High-resolution pharyngeal impedance manometry (HRPIM) is beginning to be used in clinical evaluation for dysphagia, in particular cyclopharyngeal dysphagia, which can more objectively reflect the change in UES function, so as to better explain the pharyngeal pathophysiological mechanism and find more effective treatment options [23]. The indicators commonly used to evaluate the disorder include (1) UES lumen occlusive pressure, which is the pressure of esophageal closure caused by the contraction of the UES muscles in a calm state, which should be measured before and after swallowing respectively; (2) intrabolus pressure (IBP) refers to the resistance of the UES to the food mass during swallowing. Abnormally elevated IBP indicates an increased pressure gradient at the pharyngoesophageal junction; (3) integrated relaxation pressure (IRP), that is, the pressure during the maximum relaxation period of UES; and (4) the degree and duration of UES opening. The opening of the UES comes from the pressure generated by the contraction of the pharyngeal muscles and is determined by the intensity and timing of activation of the suprahyoid and subhyoid muscles [24]. These indicators can help us identify subtle changes in early dysphagia in PD patients. An investigation of pharyngoesophageal dyskinesia in patients with early PD revealed [25] that pharyngeal and esophageal dyskinesias are present in patients with early PD before they show symptoms of dysphagia, that patients often complain of a foreign body sensation in the pharynx during swallowing and the need to swallow several times before completing a single injection, that pharyngeal manometry shows abnormalities in the peristalsis of the esophageal sphincter, and that patients with PD show an erroneous contraction timing in the pharyngeal phase in early life, and that this error occurs significantly earlier than in the case of limbic dyskinesias. This suggests that hypophagia is not simply neuronal loss and dopamine deficiency but that the body compensates for this by using oropharyngeal sensation and movement. Dysphagia severity is evaluated using fiberoptic endoscopic evaluation of swallowing (FEES) and video fluoroscopic swallowing studies (VFSS) which are considered the benchmark for diagnosing dysphagia. Both can accurately identify the penetration and aspiration of pharyngeal swallowing and the ability of the airway to clear foreign objects. Patients can be evaluated semi-quantitatively according to Penetration Aspiration Scale (PAS) grading. VFSS with video recording function can objectively record the changes in pharyngeal transit time (PTT) and the time of UES opening. For the evaluation of Parkinson’s dysphagia, it can more keenly observe achalasia of cricopharyngeus muscle disorder and distinguish it from the difficulty of injection caused by pharyngeal contraction muscle weakness. Conversely, HRPIM can identify subtle changes in the early stages of a patient’s life by quantifying the dynamic changes in pressure and time from the Eustachian tube to the UES during swallowing. It can also provide an objective basis for disease progression and improvement of treatment outcomes [26]. There is no doubt that FEES and VFSS are widely used in clinical practice due to their versatility and convenience, while HRPIM is susceptible to multiple factors, such as the age of the subject, bolus size, and bolus consistency, although HRPIM is still a very favorable diagnostic method for the evaluation of swallowing disorders in the pharyngeal stage.

At present, the pathological mechanism of PD in swallowing function is still unclear, but swallowing disorder in PD patients cannot be regarded as a simple movement disorder, and abnormal contraction timing in the pharyngeal period during swallowing may be the most important cause of aspiration. This systematic review incorporates multiple types of treatment options, including pharmacologic treatment, exercise therapy, physical factor treatment, the swallowing strategy, neuroregulatory techniques, and UES resistance reduction therapy. The purpose was to identify more sensitive diagnostic tools and more effective treatment methods, and provide supporting evidence for the establishment of larger randomized controlled trials in the future.

We systematically explored databases, such as PubMed, Web of Science, Elsevier, and Cochrane Library, to find clinical studies published in multiple languages until December, 2023. Our search involved using the terms ‘Parkinson’s disease’ AND ‘dysphagia’ OR ‘deglutition disorder’ AND ‘rehabilitation’ OR ‘physiotherapy’ OR ‘physiotherapy treatment’. In addition, we manually searched for additional studies that might be relevant.

