A Systematic Review and Meta‐Analysis on the Application of Thickened Liquids to Treat Adults With Neurogenic Dysphagia

ABSTRACT

Background

Thickened liquids have been one of the cornerstones of routine care to reduce aspiration for dysphagic patients in clinical practice. However, the evidence of this practice remains limited and uncertain.

Aims

This study aimed to systematically review and evaluate the effects of thickened liquid for adults with neurogenic dysphagia.

Methods

Five electronic databases were searched (Pubmed, Embase via Ovid, CINAHL, Web of Science and Cochrane Library) from each database's inception date until 30th July 2024. Search terms included a combination of database‐specific controlled vocabulary terms and free‐text terms relating to ‘dysphagia’ and ‘thickened food’. Study inclusion criteria focused on peer‐reviewed published articles including randomised controlled trials (RCT), cohort studies, case–control studies and case series. Only studies with neurogenic data were included. Four reviewers independently performed the search, data extraction and analysis. The outcome measure was a change in (any) relevant clinical swallowing‐related characteristic.

Results

A total of 2090 studies were identified, of which 16 met the inclusion criteria and were included in the review. The risk of bias of studies was moderate to low. Five studies either focused exclusively on different components of swallowing physiology or were unable to provide sufficient or reliable data for analysis and were thus excluded from data synthesis. Data analysis was conducted between groups (thin liquid vs. thickened liquid) concerning penetration (n = 5) and aspiration (n = 8), Penetration Aspiration Scale (PAS) scores (n = 4), unsafe swallowing (n = 7) and residue (n = 5). The results showed that thickened liquids improved swallowing safety, including reductions in aspiration events (([95% CI] = 0.49 [0.28, 0.88]; p = 0.02; I ² = 81%) in RCTs and ([95% CI] = 0.31 [0.13, 0.71]; p = 0.006; I ² = 47%) in non‐RCTs), as well as improvements in unsafe swallowing ([95% CI] = 0.27 [0.14, 0.51]; p < 0.0001; I ² = 88%) and PAS scores ([95% CI] = −1.99 [−2.59, −1.38]; p < 0.00001; I ² = 89%). However, thickened liquids did not demonstrate a significant effect in reducing penetration events ([95% CI] = 0.40 [0.13, 1.22]; p = 0.11; I ² = 88%) and were associated with increased residue in both the pharynx ([95% CI] = 1.57 [1.20, 2.06]; p = 0.001; I ² = 16%) and the oral cavity ([95% CI] = 2.87 [1.88, 4.40]; p < 0.0001; I ² = 45%).

Conclusion

The current evidence, based mainly on non‐randomised controlled trials, suggests that thickeners may help improve swallowing safety for neurogenic dysphagia, but this evidence remains weak. Further RCT evidence is needed to validate the clinical efficacy of thickeners.

Trial Registration

INPLASY International Platform for Registered Systematic Reviews and Meta Analyses Program: INPLASY202510011

Thickeners may enhance swallowing safety in neurogenic dysphagia; however, current evidence remains limited. Further randomised controlled trials are needed to validate their clinical efficacy.

1. Introduction

Dysphagia, defined as difficulty swallowing, poses a significant challenge for many adults, particularly among older populations and individuals with neurological disorders [1]. The implications of dysphagia are profound, leading to a heightened risk of aspiration, malnutrition and dehydration, as well as adverse effects on social interactions and overall quality of life [2]. Given these concerns, effective interventions are crucial for improving patient outcomes and ensuring safety during eating and drinking. Various approaches have been explored in dysphagia management, including rehabilitative exercises, compensatory strategies and sensory stimulation techniques. Recent meta‐analyses have demonstrated that a range of interventions show benefits in the management of dysphagia [3, 4, 5]. Matos et al. [6] identified conventional therapy as the most effective among 11 commonly used phonoaudiological interventions, either applied alone or with other methods. Cheng et al. [7] highlighted newer treatments—as biofeedback, pharmacological therapies, neuromodulation and soft robotics—as promising alternatives to traditional approaches. Jamil et al. [8] further emphasised that neuromuscular electrical stimulation is one of the most effective therapeutic techniques for post‐stroke dysphagia.

But, one prevalent intervention for managing dysphagia is the use of thickened liquids [9]. Thickening agents modify the viscosity of liquids, which may facilitate safer swallowing by promoting better bolus control and reducing the likelihood of aspiration [10, 11]. Clinical guidelines often recommend thickened liquids as a first‐line approach for various dysphagic populations, including stroke survivors, individuals with neurodegenerative diseases and those undergoing treatment for head and neck cancer, etc. [12, 13, 14]. Despite widespread use, the evidence regarding the efficacy of thickened liquids remains weak [14], inconsistent and sometimes contradictory [15, 16].

Previous studies have reported varying outcomes regarding the impact of thickened liquids on swallowing safety and efficiency. While some research indicates that thickened liquids significantly reduce aspiration and penetration events [17, 18], other studies suggest limited benefits or even adverse effects (such as dehydration) [19], potentially due to differences in study design, participant characteristics or assessment methodologies. For instance, Makhnevich et al. [20] found that patients receiving thickened liquids were less likely to be intubated, but they were more likely to have respiratory complications. This variability highlights the need for a comprehensive evaluation of the current evidence surrounding this intervention.

