Introduction
Facial palsy can lead to significant functional and psychological disorders due to its effects on both static and dynamic facial symmetry [1-3]. In the initial phase of complete paralysis, complications generally stem from muscle weakness in the affected area. Additionally, during partial recovery, patients may experience facial asymmetry and an inability to perform fine movements, which can be linked to prolonged paralysis or the development of post-paralytic synkinesis [4].
Facial muscles that have degenerated or atrophied due to denervation of the facial nerve can undergo re-innervation by axons from non-native muscle groups. This leads to a phenomenon known as synkinesis, which is characterized by unintentional motion in one area of the face during intentional movement in another area [5]. Synkinesis represents one of the most troubling sequelae of facial palsy. Various mechanisms have been suggested to explain synkinesis. These include aberrant regeneration in cases of third-degree nerve injuries as classified by Sunderland [6], ephaptic transmission where neighboring axons at the injury site stimulate each other [7], and hyper-excitability of the facial nucleus in the cerebral motor cortex [8].
Such synkinetic movements, regardless of the underlying mechanism, typically emerge during the neural repair process, 3 to 6 months after injury. They become particularly noticeable during spontaneous facial movements, especially during emotional expressions like involuntary blinking or smiling [9,10]. These observations suggest that nuclear hyper-excitability may significantly contribute to the development of synkinesis, in conjunction with aberrant regeneration. Numerous researchers have reported that the central cortex may also undergo reorganization as a secondary response to peripheral facial palsy [11-13]. The pathophysiology of synkinesis is highly complex and multifactorial.
In a related study, it was observed that synkinesis occurred in approximately 84% of patients when the early degeneration rate exceeded 80% [14,15]. Consequently, even with the early administration of optimal treatments such as steroids or antiviral agents, synkinesis remains an unfortunate inevitability for these patients.
Therefore, it is crucial not only to treat but also to predict and minimize synkinesis at an early stage. Synkinesis typically begins to manifest gradually between 3 and 6 months after the onset of facial palsy. Therefore, during the subacute phase, which spans from 3 to 6 months, it is advisable to engage in neuromuscular retraining therapy (NMRT). This therapy corrects improper muscle movement patterns and suppresses excessive movements, thereby preventing the development of synkinesis [16]. Early NMRT includes educating patients about potential synkinesis before it actually develops. Awareness of likely future facial changes is a crucial component of early NMRT.
Three primary treatment options are available to alleviate existing facial synkinesis: NMRT, botulinum toxin A injections, and selective neurectomy [17,18]. These methods aim to diminish the activity of synkinetic muscles and increase the activity of paralytic muscles, thus enhancing overall facial motility.
However, it is necessary to observe the detailed pattern of synkinetic hemifacial movements before determining appropriate treatment options. Synkinesis is known to manifest in various forms, including oral-ocular, ocular-oral, ocular-chin, ocular-nasal, and platysma synkinesis. These types are categorized based on the two muscle groups involved in each case [19].
In most patients with facial synkinesis, clinicians recommend botulinum toxin type A (BTX-A) injections as the first-line treatment. For those with severe facial synkinesis, selective neurectomy is considered superior [1]. However, numerous studies have shown that combining NMRT with prior BTX-A injections, rather than using BTX-A alone, significantly improves facial synkinesis and asymmetry in cases of chronic facial paralysis. These findings are supported by detailed, accurate, and objective measurement systems [20-22].
Although selective neurectomy is highly effective in eliminating synkinetic movements, it should be approached with caution due to its irreversible nature. The primary objectives of this surgery are to create a natural smile and to eliminate synkinetic movements where feasible. However, outcomes can vary significantly depending on the surgeon's experience and the patient's preferences, which are notably influenced by cultural factors [23]. This procedure can provide satisfactory outcomes in terms of facial tightness and narrowing of the eyelid aperture. For patients seeking to correct limited mouth movement, the approach should be tailored to their specific correction preferences.
Predicting Facial Synkinesis in the Acute Stage of Facial Palsy
Predicting the prognosis of patients with acute facial palsy is crucial. The prognosis for acute patients can be broadly categorized into three outcomes: complete recovery, incomplete recovery with permanent synkinesis and contracture, and no recovery, resulting in a permanently flaccid face [24,25]. When a patient first presents, predicting which of these three patterns will manifest in the coming months or years is a critical step, as it can significantly influence the treatment direction.
