Treatment of task-specific dystonia in sports: A systematic review

Highlights • We systematically reviewed treatments for task-specific dystonia in sports.• This is the first comprehensive review of treatments for task-specific dystonia in sports.• Invasive/non-invasive treatment studies require more standardized outcome measures.• More diverse treatment strategies should be trialed in a greater diversity of sports.


Introduction
Dystonia is defined as a "collection of movement disorders characterized by sustained or intermittent muscle contraction causing abnormal, often repetitive, movement, postures, or both" [1].In certain cases, dystonia is limited to a specific highly practiced skill.Known as task-specific dystonia (TSD), its exact pathophysiology is still unclear, though evidence it arise due to changes in the sensory-motor system caused by over-practicing, equipment change, or injury [2].These changes are thought to negatively impact motor-control networks specifically formed for the practiced skill, and are therefore partly etiologically distinct from other forms of dystonia [3,4].Additionally, evidence has shown maladaptive dis-inhibition of the basal ganglia and motor cortex is also involved and may result in overactivity intrahemispherically [3].This flawed motor-control circuitry and overactivity is thought to result in muscular overflow in peripheral nonactive muscles during complex motor tasks.The result is the most common symptom of TSD: excessive maladaptive muscle contractions with subsequent abnormal postures exclusive to highly skilled and repetitive movements [2].
TSD affects a broad set of motor-related pursuits as performed by typists, painters, hairdressers, and watchmakers [4,5].In skills with higher motor-cognitive demands, such as professional musicianship it is even more common [6].For this reason, it also presents in many sports, as they offer another very competitive and complex test of motorcontrol.TSD has been observed in running, pistol shooting, golf, and table tennis [7].More recently new forms have been identified including manifestations in billiards, darts, rowing, cricket and speed skating.Some sport-related forms of TSD seem to be quite prevalent, sometimes with a sport specific terminology, for example studies in golf report a prevalence of Yips (Type 1, golf specific TSD) ranging from up 22 % to almost 50 % amongst avid players [8][9][10] (results are survey-based, cursory and need more concrete epidemiological evidence).Despite the high prevalence, a recent review concluded that the study of TSD in sports is still in its infancy [11], with far fewer research studies conducted compared with musician's cramp, which has a prevalence of 1-12 %, and writer's cramp with a prevalence of 2.7 out of 1 000 000 [12,13].
As research studies are generally scarce, the optimal treatment strategy for TSD in sports has also not been elucidated, with recommendations by clinicians often being based on anecdotal evidence [14].There are few robust studies on treatments for TSD in sports, with most reviews being narrative in nature [15][16][17].Therefore, the purpose of this study was to systematically review all original research describing interventions in the management of TSD in sports, with attention for the outcome measures used.It is our aim that this review contributes to a better understanding of the heretofore under-investigated treatment protocols for TSD in sports, and raises awareness regarding the necessity for further research.

Search strategy
The PICO [18] search strategy was used employing the following search criteria: Population: subjects with TSD in sports; Intervention/ Indicator: interventions to rehabilitate TSD in sports; Comparison/ Control: none/any comparators; and Outcome: Pre and postintervention measures of rehabilitation in subjects with TSD in sports.A systematic search strategy was employed until 06/06/2022 of TSD, and all synonyms for task-specific dystonia, in sports (i.e.yips or occupational cramp) (See Appendix A for search strategy).This search in Pubmed was also translated into Embase, Web of Science, and Psychinfo (See Appendix A).Additionally, for each retrieved article, all citations were reviewed and eligible articles were retrieved.
Studies were included if they met all the following criteria: 1) Peer reviewed published studies pertaining to sports; 2) Studies with a measurement of TSD; defined as an abnormal motor activity occurring during the execution of a skilled sport-specific task.Certain studies mainly in baseball and golf refer to TSD in sports as 'type 1 yips', placing it as one subtype within a larger psycho-neuromuscular disorder (the yips) that sometimes includes dystonia (type 1 yips), and sometimes does not (type 2 yips).Articles labeled type 1 yips were considered to be TSD and included.3) Studies with an intervention focusing on rehabilitation; defined as any physiological (neurological or peripheral) or psychological intervention employed to improve symptoms of TSD in sports 4) Articles in English with no restriction on publication date.We included case reports, case studies and case-control studies.We excluded articles if they were: 1) Abstracts sans full texts, expert opinions, narrative review articles, unpublished studies and dissertations; 2) Studies exclusively relating to choking.All papers were independently screened by two authors (BN and EW) by title, abstract and by keywords using Rayyan Qatar Computing Research Institute platform [19].The level of agreement was measured with Cohen's Kappa.The full text version of articles that met the inclusion criteria were acquired and assessed.Disagreements were resolved through consensus in consultation (MT and MS).If further information was required, authors were contacted for clarification.