In our comprehensive analysis, we examined trials that investigated the impact of different therapies on dysphagia in individuals diagnosed with PD. We established specific criteria based on the PICOS principles: P (Population)—patients with PD experiencing dysphagia; I (Intervention)—any treatment aimed at addressing dysphagia; C (Comparison)—comparing experimental groups with control groups (including placebo or blank groups); O (Outcome)—evaluating HRPM, FEES, VFSS, and various swallowing assessment scales; and S (Study design) — focusing on randomized controlled trials (RCTs). Exclusion criteria: (1) duplicate studies, (2) review articles, and (3) the full article cannot be obtained.

The retrieved literature was imported into Endnote20 (Clarivate Analytics, Philadelphia, PA, USA), and the titles and abstracts were read repeatedly by two researchers to determine the literature to be included, with discussion with a third researcher in case of disagreement. Where similar lines of research existed, only the most recent trials were included.

According to the Cochrane Handbook, data were extracted from the included studies by two researchers, including the article’s first author, year of publication, title, sample size, basic information about the subjects, subgroups, intervention modality, length of intervention, diagnostic tools, and outcomes.

Two researchers evaluated the trial’s quality using the Cochrane Collaboration tools, which encompassed seven domains for assessing bias risk: random sequence generation, allocation concealment, blinding of investigator and subjects, blinding of outcome assessment, follow-up bias, selective reporting, and other sources of bias. Any disagreements were resolved through discussion with a third researcher.

Five hundred ninety-six papers matching the search strategy were retrieved from the databases. After deleting duplicate studies, 18 relevant studies were retained after reviewing article titles and abstracts. After reading the complete text, five full texts were not found, three lacked final results, 10 randomized controlled trials were obtained, and we manually searched for two more before-and-after controlled trials, as we did not retrieve any randomized controlled trials of studies related to the lowering of resistance to UES. Ultimately, our systematic review included 15 studies [28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42] (Fig. 3).

A total of 519 patients were included in 15 studies, all of whom were assessed for motor ability using the H-Y classification. Most of the staging was between 2–4, all of whom underwent a variety of treatments for dysphagia, which included oropharyngeal muscle exercise therapy [32, 38], physical factor therapy [29, 30, 35], swallowing strategies [28, 31, 33], neuromodulation techniques [36, 37], and UES resistance reduction techniques [34, 39]. FEES [31, 33, 37, 38], VFSS [28, 29, 32, 35, 36, 39], and HRPIM [34, 39] were used in the majority of studies. One or two of these three diagnostic modalities are recognized as having high confidence, which reduces the error in the study results to some extent. The general information and characteristics of the patients included in the study are shown in Table 1 (Ref. [28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42]).

We assessed the risk of bias in 15 of the included studies, three of which did not report random sequences generation, three of which did not perform allocation concealment, five of which explicitly blinded participants and personnel, and six of which blinded outcome assessment. All had a low risk of bias in terms of selective reporting and incomplete outcome data. Fig. 4 shows the assessment of risk bias for the included literature.

In a study by Ayres et al. [33], the FEES assessment did not show any difference between the three groups, while a comparison of the Functional Oral Intake Scale (FOIS) showed that the experimental group (EG) showed significant improvement in swallowing solids (p 0.001) and liquids (p 0.002), and also in the “symptom frequency” of the quality of life-related to swallowing (SWAL-QOL) (p 0.029). This demonstrates that the chin-down swallowing posture improves swallowing ability and brings sensory enhancement to the patient, but it does not improve laryngeal symptoms.

