Facial paresis is a very common post-stroke sequela, and it leads to functional and esthetic defects in patients [1]. Facial paresis results in drooping of the corners of the mouth, salivary drooling, spilling of food during meals, asymmetric smiles, and unclear speech [2]. This complication not only makes a stroke patients' daily living inconvenient but also discourages them from having a social life. Consequently, facial paresis could affect the patients' psychosocial well-being [3]. Konecny et al. [4] showed that there is a very close relationship between changes in facial paresis and Beck Depression Inventory-II scores (BDI-II), as well as a correlation between the House–Brackmann Grading Scale (HBGS) and the overall quality of life. Thus, management of facial paresis after a stroke is an important aspect of post-stroke rehabilitation.

Generally, orofacial exercises are used to rehabilitate facial paresis after a stroke, but few studies have proven the effectiveness of such exercises [25]. Moreover, no other effective therapies have yet been established.

However, in peripheral-type facial palsy, more varied treatments are being employed. One is mirror therapy. Nakamura et al. [6] used mirror therapy to prevent synkinesis after facial palsy. Azuma et al. [7] also used mirror therapy with single-dose botulinum toxin for the treatment of facial synkinesis after Bell's palsy and herpes zoster. These two studies found a significant beneficial effect of mirror therapy. In these studies, mirror therapy was conducted by simply watching the mirror while exercising. Thus, authors postulated an underlying biofeedback mechanism.

The effect of mirror therapy in stroke patients may be different from that in peripheral-type facial palsy. Neural plasticity is considered to be a main mechanism of mirror therapy in stroke patients, and it is generally considered to be mediated by mirror-induced illusion [8]. Mirror therapy has been conventionally used in the Department of Stroke Rehabilitation to treat upper-limb and lower-limb weakness [9]. In 2012, a Cochrane Review of 14 studies, which included 567 patients after stroke, found a positive effect of mirror therapy on motor function and activities of daily living [10]. To date, there has been no precedent in the rehabilitation of post-stroke central facial paresis using mirror-induced illusion.

The aim of this study was to evaluate the effect of mirror therapy on central facial paresis after stroke. To make illusional movements of the face, we used a tablet PC mirror application during orofacial exercise, expecting to find a new rehabilitation method effective for post-stroke central facial paresis.

Twenty-one patients, including 13 men and 8 women, were assessed. Mean ages were 63.1 and 55.6 years in the mirror and control groups, respectively. The baseline stroke degree, including basal MBI, K-MMSE, and NIHSS scores, was similar between the groups. Baseline facial paresis was evaluated by R-HBGS, M-dif, and M-rat; the scores were not significantly different between the two groups (Table 1).

All measurements showed significant differences between baseline and after therapy in both groups (Fig. 3). In the mirror group, the R-HBGS scores in the mid-face were 2.9±0.7 and 2.1±1.0 at baseline and after therapy, respectively. In the mouth, the R-HBGS score changed from 3.3±1.6 to 2.3±1.6. M-dif decreased from a baseline of 3.5±1.9 to 2.0±1.8 after therapy. M-rat increased from a baseline of 0.5±0.2 to 0.7±0.2 after therapy. In the control group, R-HBGS in the mid-face changed from 2.5±0.5 to 2.1±0.7, whereas that in the mouth changed from 3.5±1.1 to 2.8±1.3. M-dif decreased from 3.9±2.0 to 3.4±2.0 after

Table 2 compares the two groups. The improvement in facial movement, which is measured by the M-dif, was significantly larger in the mirror group (p=0.04). The mean difference was 1.45±0.90 in the mirror group and 0.55±1.00 in the control group. The mean change of M-rat was also larger in the mirror group, at 0.30±0.19, than that in the control group, at 0.11±0.12 (p=0.01).

Compared with the control group, the group receiving the orofacial exercise with the use of the tablet PC mirror application showed greater improvement in facial movement after stroke.

In stroke rehabilitation, mirror therapy has been demonstrated to improve the patient's motor function, especially of the upper limbs. Selvaraj et al. [12] studied 20 patients with middle cerebral artery infarction within six months after stroke onset. The study group received an additional hour of mirror therapy for 3 weeks. They found significantly greater improvements in Fugl–Meyer scores, Brunnstrom stages, and Box and Block Test scores in the study group than in the control group [13]. Recently, mirror therapy was studied in chronic stroke patients. Colomer et al. [14] conducted a randomized controlled trial including 31 patients enrolled >6 months after stroke. They found improvement in upper-limb motor function and tactile sensation. Mirror therapy was also demonstrated to improve lower-limb motor function. Sutbeyaz et al. [15] studied 40 patients without volitional ankle dorsiflexion within 12 months after stroke onset. After 4 weeks of additional 30-minute daily mirror therapy, Brunnstrom stages and Functional Independence Measure motor scores showed improvements in the study group.