Data extraction
Data was extracted from eligible articles, including participant demographics, study design, follow up period, type of intervention, type of outcome measures (standardized or subjective), statistical analysis, results and conclusion.Studies were categorized into two forms of intervention, 1) Paramedical; comprising psychological (emotional regulation or motor retraining), or physical-therapy based interventions.2) Medical; comprising botulinum toxin A (BTX-A), (oral) pharmacology or surgery.Summary statistics were aggregated from extracted data such as the frequency of different study designs, and the prevalence of standardized outcome measures.

Quality assessment
Due to the lack of cohort studies and randomized controlled trials (RCTs), we assessed the methodological quality of included studies with the McMaster Critical Review Form for Quantitative Studies (MCRF) [20].Based on the MCRF questions, summary scores were calculated to rate the quality of each study using a consistent comparison across different research designs [14].In agreement with previous studies, a percentage score of 50 % or less was perceived as a methodologically low score, while between 51 % and 75 % was considered moderate and greater than 75 % as high (See Appendix B for details).A meta-analysis and or subgroup analyses was not conducted due to the absence of homogeneous studies with sufficiently large sample sizes and standardized outcome measures.
The PRISMA 2020 (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) [21] guidelines were followed strictly in structuring this systematic review, and the protocol was registered in PROSPERO (Registration number: CRD42021261631).

Results
From all sources, 31 articles were retained for this review (complete search protocol in Fig. 1).7000 records were identified in our systematic search, of which 2802 were duplicates.Fifty-one articles met the criteria, of which three were not retrieved due to being unavailable, seven were review papers, six duplicates, four not directly sportsrelated, one was in a foreign language, one had no outcome variable, three had no intervention and one was not a TSD, leaving 25 included.Agreement between the two examiners following initial screening was a Cohen's Kappa score of 0.64, with a 99.36 % agreement, signifying substantial agreement.Additionally, 71 articles were identified from citation screening, of which 61 were duplicates and, after final assessment via group discussion, 6 remained, resulting in a total of 31 articles.

Study quality
The general quality of studies in TSD in sports was low, with a combined mean MCRF score of 49 % (STD 20 %).The non-invasive interventions with an MCRF of 59 % were more methodologically robust compared to invasive and or pharmacological interventions with an MCRF of only 42 %.Looking at individual characteristics of MCRF scores revealed many general deficiencies.All studies lacked any assessor blinding or power calculation to justify sample sizes.Three out of 31 studies presented results in terms of statistical significance [23,28,35].Only 12 of 31 studies documented ethical approval or informed consent [17,[23][24][25][26]32,33,35,[37][38][39]42].There were no properly matched control groups.Interventions were often not described in detail, hampering future replication.No studies reported power calculations.

Medical
BoNT/A showed improvement in seven out of 12 studies [17,[33][34][35]37,43,44]. Oral pharmacology seemed effective in six out of 14 studies [23,29,33,35,37,47].Ventro-oral thalamotomy and deep brain stimulation (DBS) seemed highly effective with four studies reporting improvements in patients in a wide range of sports.In three studies a total of six patients (three tennis players, a table tennis player and a juggler and a runner) reported significant improvement after ventro-oral thalamotomy [38,39,49].DBS was successful in the case of a golfer [32].

