Behavioural Interventions in People with Oropharyngeal Dysphagia: A Systematic Review and Meta-Analysis of Randomised Clinical Trials

Objective: To determine the effects of behavioural interventions in people with oropharyngeal dysphagia. Methods: Systematic literature searches were conducted to retrieve randomized controlled trials in four different databases (CINAHL, Embase, PsycINFO, and PubMed). The methodological quality of eligible articles was assessed using the Revised Cochrane risk-of-bias tool for randomised trials (RoB 2), after which meta-analyses were performed using a random-effects model. Results: A total of 37 studies were included. Overall, a significant, large pre-post interventions effect size was found. To compare different types of interventions, all behavioural interventions and conventional dysphagia treatment comparison groups were categorised into compensatory, rehabilitative, and combined compensatory and rehabilitative interventions. Overall, significant treatment effects were identified favouring behavioural interventions. In particular, large effect sizes were found when comparing rehabilitative interventions with no dysphagia treatment, and combined interventions with compensatory conventional dysphagia treatment. When comparing selected interventions versus conventional dysphagia treatment, significant, large effect sizes were found in favour of Shaker exercise, chin tuck against resistance exercise, and expiratory muscle strength training. Conclusions: Behavioural interventions show promising effects in people with oropharyngeal dysphagia. However, due to high heterogeneity between studies, generalisations of meta-analyses need to be interpreted with care.


Introduction
Swallowing disorders, or oropharyngeal dysphagia (OD), can be the result of many underlying conditions such as stroke, progressive neurological diseases, and acquired brain

Inclusion and Exclusion Criteria
The following criteria for inclusion were applied: (1) participants had a diagnosis of OD; (2) behavioural interventions were aimed at reducing swallowing or feeding problems; (3) studies included a comparison group; (4) participants were randomly assigned to one of the study arms or groups; (5) studies were published in English. Studies focussing on drooling, self-feeding, gastro-oesophageal reflux or oesophageal dysphagia (e.g., dysphagia resulting from oesophageal carcinoma or esophagitis) were excluded. Further excluded studies were those describing drug-induced swallowing problems, temporary swallowing problems caused by oedema post-surgery (e.g., anterior cervical discectomy), or swallowing problems associated with adverse effects of interventions such as inflammation and oedema resulting from recent radiotherapy (≤three months after intervention) or thyroidectomy. Studies reporting solely on feeding tube removal after intervention that did not provide data on swallowing or feeding problems, were also excluded. Studies on behavioural eating problems including bulimia, anorexia, and picky eaters, were out of scope of this review. Finally, only original research was included, thus excluding, for example, conference abstracts, doctoral theses and reviews.

Systematic Review
Methodological Quality and Risk of Bias. The Revised Cochrane risk-of-bias tool for randomised trials (RoB 2) [13] was used to assess the methodological quality of the included studies. The RoB 2 tool provides a framework for evaluating the risk of bias in the findings of any type of randomised trial. The tool is structured along five domains through which bias might be introduced into the study results: (1) the randomisation process; (2) deviations from intended interventions; (3) missing outcome data; (4) measurement of the outcome; (5) selection of the reported result.
Data Collection Process. A data extraction form was created to extract data from the included studies under the following categories: methodological quality, participant diagnosis, inclusion criteria, sample size, age, gender, intervention goal, intervention agent/delivery/dosage, intervention condition, outcome measures and treatment outcome.
Data, Items and Synthesis of Results. Two independent raters reviewed all titles and abstracts, then original articles, for eligibility. Inclusion of studies was based on consensus between raters. To ensure rating accuracy, two group sessions were held to discuss ratings of one hundred randomly selected records to achieve consensus before rating the remaining abstracts. Where consensus could not be reached between the first two raters, a third party was consulted for resolution. Methodological quality assessment was also rated by two independent researchers, after which consensus was reached with involvement of a third reviewer, when necessary. No evident bias in article selection or methodological study quality rating was present as none of the reviewers had formal or informal affiliations with any of the authors of the included studies. At this stage reviewers did not exclude studies based on type of intervention (e.g., behavioural intervention, neurostimulation).
During data collection, data points across all studies were extracted using comprehensive data extraction forms. Risk of bias was assessed per individual study using RoB 2 [13]. The main summary measures for assessing treatment outcome were effect sizes and significance of findings.

