Introduction
Facial nerves are mixed nerves consisting of motor, sensory, and parasympathetic nerves [
1]. Facial nerves are also involved in facial muscle movement, tear and saliva secretion, tympanum protection, and taste sensation by the tongue [
2].
Infra-nuclear facial nerve palsy without clear causes is referred to as idiopathic Bell’s palsy [
3] and represents the most common facial nerve disorder, with a prevalence of 11–40 cases per 100,000 persons. The incidence of Bell’s palsy does not vary among different sex and age groups, but has been reported to be higher among patients with diabetes [
2].
Vascular ischemia, viruses, genetic factors, and autoimmunity have all been highlighted as possible causes of Bell’s palsy. Thus far, vascular ischemia appears to be the most likely cause of Bell’s palsy, and cold exposure, emotional shock, and anxiety are known to cause circulatory disturbance [
4]. According to Korean medicine, Bell’s palsy occurs when the harmful energy of a cold invades the meridian system in a weak state, as well as when stasis stagnates the meridian system, obstructs qi and blood, and blocks the flow of nutrients into muscles and tendons [
3].
The major clinical symptoms of Bell’s palsy include loss of wrinkles in the forehead due to facial nerve paralysis on one side of the face, epiphora due to an inability to close the eyes, ocular discomfort, and lip distortion toward the contralateral side. Minor symptoms include impaired taste at the tip of the tongue, sound sensitivity, and ringing in the ears [
5]. Approximately 60% of patients with Bell’s palsy experience pain in the ipsilateral mastoid as a preceding symptom, and severe paralysis occurs within the next 48 hours [
6].
The muscle energy technique (MET) is a form of chuna manual treatment on fascia and is based on active muscle relaxation techniques that use the intrinsic energy of the muscles to relax and stretch stiff muscles [
7]. Compared to other manipulative therapies, MET has a wider range of applications for treating the nervous and musculoskeletal systems, proceeds smoothly, and allows patients to actively participate in the treatment process [
8]. The efficacy of MET has been proven both experimentally and clinically [
8].
While manual therapy has been undergoing a change from techniques using fast and weak twisting movements to softer techniques that focus more on soft tissues [
8], there have not been any clinical reports on the treatment of Bell’s palsy using MET. Therefore, in this study, we retrospectively compared patients with Bell’s palsy who received Korean–Western combination treatment and MET of upper trapezius and sternocleidomastoid (SCM) muscles with those who received Korean–Western combination treatment only.
Discussion
The motor nerve fibers from the motor nucleus of the facial nerve assist in controlling facial expressions and making the movements that are necessary for speech and mastication. Secretomotor fibers of the facial nerve, which originate from the superior salivary nucleus, regulate the secretory activities of the submandibular glands, sublingual glands, lachrymal glands, olfactory mucosa, and palatine mucosa. Special visceral afferent fibers of the facial nerve, which originate from the nucleus tractus solitarius, occupy two-thirds of the anterior region of the tongue and palate. They are responsible for taste sensation in the anterior two-thirds of the tongue and sensation in the anterior two-thirds of the soft palate [
1].
Facial nerve paralysis can be classified as central or peripheral [
3], and differentiating between the two types of paralysis is important. As the frontalis muscle is controlled by both hemispheres of the cerebrum, facial nerve paralysis due to upper motor neuron lesions affects the bottom two-thirds of the face only. In contrast, peripheral facial nerve lesions not only paralyze the muscles involved in the closing of the mouth and eyes but also affect the frontalis muscle, thus obscuring wrinkles on the forehead [
10]. Peripheral facial nerve paralysis can be classified as nuclear paralysis or infra-nuclear paralysis. Infra-nuclear facial nerve paralysis without a clear cause is referred to as Bell’s palsy [
3].
Bell’s palsy suddenly occurs without any evident causes or injuries. The most likely cause of Bell’s palsy is vascular ischemia. It has been hypothesized that constriction of the arterial vessels interferes with blood flow into the facial nerves, thus causing the nerves within the facial canal to swell and become pressurized, consequently leading to paralysis [
4].