In the report by Logemann et al. [28], the VFSS results of the three groups were compared. It was found that the high-thickness group had significantly fewer subjects experiencing aspiration compared with the low-thickness group (53% versus 63%, p 0.001) or the chin-down swallowing group (53% versus 68%, p 0.001). However, when considering the subjective feelings of the patients about eating, a lower proportion of subjects in the high-thickness liquids group felt that swallowing was easy and pleasurable compared with the chin-down swallowing group (29% versus 37%, p 0.05) and the low-thickness group (29% versus 37%, p 0.01). This suggests that while high-thickness liquids were the most effective strategy for eliminating aspiration, followed by low-thickness liquids and the chin-down swallowing position, they provided a negative eating experience for the patients, leading them to reject this swallowing strategy.

Heijnen et al. [30] compared the SWAL-QOL and Monroe Dunaway (MD) Anderson Dysphagia Inventory (MDADI) within each group before and after treatment. The Wilcoxon signed-rank test revealed significant changes in the symptom indices of each group. However, these changes were not statistically significant (p = N.S.), and no significant correlation was found between the scales (R 0.2). There were no discernible variations between groups in the impact of treatment and the difference between follow-up and post-therapy data, suggesting that neuromuscular electrical stimulation (NMES) does not provide therapeutic benefits for dysphagia in patients with PD.

Park et al. [35] utilized the VFSS to observe significant variations in the amplitude of horizontal (p = 0.038) and vertical (p = 0.042) movements of the hyoid bone between the test group and the placebo group. Additionally, the test group showed significant differences in the Videofluoroscopic Dysphagia scale (VDS) total score (p = 0.021), pharyngeal phase score (p = 0.028), and penetration-aspiration scale (PAS) score (p = 0.007) during the scale assessment. Conversely, the placebo group only demonstrated a significant difference in the total score (p = 0.041), indicating an improvement. Furthermore, when comparing the two groups, a substantial difference in the PAS score was found in the test group (p = 0.039).

Baijens et al. [29] discovered that the statistical significance of electrical stimulation and its location (supraglottic, subglottic, or combined) was insignificant. However, stimulation of the subglottic muscle group did show a significant difference in reducing the average time it takes for the laryngeal opening to occur compared with the combined stimulation method. Interestingly, the sham-stimulation group also exhibited the same effect. This suggests that surface electrical stimulation may impact swallowing function in PD, but it could be attributed to a placebo effect.

In a study by Manor et al. [31], the video-assisted swallowing therapy (VAST) team demonstrated a more significant enhancement in the occurrence of leftover food during FEES compared with the control group. The initial measurements were 1.73 $\pm$ 0.93 and 1.48 $\pm$ 1.06, while the immediate measurements post-therapy were 0.44 $\pm$ 0.44 and 0.95 $\pm$ 0.81, respectively. This difference was statistically significant (p 0.005). The signs of dysphagia were significantly alleviated before and after the completion of therapy and during the 1-month check-up (initial measurement 14.65 $\pm$ 5.81 and 14.27 $\pm$ 7.17, post-treatment 12.73 $\pm$ 7.6 and 13.43 $\pm$ 7.03, follow-up 9.05 $\pm$ 5.30 and 13.08 $\pm$ 7.2). It was demonstrated that VAST enhances the patient’s understanding of swallowing strategies, increases the pleasure of eating, and is very effective in helping the patient actively remove penetration/aspiration/pharyngeal residue in person.

Claus et al.’s [38] FEES study indicated that the EG exhibited a notable enhancement in residue scores (p 0.001) and maintained this improvement even after 3 months (p 0.05). Conversely, no significant improvement was observed in premature food dropping (p = 0.23) and aspiration (p = 0.68). The magnetencephalography (MEG) results revealed that the activation regions were primarily concentrated in the bilateral pericentral cortex. There was no notable distinction between the groups’ primary and secondary sensorimotor regions (p = 0.643). This demonstrates that expiratory muscle strength training (EMST) has a positive effect on treating dysphagia, but this improvement does not come from the modulation of the supra-medullary swallowing network.