Meta-Analysis
Data was extracted from relevant studies to compare the effect sizes for the following: (1) pre-post outcome measures of OD and (2) mean difference in outcome measures from pre to post between different types of behavioural interventions. All interventions were categorised into compensatory (e.g., body and postural adjustments, or bolus modification), rehabilitative (e.g., oromotor exercises or Shaker exercise), combined compensatory and rehabilitative interventions, and no dysphagia intervention. Only studies using instrumental assessment (videofluoroscopic swallow study [VFSS] or fiberoptic endoscopic evaluation of swallowing [FEES]) to confirm OD were included. Outcome measures based on visuoperceptual evaluation of instrumental assessment and clinical non-instrumental assessments, were eligible for inclusion in meta-analyses. However, if both types of data were available, instrumental assessment was preferred over non-instrumental assessment outcome data. Oral intake measures, screening tools and patient self-report measures were excluded from meta-analyses. Measures other than the authors' primary outcomes may have been selected if these measures helped to reduce heterogeneity between studies.
To compare effect sizes, group means, standard deviations, and sample sizes for pre-and post-measurements were entered into Comprehensive Meta-Analysis Version 3.3.070 [14]. If only non-parametric data were available (i.e., medians, interquartile ranges), then data were converted into parametric data for meta-analyses. Participants in studies of multiple intervention groups were analysed separately. Where studies used the same participants, only one study was included in the meta-analysis. If studies provided insufficient data for meta-analyses, authors were contacted by e-mail for additional data.
Effect sizes were calculated in Comprehensive Meta-Analysis using a random-effects model. Due to variations in participant characteristics, intervention approaches, and outcome measurements, studies were unlikely to have similar true effects. Heterogeneity was estimated using the Q statistic to determine the spread of effect sizes about the mean and I 2 was used to estimate the ratio of true variance to total variance. I 2 -values of less than 50%, 50% to 74%, and higher than 75% indicate low, moderate, and high heterogeneity, respectively [15]. Using the Hedges g formula for standardized mean difference with a confidence interval of 95%, effect sizes were calculated and interpreted using Cohen's d convention: g ≤ 0.2 as no or negligible effect; 0.2 < g ≤ 0.5 as minor effect; 0.5 < g ≤ 0.8 as moderate effect; and g > 0.8 as large effect [16].
Forest plots of effect sizes for OD outcome scores were generated for pre-post behavioural interventions. Due to blended configurations of intervention groupings across studies it was not possible to compare a homogenous behavioural intervention group against a comparison group that did not have a behavioural component. For this reason, only a subgroup between group analysis was conducted (and not an overall between group analysis) to explore effect sizes as a function of various moderators. Behavioural interventions (compensatory, rehabilitative, or combined compensatory and rehabilitative interventions) were compared with conventional dysphagia treatment (CDT), or no dysphagia therapy groups. Other subgroup analyses were conducted to compare effect sizes between selected interventions (i.e., Shaker exercise, Chin Tuck Against Resistance exercise [CTAR], and Expiratory Muscle Strength Training [EMST]), medical diagnoses, and outcome measures. Only between-subgroup meta-analyses were conducted using post-intervention data, to account for possible spontaneous recovery during the period of intervention.
Using Comprehensive Data Analysis software, publication bias was assessed following the Begg and Muzumdar's rank correlation test and the fail-safe N test. The Begg and Muzumdar's rank correlation test reports the rank correlation between the standardised effect size and the variances of these effects [17]. This statistical procedure produces tau as well as a two tailed p value; values of zero indicate no relationship, whereas deviations away from zero indicate a relationship. High standard error would be associated with larger effect sizes if asymmetry is caused by publication bias. Tau would be positive if larger effects are presented by low values, while tau would be negative if larger effects are represented by high values.
The fail-safe N test calculates how many studies with effect size zero could be added to the meta-analysis before the result lost statistical significance. That is, the number of missing studies that would be required to nullify the effect [18]. If this number is relatively small, then there is cause for concern. However, if this number is large, it can be stated with confidence that the treatment effect, while possibly inflated by the exclusion of some studies, is not nil.