In the case of Bell's palsy, which represents the majority of facial palsy cases, about 70% of patients have a favorable prognosis and achieve complete recovery [26]. However, the rest of the patients experience either incomplete recovery or, in some instances, minimal recovery at all.
One of the effective methods for predicting prognosis is the electroneuronography (ENoG) study [27-30]. This study yields the most accurate results when performed 2 to 3 days after the onset of facial palsy, coinciding with the beginning of Wallerian degeneration. It remains valid until 2 to 3 weeks post-onset, which marks the conclusion of Wallerian degeneration. After this period, the accuracy of the measurements may decrease as Schwann cell regeneration begins at the damaged portion of the facial nerve, potentially leading to false-negative or false-positive outcomes. It is important to note that the severity of nerve damage can influence the timing of Wallerian degeneration. In cases of severe nerve damage, Wallerian degeneration may start and end more rapidly, thus shortening the window for obtaining accurate measurements [31,32].
In patients with acute facial palsy, it is crucial to determine which individuals will achieve complete recovery and which will develop synkinesis. This distinction indicates whether more aggressive treatment is required or if monitoring over time is sufficient. From a pathophysiological standpoint, a tool that can differentiate between facial nerve damage of Sunderland classification grade 3 or higher and less severe damage is essential. The ENoG test meets this requirement.
Numerous studies have reported degeneration rates in ENoG tests that correlate with the development of synkinesis. Typically, a degeneration rate ranging from 60% to 80% is considered indicative of the potential for synkinesis [33-36].
Based on the author's experience, when the degeneration rate on acute ENoG tests ranges from 60% to 80%, mild synkinesis may occur. This condition is often so subtle that it is either barely noticeable or, when noticed, does not significantly interfere with daily activities. Therefore, aggressive treatment is typically not pursued in these cases. Conversely, when the degeneration rate exceeds 80%, synkinesis can significantly disrupt daily life, necessitating more aggressive treatment [16]. An accelerated and severe Wallerian degeneration, surpassing 80% as observed on ENoG and coupled with facial function inferior to House-Brackmann grade IV, is expected to correlate with widespread aberrant regeneration following Bell’s palsy. The rates of degeneration were independently quantified for each of the five branches of the facial nerve, spanning from the forehead to the neck. Evaluating each of these branches separately can provide insights into the rate of neural injury. This approach could potentially offer invaluable information regarding the specific patterns of synkinetic movements that might develop in the future within each region [16].
Minimizing Facial Synkinesis through Early-Stage Neuromuscular Retraining Therapy
Synkinesis in patients with Bell’s palsy can be minimized through early intervention with physical therapy, including NMRT, before the onset of synkinesis [16]. The timing of NMRT is crucial. Historically, NMRT has primarily been employed to address already developed synkinetic mass-like movements [37]. This therapy focuses on correcting neuromuscular abnormalities by enhancing muscle coordination, rather than increasing muscle strength.
Early NMRT involves informing patients about potential synkinesis before its onset. This awareness of anticipated facial changes is a crucial aspect that sets NMRT apart from conventional approaches. Clinicians guide patients to minimize excessive movements in areas such as the eyelids, mouth corners, forehead, nostrils, neck, and any other regions where the patient observes slight involuntary movements. The therapy encompasses soft tissue mobilization and selective stretching to strengthen the paralyzed muscles. The use of any electrical stimulation that causes mass-like twitching or movement is strictly prohibited to avoid negative impacts on synkinesis and improper muscle contractions [38-40]. It is advisable to consult a rehabilitation medicine specialist for guidance on the appropriate types and frequencies of electrical therapy for facial palsy rehabilitation.
At the onset of physical therapy, each patient is informed about the potential for synkinesis. Understanding the possibility of future facial changes is a crucial aspect of early NMRT. The therapist explains various types of synkinesis to the patients, including oral-ocular/ocular-oral, chin-oral/chin-ocular, nasal-oral/ocular-nasal, and platysmal synkinesis, which may develop 5 to 6 months after the onset of palsy. The therapist demonstrates how to prevent excessive movements in the eyelids, mouth corners, forehead, nostrils, and neck. Additionally, if a patient reports any involuntary movements in other facial areas, they are instructed on techniques to minimize these movements. In their daily routines, patients are encouraged to practice small, slow, and symmetrical facial movements to establish new movement patterns.