This systematic review aimed to synthesise existing research on the use of thickened liquids for adult neurogenic dysphagic patients. By analysing data from diverse studies, the review sought to clarify the effectiveness of thickened liquids in enhancing swallowing safety, reducing aspiration and penetration incidents and improving the overall quality of life for those affected by dysphagia. Ultimately, these findings will contribute to a better understanding of the role of thickened liquids in dysphagia management and inform clinical practices moving forward.

2. Methods

2.1. Search Strategy

The present meta‐analysis was registered in the INPLASY International Platform for Registered Systematic Reviews and Meta Analyses. The structure and the procedure of this review followed the Preferred Reporting Items for Systematic Reviews and Meta‐Analysis (PRISMA) guidelines. Five electronic databases were searched (Pubmed, Embase via Ovid, Cumulative Index to Nursing and Allied Health Literature [CINAHL], Web of Science and Cochrane Library) from each database's inception date until 30th July 2024. Search terms were as follows: (Thickener or thickened or viscosity or consistency or rheology or texture or thin or thick or gum or Xanthan or starch) AND (Deglutition Disorder? or Disorders, Deglutition or Dysphagia or Swallowing Disorder? or Oropharyngeal Dysphagia or Dysphagia, Oropharyngeal or swallowing difficulty or swallowing difficulties or swallowing dysfunction). All procedures were performed by 2 or more independent reviewers. In more detail, our search strategy and PICO are shown in the Data S1.

2.2. Inclusion Criteria and Study Selection

Study inclusion criteria comprised peer‐reviewed publications with population‐based studies that reported an association between thin liquid and any level of viscosity of thickened liquid in (neurogenic) disease. Case reports, review papers, meta‐analyses, organisational guidelines, editorial letters, expert opinions and conference abstracts were excluded. Studies containing data from patients with neurogenic and non‐neurogenic dysphagia were included, as long as the data for neurogenic dysphagia can be extracted for further analysis. Studies were selected using the following inclusion criteria: (1) investigation of adults with neurogenic dysphagia, (2) use of thin liquid as comparison, either with or without additional dysphagia treatment, (3) written in English, and (4) access to full text.

Studies were classified according to five main levels of evidence according to Oxford Centre for Evidence‐Based Medicine: Levels of Evidence (March 2009) (https://www.cebm.ox.ac.uk/resources/levels‐of‐evidence/oxford‐centre‐for‐evidence‐based‐medicine‐levels‐of‐evidence‐march‐2009). All classifications were based on the agreement of at least two authors and disagreements were resolved by discussion.

2.3. Data Extraction

The data extracted included: demographic information of participants (age and patient characteristics), intervention protocol, outcomes (mean [standard deviation; SD] or events) and sample sizes. If data were presented in figures and raw data was not provided by the authors, an online plot digitaliser program (https://plotdigitizer.com/app) was used to extract graphic data. Outcome measurement ‘Unsafe swallowing’ was defined as either a Penetration Aspiration Scale (PAS) score ≥ 2 or the presence of penetration and aspiration events.

2.4. Risk of Bias Assessment

The Joanna Briggs Institute (JBI) checklists were used to assess the risk of bias of the randomised controlled trials (RCT) and non‐RCT (NRCT) studies included in this review. We applied the ‘Checklist for the Randomized Controlled Trial’ to three RCT studies [10, 15, 16] and the ‘Checklist for quasi‐experimental studies’ to the 13 NRCT studies [17, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31]. For each question, a positive, negative or unclear answer was given. Then, each study was categorised depending on the percentage of positive answers: more or equal to 70% indicated a low risk of bias, < 70% indicated a moderate risk of bias and < 50% indicated a high risk of bias [32].

2.5. Statistical Analysis

All statistical analyses were performed by Review Manager 5.3 software program (RevMan; Cochrane Collaboration, Oxford, UK). Data from thickened liquids were classified as the ‘experimental’ group, while data from thin liquids were classified as the ‘control’ group. Heterogeneity was assessed using the I ² statistic, with thresholds of < 25% (low), 25%–50% (moderate) and > 50% (high). Sensitivity analysis was conducted for I ² > 50%, using leave‐one‐out methods. A random‐effects model was used throughout the meta‐analysis. Statistical significance was set at p < 0.05. Odds Ratios were calculated for dichotomous data, and Mean Differences for continuous data.

3. Results

3.1. Study Selection

Two thousand six hundred and seventy studies were imported for screening in Covidence, and 581 duplicates were removed (see Figure 1), leaving 2091 studies to be screened. One study was identified from a systematic review. After screening, 62 full texts were assessed for eligibility. Finally, 16 studies met the inclusion criteria and were included in the systematic review, and 11 studies provided sufficient data for targeted outcome measurements and thus were included in the quantitative analysis.

FIGURE 1.

Flow chart for study inclusion and exclusion process.

3.2. Study Characteristics

Detailed descriptions of key characteristics for the 16 included studies (3 RCTs and 13 NRCTs) are shown in Table 1 (Detailed thickener formulations of studies are shown in Data S2). Regarding the classification based on evidence level, of the 16 selected studies, one study was classified as level 1b, nine studies were classified as level 2b, five studies were classified as level 3b, and one study was classified as level 4.

TABLE 1.

Characteristics of included studies.