Study Selection
A total of 8059 studies were retrieved across four databases: CINAHL (n = 239), Embase (n = 4550), PsycINFO (n = 231), and PubMed (n = 3039). After removal of duplicate titles and abstracts (n = 1113), a total of 6946 records remained. After assessing titles and abstracts, 261 original articles were identified. Full-text records were accessed to verify all inclusion criteria. During full-text assessment, articles were divided into different types of interventions, as this systematic review reports on behavioural interventions only. Based on the inclusion criteria, 36 articles were included, after which one study was identified through reference checking of the included articles. Figure 1 presents the flow diagram of the article selection process according to PRISMA.

Description of Studies
All 37 included studies are described in detail in Tables 2 and 3. Table 2 reports on study characteristics, definitions and methods of diagnosing oropharyngeal dysphagia, and details on participant groups. Information such as medical diagnosis, sample size, age and gender, is provided on all study groups. Table 3 presents intervention goals, intervention components, outcome measures and treatment outcome of each included study.
Participants (Table 2). The 37 studies included a total of 2656 participants (mean = 72; SD = 124.5), with the sample sizes across studies ranging from 10 [30] to 742 participants [38]. All but two studies reported the mean age of participants [38,49], which was 65.6 years (SD = 8.8). Participant age range was only reported in five studies, ranging between 55 [36] and 95 [38] years. The mean percentage of male participants across all studies was 55.8% (SD = 13.7).