The study of Byeon et al. [32] conducted in 2016 demonstrated that after treatment, the videofluoroscopy (VFS) scale scores decreased in both groups. However, the combined intervention group exhibited a more significant decrease than the EMST group (p 0.05). This finding provides evidence that EMST can improve patients’ swallowing function, mainly by activating the supraglottic muscle groups, which positively impacts the activation of the supraglottic muscles. The simultaneous use of postural compensatory strategy and EMST can give patients safe swallowing and strengthen swallowing muscles, synergistically improving dysphagia.

Pflug et al. [37] demonstrated that there was no notable alteration in the average measurements of swallowing capacity at the start when compared with the average measurements of PAS and residues under the stimulation modes of deep brain stimulation with Subthalamic Nucleus (STN-DBS) or deep brain stimulation with Subthalamic Nucleus and Substantia nigra (STN+SNr-DBS), as determined by Friedman tests. The research findings indicated that STN+SNr-DBS does not provide any additional impact on the swallowing functionality compared with STN-DBS. If the application of STN+SNr-DBS is intended for enhancing axial symptoms and gait disorders in PD patients, it can be deemed safe with dysphagia.

In a study by Wu et al. [39], the comparison between the patient’s UES integrated relaxation pressure (IRP) before and after surgery showed a decrease from 13.7 mmHg to 3.6 mmHg, with a mean difference of –10.1 mmHg (95% confidence interval (CI) [–16.3, –3.9], p = 0.007). Additionally, the average hypopharyngeal intrabolus pressure (IBP) decreased from 23.5 mmHg to 10.4 mmHg, with a mean difference of –11.3 mmHg (95% CI [–17.2, –5.4], p = 0.003). It was demonstrated that cricopharyngeal peroral endoscopic myotomy (C-POEM) could induce the relaxation of the UES, thus reducing the resistance during swallowing and promoting the improvement of swallowing function.

Triadafilopoulos et al. [34] conducted a study involving three individuals who received botulinum neurotoxin (BoNT) injections for dysphagia in their UES. All three patients showed enhanced swallowing ability, and the average duration of the impact was 8.3 months (ranging from 1 to 2 to 3 months, with a 95% CI of 2–13.6 months). In patients with PD, injections of BoNT into the esophageal sphincter have demonstrated efficacy in alleviating dysphagia. Moreover, this treatment is considered feasible, well-tolerated, and safe for repeated use.

In the study by Khedr et al. [36], a two-way repeated measures analysis of variance was conducted for each Arabic-Dysphagia Handicap Index (A-DHI) score. The main factors were treatment condition (real repetitive transcranial magnetic stimulation (rTMS) versus sham rTMS) and time (baseline, post-treatment, after 1, 2, and 3 months). The results indicated that the average decrease in the A-DHI was significantly more significant in the real group (14.4 $\pm$ 9.9) compared with the sham rTMS group (0.9 $\pm$ 3.0) (p = 0.0001). A comparison of the VFSS between the two groups revealed a significant improvement in hyoid maximal elevation time and pharyngeal transit time (PTT) in the rTMS group when swallowing liquids (p = 0.04 and p = 0.03). Regarding eating solids, two-way repeated-measures analysis of variance (ANOVA) showed that the rTMS group exhibited a significant difference (p = 0.007 and p = 0.03, respectively). While comparing the two groups regarding Penetration Aspiration (PA) scores and residue (including three types of food: liquid, semi-solid, and solid), there was no difference (p = N.S.). Thus, rTMS improves dysphagia in PD patients; this effect is evident from the end of treatment and can be maintained for 3 months. In addition, five monthly supplemental therapies may have a more prolonged maintenance effect.

A study by Restivo et al. [41] included only four patients. After percutaneous injection of BoNT into a bilateral cricopharyngeal muscle, the patient showed significant improvement in swallowing function 48 hours later, and this effect lasted for more than 20 weeks. Meanwhile, VFSS showed that the coordination between the cricopharyngeal muscle and the pharyngeal constrictor muscle was normal, the symptoms of cricopharyngeal muscle spasm were significantly improved, and the patient’s body weight was significantly increased.

