Dysphagia is a common condition and a life-threatening symptom during the acute phase after the onset of stroke; ~80% of patients with stroke experience dysphagia (1). Despite the reduced incidence of dysphagia over time after stroke (2), almost half of all patients continue to experience the symptoms of dysphagia during the convalescence stage (3). Dysphagia can result in a significant increase in the incidence of complications, including aspiration pneumonia, malnutrition, and dehydration, thus leading to a prolonged hospital stay and high medical expenses (4–6). Moreover, post-stroke complications can delay functional recovery and degrade the quality of life when patients cannot eat or drink (7). Therefore, by improving the deglutition function of patients with stroke, we will be able to increase their quality of life.

Mirror therapy, as a non-invasive therapeutic and rehabilitative intervention, has been widely used to improve functional recovery after stroke (8–10). In contrast to other interventions, which tend to employ somatosensory input to assist motor recovery (11), mirror therapy is based on visual stimulation. During the general mirror therapy, a mirror is placed on the patient's midsagittal plane, thus reflecting the non-paretic side as if it were the affected side (12–14). Some authors have recently described “mirror-like” videos or computer graphic systems, in which a video or computer graphic image of the moving limb is presented (15, 16). The main characteristic of mirror therapy is to establish internal behavioral representations and external observations by imitating these events in person (17). In addition, mirror therapy is a relatively simple to administer and represents a convenient the possibility for self-administered home therapy, even for patients with severe motor deficits (18). The neural mechanisms underlying mirror therapy predominantly include the promotion of motor network connections and activation of the primary motor cortex in the brain (19, 20). A previous study indicated that mirror therapy plays a vital role in the recovery of swallowing function (21). Unlike the use of mirror therapy for upper or lower limb functional restraint, the mode of application for mirror therapy when managing dysphagia is to provide visual cues to stimulate swallowing. Consequently, researchers have already investigated the effect of mirror therapy on neuronal activity during swallowing (22). Thus far, mirror therapy has been widely used to improve swallowing function after stroke in China. However, there have been no specific reviews conducted on the specific effect of mirror therapy on dysphagia after stroke. Therefore, there is an obvious and urgent need to determine the efficacy and safety of mirror therapy for post-stroke dysphagia. In this study, we conducted a systematic review and meta-analysis to ascertain the efficacy and safety of mirror therapy for post-stroke dysphagia.

The results of this study indicated that mirror therapy may play a positive role in treating post-stroke dysphagia. According to subgroup analysis, mirror therapy during the acute and convalescent phases after a stroke was associated with a positive effect size in terms of clinical effectiveness rate. In terms of the water swallowing test, mirror therapy during the acute phase of stroke showed a positive effect size. However, mirror therapy during the convalescent phase of stroke had no positive effect size. Sensitivity analysis showed that this evidence was stable. In addition, there was no evidence of any publication bias. To the best of our knowledge, this is the first study to investigate the efficacy and safety of mirror therapy on post-stroke dysphagia. Consequently, our results are valuable and important for both the patient and clinic.

Due to the lack of a systematic review or meta-analysis focused on the synergic effects of mirror therapy, our present findings were compared to similar, but not specific, research studies. Most previous studies focused on the hand-mirror system (38). In addition to the activation of corresponding mirror neurons by hand movements, mirror neurons can also respond to the stimulation of mouth movement and become active when individuals perform or observe lip activities related to feeding function (39). In addition, the firing of mirror neurons was temporally and closely associated with grabbing, lip movement, crushing, or sucking (40). A previous study using functional magnetic resonance imaging showed that the visual and audiovisual stimuli associated with swallowing motion can activate cortical areas related to the swallowing motor, such as the auxiliary motor area, premotor area, primary motor area, and cingulate gyrus (41). Another previous study used magnetoencephalography to detect a positive relationship between the activated regions of the cerebral cortex and the mirror neuron system during swallowing (21). Based on the existing evidence, we were able to provide answers to key research questions, especially with regard to swallowing function and the incidence of pneumonia. The positive results for clinical effectiveness rate were consistent with those for the water swallowing test. To a certain extent, the positive effects identified here suggest that mirror therapy as a non-invasive therapeutic and rehabilitative intervention can improve the swallowing ability of patients after stroke. However, these results are limited because our subgroup analysis indicated that the treatment effects between the acute and convalescent phases of stroke were not uniform.