Study a	Level of evidence	Sample size	Age in years (mean)	Gender (F/M)	Aetiology	Dysphagia duration	Methods	Liquid viscosity/thickeners	Outcomes	Duration/follow up	Results
Randomised controlled trials
Logemann 2008 b	2b	711	50–79 (294) 80–95 (417)	213/498	Dementia–Alzheimer's (n = 109); Dementia–Single/multi‐stroke (n = 109); Dementia–Other (n = 133); Parkinson's disease–No dementia (n = 228); Parkinson's disease–Dementia (n = 132)	Not mentioned	All patients received all three interventions in a randomly assigned order (chin‐down; nectar thickened liquids; honey‐thickened liquids)	Thin liquid, 15 cps; nectar, 300 cps; honey, 3000 cps./not mentioned	Aspiration frequency	Immediate	Immediate elimination of aspiration on thin liquids occurred most often with honey‐thickened liquids for patients in each diagnostic category, followed by nectar‐thickened liquids and chin‐down posture. Patients with the most severe dementia exhibited the least effectiveness on all interventions.
Robbins 2008 c	1b	515	80.5	156/359	Dementia—Alzheimer's disease (n = 88), Single or multi‐stroke (n = 75), Other (n = 97); Parkinson disease—No dementia (n = 154), Dementia: (n = 101)	Not mentioned	Thin liquid with chin down (n = 259) vs. nectar thick (n = 133) vs. honey thick (n = 123) in head neutral position	Thin liquid, 15 cps; nectar, 300 cps; honey, 3000 cps./not mentioned	Primary outcome: definite pneumonia/Suspected pneumonia; secondary outcome: definite pneumonia or death	3 months	The 3‐month cumulative incidence of pneumonia was 0.098 and 0.116 in the chin‐down posture and thickened‐liquid groups, respectively (hazard ratio, 0.84 [95% CI, 0.49 to 1.45]; p = 0.53). The 3‐month cumulative incidence of pneumonia was 0.084 in the nectar‐thick liquid group compared with 0.150 in the honey‐thick liquid group (hazard ratio, 0.50 [CI, 0.23 to 1.09]; p = 0.083).
Diniz 2009 b	2b	61	63.36 (±13.35)	21/40	Stroke	Acute stroke (n = 19): median 4 days (range, 3–7 days) Prior stroke (n = 42): 150 days (range, 9 days to 4.93 years)	All patients received both liquid (5/10/20 mL) vs. spoon thick (initially 2.5‐); the first consistency was randomly chosen	Spoon‐thick (pudding‐like): 70 mL water+0.04 g Clight lemon juice, 4.4 g Thick&Easy instant food thickener, and starch compound/Starch (Hormel Health Labs, Savannah, GA)	Penetration; aspiration	Immediate	Aspiration occurred in only 3 patients with the spoon‐thick consistency vs. 21 with the liquid consistency (relative risk = 0.13; 95% confidence interval = 0.04–0.39; p < 0.001).
Non‐randomised controlled trials
Ahn 2022 c	3b	20	73.5 (median)	0/20	Ischemic stroke (n = 13), hemorrhagic stroke (n = 3), brain tumour (n = 2), and traumatic brain injury (n = 2)	42 days	Thin liquid group (IDDS 0–2) vs. thick liquid group (IDDS 3–4)	Thin liquid (IDDS 0–2); thick liquid (IDDS 3–4)/not mentioned	Primary outcome: the viscosity of thickened fluid; medical records: incidence of pneumonia/urinary tract infection/constipation/dehydration status (blood urea nitrogen)	1 week/1 month	Constipation (p = 0.338) and dehydration status (p = 0.202) were not significantly different between the 2 groups. In 2 evaluations for 20 patients, 40.0% of the cases did not follow the educated viscosity, and the number gradually increased in the follow‐up evaluation.
Bhattacharyya 2003 b	2b	55	60.2	27/28	Unilateral vocal cord paralysis (UVCP)	VFS was obtained 7.9 weeks after the initial onset of UVCP.	Each subject received 1 mL/3 mL thin and paste	Not mentioned/not mentioned	PAS scores; residue	Immediate	The mean PAS scores for liquid and paste bolus were 3.1 vs. 1.5, respectively (p < 0.001). The liquid bolus penetrated in 19 (34.5%) patients and was aspirated in 11 (20%) patients. In contrast, the paste bolus penetrated in 12 (21.8%) cases and was aspirated in 0 cases (p < 0.001). Patients were more likely to exhibit pharyngeal or hypopharyngeal residue with thicker bolus consistencies, primarily at the base of the tongue and vallecula (p = 0.018 and p = 0.031, respectively).