Description of Studies
All 37 included studies are described in detail in Tables 2 and 3. Table 2 reports on study characteristics, definitions and methods of diagnosing oropharyngeal dysphagia, and details on participant groups. Information such as medical diagnosis, sample size, age and gender, is provided on all study groups. Table 3 presents intervention goals, intervention components, outcome measures and treatment outcome of each included study. Outcome measures (Table 3). Many different outcome measures were used across the included studies targeting different domains within the area of OD. The most frequently used measures were the Penetration Aspiration Scale (PAS; 15 studies), the Functional Oral Intake Scale (FOIS; 8 studies), various water swallow tests (4 studies), and the Mann Assessment of Swallowing Ability (MASA; 3 studies). All other outcome measures were used in one or two studies only, confirming the substantial heterogeneity in outcome measures.
Interventions (Table 3). The included 37 studies comprised a range of behavioural interventions, delivered by various health professionals. The interventions were most frequently implemented by single allied health disciplines: occupational therapists in ten studies, speech pathologists in eight studies, physical therapists in two studies [36,48], and nursing staff in one study [55]. In five studies, more than one discipline was involved [23,27,28,33,48], and two studies reported caregivers as the intervention agent either as a single agent [24] or in addition to occupational therapists [22]. Nine studies did not specify disciplines involved in providing the interventions. The intervention dosage varied greatly, ranging from one training session [54] to exercise 3 times daily, 7 days per week for 42 days [25].
Behavioural intervention groups (Table 3). Of the 37 included studies, seven studies comprised three participant groups [19][20][21]23,25,26,38], whereas all other studies included two groups. Based on authors' description of therapy contents, all intervention groups were categorised into compensatory, rehabilitative, and combined compensatory and rehabilitative interventions. Ten studies included different types of intervention groups (i.e., compensatory, rehabilitative and/or combined compensatory and rehabilitative intervention groups). Five studies included only compensatory groups [20,24,38,39,55], ten studies included only rehabilitative groups, and thirteen studies included only groups combining compensatory and rehabilitative interventions.     Guillén-Solà, et al. [26] • Spain Hägglund, et al. [27] • Hägglund, et al. [28] • Sweden Hwang, et al. [29] • Korea   Jang, et al. [31] •  Mepani, et al. [40] • USA OD: Post deglutitive dysphagia, pharyngeal phase dysphagia, VFSS to confirm Diagnosis: Stroke or chemoradiation for head and neck cancer Inclusion: Pharyngeal phase dysphagia, incomplete UES opening and postdeglutitive aspiration, hypopharyngeal residue, able to comply with protocol, dysphagia with aspiration of at least 3 month duration Exclusion: History of pharyngeal surgical procedures excluded.   Park, et al. [42] •   • Turkey    (21) Woisard, et al. [54] • France  Ayres et al. [19] To verify the effectiveness of a manoeuvre application in swallowing therapy in patients with PD. Three groups: Experimental group: Chin-down posture manoeuvre (patient instructed to 'swallow lowering the head until chin touches in the neck'). Patients performed manoeuvre twice a day, swallowing saliva, during meals, throughout the week, at home. Patients were given a form to record the number of times the manoeuvre was performed at home. Patients also given instructions for optimal feeding and swallowing related to 'swallowing orientations': (1) environment during feeding (2) posture (3) meal-time (4) oral hygiene. Written instructions given.
Orientation group: Patients also given instructions for optimal feeding and swallowing related to 'swallowing orientations': (1) environment during feeding (2) posture (3)  Primary outcomes: return to pre stroke diet < 6 months Secondary outcomes: time to return to normal diet, proportion recovered, functional swallowing, dysphagia-related complications, died, were institutionalised, or dependent in daily living 6 months post stroke.
Compared with usual care and low-intensity therapy, high-intensity therapy was associated with an increased proportion of patients who returned to a normal diet (p = 0.04) and recovered swallowing (p = 0.02) by 6 months.
Carnaby et al. [21] Effectiveness and safety of exercise based swallowing therapy and neuromuscular electrical stimulation for dysphagia   Guillén-Solà et al. [26] Effectiveness of inspiratory/expiratory muscle training (IEMT) and neuromuscular electrical stimulation (NMES)  Hägglund et al. [28] To determine the effects of neuromuscular training on swallowing function in patients with stroke and dysphagia.
Intervention agent: Discipline NR Dosage: Neuromuscular training = 3 times per session and 3 times daily before eating Orofacial sensory vibration stimulation was performed 3 times daily before meals. 5 weeks of training in total.
Group A-Orofacial sensory-vibration stimulation: Patients received 5 weeks of continued oro-facial sensory vibration stimulation using an Oral B ® electric toothbrush. Instructions given on how to stimulate the buccinator mechanism, lips, external floor, tongue. Group B-Orofacial sensory-vibration stimulation + oral neuromuscular training (Muppy ® ): Patients received oral neuromuscular training for 5 weeks + oro-facial sensory vibration stimulation 1) Oral device (Muppy ® ) was used for oral neuromuscular training that aims to stimulate sensory input and strengthen facial, oral, pharyngeal muscles. Muppy ® is placed pre-dentally behind closed lips and pt sits in upright position. Patients hold device against a gradually increasing horizontal pulling force for 5-10 s whilst trying to resist the force by tightening the lips (2) oro-facial sensory stimulation of buccinator using electric toothbrush. Verbal, practical and written instructions about training given. Patient/caregiver reported training in a log-book. All patients in both groups self-administered or were assisted by relatives or ward staff in oro-facial sensory vibratory stim.  Study group significant improvement in functional dysphagia score-nasal penetration degree. Nasal penetration degree and peak cough flow showed greater improvement in study vs. control group.
Jeon et al. [32] To investigate the effects of NMES plus upper spine cervical mobilisation on forward head posture, and swallowing in stroke patients with dysphagia. Patient instructed to look at target object in a direction 15 degrees diagonally to the right side 3.
Tester then initiated given exercises by moving the patient's neck in a diagonal direction opposite to the direction specified 4.
Patient instructed to 'draw your jaw inward' and tester applied a level of resistance to the patients jaw to fully activate neck flexor below jaw (rotation to the right) 5.
Same exercises applied in opposite direction.
Control group: Shaker exercise 1. Isometric exercises: Patients lay on bed and raised their heads without moving shoulders off the bed, looked at ends of their feet for 60 s, and then lowered heads back on the bed. If patient had difficulty raising his/her head, they were asked to perform same exercise for 3 times for as long as they could. Isotonic exercises: Patients raised their head in same posture and looked at the ends of their feet 30 consecutive times.  Swallowing reflux, Cough and voice quality and swallowing time, number of swallows and SpO2 All Statistically significant differences between groups after therapy (p = <0.01).
Kyodo et al. [37] To evaluate the effectiveness of puree diets containing a gelling agent for the prevention of aspiration pneumonia in elderly patients with moderate to severe dysphagia.

Intervention agent:
Gastroenterologists experienced in transnasal endoscopy along with a speech therapist evaluated swallowing. Discipline who created gelling agent (intervention) NR.

Dosage (average): NR
Patients underwent endoscopic swallowing evaluation while sitting in a chair/sitting up in bed. Images of oropharynx and larynx were displayed on a monitor and recorded on digital video recorder. Pureed diet without gelling agent was made by mixing 100 g of white rice and 50 mL of water with a blender for one minute. Texture characteristics (IDDSI Level 4) were: hardness, 1760 ± 125 N/m 2 ; cohesiveness, 0.59 ± 0.03; adhesiveness, 224 ± 56 J/m 3 .