Huang et al. [42] described 38 PD patients with dysphagia that were treated with high-frequency rTMS for 10 days. After treatment, all patients reported subjective improvement in swallowing function. Functional magnetic resonance imaging (fMRI) examination was performed before and after treatment to observe the brain activation of patients when swallowing saliva, and compared with fMRI of 30 healthy people. A general linear model was used to make a statistical parameter graph for each individual subject, and the two-sample t test was used to compare the activation difference of rTMS between the EG and orientations group (OG) and within the EG before and after treatment. The results showed that, compared with the OG, the activation degree of the anterior central gyrus, auxiliary motor area, and cerebellum was higher in the EG before treatment, while the activation degree of the parahippocampal gyrus and caudate nucleus was lower. Activation of the parahippocampal gyrus and caudate nucleus increased after treatment, suggesting that high frequency rTMS may induce neural plasticity.

Four patients were included in Born et al.’s [40] study. HRPIM examination proved that the patients had cyclopharyngeal dysfunction. After C-POEM surgical intervention, the patients all showed significant and lasting improvement in swallowing function. Therefore, for dysphagia in Parkinson’s patients, we should carefully evaluate the patients’ cyclopharyngeal function. When patients have obvious cyclopharyngeal dysfunction, C-POEM surgery can play an irreplaceable role in other treatments.

Given that pharmacological treatment is always present throughout PD, this systematic review did not include drug-related studies. Despite the widespread use of dopaminergic medications, particularly levodopa, in clinical settings, these drugs can provide external dopamine supplementation to patients and effectively alleviate motor symptoms. In advanced cases of PD, where motor function fluctuates with changes in dopamine levels, the stimulation of levodopa can significantly enhance the body’s compensatory ability to improve airway protection and subsequently lead to a notable enhancement in swallowing function [43]. Despite extensive research backing the enhancement of swallowing ability through levodopa, its impact on individuals in the early stages of PD remains inconclusive [44]. Swallowing difficulties in PD are believed to stem from a combination of dopaminergic and non-dopaminergic degeneration.

Exercise therapy is now widely used to improve the movement disorders in PD patients. Exercise therapy has achieved satisfactory results in both animal experiments [45, 46, 47] and human trials [48, 49, 50], especially high-intensity exercise, which can improve the plasticity of motor nerves. The targeted training of relevant muscles in PD patients to solve the problem of dysphagia has also been widely used by swallowing therapists in clinical practice. In particular, with EMST therapy, the improvement of expiratory muscle strength can significantly improve the patient’s coughing, i.e., airway clearance, thus helping to reduce aspiration or leakage. On the other hand, EMST induces the activation of the sub-chin muscles, leading to the elevation of the tongue-laryngeal complex. One study has shown that [51], after 4 weeks of EMST intervention, the electromyography (EMG) signal of the expiratory muscle showed longer duration, higher peaks, greater mean amplitude, and a significant improvement in PAS.

Although physical factor therapy is now widely used for functional recovery after CNS lesions, the role it plays in PD remains debatable, so more research is needed to elucidate the therapeutic effect and mechanism of action of physical factors for dysphagia in PD.

