Modern concepts in facial nerve reconstruction

EMG plays a central role in the evaluation of the patient (Figure 1). Muscular damage leads to alterations of the insertion potentials during needle EMG. EMG allows a prognosis on the probability of spontaneous healing [5]. In congenital palsy or in chronic palsy EMG allows an assessment, if musculature (still) is existing and to what degree and in which regions of the face spontaneous regeneration with defective healing took place. In lesions proximal to the stylomastoid foramen disturbance of the lacrimal function and taste, or hyperacusis can be observed. In patients with regeneration and defective healing the clinical examination together with EMG allow the physician to evaluate the severity of dyskinesia, synkinesia, and autoparalytic syndrome [6].

Magnetic resonance imaging (MRI) is preferred method of choice in order to localize a lesion of the facial nerve in the brainstem, the cerebellopontine angle and in the intratemporal course of the nerve [7]. MRI is much more accurate than classical topodiagnostic methods like Schirmer's test, stapedial reflex test, and taste function testing [8]. MRI also helps to evaluate the vitality of the mimic musculature in cases with long-term denervation. Muscle atrophy and fibrosis leads to an asymmetry of muscle volume in relation to the healthy side visible in MRI [9]. Such detailed analysis accounting for the patient's wishes and the life-expectancy of a comorbid patient should lead in an individual concept for the surgical rehabilitation of each patient.

In the first two months after trauma the nerve stumps can normally be dissected without hindering scar formation and best possible functional results can be achieved [6]. A direct co-adaptation of the facial nerve stumps is only possible, if the stumps are sharp-edged, i.e. after direct trauma, immediately within 24 hours after onset of the lesion.

Later, when the nerve stumps have to be freshened or if a gap of more than 1 cm is observed, an interpositional graft is needed to guarantee a tension-free nerve suture [3].

Well-proven donor nerves are the greater auricular nerve and the sural nerve. The use of biodegradable nerve tubes as alloplastic alternative can not be recommended for regular use as to date only case reports on their application are published [10].

Particularly after tumor resection the extratemporal resection defect can be very large in size. In such a situation a combined approach makes sense: The upper face is reconstructed with the proximal facial nerve and the lower face with a hypoglossal-facial-jump-nerve anastomosis. The separated reanimation of upper and lower face offers the advantage of prevention of synkinesia between both areas [6].

Because the first clinical signs of a successful regeneration do not occur before a time of six months and the finial results even needs twelve to 18 months, nerve suture is often combined with static reanimation of the eye closure using a upper lid weight [11, 12]. If lid weight is not effective, the first alternative is a palpable spring. This surgery is typically performed by an ophthalmologist [13]. If the lower lid is suspended due to loss of facial tone, it is recommended to combine upper eye lid surgery with a lower lid plasty [14].

An alternative technique for the restoration of eye closure is to use a dynamic temporalis muscle plasty [15]. In individual cases, it could be reasonable to reanimate the angle of the mouth with a dynamic muscle plasty, too. But the surgeon has to take care not to injure the very thin facial nerve branches entering the orbicularis oris muscle. If the patient wishes a very fast solution or if life expectancy is low, a dynamic muscle plasty can also be performed as a single procedure without nerve reconstruction. Here, the temporalis muscle or the masseter muscle is used for perioral reconstruction in combination with upper lid weight for eye restoration [16]. Informed consent is necessary that the geometrical vectors of this kind of muscle plasties are limited. Muscle plasties only allow a few restored movements. A digastric muscle plasty is indicated for restoration of the depressor of the corner of the mouth in cases of isolated palsy of the marginal mandibular branch or congenital aplasia of the depressor anguli oris muscle [17].

Even a dynamic muscle plasty can be technically impossible in cases of extended tumour surgery. As third choice static slings are part of the surgical arsenal. Slings allow restoration of the resting tone and improvement of facial asymmetry at rest in direction of the inserted sling. Autologic material like fascia lata or the tendon of the palmaris longus muscle is first choice in front of alloplastic material. Complications, especially wound healing problems, are seen more frequently with alloplastic material [18].

First choice for cross-nerve suture is the hypoglossal-facial jump nerve anastomosis (Figure 2 and 3). The classical type of hypoglossal-facial nerve anastomosis using the entire proximal hypoglossal nerve should be avoided nowadays. Classical hypoglossal-facial nerve anastomosis leads to unpleasant long-term sequelae, because the unilateral tongue atrophy produces permanent speech and swallowing problems. The hypoglossal-facial jump nerve anastomosis using only part of the hypoglossal nerve avoids tongue atrophy and the success rate is comparable to the classical type. Hyperkinesia, often seen after the classical technique, is avoided by the jump technique, because less nerve fibres regenerate to the periphery.