Postauricular pain is the most common early symptom of Bell’s palsy [
11]. When the patient tries to firmly close the eye on the paralyzed side of the face, the pupil of the eye moves abnormally upward relative to the pupil of the other eye; this symptom is referred to as Bell’s phenomenon [
10]. Patients with Bell’s palsy may not be able to wrinkle their forehead or whistle, and food may collect within the mouth on the ipsilateral side such that fluids leak from the mouth. If the chorda tympani nerve becomes affected, taste sensation may become impaired in two-thirds of the tongue tip. If the stapedius muscle becomes affected, sound sensitivity may also occur.
Aside from acupuncture and herbal medicine, Korean medical treatments include electroacupuncture [
12], pharmacopuncture [
13], embedding therapy [
14], acupotomy [
15], and chuna manual therapy [
16,
17]. These therapies have been clinically practiced, and their efficacy has been clinically proven. In Western medicine, the administration of steroids and antiviral agents, physical therapy, and surgery are used [
5].
MET stretches shortened muscles to relieve tenderness and pain, and to recover the original state of the muscles. This not only removes trigger points within the muscles, but also relieves pain in the ligament or periosteum at the attachment point. In a study by Lewit and Simons, MET was effective for 330 out of 351 muscles, muscle relaxation was observed in 63% of all patients in the second examination 3 months later, and pain relief was achieved in 23% of muscles [
18].
With regard to the mechanism underlying MET, during isometric exercise, in which the forces applied by the therapist and the patient onto the agonistic muscle match, reciprocal inhibition occurs in the antagonistic muscle, and post-isometric relaxation occurs in the agonistic muscle due to physiological and neurological reactions. Thereby, contracted muscles and tissues can be stretched and relaxed.
MET resembles chuna manual therapy for the meridian muscle in Korean medicine [
19], which applies stimuli onto the surface of the patient’s body to control the meridian system, to prevent and treat diseases. Therapeutic effects are achieved through correction of particular parts of the body such as joints and the skeletal structure using manual force [
20].
The effectiveness of chuna manual therapy for treating Bell’s palsy has been reported in a clinical study by Park et al [
16], in which the danmuji anchu traction technique was performed, and in a clinical study by Jeong et al [
17], in which JS supine cervical distraction, supine cervical manipulation, and cranial base release were performed. These chuna manual therapy techniques for Bell’s palsy fall under the areas of chuna joint distraction therapy, chuna spine and joint manipulation therapy, and chuna craniosacral therapy. However, clinical research on fascia chuna therapy for Bell’s palsy is currently lacking.
The Department of Acupuncture & Moxibustion, Korean Medicine Hospital Dong-Eui University has been performing MET of upper trapezius and SCM muscles associated with facial nerve disorders to treat Bell’s palsy. Significantly better treatment outcomes have been observed in patients who receive Korean–Western combination treatment and MET compared to patients who receive Korean–Western combination treatment only. In this retrospective analysis of the medical records of 34 patients who were diagnosed with Bell’s palsy and who met the criteria for inclusion into the study, overwork and stress were the most common causes of paralysis, followed by cold exposure, unknown causes, and the common cold. Postauricular pain was the most common early symptom at onset, followed by lacrimation, scheroma, dysgeusia, tinnitus, hyperacusis, dizziness, and hypoacusis.
In Group A (patients who had received Korean–Western combination treatment and MET of upper trapezius and SCM muscles), Yanagihara scores were significantly improved at all time intervals (P01, P12, P23, P34). Yanagihara scores were also significantly improved in Group B (patients who had received Korean–Western combination treatment only) at all time intervals except for P01. For each of the time periods P01, P02, P03 and P04, the improvements observed in Yanagihara score were significantly greater in Group A than in Group B. Based on these results, the treatment of Bell’s palsy with MET of upper trapezius and SCM muscles may be beneficial. The theoretical background on the application of MET on these two muscles is given below.