Primary outcome:
Presence of material in throat using endoscopic cyclic ingestion score (0 to 4) Secondary outcomes: Sense of material remaining in the throat after swallowing of pureed rice and/or test jelly; degree of dysphagia using Hyodo-Komagane score (0 to 12: mild 0-3; moderate 4-7; severe [8][9] Residuals in throat were significantly less likely with pureed rice with than without the gelling agent (median cyclic ingestion score (range); 1 (0-4) vs. 2 (0-4); p = 0.001. Irrespective of presence or absence of the gelling agent, the sense of materials in the throat was significantly less frequent in older patients (p = <0.01). No adverse events occurred.
Intervention agent: Speech pathologist Dosage: NR Chin-down intervention: chin to the front of the neck, three swallows of 3 mL of thin liquid from a spoon and three swallows of the same liquid from an 8-oz cup filled with 6 oz of liquid. Nectar or Honey-thickened liquids: on the two thickened liquid interventions, three swallows of 3 mL of thickened liquid from a spoon and three self-regulated swallows, performed as separate swallows, each from an 8-oz cup filled with 6 oz of the thickened liquid.   Park et al. [46] Effect of Resistive Jaw Opening Exercise (RJOE) on hyoid bone movement, aspiration, and oral intake level in stroke patients.
Experimental group: RJOE device to provide resistance to suprahyoid muscles. Isometric exercise, 30 s with device resistors pressed downward (3 times, 30-60 s of rest). Isotonic exercise repeatedly depressed by RJOE by holding device resistance down for 2-3 s then returned to original state (10 reps, 3 sets) with 30 s rest. Placebo group: RJOE using 1-mm thick device with almost no resistance to suprahyoid muscles. Exercise type and frequency of RJOE same as experimental group. Both groups received conventional dysphagia therapy after intervention, which involved orofacial muscle exercises, thermal tactile stimulation and therapeutic or compensatory manoeuvres.
Primary outcomes: Hyoid bone movement -by two-dimensional analysis of anterior and superior motion on VFSS. Aspiration-PAS Oral intake level-FOIS.
Both groups significant differences in hyoid movement, PAS, FOIS (p< 0.05). No significant difference between groups except for liquid type, PAS. Effect sizes (Cohen's d) 0.6-1.1 for anterior, superior movement of hyoid bone, semisolid and liquid type of PAS, and FOIS respectively.
Ploumis et al. [47] Evaluate cervical isometric exercises in dysphagic patients with cervical spine alignment disorders due to hemiparesis after stroke.
All patients -inpatient program including physiotherapy, occupational and speech therapy. Speech included deglutition muscle strengthening, compensatory techniques. Experimental group: +plus cervical isometric strengthening exercises contract neck muscles under resistance forward-backward-sidewards).
Control group: Regular speech therapy plus sitting balance.
Experimental group-more pronounced correction (p < 0.01) of cervical alignment in both planes and greater improvement (p < 0.05) of deglutition too, than control group. Steele et al. [49] Compare outcomes of two tongue resistance training protocols Tang et al. [50] Effect of rehabilitation therapy on radiation-induced dysphagia and trismus in nasopharyngeal carcinoma (NPC) patients after radiotherapy.
Both groups routine treatment. Rehabilitation group: training by therapists at hospital, continued at home post-discharge by exercise booklet, guardian oversight and calendar Exercises: Tongue-range of motion exercises included passive and active movement exercises. Pharynx and Larynx-exercises changing body position to maximize swallow function and minimize aspiration. Swallow manoeuvres included effortful swallow and Mendelsohn manoeuvre. Sensory procedures utilizing pharyngeal cold stimulation performed by therapists. Exercise for Trismus-Active jaw movements-opening/closing mouth repeatedly, opening mouth slightly, moving lower mandible to left and right, stretched chin downward and forward and a range of passive jaw movements. Control group: No rehabilitation exercises Both groups received routine treatment (e.g., anti inflammatory treatment for aspiration pneumonia) Primary outcomes: Severity of dysphagiawater swallow test Trismus-LENT/SOMA score and the interincisor distance (IID).
Rehabilitation group only significant improvement in swallowing function. Percentage of patients with effective results in rehabilitation higher than control (p = 0.02). Control IID significantly decreased at Post (p = 0.001), both groups decreased at 3 months, rehabilitation group less than controls (p = 0.004). Trismus in rehabilitation higher vs. control (p = 0.02).
Tarameshlu et al. [51] Effects of Traditional Dysphagia Therapy (TDT) on swallowing function in Multiple Sclerosis (MS) patients with dysphagia.
Traditional Dysphagia Therapy (TDT): Includes oral motor control, range of motion exercises, swallowing manoeuvres, strategies to heighten sensory input. Usual care (UC): postural changes, modifying volume and speed of food presentation, changing food consistency and viscosity, and improving sensory oral awareness.  Effects of resistance training of swallowing muscles in community dwelling older individuals with dysphagia.
Intervention agent: Research co-workers Dosage: intervention exercises for 10 s; 1 set = 10 reps. 2 sets per day 3× per wk × 3 months Control/both groups: dysphagia brochure (about oral hygiene, tongue resistance exercise, head flexion exercise against manual resistance, nutrition, and food modifications).
Intervention: resistance exercises for swallowing muscles involving tongue resistance exercise and head flexion against manual resistance. Research co-workers instructed participants once how to perform resistance training.
Woisard et al. [54] Effect of a personalised transportable folding device for seating on dysphagia Intervention agent: Occupational therapy Dosage: 1 x training session with device (D+ group) and without device (D-group).
D-/All groups: All patients training session: evaluation of needs, impact of head positioning on swallowing, adapted position of head through body positioning, practice using occupational therapy cushions or personalised transportable folding device for seating (DATP) according to randomisation. D+ group: in charge to determine characteristics of the device required so they could have them during the training session. Instruction for patients was to put the personalised instructions into practice by using the device.
Primary outcomes: quality of swallowing Secondary outcomes: posture, device acceptability, QoL. Measurement of hyoid bone movement during swallowing. VFSE and questionnaire.
Significantly better posture both groups (p < 0.001), more hyoid bone motion in D+ group. Significant mean difference for D+ group vs. D− group, for horizontal and vertical movement. Other swallowing markers not significant.
Zhang and Ju [55] Clinical improvement of nursing intervention in swallowing dysfunction of elderly stroke patients.  Most studies (n = 23) included a comparison group that received a type of dysphagia treatment often referred to as traditional therapy, standard swallow therapy, or conventional dysphagia treatment (CDT). Some studies also used the term usual care for CDT groups. CDT treatment could include counselling and the provision of information about swallowing and dysphagia, compensatory strategies (e.g., bolus modification and adjusted head positioning), rehabilitation, oromotor exercises and/or thermal stimulation. Three studies included a comparison group receiving medical standard care without dysphagia treatment [20,51,56]. In three studies, patients underwent sham dysphagia training [36,43,53]. Several studies compared two or three behavioural interventions without having a CDT or medical standard care group included [33,34,46,49,50,55].

