the highest number of idiosyncratic symptoms (12 – which is 21% of items in this scale). CDSS is the only instrument that has no idiosyncratic symptoms. CDSS also analyses the lowest percentage of symptoms – 8% of 91 distinct symptoms. On the other hand, CDIP-58 examines the highest percentage of symptoms – 63%.

TWSTRS is the instrument that analyses the highest number of symptoms in the motor symptoms category – 12 (67%) and in the disability category – 10 (91%). CDIP-58 captures the highest number of sensory symptoms, sleep disturbances and gait disturbances (six (43%), four (100%) and nine (100%), respectively). CDQ-24 consists of the highest number of symptoms related to emotions and social interactions (17 (94%) and 13 (76%), respectively). Detailed information about the symptom categories can be found in Table 4.

The mean overlap among all scales is 0.17 which is interpreted as a very weak similarity between the instruments [24]. Mean overlaps of single scales range from 0.09 (BFMDRS) to 0.22 (CDIP-58). The mean overlap is very weak for all scales apart from CDIP-58 (weak mean overlap). The overlap among specific scales ranges from 0.00 to 0.64 (Table 5).

The correlation between the mean Jaccard coefficient of each scale (the mean overlap a scale with all others) and the length of the scale is 0.61 for the number of specific symptoms captured and 0.54 for the adjusted scale length.

In our study, we have conducted a literature search and gathered 196 studies from which we have chosen the seven most commonly used instruments examining CD symptoms. According to the literature, TWSTR is the most popular rating tool used in CD research (82.7%). The second one is Tsui scale (20.4%). 91 disparate symptoms were identified. The mean overlap among all tools was very weak (0.17). As presented in Table 2 38% of symptoms were present in only one instrument while none of the symptoms appeared in all of them. What is more, we conducted the analysis of the categories of symptoms which revealed that some scales do not mention specific categories. It can be found that the most popular scale (TWSTRS) mainly focuses on motor symptoms and disability while there are no items evaluating psychological status. Tsui scale and CDSS also analyse only motor disturbances. On the other hand, BFMDRS focuses mainly on gait and disability. There is also a group of scales (CDIP-58, CDQ-24 and DNMSQuest) focusing on NMS, omitting the motor symptoms. Furthermore, we showed there is a moderate correlation between the Jaccard coefficient and the length of the scale showing that scales with a bigger amount of items have higher level of similarity to other questionnaires.

To our best knowledge, this is the first study to analyse the content overlap of the scales used in CD research. Previously published comparisons of scales focused on the clinical applicability, validity and descriptive analysis of the symptoms measured by the scales [28‐30]. In some studies, CD symptoms were examined by more than one scale. For instance, Tarsy used TWSTRS and Tsui scale to assess the dystonia severity for the same patients in the study about botulinum toxin efficacy and found a significant positive correlation between the reduction of two scales scores after the treatment [31]. Considering our analysis, those scales analyse mainly motor symptoms which might explain a similar result, regardless of the weak overlap of the questionnaires (0.31). On the other hand, Tomic et al. presented a significant correlation between disability measured by TWSTRS and subscales of CDQ-24 [32]. However, considering the very weak overlap between TWSTRS and CDQ-24 (0.15) the symptoms assessed by CDQ-24 might be overlooked while using only TWSTRS. As was shown by Jost et al. TWSTRS is the main scale used in randomized controlled trials regarding botulinum toxin efficacy which is in line with our literature review in which it was the most commonly used scale [30]. It indicates that the NMS assessed by other scales are not taken into consideration in such trials regardless of their probable influence on the results. Even though TWSTRS analyse motor symptoms extensively, it captures only 30% of symptoms overall.

BFMDRS has the lowest mean overlap (0.09), simultaneously having very weak or weak overlap with scales analysing mostly motor symptoms (e.g. TWSTRS) or NMS (e.g. DNMSQuest). Therefore, this scale should not be used alone but with e.g. Tsui scale or CDSS which evaluate most of motor symptoms while not considering disability assessed in BFMDRS. Instruments with very weak overlap, especially with an overlap which equals zero (e.g. Tsui scale and BFMDRS, Tsui scale and CDQ-24, CDQ-24 and CDSS) should not be used interchangeably as they examine entirely different symptoms.

Taking into consideration our original analysis of symptom categories, three types of scales might be identified – analysing motor symptoms, NMS or both. The first type consists of TWSTRS, Tsui scale and CDSS; the second – CDQ-24 and DNMSQuest; the third – CDIP-58 and BFMDRS. TWSTRS examines the highest number of motor symptoms and disability symptoms; CDIP-58 – of sensory symptoms, sleep disturbances and gait disturbances; CDQ-24 – of emotions and social interactions (however the differences between the number of symptoms analysed by CDQ-24 and CDIP-58 in those categories are small). CDIP-58 is the only scale that examines at least one symptom in each category. The suggested categorization is also supported by specific content overlap analysis. Motor symptoms scales have none or very weak overlap with nonmotor scales, while the overlap score is greater inside the type.

Based on our analysis, CDIP-58 examines the largest number of symptoms and has the highest mean overlap with other scales. Thus, it might be a valuable clinical tool. Furthermore, there is a moderate correlation between the Jaccard index and scale length, and CDIP-58 is the scale consisting of the greatest number of items in our study. However, it analyses a low number of specific motor symptoms. TWSTRS on the other hand is well validated and most commonly used to assess motor symptoms [30]. That is why in our opinion CDIP-58 should be combined with TWSTRS for complex assessment of CD.