Several modifications of the hypoglossal-facial jump nerve anastomosis are described. Mostly used are a side-to-end nerve suture at the side of the proximal hypoglossal nerve and an end-to-end nerve suture to the distal facial nerve using a nerve graft in-between the hypoglossal and facial nerve. The hypoglossal nerve is incised to about 30%. Thereby, the nerve opens itself wedge-shaped to house the graft for the end-to-side nerve suture. Rarely, it is possible to bring together hypoglossal and facial nerve tensionless without using an interpositional graft. Other donor nerves for cross-nerve suture (motoric trigeminal nerve, accessory nerve, parts of the cervical plexus, ansa nervi hypoglossi) cause more morbidity in the donor region and show less satisfactory results [20].

The best alternative to hypoglossal-facial jump nerve anastomosis is a cross-face facial nerve suture: Peripheral facial nerve branches distal to the parotid gland are dissected on the contralateral healthy side. Even when electrostimulation is used to select two to four nerve branches to restore a selective symmetrical reinnervation of the ipsilateral lesioned side some additional palsy on the healthy side has to be accepted. To create a balance between these two aspects is difficult. The branches must be cut as distal as possible to minimize weakness on the healthy side. Long and several interpositional grafts are needed. Therefore, the suralis nerve is best choice. The suralis nerve is divided into several pieces. These pieces are pulled through the midface from the healthy to the lesioned side. The sural nerve grafts are sutured end-to-side to the facial nerve donor branches on the healthy side and end-to-end to selected peripheral facial nerve branches or to the main facial nerve trunk on the lesioned side[20].

Of course, depending on the individual situation, all kind of muscle plasties and sling procedures described above belong to the reanimation repertoire also in the situation of an early reconstruction in case of intratemporal lesion, more proximal facial nerve lesion or no possibility for extratemporal reconstruction.

If a cross-face facial nerve suture is chosen, even more time is needed because the grafts and therefore the distance to be reinnervated are much longer. To overcome this situation, the facial musculature of the lesioned side can be reanimated additionally by a so called babysitter procedure: Parallel to the cross-face surgery the facial musculature is reanimated by a hypoglossal-facial jump nerve anastomosis [21]. Recently, the babysitter procedure has also been described using the masseteric branch of the trigeminal nerve [22]. If the denervation time is longer than 6 months the proceeding fibrosis of the peripheral facial nerve could hinder the direct connection of the cross-face nerve suture to the target musculature. In such a situation, a different, two-step procedure is necessary: Nine to twelve months after the first step, when the nerve grafts are completely passed by the regrowing axons, the distal side of the grafts are connected to a free muscle transplant on the lesioned side (see below). A single step procedure, i.e. suture of the cross-face interpositional grafts and free muscle transfer at the same time in one surgical session, cannot be recommended as standard procedure as only limited data is published on this technique [23, 24].

Free microvascular muscle transfer in combination with cross-face nerve suture is therapy of choice in patients with congenital facial nerve palsy (for instance in children with Moebius syndrome). Here, often the nerve and the mimic musculature do not exist [25]. The most frequent muscles used are the gracilis muscle and the pectoralis minor muscle [15, 26]. In case of bilateral congenital palsy the reanimation of the free muscle transplant can be restored with bilateral hypoglossal-facial jump nerve anastomosis.

Especially in adult patients after tumor surgery, the use of dynamic muscle transfer (see above) is a good alternative to elaborate nerve reconstructions.

Dyskinesia and synkinesia as result of effective nerve regeneration can be reduced effectively by botulinum toxin injections (Figure 4) [27]. The reversibility of the botulinum toxin effect allows an individual adoption of necessary treatment. Since the introduction of botulinum toxin for this indication, definitive selective myectomies or neurectomies are no longer necessary. These irreversible and rough procedures should only be discussed if botulinum toxin is not effective. In facial areas with permanent weakened movements the asymmetry to the contralateral facial side is even amplified by overuse of the contralateral healthy side. In such a case, botulinum toxin can also be applied on the healthy side to reduce the muscle movements in the overused mimic areas. On the healthy side, botulinum toxin is most often used to reduce the function of the depressor anguli oris muscle [28].

Mimic therapy should start at best when the first reinnervation signs are visible by EMG or are at least when reinnervation is clinically visible in the mimic musculature after nerve reanimation surgery. Before, mimic therapy only is frustrating for the patients, because it will not result in voluntary movements. In case of hypoglossal-facial jump nerve anastomosis, the training must focus first on intended tongue movements to induce facial mimic movement. The patient will learn which kind of intended tongue movements lead to which kind of facial movement. With time, the patient will move his face without thinking on tongue movements anymore.

Systematic controlled studies on the role of physical therapy and also on the role of electrostimulation therapy are lacking [29, 30]. It is imaginable that physical therapy could help to reduce the degree of muscle atrophy in the first time after nerve suture to bridge the time before the regrowing axons have reached the mimic musculature. In patients with muscle transfer physical therapy could start after wound healing and help the patient to train the transferred muscle for his new function [31].