Keywords
facial nerve classification - facial paralysis injury - nerve - reconstruction - surgical oncology
The facial nerve, the seventh cranial nerve, takes a long anatomical course from the cerebral cortex, where the facial nucleus originates within the pons. The motor, sensory, and parasympathetic branches arise from three different nuclei.[1] The intratemporal portion of the facial nerve divides into four segments: meatal, labyrinthine, tympanic, and mastoid. The labyrinthine segment extends from the internal auditory meatus to the geniculate ganglion, where the cell bodies of special visceral afferent neurons carrying taste from the anterior two-thirds of the tongue are located. At the geniculate, the nervus intermedius joins the facial nerve proper, and the greater superficial petrosal and lesser petrosal nerves exit the facial nerve. The tympanic segment of the nerve extends posteriorly from the geniculate ganglion to the second genu, where the facial nerve turns inferiorly, transitioning to the vertical or mastoid segment, giving off the stapedius nerve and the chorda tympani. The facial nerve exits the temporal bone through the stylomastoid foramen, yielding the extratemporal portion of the nerve, which runs within the parotid gland and then innervates all muscles of facial expression.[2]
Almost a fifth of all facial nerve palsy cases are found in the setting of oncologic pathology, with acoustic neuroma (10%) being the second most common cause overall, followed by head and neck cancers (7%).[3] In these instances of acute facial paralysis, it is paramount to select an adequate method of reconstruction to optimize the clinical outcomes. Reanimation methods depend on several factors, including timing, age, location, etiology, available surgical options, and surgeon experience. With numerous surgical therapy options available, authors advocate consideration of the level of injury as a framework for systematically approaching the viable options of reconstruction. Herein, the authors classified the oncologic facial nerve injuries in three levels in relation to their nerve reconstruction methods of choice or strategies ([Fig. 1]): Level I, II, and III.
Fig. 1 Course of the facial nerve as it originates from the pons, passes through the temporal bone and parotid gland, and terminates at its muscular motor branches. Levels of injury (I, II, and III) are labelled, and borders are defined.
Level I (intracranial to intratemporal) is defined as facial nerve lesions that occur at interval from the origin of the facial nucleus to the exit of the nerve through the stylomastoid foramen without an available proximal nerve stump to graft to. Level II (intratemporal to extratemporal and intraparotid) interval is defined as the proximal nerve stump being available through the exit point of the stylomastoid foramen to the anterior parotid gland. Level III (extratemporal and extraparotid) interval is defined as nerve injuries that occur distal to the parotid gland. This classification serves as a reference to the authors to indicate the available options and methods of reconstruction; however, this needs to be individualized to the patient's needs.
Clinical features, common clinical scenarios, donor nerves available, recipient nerve, and priorities of reconstruction will be presented at each level. Additionally, examples of clinical cases will be shared to illustrate the utility of framing acute facial nerve injuries within injury levels. The clinical case studies were approved by the ethical institutional committee in Sweden (Dnr:2020-03492 with amendment 2022-04681-02 and 2023-01156-02). Signed letters of informed consent were collected from patients for participation in the study, and additional oral and written consent regarding the publication of photos and/or video material was recorded and collected.
Level I: Intracranial to Intratemporal
Level I: Intracranial to Intratemporal
An injury that occurs along the course of the facial nerve from its intracranial origin at the facial nuclei located within the pons along the course to the intratemporal portion, where a proximal nerve stump cannot be retrieved and is not available for grafting is considered a Level I injury ([Fig. 2]).
Fig. 2 Level I injury of the facial nerve, the authors preferred nerve transfers indicated with green labels and blue arrows—alternative nerve transfers labelled in white. Central nervous system lesions or intratemporal lesions render the proximal facial nerve stump unavailable for grafting. The authors prefer to use a masseter nerve to the upper trunk of the facial nerve eye and smile reanimation and mini-hypoglossal nerve transfer for lower lip reanimation. Ansa cervicalis can also serve as a first-line transfer. Contralateral CNVII and ipsilateral spinal accessory are considered alternatives.