Risk of Bias Assessment
The Begg and Mazumdar rank correlation procedure produced a tau of 0.305 (twotailed p = 0.113), indicating there is no evidence of publication bias. This meta-analysis incorporates data from 15 studies, which yield a z-value of 7.528 (two-tailed p < 0.001). The fail-safe N is 207. This means that 207 'null' studies need to be located and included for the combined two-tailed p-value to exceed 0.050. That is, there would need to be 13.8 missing studies for every observed study for the effect to be nullified. Both of these procedures (i.e., Begg and Mazumdar rank correlation and fail-safe N) indicate the absence of publication bias.

Methodological Quality
Risk of bias of the included RCTs was assessed using the RoB 2 tool. Figures 2 and 3 present the risk of bias summary per domain for individual studies and for all included studies. Most studies showed low risk of bias per domain, but more than half of the included studies (19/37) scored overall as having some concerns, with three studies identified as being at high risk.

Risk of Bias Assessment
The Begg and Mazumdar rank correlation procedure produced a tau of 0.305 (twotailed = 0.113), indicating there is no evidence of publication bias. This meta-analysis incorporates data from 15 studies, which yield a z-value of 7.528 (two-tailed p < 0.001). The fail-safe N is 207. This means that 207 'null' studies need to be located and included for the combined two-tailed p-value to exceed 0.050. That is, there would need to be 13.8 missing studies for every observed study for the effect to be nullified. Both of these procedures (i.e., Begg and Mazumdar rank correlation and fail-safe N) indicate the absence of publication bias.