Makhnevich 2024 c	2b	8916	85.7 ± 8	4829/4087	Alzheimer's disease and related dementias (ADRD)	Not mentioned	Thin diet group (n = 4458) vs. thick diet group (n = 4458)	Not mentioned/not mentioned	Mortality (primary outcome), respiratory complications (e.g., pneumonia), intubation, hospital length of stay	Immediate	No significant difference in hospital mortality between the thick liquids and thin liquids groups (hazard ratio, 0.92; 95% CI, 0.75–1.14]; p = 0.46). Compared with patients receiving thin liquids, patients receiving thick liquids were less likely to be intubated (odds ratio [OR], 0.66; 95% CI, 0.54–0.80), but they were more likely to have respiratory complications (OR, 1.73; 95% CI, 1.56–1.91).
McGrail 2015 c	2b	39	Thin liquid group (n = 21, age 66.6 ± 21.4); thickened liquid group (n = 18, age 73.2 ± 12.2)	17/22	Post‐ischemic stroke	Not mentioned	21 thin liquid vs. 18 thick liquid	Not mentioned/commercial thickened liquids (Hormel Healthlabs Inc., Savannah, GA) and powder thickener (Starch; Novartis Nutrition Corporation, Minneapolis, MN)	Fluid intake	Immediate	Patients receiving thin liquids consumed significantly more than patients receiving thickened liquids (1405.45 ± 727.1 mL) (mean ± SD) vs. 906.58 ± 317.4 mL (mean ± SD); p = 0.0031; however, they were also offered significantly more fluids (mean = 2574.7 mL vs. 1588.9 mL, p = 0.0002).
Morishita 2022 b	3b	13	79.6 ± 9.6	5/8	Cerebral infarction (n = 3); Parkinson's disease (n = 2); deconditioning (n = 8)	Not mentioned	Thin vs. C thin vs. thick vs. C thick; in a random order	Thick: 100 mPa·s/xanthan gum	(PAS) score; residue; Swallowing reflex initiation; patient's subjective difficulty in swallowing	Immediate	PAS showed significant differences between liquids (p < 0.01). YPR‐SRS (Vallecula residue) showed no significant differences between liquids. YPR‐SRS (Pyriform sinus residue) showed significant differences between liquids (p < 0.01) but not in post hoc analysis. Swallowing reflex initiation showed a significant difference between liquids (p < 0.01).
Nakao 2022 b	3b	18	67.1 ± 12.8	0/18	Post‐stroke	Not mentioned	Each patient received thin vs. moderately thick (3 mL)	Thin: IDDSI level 0; thick: IDDSI level 3/xanthan gum (Softia‐1SOL, Nutri Co. Ltd., Mie, Japan)	PAS score; residue valleculae (NRRS V) and pyriform sinuses (NRRS P)	Immediate	Laryngeal peak and mean velocity, laryngeal movement distance, and iEMG were all significantly greater while swallowing moderately thick liquids compared to thin liquids. Compared to thin liquids, moderately thick induced an increase in laryngeal movement velocity and in suprahyoid muscle activity.
Vilardell 2016 b	2b	122	MS group (n = 46, 75.63 ± 8.40) XG group (n = 76, 74.83 ± 10.86)	49/73	Post‐stroke	Chronic MS group (n = 46, 7.45 ± 8.97 months) XG group (n = 76, 27.32 ± 25.32)	46MS thickening agent vs. 76 XG thickening agent	1–50 mPa s for thin liquids; 51–350 mPa s for nectar; and > 1750 mPa s for spoon thick/Modified Starch (Resource ThickenUp, Nestle´ Health Science, Vevey, Switzerland) & xanthan gum (Resource ThickenUp Clear, Nestle´ Health Science, Lausanne, Switzerland)	Clinical signs: fractional swallow, oropharyngeal residue, PAS scores; VFS signs: oral residue, pharyngeal residue, penetration, aspiration	Immediate	Both thickeners similarly improved the safety of swallowing. Prevalence of safe swallowing significantly increased with enhanced viscosity (p < 0.001 vs. liquid), increasing bolus viscosity with either thickener increased prevalence of safe swallows (p < 0.001 vs. liquid), PAS score was significantly reduced with increased viscosity with both thickeners.
Park 2013 b	4	30	70.5 ± 6.9	14/16	Post‐stroke	Not mentioned	Each patient received thin/nectar‐thick/puree (5 mL), compared in aspirators vs. non‐aspirators	Not mentioned/not mentioned	Swallowing physiology (oral and pharyngeal transition time)	Immediate	Both the oral and pharyngeal transitions differed significantly for the puree compared with the thin and nectar thick liquids.
Oommen 2011 b	3b	52	67.5	Not mentioned	Stroke	Not mentioned	Each patient received thin/nectar‐thick (5/10 mL), comparisons made between aspiration group vs. non‐aspiration group	Thin: 14 cp; nectar thick: 187 cP/not mentioned	Swallowing physiology: Stage transition duration (STD), Laryngeal closure duration	Immediate	Bolus consistency (thin versus nectar thick liquids) did not significantly affect STD [F (1, 265) = 0.27, p = 0.87]. Bolus volume did not significantly affect STD [F (1, 265) = 1.08, p = 0.30].