Clinical Presentation
Common oncologic clinical entities that result in a level I injury include intracranial benign or malignant tumors, cholesteatoma, vestibular schwannoma, and postresection tumors.[4]
[5] This presents as either an upper motor neuron (UMN) lesion or a lower motor neuron (LMN) lesion, two clinically distinct entities. Due to bilateral cortical innervation of the muscles of the upper face, only LMN lesions will result in complete facial palsy. Therefore, raising the eyebrow is the most clinically useful distinguishing tool to assess UMN versus LMN in facial palsy.[4] Complete facial palsy presents with the following classic features: brow depression, lagophthalmos, paralytic ectropion, nasal deviation away from the affected side, flattened nasolabial fold, asymmetry of the mouth, in additional psychosocial and physical dysfunction related to mastication and speech.
Priorities
Priorities of reconstruction for LMN level include eye closure, lip elevation-smile, and lower lip reanimation. Paralytic lagophthalmos is characteristic of LMN facial palsy. Symptoms of paralytic lagophthalmos include reflex tearing and ocular sicca, which may progress to exposure keratopathy and corneal vision loss.[6]
[7] In oncologic pathology where paralytic lagophthalmos exists in the setting of discontinuity of the facial nerve where no functional recovery can be expected, therapeutic management should focus on reestablishing motor neuron input to the oculi muscle.[8] No gold standard exists today for the reconstruction of the orbicularis oculi muscle due to the delicate and specialized nature of the muscle. Therefore, in the acute setting of oncologic resection of the facial nerve or nerve damage, a concerted effort should be made to salvage these muscles to avoid the sequela of lagophthalmos and exposure keratopathy. Additional priorities of this level should include competency of the oral sphincter, which prevents drooling and enables mastication, drinking, and speech, in addition to facial movements that convey nonverbal communication and expression.[3] Consequences of complete facial paralysis also result in facial asymmetry leading to functional and psychosocial dysfunction; efforts to mitigate asymmetry and maximize aesthetic outcomes are of functional importance in this patient population.
Authors' Reconstructive Method of Choice
The masseter to facial nerve transfer has proven utility to reinnervate paralytic muscles of the midface and perioral region, resulting in a natural symmetric smile. Advantages include minimal donor deficit, a high density of axons within the masseter nerve resulting in rapid recovery within 6 months and good resting tone.[9] The masseter nerve is our first option for reinnervation of the upper trunk before it has branched into its terminal branches, usually consisting of the temporal, zygomatic, and buccal branches ([Fig. 2]). Given the reinnervation of two distinct functions (eye closure and smile) from the same source of neurotization, it is expected to produce some degree of synkinesis in most patients. Synkinesis can be addressed later if needed using conservative management with botulinum toxin. Severe synkinesis requiring surgical correction is very unusual. Our preference for lower lip depressor innervation is to use a Mini-Hypoglossal transfer to the marginal mandibular nerve ([Fig. 2]). Terzis and Tzafetta describe a case series where 29 patients over 18 years had a mini-hypoglossal nerve transfer or negotiation to the lower lip depressors, with 80% achieving a moderate result or greater.[10] Branches of the ansa cervicalis chain can also serve as a donor for transfer to the lower facial nerve branches,[11] but require extensive dissection. The authors prefer a mini-hypoglossal nerve transfer side-to-end as the exposure can be made through a smaller incision. The hypoglossal nerve is approached through a 3- to 4-cm submandibular incision, careful dissection is performed to identify the posterior belly of the digastric muscle. Once the posterior digastric is freed circumferentially, it is critical to identify the hypoglossal nerve with a nerve stimulator before transection of the posterior digastric. The marginal mandibular nerve can be identified in the same incision, exiting at the lower boarder of the parotid gland. Once identified, the marginal mandibular nerve can be transected proximally brought inferiorly. A small partial transection is made in the hypoglossal nerve to accommodate the proximal cut end of the marginal mandibular nerve. A side-to-end coaptation is then performed with the hypoglossal nerve. A recent systematic review and meta-analysis representing 961 patients undergoing hypoglossal nerve transfer showed significantly less tongue atrophy among the groups where the hypoglossal nerve was partially transections versus an end-to-end anastomosis.[12]