The mechanism of the effect of mirror therapy is considered to be through neural plasticity, which is the principal concept of recovery in stroke patients. It refers to the capacity and resiliency of the brain to change structure and function, enabling the brain to adapt to different conditions after injury. To reveal the impact of mirror therapy on neural plasticity, several studies were conducted with neuroimaging. Guo et al. [16] used functional magnetic resonance imaging (fMRI) to study mirror-induced ankle movements. They investigated five stroke patients performing contralateral ankle flexion/extension while being evaluated by fMRI, and found more significant activation of the ipsilateral sensorimotor cortex during mirror use than with non-mirrored movement. Bhasin et al. [17] also compared pre-treatment and post-treatment fMRIs in 20 stroke patients receiving 8 weeks of mirror therapy. There were significant improvements in the laterality index (LI) of the ipsilateral primary motor cortex and supplementary/premotor cortex in all patients. They postulated the ‘restitution’ principle of neural plasticity as the underlying mechanism.

The concept of mirror-induced illusional visual feedback sufficiently explains the way mirror therapy triggers neural plasticity. Mirror therapy on the paretic limb was commonly employed using the following method: place the mirror in the patients' mid-sagittal plane, so that the patient watches the reflected movement of the nonparetic limb as if it were the paretic limb. As a result, it creates a visual illusion of intact movement of the impaired limb. This mirror-made visual illusion facilitates cortical activation and reorganization, thereby augmenting the recovery.

We assumed that the same mechanism could underlie the amelioration of facial muscle weakness after stroke. However, creating illusions of facial movement was a problem. There had been no previous modes of mirror therapy for facial paresis, because placing a mirror in the mid-sagittal plane was impossible for the face. Thus, use of mirror therapy for facial paresis was challenging. We designed a tablet PC application-based mirror therapy to overcome this restriction. Any application converting images from right to left could be applicable. With the application, the patients could watch the intact illusion of the impaired face. As a result, our outcome, effective mirror therapy in facial paresis, was following previous studies with peripheral limbs. Thus, we think it was a meaningful attempt to develop a new rehabilitation method for post-stroke facial paresis.

This study has some limitations. First, the sample size was small; hence, larger studies are needed to validate our findings. Second, the results showed minimal changes and large deviations, which might be considered the effect of measurement error. However, to minimize the error, the patients were assessed three times each by two psychiatrists with the same measuring tape. Third, outcome measurements were made by two individuals, one of them being the director of the study; this increases the possibility of observer bias. To decrease this bias, one other physiatrist who did not know which patients were in the study group also measured the outcome, and the mean value was used for data analysis. Fourth, in previous studies, the distance between the corner of the mouth and earlobe was measured only by two-dimensional (2-D) video analysis [24]. However, since we thought three-dimensional (3-D) measurement would be much more reliable for the face, we adopted direct measurement of facial movement on the patient's face. After we launched this study, Katsumi et al. [18] introduced a 3-D video analysis to evaluate facial nerve palsy, because 3-D analysis could measure anteroposterior mouth movement, whereas 2-D analysis could not. Although our method was also three dimensional and practical, its validity and reliability was unproven and limited. Finally, we did not use neuroimaging examinations in the study; hence, further study will be needed to elucidate the underlying mechanism.

Despite these limitations, this study has important strengths. From clinical experience, facial paresis is particularly challenging in stroke patients. There are few treatment methods with proven effectiveness; hence, rehabilitation of central facial paresis is limited and difficult. Although mirror therapy has been proven effective for stroke rehabilitation, it had not been applied to central facial paresis, because making an illusion of the face was not feasible. We found a way to overcome this limitation by designing a tablet PC application-based mirror therapy. This study is the first to introduce mirror therapy for post-stroke central facial paresis. We have shown positive outcomes after employing this new rehabilitation method. We expect physiatrists to start using mirror therapy on central facial paresis, and thus a larger study would be possible in the future.

Though this study had several limitations, the results showed that mirror therapy using a tablet PC might be an effective tool for treating central facial paresis after a stroke. This study is challenging, as the first to introduce mirror therapy for central facial paresis. Our sample size was small, so larger studies are needed to confirm the effectiveness of this new rehabilitation method for post-stroke facial paresis.