Rofes 2014 b	3b	120	74.4 ± 12.4	55/65	Neurodegenerative diseases (n = 13), stroke (n = 66), elderly (n = 41)	Not mentioned	VVST for each patient (5, 10, and 20 mL) and viscosities (thin liquid, nectar‐like, spoon thick)	1–50 mPa s for thin liquid, 51–350 mPa s for nectar‐like, 351–1750 mPa s for honey and conservative spoon thick and > 1750 mPa s for spoon‐thick viscosity/Xanthan gum (Resource ThickenUp Clear, Nestle Health Science)	Clinical signs: Voice changes, Cough, Oxygen desaturation > 2%, Safe swallow, Effective lip closure, Oral residue, Pharyngeal residue, Piecemeal deglutition; VFSS signs: Penetrations, Aspirations, Safe swallow, Oral residue, Vallecular residue, Pyriform sinus residue		The prevalence of aspirations was 12.7% with thin liquid, 7.7% with nectar‐like (p < 0.01), and 3.4% with spoon‐thick (p < 0.01) viscosities. PAS was reduced from 3.24 ± 0.18 at thin liquid to 2.20 ± 0.18 at nectar‐like (p < 0.001) and to 1.53 ± 0.13 at spoon‐thick (p < 0.001) viscosities; (ii) did not enhance pharyngeal residue; (iii) nectar‐like viscosity did not affect bolus velocity nor timing of swallow response and (iv) spoon‐thick viscosity reduced bolus velocity.
Ortega 2020 b	2b	128	Older (82.96 ± 1.24) HNC (68.29 ± 1.39) Parkinson's (72.34 ± 1.92) Stroke (79.42 ± 1.36)	51/77	Older (n = 36); head/neck cancer (HNC) (n = 31); Parkinson's disease (n = 30); chronic post‐stroke (n = 31)	Not mentioned	Each patient received 4 levels of shear viscosity of liquids (< 50, 250, 1000 and 2000 mPa·s) (5/20 mL); terminated when aspiration was observed	< 50, 250, 1000 and 2000 mPa·s/combination of xanthan gum, modified starch (Fresubin Clear Thickener (Fresenius‐Kabi Deutschland GmbH, Bad Homburg, Deutschland))	PAS scores, safe &unsafe swallowing	Immediate	Patients had a high prevalence of VFS signs of impaired efficacy (98.44%) and safety (70.31%) of swallow with a severe PAS score (4.44 ± 0.20). Fresubin Clear Thickener (FCT) showed a strong therapeutic effect on the safety of swallow at a range between 250 and 1000 mPa·s (74.19%–96.67%, safe swallows in G1, G3, G4, and 58.06% in G2), without increasing pharyngeal residue.
Bolivar‐Prados 2023 b	2b	85	83 ± 6.93	40/45	Aging (n = 38), stroke (n = 31), other neurodegenerative diseases (n = 16)	Not mentioned	6 levels of viscosity, terminated when aspiration was observed	< 50 mPa·s, 100, 200, 400, 800 and 1600 mPa·s/xanthan gum (Tsururinko Quickly)	PAS scores; residue; Oropharyngeal; Swallowing Response: laryngeal vestibule closure, upper oesophageal sphincter opening	Immediate	At < 50 mPa·s, only 16.25% of patients swallowed safely, 45% had penetrations (PAS 3–5), and 38.75% had aspirations (PAS 6–8). Thickened fluid greatly increased the prevalence of safe swallowing from 62.90% at 100 mPa·s to 95.24% at 1600 mPa·s in a shear‐viscosity‐dependent manner. The penetrations and aspirations were significantly reduced to 3.60% and 1.19%, respectively, at 1600 mPa·s. The threshold viscosity was 100 mPa·s and the increasing viscosity above 800 mPa·s did not further improve the therapeutic effect significantly. Increasing the shear viscosity significantly reduced the time to laryngeal vestibule closure (−16.7%), increased the time to upper oesophageal sphincter opening (+26.88%), and reduced the pharyngeal bolus velocity (−31.62%) without affecting the pharyngeal residue.
Clave p 2006 b	2b	92	51.92 ± 2.41	34/58	Stroke (n = 24), traumatic brain injury (n = 22), amyotrophic lateral sclerosis (n = 20), multiple sclerosis (n = 16), myopathies (n = 10)	64.43 ± 96.13 (months)	Nectar vs. liquid vs. pudding (3, 5, 10, 15, 20 mL). If the patient presented VFS signs of aspiration on two consecutive increased volumes during nectar or liquid viscosity studies, the series was interrupted, higher volumes were not tested, and a ‘safer’ series was assessed.	Liquid (20.4 mPa·s), nectar (274.4 mPa s) and pudding (3931.2 mPa·s)/Starch (Resource ThickenUp; Novartis Consumer Health SA, Barcelona, Spain)	Penetration/aspiration events	Immediate	Brain damage patients presented: (i) 21.6% aspiration of liquids, reduced by nectar (10.5%) and pudding (5.3%) viscosity (p < 0.05) and (ii) 39.5% oropharyngeal residue. Neurodegenerative patients presented: (i) 16.2% aspiration of liquids, reduced by nectar (8.3%) and pudding (2.9%) viscosity (p < 0.05) and (ii) 44.4% oropharyngeal residue. Both groups of patients presented prolonged swallow response ≥ 806 ms) with a delay in laryngeal closure ≥ 245 ms), and weak bolus propulsion forces (≤ 0.20 mJ).