Other Available Options
In the setting of a level I injury with no proximal facial nerve stump available, the possible options include ipsilateral spinal accessory (cranial nerve XI) and contralateral facial nerve branch with interposition nerve graft (selective cervical or zygomatic branches; [Fig. 2]). However, these options are inferior for optimizing a maximum number of axons and minimizing morbidity. Contralateral nerve grafting requires long nerve grafts to reach their targets, and it is estimated that 50% of axons are lost at each neurorrhaphy site.[13] Complete sectioning of the spinal accessory nerve can produce significant shoulder weakness, and the transfer lacks the “physiologic dynamism” other cranial nerves provide.[14]
Level I Case
A 55-year-old woman, otherwise healthy, underwent the removal of intracranial vestibular schwannoma, including the facial nerve intracranially, with a sequela of complete hemifacial paralysis ([Fig. 3a–d]).
Fig. 3 (a–d) Level I preoperative physical examination.
This patient was defined as a level I injury according to our clinical classification scheme, and 2 months after initial surgery, she underwent masseter nerve transfer to reinnervation of the upper trunk and mini-hypoglossal transfer (side-to-end) to the marginal mandibular nerve for reinnervation of the lower lip and fascia lata sling for static support of the midface ([Fig. 4]). The patient received adjuvant treatment to improve the outcomes, such as a gold-weight implant to improve eye closure and botulinum toxin in the platysma and depressor anguli oris (DAO) muscles due to synkinetic movements.
Fig. 4 Level I injury intraoperative photos identifying all branches of the facial nerve and transfer nerves (masseter and hypoglossal).
Long-term follow-up 2 years postnerve transfer pictures ([Fig. 5]) showed excellent symmetry at rest, correction of ectropion, effortless smile with activation of masseter nerve and lower lip symmetry, tonicity and depression with tongue activation.
Level II: Intratemporal to Extratemporal and Intraparotid
Level II: Intratemporal to Extratemporal and Intraparotid
A level II injury occurs along the course of the facial nerve from its exit of the temporal bone at the stylomastoid foramen, where the proximal nerve stump is retrievable and available for nerve grafting, to the anterior border of the parotid gland ([Fig. 6]).
Fig. 5 (a, b) Level I injury patient 2 years postop; marginal mandibular activation via hypoglossal (a) and smile activation using masseter (b).
Fig. 6 Level II facial nerve injury, authors preferred nerve transfers indicated with green labels and blue arrows—alternative nerve transfers are labelled in white. Intratemporal and extratemporal intraparotid lesions render the proximal facial nerve stump available for grafting. The authors prefer dual innervation, using the ipsilateral facial nerve with nerve graft to the upper trunk temporal and zygomatic branches for eye and forehead reanimation. Masseter nerve to the buccal and zygomatic branches for smile and nose reanimation. Ansa cervicalis or mini-hypoglossal transfer to marginal mandibular branch and lower lip reanimation. Ipsilateral spinal accessory as an alternative transfer.
Clinical Presentation
Clinical scenarios common for a level II injury are primarily parotid gland tumors or lateral temporal bone resection. Parotid tumors account for 80% of all salivary gland tumors, and 20% are malignant and often invasive to the facial nerve that passes through the parotid gland.[15] Often, the nerve is resected to mitigate the chance of recurrence and improve prognosis. Tumors at this level involving the facial nerve result in complete facial paralysis with or without skin defects and often result in long nerve gaps. These are exclusively LMN injuries at this level, resulting in the classical presentation of complete palsy. Often, a soft tissue defect, either externally or a loss of tissue bulk, presents with this level of injury.