Methodological Quality
Risk of bias of the included RCTs was assessed using the RoB 2 tool. Figures 2 and 3 present the risk of bias summary per domain for individual studies and for all included studies. Most studies showed low risk of bias per domain, but more than half of the included studies (19/37) scored overall as having some concerns, with three studies identified as being at high risk.

Meta-Analysis: Effect of Interventions
Twenty-one studies were included in the meta-analyses [21,22,24,25,[28][29][30][31]34,35,[40][41][42][43][44][45][46]49,51,52,54]. All study groups were categorised into compensatory interventions, rehabilitative interventions, combined compensatory and rehabilitative interventions, and no dysphagia intervention. Seventeen studies were excluded from meta-analyses: one study included patients with self-reported swallowing difficulties without confirmed OD diagnosis by instrumental assessment (VFSS or FEES) [48], four studies did not report on instrumental or clinical non-instrumental outcome data [20,28,37,40], ten studies provided insufficient data for meta-analysis [21,24,27,34,38,39,48,51,56,57], and two studies were excluded to reduce heterogeneity between studies [32,53].  . Note. If one or more yellow or red circles (domains) have been identified for a particular study, the Overall score (last column) shows an exclamation mark, indicating that either the study shows some concerns (yellow circle with exclamation mark) or is at high risk (red circle with exclamation mark).  . Note. If one or more yellow or red circles (domains) have been identified for a particular study, the Overall score (last column) shows an exclamation mark, indicating that either the study shows some concerns (yellow circle with exclamation mark) or is at high risk (red circle with exclamation mark).

Meta Analysis
Between subgroup analyses. Subgroup analyses (Table 4) were conducted comparing different types of interventions: behavioural interventions were compared with conventional dysphagia treatment (CDT), or no dysphagia therapy groups ( Figure 5). Both behavioural interventions and CDT were categorised into mainly compensatory, rehabilitative, and combined compensatory and rehabilitative interventions. Overall, significant treatment effects were identified favouring behavioural interventions. In particular, large effect sizes were found when comparing rehabilitative interventions with no CDT, and combined interventions with compensatory CDT. When comparing selected interventions based on commonalities across studies against CDT, significant, large effect sizes were found in favour of Shaker exercise, chin tuck against resistance exercise (CTAR), and expiratory muscle strength training (EMST). Most studies were conducted in stroke populations and showed significant, moderate effect sizes. Comparisons between outcome measures indicated at significant effects for PAS only. treatment effects were identified favouring behavioural interventions. In particular, large effect sizes were found when comparing rehabilitative interventions with no CDT, and combined interventions with compensatory CDT. When comparing selected interventions based on commonalities across studies against CDT, significant, large effect sizes were found in favour of Shaker exercise, chin tuck against resistance exercise (CTAR), and expiratory muscle strength training (EMST). Most studies were conducted in stroke populations and showed significant, moderate effect sizes. Comparisons between outcome measures indicated at significant effects for PAS only.   [21,22,25,[29][30][31]34,35,[41][42][43][44]46,52]. Note. Refer to Table 2 Figure 5. Between subgroup meta-analysis for different types of interventions: behavioural interventions compared with conventional dysphagia treatment (CDT) or no dysphagia therapy [21,22,25,[29][30][31]34,35,[41][42][43][44]46,52]. Note. Refer to Table 2 for explanation of the subgroups.

Discussion
This systematic review aimed to determine the effects of behavioural interventions in people with OD based on the highest level of evidence (RCTs) only. Findings from the literature were reported using PRISMA and meta-analysis procedures.