We found that the use of mirror therapy to treat swallowing function has a positive effect size during the acute phase of stroke but not during the convalescent phase. This difference may be related to several factors.

First, it is known that mirror therapy can be applied to different phases of patients with stroke (acute, subacute and chronic stroke) (42–44). One previous study reported that early intervention was promising with regard to the treatment of swallowing function after stroke (45). This might explain the different treatment effects between the acute and convalescent phases of stroke. It is possible that the inconsistencies between the pooled and subgroup analyses may be attributable to significant heterogeneity between subgroups. Moreover, it is possible that the subgroup analysis may not be reliable because only a small number of trials were included in each subgroup and that multiple subgroup analyses can lead to spurious results. Further research is required to investigate these factors further. Despite the detection of positive results, several problems are evident in the five studies analyzed here. Although mirror therapy is extensively used to treat post-stroke dysphagia in clinical practice, the application of mirror therapy still remains limited. All the trials were performed in China; no similar work has been reported for the treatment of post-stroke dysphagia in any other country. Because specific stroke subtypes were not specified the studies included here, these results cannot be universally applied, even in the Chinese population. In addition, there are no unified criteria for applying mirror therapy at present (18, 46). The implementation of treatment schemes (such as video content setting, observation and execution time, duration of treatment, and evaluation indicators) that were described across different publications is known to vary in clinical practice. In addition, mirror therapy is a therapeutic option based on the integration of observation, imagination, and imitation; thus, the strategy relies upon the cognition and vision of the patients being tested, thus limiting the scope of application.

Second, we generated funnel plots and performed Egger's test for publication bias (32). To our surprise, Egger's test for publication bias was not significant (p > 0.05), thus implying that the five studies included in our analysis were unlikely to feature significant publication bias. Even though Egger's test suggested that there was no publication bias, the limitations imposed by potential publication bias should be considered because it is easier to report positive results (47, 48). Furthermore, we should not ignore the fact that most studies evaluating patient-reported outcomes did not use a placebo-controlled design. Placebos serve as a control to discriminate the effects of an active treatment from the non-specific components of the treatment (49). With regard to placebo, expectancy is usually defined as the subjective probability of the occurrence of a specific clinical outcome (50). However, only one study included used sham mirror therapy (33). As a result, we cannot rule out the potential confounding effect of placebo on our findings.

Finally, all of the assessment scales used in this study were indirect assessment methods and therefore lacked direct observation methods and imaging detection; this may have had an effect when determining the specific impact on the effective rate of treatment. It is well-known that the gold standard methods for evaluating swallowing functions are flexible endoscopic evaluation of swallowing (FEES) and video-fluoroscopic swallowing studies (VFSS) (51, 52). However, no datasets are available yet for these methods. Thus, potential clinical heterogeneity could not be excluded. Therefore, we cannot draw definite conclusions based on the results from the studies analyzed herein.

Several specific limitations need to be considered with regard to this study. First, the currently available evidence related to the use of mirror therapy for post-stroke dysphagia originated from a relatively sample size; furthermore, there were limited numbers of subjects in each trial and limited analysis of outcome data. Therefore, there is a need to carry out further studies with a larger sample size using more objective indicators. Second, safety indicators may not be optimal and follow-up data are lacking. Hence, we could not fully evaluate the safety profile of mirror therapy or the long-term efficacy of this technique in the target patient population. Third, although we performed specific searches in a wide range of databases, all of the included trials were performed in China; as such, our results may not be representative on a wider scale.

Mirror therapy may have a positive role in the management of post-stroke dysphagia but has yet to be specifically confirmed. In addition, data from existing trials suggest that evidence for the safety of mirror therapy in patients with post-stroke dysphagia is not yet sufficient.

The original contributions presented in the study are included in the article/supplementary materials, further inquiries can be directed to the corresponding author/s.

KH conceived the study and drafted the paper. LW and FN developed the search strategies. XL and KL performed literature selection and data extraction. RM conceived the study, designed the protocol, and assisted with data acquisition, analysis, and drafting the paper. All authors contributed to the article and approved the submitted version.

This work was supported by the Chinese Medicine Research Program of Zhejiang Province (Nos. 2020ZX011, 2022ZQ047, and 2022ZB198), the Zhejiang Chinese Medical University Research Fund (No. 2018ZY17), and the Traditional Chinese Medicine of Zhejiang Province Science and Technology Plan Project (No. 2018ZQ027).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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