Priorities
Priorities of reconstruction for this level reflect that of a level I injury because the clinical morphology is similar. Eye protection tops the list for level II injury, followed by oral competence, smile, and facial symmetry. Reconstruction strategies must be aimed at providing high numbers of axons to the distal facial nerve branch for reanimation. In the instance of a level II lesion, the availability of a proximal facial nerve stump lends itself to expanding reconstructive options. Traditional approaches have used only interposition nerve grafts between the proximal and distal nerve stumps; however, due to suboptimal conditions for nerve regeneration (i.e., radiation, long nerve gaps, advanced age), nerve gaps are only indicated in short nerve gaps (<5 cm).[16] In the scenario of a skin deficit, emphasis is placed on restoring adequate skin bulk to minimize contour deformity and coverage of vital structures with supple, well-vascularized soft tissue coverage.
Authors' Reconstructive Method of Choice
Our method of choice is what we refer to as “triple innervation of the facial nerve” to maximize the number of axons to every facial target: eye closure, midface reanimation, and lower lip reanimation. The facial nerve stump is used with a nerve graft for eye closure and forehead reanimation to temporal plus or minus zygomatic branches ([Fig. 6]). Our standard is to use a sural nerve graft. However, when a free flap is required to reconstruct the defect, in the case of vascularized or nonvascularized nerve grafts, we use the vastus lateralis motor nerve from the anterolateral thigh flap donor site to minimize donor site morbidity. Oral competence, smile, and nose reanimation are achieved through masseter nerve transfer to zygomatic and buccal branches. Lower lip depressors are prioritized as a secondary goal for reanimation. Typically, oncologic surgeons have exposed the neck in the clinical scenario of a level II injury, making selective nerve branches from the ansa cervicalis retrievable and readily available for transfer. For this reason, the ansa cervicalis is our first choice for reinnervation of lower lip depressors in a level II injury ([Fig. 6]). In comparison to Level I injuries the branched of the ansa cervicalis are not readily exposed, our choice would then be to use the hypoglossal nerve due to a small incision required to perform the transfer. Mini-hypoglossal transfer (side-to-end) to the marginal mandibular branch can also achieve lower lip reinnervation at this level. Both mini-hypoglossal and ansa cervicalis nerve transfers are options to secure powerful reinnervation of the lower lip depressors. However, they both can create synkinetic movements of the DAO and mentalis.[11]
Other Available Options
Alternatives in the setting of a level II injury include spinal accessory nerve branch transfer. However, this option is inferior for minimizing morbidity concerning shoulder weakness.[14] In the clinical scenario of soft tissue loss, options for reconstruction expand to incorporating a vascularized nerve graft with a free anterolateral thigh flap.[17] Vascularized nerve grafts have several benefits when compared to their nonvascularized counterparts, including an increase in the number of myelin fibers, thickness, axon diameter, faster axonal regeneration, and higher axonal count.[18] Vascularized grafts are not always feasible to retrieve. However, when the patient requires soft tissue reconstruction of a level II injury, the vastus lateralis motor nerve can be harvested with the vascularized pedicle of the lateral femoral circumflex descending branch ([Fig. 7]). A vastus lateralis motor branch graft provides minimal donor morbidity due to its multiple branches and sufficient length for grafting in a level II injury zone of the facial nerve.[18]
Fig. 7 Vascularized vastus lateralis motor nerve graft with anterolateral thigh flap.
Level II Case
A 28-year-old male presents with a mammary analogue secretory carcinoma in the left parotid gland. He underwent parotidectomy with sacrifice of the left facial nerve with reconstruction ([Fig. 8]) and postoperative radiotherapy.
Immediately postop he exhibited complete left facial palsy with lagophthalmos, brow ptosis, and complete loss of commissure excursion. Eighteen months postop the patient attained complete resolution of lagophthalmos, and near complete resolution of brow ptosis, and upper and lower lip excursion ([Fig. 9a–h]).