First, the effects of treatment on the SCM muscle on Bell’s palsy can be considered from an anatomical perspective. To begin with, its effects on the autonomic nervous system can be considered. The SCM muscle is anatomically closely associated with the stellate ganglion, which is a central ganglion through which sympathetic nerves that travel to the head, neck, and chest pass [
21], and carotid arteries and the SCM muscle are ventrally distributed [
22]. The Gisa (ST11) of the stomach meridian is an acupuncture point located on the inner side of the SCM muscle and corresponds anatomically to the stellate ganglion. Trigger points of the SCM muscle cause symptoms of autonomic nerve disorders such as headache, dizziness, and nausea [
23], possibly by contracting to stimulate the stellate ganglion. In Western medicine, blockade of the autonomic nervous system alleviates hyperactivation of the autonomic nervous system and subsequently increases peripheral blood flow, and stellate ganglion block is used in the treatment of facial nerve paralysis [
22]. Murakawa et al [
24] performed stellate ganglion block in 35 patients with Bell’s palsy and observed significant improvements in the blood flow of the common carotid arteries. Since Bell’s palsy is known to develop as a result of reduced microcirculation in the facial nerves, a stellate ganglion block may treat Bell’s palsy by improving blood flow in the common carotid arteries. Lim et al [
25] found significantly higher low frequency/ high frequency ratios in patients with Bell’s palsy than in normal patients. The low frequency/high frequency ratio is used as a marker of regulation in the autonomic nervous system. It is directly proportional to the activity of the sympathetic nervous system and inversely proportional to the activity of the parasympathetic nervous system. Kim et al [
26] reported improvements in facial blood circulation after performing acupuncture and manual therapy on the SCM muscle. In summary, relaxing the SCM muscle reduces stimulation of the stellate ganglion, alleviates hyperactivation of the sympathetic nervous system, and increases peripheral blood flow in the face, representing an effective method for treating Bell’s palsy.
Furthermore, because the SCM muscle is anatomically associated with cervical arteries, veins, and lymph, constriction of the SCM muscle may reduce blood flow to the head and neck by applying pressure to the cervical arteries, veins, and lymph, causing facial swelling [
23], while relaxation of the SCM muscle may promote blood circulation in the head and neck.
In addition, the sternal and clavicular branches of the SCM muscle both attach to the mastoid [
27], and facial nerves exit the cranial cavity through the stylomastoid foramen located on the anteromedial side of the mastoid [
28]. Around this location, they communicate with the vagus nerve, the auricular branches of the glossopharyngeal nerve, the internal carotid plexus, and the trigeminal nerves [
1]. The posterior auricular nerve separates from the facial nerves as it exits from the stylomastoid foramen. After passing through the anterior margin of the SCM muscle, it travels upward to receive sensory information from the mastoid surface and the auricle, and communicates with the cervical plexus through the greater auricular nerve and the lesser occipital nerve. The lesser occipital nerve travels along the posterior margin of the SCM muscle, curves and travels upward, and distributes throughout the retroauricular area. The greater auricular nerve exits the posterior margin of the SCM muscle, travels upward along the posterior region of the external jugular vein, and is distributed to the surface of the parotid gland, auricle, and mastoid [
29]. If the mandible is pushed back due to problems with the SCM muscle, the Wangol (GB12) area can become compressed. Therefore, because tension in the SCM muscle directly and indirectly applies pressure on the nerves around the mastoid, problems with the SCM muscle must be considered a symptom preceding Bell’s palsy when patients complain of pain in the Wangol (GB12) area [
23].
Second, the effects of MET of upper trapezius and SCM muscles can be understood in terms of its cervical alignment. The upper fibers of the trapezius muscle are controlled by the same accessory nerves that control the SCM muscle and are located on the agonist and antagonist muscles. Constriction of these muscles leads to neck pain, and chronic stiffness can cause degenerative changes to the cervical spine [
23].
In addition, some of the nerve fibers of the trigeminal nerve enter the pons, form the spinal tract that travels down toward the spinal cord, and form a synapse at the spinal nucleus located at the medial side of the spinal tract. The spinal nucleus of the trigeminal nerve is located immediately below the principal sensory nucleus of the pons and extends to the C2 spinal cord level [
30].