Abbreviations: C thick, carbonated thick drink; C thin, carbonated thin drink; cps, centipoise; iEMG, electromyography; mL, millilitre; mPa·s, millipascal‐seconds; ms, milliseconds; MS, modified starch; NRRS, normalised residue ratio scale; VFS/VFSS, videofluoroscopic; V‐VST, Volume‐Viscosity Swallow Test; XG, xanthan gum; YPR‐SRS, Yale Pharyngeal Residue Severity Rating Scale.

^a Study design.

^b Crossover group.

^c Parallel group.

3.3. Participant Characteristics

In total, 1287 individuals were included in the RCTs and 9690 in the NRCTs. Participants were primarily older adults, and the majority of studies included participants with a primary dysphagia aetiology of cerebrovascular accident/stroke or brain injury. Other common aetiologies included dementia and Parkinson's disease. Five studies did not report full information on age and one did not report on sex; head and neck cancer (HNC) patients were also included in Ortega et al. [28] but these patients were not included in the data synthesis and their data were not analysed in our study.

3.4. Viscosity of Thickened Liquid

The majority of included studies provided sufficient information regarding tested viscosity or International Dysphagia Diet Standardisation Initiative (IDDSI) levels to identify each liquid. The remaining studies described liquids either using ambiguous terms or were not mentioned. The viscosities used in each study are listed in Table 1

3.5. Risk of Bias

Risk of bias evaluation of the 3 RCTs included showed that one was low [15] and the others moderate [10, 16]. Regarding the NRCTs, only one out of 13 studies showed a low risk of bias [20] while the rest remained of moderate risk. The negative or unclear items mostly concerned the absence of a control group, pre and post outcome measurements, and follow‐up evaluations. Unlike other pre‐post interventional studies, most studies used immediate outcome measurements and own controls which lacked both pre and post measurements. A summary of the risk of bias assessment is presented in Figure 2.

FIGURE 2.

Risk of bias assessment of included RCTs and NRCTs.

3.6. Outcome Measures

The outcome measures used varied across studies. The most used outcome measure was aspiration incidence, followed by penetration incidence, PAS scores, safe swallows and residue. Additionally, studies by Park et al. [29] focusing on swallowing physiology, McGrail et al. [24] addressed fluid intake, Ahn et al. [21] investigated the incidence of pneumonia, urinary tract infection, constipation, etc., and Makhnevich et al. [20] examining mortality, respiratory complications, intubation and length of hospital stay were excluded from the data analysis. Due to insufficient data for analysis, these variables were not further quantified. Although outcome measurements in Clave [23] were available, sufficient and reliable data could not be extracted and thus were not included in the data analysis either.

3.7. Data Synthesis and Meta‐Analysis

Among all thickened liquid studies, aspiration (n = 8 studies) and unsafe swallowing (n = 7 studies) were studied most extensively. Data analysis was conducted between groups (thin liquid vs. thickened liquid) concerning penetration (n = 5) and aspiration (n = 8) events, PAS scores (n = 4), unsafe swallows (n = 7) and residue (n = 5). However, data from the carbonated liquid groups in Morishita [25] were not included, as carbonated liquids may introduce a distinct variable affecting swallowing outcomes. Data analysis was performed separately for RCTs and NRCTs when sufficient data were present.

3.7.1. Effects of Thickened Liquid on Aspiration Events From RCTs

Three datasets from RCTs and five datasets from NRCTs were merged to yield a pooled effect size regarding how thickened liquid affects aspiration incidence. The analysis demonstrated a medium pooled effect size for the impact of thickened liquids in reducing aspiration events from RCTs ([95% CI] = 0.49 [0.28, 0.88]; p = 0.02; I ² = 81%) (Figure 3) and a small pooled effect size from NRCTs ([95% CI] =0.31 [0.13, 0.71]; p = 0.006; I ² = 47%) (Figure 4b). Within the RCTs analysis, both Logemann 2008 and Robbins 2008 conducted multi‐centre large‐scale studies while Diniz 2009 included a competitively small group of people, which likely contributed to the substantial heterogeneity when merged. Notably, the heterogeneity was significantly reduced after removing the study by Diniz 2009 (I ² = 0%). By contrast, the test for heterogeneity within NRCTs was not significant (Tau² = 0.35; I ² = 47%, p = 0.11).

FIGURE 3.

Effects of thickened liquid on Aspiration events in RCTs. Forest plot indicates significant effects of thickened liquid in reducing aspiration events.

FIGURE 4.

Effects of thickened liquid on swallowing safety in NRCTs. Forest plot indicates significant effects of thickened liquid in reducing (a) unsafe swallowing, (b) aspiration events, and (d) PAS scores, while shows no significant effect on (c) penetration events.

3.7.2. Effects of Thickened Liquid on Unsafe Swallowing

As shown in Figure 4a, 6 datasets were merged into a pooled effect size. The analysis revealed a small pooled effect size for the impact of thickened liquids in reducing unsafe swallowing ([95% CI] = 0.27 [0.14, 0.51]; p < 0.001; I ² = 88%). Additionally, the result showed that unsafe swallowing was more likely to occur when swallowing thin liquid ([95% CI] = 3.73 [1.98, 7.03]; p < 0.001; I ² = 88%). The test for heterogeneity was significant (Tau² = 0.51; p < 0.001; I ² = 88%) and was reduced from 88% to 0% when the study by Rofes et al. [30] and Bolivar‐Prados et al. [22] were excluded. This reduction may be attributed to differences in study methodologies, as Rofes et al. utilised the Volume‐Viscosity Swallow Test (V‐VST) strategy, and Bolivar‐Prados et al. employed a similar approach, terminating the test when aspiration was observed. Importantly, the effect size remained significant after adjustment.

3.7.3. Effects of Thickened Liquid on Penetration Events

Four datasets from NRCTs were merged to yield a pooled effect size regarding how thickened liquid affects the penetration incidence in dysphagia patients. The results showed that there was no significant difference in penetration incidence between thin liquid and thickened liquid ([95% CI] = 0.40 [0.13, 1.22]; p = 0.11; I ² = 88%) with substantial heterogeneity (Figure 4c). Sensitivity analysis was thus conducted. Regarding penetration incidence, the heterogeneity was significantly reduced (I ² = 0%) when the study by Vilardell et al. [31] which performed a retrospective study where the exclusion criteria were not mentioned (and had a high risk of population similarity bias) was excluded. However, the difference in penetration incidence between the thickened and thin liquid groups remained insignificant ([95% CI] = 0.68 [0.42, 1.10]; p = 0.11; I ² = 0%). Therefore, the result suggested that thickened liquid does not help in reducing penetration incidence.