The swallowing strategy is currently the most widely used and cost-effective intervention for patients with PD, which achieves safe swallowing by utilizing safer swallowing postures, more adapted food forms, or more potent sensory stimuli. Although the swallowing strategy does not improve swallowing function, it can provide patients with a more soothing subjective eating experience. Commonly used swallowing postures include chin-down swallowing, lateral swallowing, and semi-sitting swallowing. One effective technique for PD patients is chin-down swallowing, which involves bringing the chin closer to the neck. This action shortens the distance between the tongue’s base and the throat’s back while preventing food from prematurely falling into the pharynx. The increase in viscosity of the food mass results in higher internal tension, making it easier for the food mass to gather. This allows the patient to have better control over the food mass during oral and pharyngeal phases, reducing the risk of aspiration. In clinical settings, thickener is commonly used to prepare liquid that matches the patient’s feeding consistency. This method is widely used in conjunction with postural compensatory training. However, it is essential to note that prolonged consumption of highly thickened liquid may lead to dehydration. It should only be considered a temporary solution during improved swallowing function rather than a long-term strategy. Considering the adverse feeding experience associated with thickened liquids, the subjective choice and compliance of the patient are also things we need to consider when looking for a swallowing strategy that applies to the patient. Furthermore, the more fluid the consistency, the greater the patient’s ability to control the doughnut and the safer the swallowing process. However, this also places higher demands on the patient’s tongue muscles as they need to exert pressure to propel the doughnut into the pharynx. Therefore, the appropriate liquid consistency should be determined based on the patient’s circumstances. VAST is a form of sensory stimulation that complements the swallowing action. By observing a video of themselves swallowing, patients can enhance their visual perception and ensure the successful completion of the swallowing process. This approach can improve the patient’s feeding and swallowing experiences. Completing the swallowing action can increase the fun of the patient when eating. It is essential to mention that VAST might only be beneficial for individuals with mild to moderate dysphagia who have PD, and there is insufficient evidence to support the therapeutic benefits of VAST in patients with advanced stages of PD.

Neuromodulation is a rapidly evolving field and, as we delve deeper into the nervous system, it has transformed into a therapy that can exert a targeted effect on specific symptoms. In the case of dysphagia, whether it is deep brain stimulation (DBS), transcranial magnetic stimulation (TMS), or transcranial direct current stimulation (tDCS), all these methods have demonstrated effectiveness. However, there is a need for authoritative therapeutic protocols encompassing the stimulation’s target point, mode, intensity, and duration, which should be validated through numerous clinical trials to establish their superiority. DBS, commonly known as “brain pacemaker”, is a minimally invasive, reversible, and adjustable surgery on the brain in which electrodes are implanted into specific locations in the brain area, and the electrical impulses emitted by the electrodes are used to regulate abnormal brain discharges, thus achieving the purpose of treating PD. DBS is often applied to PD patients who develop severe resistance to dopaminergic drugs. There is a need for more reliable treatment guidelines for DBS, with the commonly targeted areas being the STN and the globus pallidus internum (GPi). The treatment of DBS in the advanced stages of the disease focuses on reconstructing the motor control loops, particularly the STN and pedunculopontine tegmental nucleus (PPN). The PPN is connected to the GPi and substantia nigra (SNr) through parallel motor loops [52]. Additionally, the PPN serves as a relay station for transmitting inputs to the nucleus of the solitary tract and plays a crucial role in the swallowing center. However, it is still unclear whether DBS is beneficial or detrimental to dysphagia [53]. A recent study [37] have shown that combined STN+SNr stimulation improves dyskinesia without negatively affecting swallowing function. Therefore, it can be considered a viable stimulation target. TMS is widely used for dyskinesia, cognitive deficits, and dysphagia resulting from neurological injuries, making it a promising therapeutic approach. It restores the temporal sequence of swallowing movements by bi-directionally modulating cortical excitability, re-establishing bilateral hemispheric balance so that commands from each hemisphere can be effectively delivered to both sides of the brainstem, integrating the cranial nerves utilized in the swallowing process.