Main findings
The aim of this study was to systematically review all original research describing interventions in the management of TSD in sports.Thirty-one studies involving ten different sports were found treating a  total of only 74 affected participants.Twenty-one studies were medical: invasive and/or pharmacological and 13 were paramedical.Despite the wide range of sports involved and differing intervention strategies, average quality of studies was low (an MCRF score of under 50 %) making any quantitative assessment of treatment effectiveness difficult.The principal deficiency was a lack of quantitative outcome measures and statistics to test the efficacy of treatments, and instead relying on anecdotal evidence.An additional complication was the heterogeneity of interventions, where a large number of different interventions have been trialed, but few have been replicated across multiple forms of TSD.In these low quality studies, the effectiveness of interventions was generally in line with other form of TSD, showing generally positive results for paramedical interventions and mixed results for medical studies.Taken together, our results showed there is no conclusive evidence for an effective treatment strategy for TSD in sports, and more standardized measures of treatment outcomes are needed.

Study quality
TSD in sports is a troublesome condition for skilled athletes that has been insufficiently studied using standard outcome measures and statistics compared to other forms of TSD.This is partly due to the difficulty in finding subjects in rarer forms of TSD.However as seen in studies in other forms of TSD that may be equally rare (musician's dystonia) [12,13], it is possible to collect more robust cohorts and use objective measures and statistics more often [54][55][56].For instance a review found eight instances of SMR trialed in musicians [54], whereas we found only two in sports where only one completed the protocol.Another recent systematic review found 75 studies of botulinum toxin injection in writers and musicians, of which three were high quality double blind RCTs [57][58][59] and nine open label trials [55].Comparatively in the 12 sports-related studies we found, there were zero RCTs and only one open-label design [23].This re-emphasizes the low quality of intervention studies in TSD in sports in comparison.This over reliance on subjective and anecdotal evidence thus results in low reliability and   generalizability, and highlights the need for more and better research studies.

Effectiveness
The reported effectiveness of emotional regulation studies was high with all reviewed interventions improving symptoms of TSD in seven studies.It is important to note that these studies are exclusively on golfers.Reviews have noted choking is a common performance deficit in golf, and sometimes interacts with TSD, making it unclear whether these emotional regulation techniques would work equally well in other sportrelated TSD [11].Other paramedical interventions seemed also successful, but these studies were too few to draw broader conclusions.It is notable that SMR was only trialed twice in TSD in sports [27,35], indicating it is under-represented considering the technique has been especially effective in musician's dystonia [54].
The effectiveness of invasive and/or pharmacological studies in athletes were similar to studies in musicians and lower than in writers.Seven out of 12 (58 %) of studies of botulinum toxin in sports were effective, which was less than the 73 % efficacy shown in RCTs of 139 patients with occupational, writer's and musician's dystonia [60].This lower effectiveness in sports may be due to the intricate movement demands and strength requirements compared to other forms of TSD.A similar reduced efficacy (54 %) [61] has been shown in musicians specifically, presumably for the same reason [61][62][63].Pharmacological interventions were effective in 6/14 [23,29,33,35,37,47] studies.Unlike BTX-A, evidence for pharmacology in other forms of TSD is scarcer with no systematic reviews.Narrative reviews mention inconsistent results frequently accompanied by intolerable side effects [4,61,64], which may partly explain the low effectiveness in sports.Surgery was highly effective in sports, reporting unanimous positive results for both ventrooral thalamotomy and DBS.This agrees with results of successful surgeries in other forms of TSD, particularly in musicians [49].Although highly effective, these treatments carry significant risk of complications like temporary or permanent dysarthria and other motor deficits [5,65,66].Despite this, recent reviews have suggested favoring surgery in highly coordinated sports, arguing that botulinum toxin may cause excessive losses in coordination making its use untenable for expert performers [55].