Fig. 8 Level II injury intraoperative photos; ipsilateral nerve graft for eye and forehead reanimation, masseter nerve for midface reanimation, ansa cervicalis nerve transfer for lower lip reanimation.
Fig. 9 (a–h) Level II patient physical exam immediately postop (left-sided column; [a–d]) versus 18 months postop (right-sided column; [e–h]).
Level III: Extratemporal and Extraparotid
Level III: Extratemporal and Extraparotid
A level III injury occurs along the course of the facial nerve from where the branches exit the parotid gland and distal, extratemporal and extraparotid ([Fig. 10]).
Fig. 10 Level III facial nerve injury, authors preferred nerve transfers indicated with green labels and blue arrows. Alternative nerve transfers are labelled in white. Extratemporal and extraparotid nerve injuries or distal nerve injuries render incomplete facial palsy with injury to selective nerve branches depending on clinical scenario. Anatomical reconstruction through direct nerve repair using nerve grafts for short gaps (<5 cm) or interfacial nerve transfer in the scenario of large nerve gap (>5 cm), for example, the cervical branch to marginal mandibular nerve.
Clinical Presentation
Common clinical scenarios at this level are selective oncological procedures requiring soft tissue excision that may involve the facial nerve (i.e., neck biopsy, submandibular gland, skin cancer excision) or soft tissue malignancies such as a sarcoma requiring a wide excision. Therefore, only partial facial palsy is encountered, and generally, only selected branches are affected; typically, the frontal or the marginal mandibular nerve branches.
Priorities
Ideally, for a given injury at this level, the priority is to restore continuity and functionality of the injured facial nerve branch(es) using ipsilateral facial nerve branches or interfacial nerve transfer. The spontaneity of movements can be achieved due to the availability of proximal nerve stumps and should be prioritized for this level.
Authors' Reconstructive Method of Choice
The authors advocate for a method of anatomical reconstruction to restore the continuity of the facial nerve branches by direct repair or interposition of nerve grafts to ipsilateral facial nerve branches or interfacial nerve transfer such as platysma motor branch to the marginal mandibular nerve[19] ([Fig. 10]). In case of long nerve gaps (>5 cm), interfacial nerve transfers can be performed by using expendable neighbor branches for neurotization, such as the cervical branch to the marginal mandibular branch.
Other Available Options
Alternatives in a level III injury setting include cross-face nerve grafting from contralateral cranial nerve VII and ipsilateral nonfacial cranial nerve donors as second-line options. Long nerve grafts are suboptimal in this setting due to the loss of axonal load over two coaptation sites.[13] Ipsilateral nonfacial cranial nerves provide a large axonal load but do not provide spontaneous reinnervation and require relearning. Like level II injuries, a large soft tissue defect may be encountered. Options for reconstruction expand to the incorporation of a vascularized nerve graft with a free anterolateral thigh flap.[17]
Level III Case
A 66-year-old male presents with a malignant fibrous histiocytoma that needed extensive soft tissue resection with the sacrifice of the marginal mandibular nerve. Nerve reconstruction was performed by transferring the platysma motor nerve or cervical branch of the facial nerve to the distal stump of the marginal mandibular nerve ([Figs. 11a–c] and [12a–d]).
Fig. 11 (a–c) Level III injury intraoperative platysma motor nerve transfer plus ALT for coverage.
Fig. 12 (a–d) Level III injury patient immediately postop and 3 years postop, showing excellent symmetry and normal functional recovery of the lower lip muscles.
Conclusion
Early facial nerve reconstruction should be offered in every patient with oncological resections of the facial nerve. The selection of donor nerves is key in successful facial nerve reconstruction in oncological patients. Combination of facial and nonfacial donor nerves (hybrid) is usually necessary to achieve maximal reinnervation of the mimetic muscles. Our proposed classification of three levels of facial nerve injuries provides a guide to selection of method of nerve reconstruction in relation of the level of injury in oncologic patients while prioritizing functional outcomes.