Systematic Review Findings
In total, 37 behavioural RCTs in OD were identified. Considering the high prevalence [3] and severe impact of OD on health [57], quality of life [5,58], and health-economics [59], the limited number of high-level evidence studies is concerning. RCTs are costly and usually require extensive funding [60]. Possibly, the general lack of awareness of OD [61] might place funding applications in this research area at a disadvantage when competing with well-known, life-threatening diseases such as cancer or stroke. Although OD is a symptom of these diseases, and many other underlying conditions, limited public knowledge persists, resulting in reduced understanding and recognition of the devastating consequences of OD, in both health-care and non-health-care practitioners [61].
Further, although RCTs are characterised by random allocation and allocation concealment, few of the included studies included sufficient reporting on the processes of randomization and blinding. These finding are in line with current literature on quality assessments of RCTs [62,63], confirming that the risk of selection bias [63] and the success of blinding methods in RCTs [62] can often not be ascertained due to frequent poor reporting.
When comparing behavioural RCTs in OD, several methodological challenges arise. Authors may use different definitions for OD or fail to provide sufficient details when reporting on the swallowing problems of the included patient populations. Also, several studies used non-instrumental assessments (i.e., patient self-report or a screening tool) to identify or confirm OD, making the comparison between studies precarious. The use of a screening tool is especially problematic in identifying OD and cannot act as confirmation of OD. A screening tool's purpose is merely to identify patients at risk of OD, after which further assessment may confirm or refute the diagnosis [2]. Additionally, although instrumental assessment is considered the optimal tool for confirming OD diagnosis, VFSS and FEES protocols may differ (e.g., using different numbers of swallow trials, viscosities, and volumes).
Studies used a wide range of outcome measures to evaluate treatment effects. Since OD is a multidimensional phenomenon [64], different dimensions of OD may result in different therapy outcomes. For example, changes in dysphagia-related quality of life or oral intake do not necessarily correlate with findings from instrumental assessment. As such, to reduce heterogeneity in meta-analyses, patient self-report and oral intake measures were excluded. Also, some studies included outcome measures with poor or unknown psychometric properties, which in turn undermines the interpretation of treatment effects as data may not be valid or reliable. In addition, measures with weak responsiveness characteristics are not sensitive to treatment changes and should therefore be avoided as outcome measures aiming to determine intervention effects [2].
Most studies included a combined rehabilitative and compensatory intervention group or a rehabilitative intervention group, with only a few studies including exclusively compensatory groups. As the interventions classified as CDT comparison groups showed large variation as well, CDT comparison groups were categorised into similar group types (compensatory and/or rehabilitative CDT). Overall, terminology in the literature referring to CDT comparison groups was varied and complex. This was especially pertinent when interventions were not described in sufficient detail and descriptive terms such as "usual care" or "traditional therapy" did not provide further clarity on the type or content of CDT provided. Despite using categories to group different types of interventions, some degree of heterogeneity was inevitable. Interventions used different types of exercises or care, in distinct dosages, and were applied by different health care professionals. Therefore, it is challenging to identify the "active" ingredients of individual interventions, especially as most studies combined the use of different treatment strategies.

Meta-Analysis Findings
When considering meta-analyses for behavioural interventions, overall significant treatment effects were identified as favouring behavioural interventions over CDT and withholding dysphagia therapy. Most promising intervention approaches were rehabilita-tive interventions, which were associated with large effect sizes. Additionally, rehabilitative interventions such as Shaker exercise, CTAR exercise, and EMST showed significant, large effect sizes. However, since most studies included in the meta-analysis provided data on stroke patients only, future research still needs to confirm these findings in other diagnostic populations such as Parkinson's disease, acquired brain injury or patients with head and neck oncology. As stated above, patient self-report and oral intake measures were excluded from meta-analyses to increase homogeneity between studies. Though self-report and oral intake data might be interesting for future meta-analyses, this would require additional RCTs to be published, as currently there is limited data available in the literature. Finally, future studies should report on treatment dosage and duration in more detail. Due to high heterogeneity between studies and incomplete reporting, no subgroup meta-analyses could be conducted for these variables.

Limitations
Although reporting of this review followed the PRISMA guidelines to reduce bias, some limitations are inherent to this study. As only RCTs published in English were included, some RCTs may have been excluded based on language criteria. In addition, meta-analyses were restricted because of heterogeneity of the included studies. As such, comparisons across studies are challenging and, generalisations and meta-analyses results should be interpreted with caution.

Conclusions
Meta-analyses for behavioural studies in oropharyngeal dysphagia identified an overall, significant, large pre-post interventions effect size. Significant treatment effects were identified favouring behavioural interventions over conventional dysphagia treatment. Notably, large effect sizes were found when comparing rehabilitative interventions with no dysphagia treatment and combined interventions with compensatory conventional dysphagia treatment. Selected interventions compared with conventional dysphagia treatment showed significant, large effect sizes in favour of Shaker exercise, CTAR, and EMST.
Behavioural interventions show promising effects in people with oropharyngeal dysphagia. Still, generalisations from this meta-analysis need to be interpreted with care due to high heterogeneity across studies.