3.7.4. Effects of Thickened Liquid on PAS Scores

As shown in Figure 4d, 4 datasets were merged into a pooled effect size. The analysis revealed a large pooled effect size for the impact of thickened liquids on PAS scores with substantial heterogeneity ([95% CI] = −1.99 [−2.59, −1.38]; p < 0.00001; I ² = 89%). The heterogeneity was reduced from 89% to 76% when the study by Bolivar‐Prados et al. [22] was excluded as the test was terminated when aspiration was observed in their study. However, the effect size remained significant after adjustment.

3.7.5. Effects of Thickened Liquid on Residue

As shown in Figures 5 and 4 datasets were merged into a pooled effect size. The analysis demonstrated a large pooled effect size for the impact of thickened liquids on both pharyngeal residue ([95% CI] =1.57 [1.20, 2.06]; p = 0.001; I ² = 16%) and oral residue ([95% CI] =2.87 [1.88, 4.40]; p < 0.0001; I ² = 45%). The heterogeneity for both analyses was not significant (pharyngeal residue: Tau² = 0.01, p = 0.31, I ² = 16%; oral residue: Tau² = 0.08, p = 0.14, I ² = 45%).

FIGURE 5.

Effects of thickened liquid on residue in NRCTs. Forest plot indicates thickened liquid significantly increases the likelihood of (a) pharyngeal residue and (b) oral residue.

4. Discussion

This systematic review evaluated the efficacy of thickened liquids in adult patients with neurogenic dysphagia and yielded several important findings. Among all thickened liquid studies, aspiration and unsafe swallowing were the most extensively studied. Both results from RCTs and NRCTs showed that thickening liquid helped to reduce aspiration. Results from NRCTs showed improvement for thickeners in reducing unsafe swallowing in dysphagic patients. However, thickeners have a limited impact on penetration incidence and are associated with increased residue [33, 34]. Our findings indicate that while thickened liquids may be effective for patients with neurogenic dysphagia, their utility should be carefully weighed against potential drawbacks such as residue accumulation.

The findings of this systematic review contribute to the growing body of literature on the use of thickened liquids for managing dysphagia. Previous systematic reviews of related topics focused exclusively on RCTs to ensure the robustness of their findings. That review included a total of 2 RCTs (3 in their latest update), both of which weakly recommended against the thickening of liquid due to the quality of evidence evaluation [14]. Importantly, those individual studies focused on outcomes such as mortality, pneumonia, quality of life, nutritional status or oral intake in individuals with oropharyngeal dysphagia. Notably, the RCT included in their update focused on a maxillectomy population [35], which was believed to significantly impact swallowing physiology and therefore was not included in our study.

In line with earlier studies, our results indicated that thickened liquids may help reduce aspiration events and increase the prevalence of safe swallowing in neurogenic dysphagia, suggesting that they could potentially improve swallowing safety by changing the flow dynamics during swallowing [2, 33, 36]. While the study by Diniz et al. [10] suggested that thickened liquid reduced the incidence of penetration (observing penetration in 13.1% of patients receiving water compared to none in those receiving the spoon‐thick (pudding‐like) consistency), our meta‐analysis result showed that there was no difference in penetration incidence between thin liquid and thickened liquid. This could be attributed to the fact that penetration primarily depends on laryngeal sensation and the timing of swallowing reflexes, which are not significantly altered by increasing liquid viscosity [23]. Thickened liquids may slow the bolus, but this does not necessarily enhance the mechanisms required to prevent material from entering the supraglottic space. Moreover, previous studies also suggested that increased bolus viscosity may also result in increased post‐swallow oral and pharyngeal residue, which could subsequently contribute to penetration during subsequent swallows [33, 34]. Notably, pharyngeal or vallecular residue may serve as a source for secondary aspiration, especially in patients with impaired airway protection or delayed swallow response. A previous fluoroscopic study including over 2000 dysphagic patients found that 56% of aspiration occurred on residue remaining after swallowing [37]. In addition, residue increases the effort of swallowing requiring additional clearing swallows and negatively impacts pleasure associated with eating, which may reduce the quality of life [38]. These risks highlight the importance of evaluating residue not only as an isolated observation but also in relation to patient safety and overall health outcomes. These findings support the notion that thickened liquids alone may not address penetration effectively and need additional interventions, such as techniques that target laryngeal closure or sensory stimulation, to complement their use.

While thickened liquids are commonly used to improve swallowing safety, their clinical application is often challenged by issues such as poor patient adherence, increased risk of dehydration and reduced quality of life. In a previous study involving 20 patients, 40% did not follow the educated viscosity, and the number gradually increased in the follow‐up evaluation [21]. Robbins et al. found that more patients assigned to thickened liquids than those assigned to the chin‐down posture intervention had dehydration (6% vs. 2%), urinary tract infection (6% vs. 3%) and fever (4% vs. 2%) [15]. Nevertheless, given that dysphagia is closely associated with age, physical function, frailty, polymedication and multimorbidity [39], these results may have been affected by the backgrounds of patients in these various study facilities [40] and should be considered when interpreting the results.

Several methodological limitations were identified across the included studies, which may have contributed to the overall risk of bias. Lack of blinding was a common issue in multiple studies, where either participants, therapists or assessors were not blinded [10, 15, 20, 22, 23, 24, 26, 27, 28, 29, 30, 31, 41]. This introduces potential performance and detection bias. Absence of randomisation or unclear randomisation procedures [10, 25, 41] was also observed, increasing the risk of selection bias. Small sample sizes, as seen in Ahn et al. (n = 20) [21], Morishita et al. (n = 13) [25] and Nakao et al. (n = 18) [26], may reduce statistical power and limit the generalisability of findings. Despite these weaknesses, these studies were included due to the scarcity of high‐quality data in this research area. Their limitations have been considered when interpreting our findings, and caution is advised in generalising the conclusions.