Therapies to reduce the UES resistance are now progressively being used to treat cricopharyngeal dysphagia in PD. According to a study related to pharyngeal manometry, at least 20–30% of all swallowing in PD patients is associated with UES loss of relaxation [54]. The specific manifestations are increased resistance to the passage of the esophageal mass through the UES, decreased UES opening, and reduced duration of the UES opening due to the dysregulation of the swallowing time series or the increased tension of the UES itself, leading to a sense of obstruction during pharyngeal swallowing, which is often compensated for by forceful swallowing to alleviate the symptom and to generate more pressure by increasing the strength and duration of the pharyngeal reflexes, but this may reinforce this mis-sequencing [55]. Although forceful swallowing has been proven effective in the short term, it may not be a suitable method in the long run as it can speed up the transformation of the PC muscle from fast twitch to slow twitch fiber, resulting in weaker benefits compared with the harms it causes. Conversely, reducing the UES can have the opposite effect by decreasing the pressure on the PC muscle by lowering the resistance of the CP, thus aiding in restoring the timing of pharyngeal muscle contractions. C-POEM has been demonstrated to be a less intrusive and safer alternative. C-POEM is less invasive, has fewer adverse effects, helps relax the UES, and reduces swallowing resistance during swallowing, thus avoiding the occurrence of pharyngeal retention, infiltration, and aspiration and improving swallowing function. In PD, due to the lack of dopamine, the lack of inhibition of acetylcholine stimulation results in the occurrence of muscle tremors and bradykinesia. The injection of BoNT in the local area can prevent the release of acetylcholine from the neuromuscular junction, effectively stopping the transmission of nerve signals and muscle movement. This method is extensively employed to treat hypertonia caused by neurological disorders. This therapy is less invasive and can be used multiple times. It may cause some local side effects. BoNT has proven to be very effective in treating various non-motor symptoms of PD [56], such as excessive saliva, excessive sweating, digestive problems, urinary problems, and dysphagia. It can be injected directly into the UES to reduce resting UES pressure, making it a potential substitute for C-POEM due to its safety and effectiveness.

It is important to note that, unlike cerebrovascular lesions such as stroke, PD is a degenerative neurological disease and swallowing disorders gradually worsen as the disease progresses. Exercise therapies and swallowing strategies are the most potent guarantees for safe swallowing in early-stage PD patients. Still, this compensatory mechanism gradually fails as the disease progresses, and they cannot cure it alone. When selecting assessment tools, swallowing scales excessively emphasizes the patient’s subjective perception. Conversely, the FEES and VFSS primarily aim to depict the characteristics of symptom onset, such as premature food drop, pharyngeal retention, leakage, and aspiration. Although these tools can assist in comprehending swallowing movements over time when combined with video recording and processing software, their focus is limited to the area above the UES. There is a lack of quantitative assessment of the entire pharyngeal contraction timing, so the examiner’s skill level and subjective opinion can disturb the results. The insensitivity of the assessment tool thus leads to a significant overstatement of the role of conventional swallowing therapy; the use of HRPIM would be a perfect complement to both and has a higher sensitivity for early cricopharyngeal dysphagia. This systematic review includes four study on UES resistance reduction therapy. Patients all showed effective and continuous improvement of swallowing disorder in the pharyngeal stage after treatment, and the IRP and IBP of patients showed significant and continuous reduction after HRPIM examination, which proved that cyclopharyngeal dysfunction may be the primary cause of swallowing disorder in Parkinson’s patients. Therefore, UES resistance reduction therapy may be more effective than other treatments.

This study confirmed the beneficial effects of various treatment schemes of swallowing disorders in PD patients. However, due to the small number of included studies, insufficient sample size, heterogeneity of interventions and main outcomes (such as HRPIM and FESS), and differences at the individual level (such as age, sex, and course of disease), we did not conduct a meta-analysis. Therefore, more high-level evidence is needed to prove the effectiveness of the treatment. In addition, we suggest that HRPIM should be more included in the assessment of swallowing disorders, because it can identify the early decline of cyclopharyngeal function in PD patients earlier and is more suitable for horizontal comparison of the effectiveness of various treatments, which can make the data more reliable. In addition, since there is an on-off phenomenon in patients with advanced PD due to fluctuations in dopamine levels in the body, it is necessary to report in which state the study was carried out, especially for therapeutic intervention in patients with advanced PD. In the future, studies with larger sample sizes and blinded as much as possible are needed to determine the efficacy of different treatments for swallowing disorders in PD patients.