Within the face, heterogeneous nervous communication occurs between facial nerves and the trigeminal nerve. Facial nerves meet the supraorbital nerve, which branches off from the ophthalmic nerve (V1), the zygomatic nerve and the infraorbital nerve of the maxillary nerve (V2), and the auriculotemporal nerve, buccal nerve, and lingual nerve of the mandibular nerve (V3). The connections between facial nerves and the trigeminal nerve provide afferent pathways that carry proprioceptive information necessary for sophisticated movements of complex facial muscles and play a physiological role in the recovery of damaged nerves [
31].
In summary, treatment of the upper trapezius and SCM muscles can contribute to the recovery of a normal cervical alignment, and affects activation of the spinal nucleus of the trigeminal nerve that extends down to the C2 spinal cord level. An activated trigeminal nerve may play a physiological role in the recovery of damaged facial nerves.
Third, the association between the face and the upper trapezius and SCM muscles may be considered in terms of the meridian muscle. Citing the anatomical reports on the three yang meridian muscles of the hand by Park et al [32–34], the anatomical association between the neck and face can be explained through the routes in which the meridian muscle travels. The large intestine meridian muscle connects the trapezius muscle (LI16), the SCM muscle (LI17–18), the greater auricular nerve (LI18), the accessory nerve (LI18), the SCM branches of the external carotid artery (LI18), the orbicularis oris superior muscle (LI19–20), the infraorbital nerve (LI19–20), the facial artery (LI19–20), and facial nerve (LI20) [
32]. The arm greater yang small intestine meridian muscle connects the trapezius muscle (SI12–15), the accessory nerve (SI12–17), the SCM muscle (SI16–17), the external jugular vein (SI16–17), the facial nerve (SI17), the sympathetic trunk (SI17), the posterior auricular artery (SI17), the internal jugular vein and internal carotid artery (SI17), the zygomatic muscle (SI18), the masseter muscle (SI18), the zygomatic branch of the facial nerve (SI18), the transverse facial artery and vein (SI18), the maxillary artery branch (SI18), and the temporal branch of the facial nerve (SI19) [
33]. The triple energizer meridian muscle connects the trapezius muscle (TE15), the accessory nerve (TE16–17), the SCM muscle (TE16–18), the facial nerve (TE17–18), the internal jugular vein (TE17), the temporal branch of the facial nerve (TE20–23), the orbicularis oculi muscle (TE23), and the zygomaticotemporal artery and vein (TE23) [
34]. Therefore, MET performed on the upper trapezius and SCM muscles affects activation of the facial structures that are connected through meridian muscles and may have significant therapeutic effects against Bell’s palsy.
Lastly, the effects of MET of upper trapezius and SCM muscles can be understood in terms of the similarity between symptoms of Bell’s palsy and pathology of the two muscles. The trigger points of the upper trapezius muscle and of the SCM muscle cause referred pain in the face, which tends to be misdiagnosed as atypical facial pain. Pain from the sternal branch of the SCM muscle occurs in the cheek, the temporal region, and the orbit. Pain from the clavicular branch of the SCM muscle occurs in the frontal lobe, deep inside the ipsilateral ear, and on the back of the ear or on the auricle. When trigger point number 1 of the upper trapezius muscle fiber becomes activated, pain travels along the lateral surface of the neck on the ipsilateral side up to the mastoid and may spread to the masseter muscle. In addition, the autonomic symptoms of the sternal branch of the SCM muscle may cause excessive tearing on the ipsilateral side, and proprioceptive symptoms of the clavicular branch can cause positional dizziness, vertigo, and hearing impairment on the ipsilateral side [
27]. Therefore, referred pain in the facial area indicates that the face is affected by the upper trapezius muscle and the SCM muscle, and can be seen to share similar symptoms with Bell’s palsy such as excessive tearing, hearing impairment, and dizziness.
In this study, it was observed that MET of upper trapezius and SCM muscles had positive effects on Bell’s palsy. However, due to the retrospective nature of the study, we could not systematically control for the effects of selection bias. Furthermore, due to the small sample size of 34 patients and the limited treatment period of 4 weeks, we could not investigate post-treatment progress or complications. There is a need for a systematic randomized controlled trial of MET for the treatment of Bell’s palsy, with sufficient sample size and long-term patient monitoring.