After informing the patients that synkinesis involves uncoordinated, inappropriate movements, the therapist explains that the main purpose of NMRT is to learn new patterns to minimize synkinesis (Table 1).
NMRT is commonly prescribed for facial palsy patients who already exhibit synkinesis. However, initiating NMRT early, as soon as movement returns to the affected side after recovery from Bell's palsy, can provide insights into the potential for synkinesis and awareness of possible future facial changes.
However, it is essential to comprehend that all efforts to minimize synkinetic movements are predicated on the assumption that critical interventions are performed during the acute phase. These interventions include the timely administration of steroids or antiviral agents and, if necessary, the implementation of facial nerve decompression surgery. These measures are crucial during the acute phase and establish the groundwork for all subsequent efforts.
Two Treatment Options: Chemo-Denervation Combined with Neuromuscular Retraining Therapy and Surgical Denervation
NMRT normalizes resting muscle tone and enhances facial expressions by inhibiting synkinetic movements [41]. During NMRT, patients engage in subtle facial exercises to enhance facial symmetry [42]. Since facial muscles lack sensory receptors, NMRT aids in increasing self-awareness of facial posture and movements. A systematic review demonstrated that biofeedback with electromyography is an effective treatment for synkinesis and asymmetry following facial palsy [43]. Similarly, biofeedback utilizing a mirror has proven to be an effective treatment strategy [17]. Individualized facial neuromuscular re-education has shown greater effectiveness in improving facial symmetry in patients with Bell’s palsy compared to conventional treatments, such as electrical stimulation, gross facial expression exercises, and massage [44].
Although NMRT is the cornerstone treatment for synkinesis, BTX-A has shown superior efficacy in reducing facial hyper-contractions during both spontaneous and rehabilitation-induced recovery of facial movements. Therefore, the effectiveness of BTX-A may be enhanced by integrating NMRT [20,45,46].
The combination of NMRT and BTX-A, specifically targeted at the affected synkinetic regions, has proven to be more effective than using NMRT alone [20]. A previous review indicated that NMRT and BTX-A work synergistically to treat synkinesis [41]. Our earlier research confirmed that combining NMRT and BTX-A is advantageous for patients with mild facial synkinesis localized to the oral-ocular region who also have a high score on the Sunnybrook Facial Grading System [19].
In most patients with facial synkinesis, the initial treatment typically involves BTX-A injections and/or NMRT. However, for those with severe facial synkinesis that does not respond to BTX-A and/or NMRT, selective neurectomy may be considered as an alternative [47-49]. The primary goals of selective neurectomy are to create a natural smile and eliminate synkinetic movements. It is important to note that surgical outcomes may vary based on the surgeon's experience and the patient's preferences.
Selective neurectomy is effective in eliminating synkinetic movements, but it requires careful execution as it is an irreversible surgical procedure. The most important consideration is identifying which branch of the facial nerve should be removed. Performing excessive neurectomy can compromise facial expression, while insufficient removal may result in persistent synkinetic movements. Furthermore, to prevent recurrence due to axonal sprouting or regeneration post-surgery, the neurectomy site is reinforced by applying hemoclips in two to three layers at the proximal and distal cut ends of the nerve branches [23].
Surgical intervention can yield gratifying results in alleviating facial tightness and abnormal eyelid constriction. However, for individuals seeking to improve restricted mouth mobility, the approach should be customized based on the patient's preference for correction. While the vertical tilt of the mouth corner can see substantial improvement, the enhancement of horizontal angles may be less than optimal.
Conclusion
It is well established that the most critical factor for patients with acute facial palsy is the timely administration of steroids and antiviral agents. However, patients with relatively severe facial palsy, as indicated by an ENoG degenerative rate exceeding 80% or a House-Brackmann grade higher than IV, are likely to develop synkinesis despite appropriate initial therapy. Therefore, it is crucial to suppress the intensification of synkinesis and provide re-education through early NMRT in patients predicted to develop synkinesis. In cases where synkinesis has already strongly formed, it is recommended to initiate treatment with a combination of BTX-A and NMRT. If synkinesis is so severe that it significantly impacts daily life, or if there is no significant improvement with BTX-A or NMRT, selective neurectomy can be considered as a viable surgical alternative.
PDF Links
PubReader
ePub Link
XML Download
Full text via DOI
Download Citation