Implications for future research
The majority of studies in our review were single cases, indicating a need for prospective studies in larger cohorts.This is challenging as TSD is rare making RCT designs often unfeasible.An alternative is crossover RCTs, where groups are exposed to different treatments regiments sequentially.In instances where only single-case designs are feasible, we recommend using new forms of Bayesian analysis that allow for statistical inferences to be made, even n = 1 style designs [67].Finally in all studies, interventions should not be trialed simultaneously [33,42].
Irrespective of study design, researchers should consider using standardized outcome measures on a sport by sport basis due to the diverse physical demands that are specific to different sports.For example, in a case of TSD in baseball botulinum toxin lead to performance deficiencies [41] (likely due to the high strength requirements of the sport), while in a case of billiards it was very effective [43].To investigate inter-sport differences in effectiveness, outcome measures could include biomechanics (EMG, kinematics), performance (sportspecific metrics such as successful putts in golf, shots on target in darts), and surveys (partly using standardized improvement scales with addended sections that are sport-specific).Therefore we also recommend the use of protocols to reduce heterogeneity in study design and interventions, so that researchers will be more able to measure their efficacy in a standardized manner.This would be a key improvement over clinician's current reliance on treatment protocols based on studies of other sports, writers and musicians.
Interventions for different forms of TSD should not cluster around particular skills, such as sensory motor retraining being rare in sports, whereas it has been successful in musicians and writers [2,54].Conversely, SFGI has been effective in golfers [24][25][26], but not adopted to treat other sports or TSD more broadly.These gaps in the research offer a promising opportunity for future treatments.For example, based on aforementioned results, a sequential multiple assignment randomized trial in a large group of golfers, with botulinum toxin and sensory motor training to measure there efficacy separately.

Limitations
It is important to recognize limitations to current research in TSD interventions in sports, specifically in the yips.The yips is a movement disorder where there are two distinct forms of pathophysiology, 1) psychological, where fear causes a loss of ability and 2) physiological, where multiple factors lead to dystonic symptoms.Although there is also significant overlap as remarked by Clarke et al. [11], it remains a challenge to differentiate them diagnostically at the current state of art.We attempted to exclude psychological yips by excluding all chocking articles where dystonia was not mentioned, however the authors concede this still leaves the possibility for confusion as to whether psychology was a factor among many in developing dystonia (as is true in task-specific dystonia), or was the only factor (as is thought to be possible in the yips).Due to the importance of the further study of dystonia in sports we decided to accept this limitation and include yips articles.Finally it is important to address the likely risk of publication bias towards positive results in the literature, as a further limitation to this review's accuracy, and the likely strength of evidence for current treatments.

Conclusion
This systematic review showed a lack of evidence for any specific treatment of TSD in sports.The 31 studies reviewed, were of low quality and did not use standardized outcome measures and appropriate statistical analyses.A descriptive synthesis revealed emotional regulation was quite effective, but only applied to golfers.Almost no formal evidence to support the use of botulinum toxin or pharmacology was found.Future well designed studies in higher numbers of athletes and with sport specific outcome measures are essential to increase our understanding and improve treatment of TSD in sports.

Author declaration
The authors affirm that the submission of this manuscript is in compliance with the following ethical requirements: 1.This study was conducted in strict accordance with the guidelines of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA), and was submitted to PROSPERO (Registration number: CRD42021261631).2. Informed consent was not necessary for this work.3. We confirm that we have read the position of Parkinsonism and Related Disorders on ethical publication and affirm that this work is consistent with those guidelines.The authors declare that the manuscript is original, it is not being considered for publication elsewhere and will not be submitted elsewhere while under consideration for Parkinsonism and Related Disorders or after it has been accepted by Parkinsonism & Related disorders.Authors declare there was also no ghost writing by anyone not named in the author list.4. The authors have seen and approved the manuscript in the form it is being submitted to the journal.The authors declare that they have conformed to the highest standards of ethical conduct in the submission of accurate data and that they acknowledge the work of others when applicable.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.opean Fund for Regional Development from the European Union (01492947) and the province of Friesland, the Stichting Wetenschapsfonds Dystonie and unrestricted grants from Actelion and Merz.

Appendix A
Search Protocol for Search Engines: ¶

B
.Nijenhuis et al.

Label SCS¼ Single Case Study, MCS¼ Multiple Case Study, CR ¼ Case Report, CS¼ Case Series NA¼ Not Applicable, NR¼No Response.
Low OL=Open, Label SCS= Single Case Study, MCS= Multiple Case Study, CR = Case Report, CS= Case Series NA= Not Applicable, NR= No Results (continued )