Considering studies not included in our data analysis, Clave et al. [23] supported the use of thickened liquids for managing dysphagia, aligning with the majority of literature on this topic. Clave et al. found that thick liquids significantly reduced the risk of aspiration compared to thin liquids, enhancing swallowing safety. However, they noted that thickened liquids could lead to decreased fluid intake and patient dissatisfaction due to changes in taste and texture. Park et al. [29] tried to determine whether oral transit time and pharyngeal transit time were different across varying bolus viscosities. They found that both the oral and pharyngeal transitions differed significantly for the puree compared with the thin and nectar thick liquids. Ahn et al. [21] investigated 20 dysphagia patients with brain lesions who were recommended to use thickeners. The authors reported that there were no significant differences in the incidence of adverse effects, such as pneumonia or dehydration, according to the fluid viscosity, emphasising the need to develop detailed criteria for thickener use in dysphagia patients with brain lesions. Makhnevich et al. [20] contributed to the discussion with a large cohort study focusing on older patients with Alzheimer's disease and related dementias. They reported no significant difference in hospital mortality between patients on thick versus thin liquids but noted that those on thick liquids had lower intubation rates, albeit with a higher incidence of respiratory complications. This highlights the need for careful consideration of the benefits and risks associated with thickened liquids, particularly in vulnerable populations. McGrail et al. [24] found that patients consuming thin liquids had significantly higher fluid intake compared to those on thickened liquids. This raises important questions about hydration adequacy when implementing thickened diets. Their findings suggest that while thickened liquids may enhance swallowing safety, they could inadvertently restrict overall fluid consumption if not managed appropriately.

One limitation of this review is that we only searched in ‘title/Abstract’ or ‘topic’ fields. This was with a view to trying to keep our review more focused. However, there is the possibility that the search terms we used may have resulted in some relevant papers being excluded. Moreover, only English studies were included for analysis. Further, the included studies were highly heterogeneous due to the limited number of RCTs and methodological diversity and bias of the NRCTs. Variability in study designs, participant characteristics and assessment methodologies often lead to differing outcomes, complicating the synthesis of evidence regarding treatment effectiveness. Therefore, our results must be interpreted with some caution.

A further potential limitation of this study is that, for some included studies, numerical data were not directly available and had to be extracted from figures using an online plot digitizer tool (https://plotdigitizer.com/app). Two independent reviewers verified the extracted data to ensure its accuracy, though minor estimation errors cannot be completely ruled out.

While our review highlights the potentially significant effects of thickened liquids for the management of neurogenic dysphagia, it needs to be interpreted with care as many of the findings related to aspiration, penetration, and swallowing safety were driven primarily from non‐RCTs, which limits the overall strength of the evidence. It is also worth noting that other therapeutic approaches have also demonstrated meaningful effects on swallowing function in patients with dysphagia. For instance, a meta‐analysis on oropharyngeal muscle strength training showed significant improvements in both swallowing safety and oral intake: the training reduced PAS scores compared with conventional therapy (mean difference = −0.98, 95% CI −1.34 to −0.62, p < 0.0001) and increased FOIS scores (mean difference = 1.04, 95% CI 0.55 to 1.54, p < 0.0001) [5]. Similarly, another meta‐analysis reported a moderate effect size favouring neuromuscular electrical stimulation (NMES) over traditional therapy in improving PAS scores (standardised mean difference = −0.56, 95% CI −1.01 to −0.10, p = 0.02) [3]. Thickened liquids appear to provide some benefits, such as reducing aspiration risk; however, our study indicated that improvements in penetration were not significant. This raises important questions about the need for individualised treatment approaches. In addition, the meta‐analysis presents a high level of heterogeneity, which weakens the generalisability of the results. Despite our sensitivity analyses not substantially altering the findings, the overall meta‐analysis presents a considerable degree of heterogeneity that limits the strength and applicability of the effectiveness of thickeners. Moreover, potential confounding factors such as patient comorbidities, variability in dysphagia severity, and differences in thickener formulations could not be adjusted, as this information was inconsistently reported across the included studies, which also limits the interpretability of the results.

Future research should prioritise the use of standardised outcome measures to enable more accurate comparisons across studies. Well‐designed, multicentre randomised controlled trials are also needed, with consistent thickener formulations and appropriate stratification based on dysphagia severity. In addition, comparative studies evaluating thickened liquids alongside other dysphagia management strategies could provide valuable clinical insights. Finally, personalised interventions tailored to patients' specific etiologies and individual needs will be essential for optimising treatment outcomes.

Overall, our findings reinforce the necessity for further large‐scale research to validate the use of thickened liquids across diverse dysphagia populations and to develop comprehensive treatment protocols tailored to individual patient characteristics.

Author Contributions

S.H.: conceived the review; W.‐Q.L., I.C., A.S. and M.D.: researched the literature; W.‐Q.L.: wrote the manuscript; W.‐Q.L., I.C., A.S. and S.H.: revised the manuscript. All authors contributed to the article and approved the submitted version.

Conflicts of Interest

Shaheen Hamdy is the chief scientific officer and stocks/shares holder of Phagenesis Ltd., a company involved in neuromodulatory dysphagia treatment. The other authors declare no conflicts of interest.

Supporting information

Data S1.

Data S2.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.