Facial Nerve Repair – Indications, Contraindications

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Facial Nerve Repair there are three main options: direct end-to-end coaptation, coaptation with an interposition graft, and nerve transfer. Nerve transfer is not a nerve repair procedure, per se. However, it deserves mention because its goal is similar to nerve repair: reinnervation of the denervated tissue. Early after nerve injury, when the site of injury is known (such as direct trauma or iatrogenic nerve transection), a tension-free direct end-to-end coaptation is the best treatment option. When there is a discontinuity of the nerve and approximation without tension is not possible, the next best option is an interposition graft. When these two procedures are successful, a spontaneous smile usually can be recovered. The success rate of these procedures varies greatly (5% to 86%) between series with improvement to a House Brackmann (HB) grade 3. For the majority of the series, this number is close to 50% of cases reaching HB 3, which is the best outcome expected from this procedure. When only the distal nerve stump is surgically accessible and/or more than one year from the lesion has occurred, a nerve transfer should be considered.

This could be the case with lesions near the brainstem or when there is a malignant invasion of the proximal end of the nerve (malignancies of the petrous temporal bone, for example). This requires another cranial nerve neurotmesis of a division or the entire nerve to maintain the tone of the affected facial muscles and with training produce a smile upon volitional movement of the muscle group functionally associated with the donor nerve (tongue in the case of hypoglossal nerve transfer, masseter muscle in the case of masseteric nerve transfer).

Anatomy and Physiology

The facial nerve, or seventh cranial nerve, has its origin in the lower pons and curves in the CPA to enter the internal auditory meatus along the vestibulocochlear nerves. Inside the petrous bone, it has a tympanic and petrous course where it gives off the greater superficial petrosal nerve (GSPN) with presynaptic general somatic efferent axons to the lacrimal gland and nasal/palatine mucosal glands (synapse at the pterygopalatine ganglion). It then turns as the geniculate ganglion (contains cell body of special somatic sensorial pseudounipolar neurons for taste sensation) after which the nerve gives off a stapedial branch (stapes muscle) and divides into the chorda tympani (presynaptic axons to the submandibular and sublingual glands, with synapsis at the submandibular ganglion) and the motor branch of the facial nerve.

The motor axons of the facial nerve originate from the motor nucleus of the facial nerve in the pons. After originating from their nucleus, the motor axons of the facial nerve course upwards and backward to then turn around the abducens nucleus forming the facial colliculus before exiting the brainstem. All the afferent fibers and all the visceral efferent fibers course form the nervus intermedius which runs between the facial nerve and the vestibulocochlear nerve at the CPA.

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The symptoms associated with an injury of the facial nerve will be directly related to the site of the injury along the course of the nerve. The resulting injury from transection of the facial nerve can be predicted by considering the type of axon that is coursing the nerve in a particular segment (takeoff from pons, after the origin of the GSPN, or isolated distal motor division. This arrangement also explains why lesions proximal to the takeoff of the GSPN can lead to the crocodile tears phenomenon; if a facial nerve is injured before the takeoff of the GSPN, there might be aberrant reinnervation of the pterygopalatine ganglion (and lacrimal gland) with axons originally destined to the submandibular gland causing a patient to cry due to a stimulus that normally provokes salivation.

Indications of Facial Nerve Repair

Indication for facial nerve repair is a facial paralysis or near-complete facial paralysis (HB grade 5 or 6) due to direct injury to the facial nerve axons. In cases of facial coaptation, with or without interposition graft, there must be a viable distal and proximal segment of nerve. This excludes lesions very close to the pons and lesions due to demyelinating disorders with direct damage to the upstream facial circuitry (nuclei, tracts, and cortex). For both nerve transfer and coaptation, there should be insertion and/or fibrillation potentials present on electromyography (EMG), indicating the preservation of neuromuscular junctions.

  • If the proximal and distal nerve stumps can be approximated without tension, direct end-to-end coaptation is indicated.
  • When both stumps are accessible, but it is impossible to approximate them without tension, an interposition nerve graft (most commonly the greater auricular nerve or the sural nerve) is indicated. Interpositional graft can only be used in cases of facial paralysis of 1 year or less.
  • Paralysis lasting longer than one year should be treated with hypoglossal or masseteric nerve transfer.

Contraindications of Facial Nerve Repair

  • Absence of any electric activity on preoperative EMG of the target muscles: Lack of insertion or fibrillation potentials indicates atrophy of the neuromuscular junction with possible fibrosis of the muscles.
  • Short life expectancy: patients with a disseminated malignancy might not live to experience the benefits of the procedure; thus, it may expose them to the surgical risk without any benefit.
  • Poor functional neurological outcomes: patients with a vegetative state or severe dementia will not be able to comply with facial retraining or benefit in any way from any of these procedures.
  • Preexisting dysphagia is a contraindication for hypoglossal nerve transfer: the procedure will probably make it worse.

Equipment

Following are the requirements to carry out facial nerve repair:

  • Surgical microscope
  • Microsurgical instruments
  • 8-0 to 10-0 nylon sutures
  • Fibrin glue

There has not been any evidence of a significant difference between the use of fibrin glue or nylon sutures; however, there is a tendency to favor the use of suture. The technical challenge of using sutures in confined spaces and proximity to the brainstem has lead to the preferred use of fibrin glue for intradural and transdural coaptation. Despite being technically demanding, sutures confer additional stability and have been used safely and successfully even close to the brainstem.

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Personnel

The nerve coaptation is usually carried out by a neurosurgeon, head and neck surgeon, or a plastic surgeon. In the case of a direct hypoglossal nerve transfer, the involvement of a neuro-otologist is highly recommended, as this procedure may require a cortical mastoidectomy.

The importance of operating room staff that is familiar and engaged as a team with the procedure cannot be underestimated.

Preparation

Patient education regarding the goals of the procedure is essential as facial reanimation takes several months to demonstrate any noticeable changes, and to maximize the results requires facial rehabilitation and, very often, the addition of adjuvant procedures. Patients with some degree of preoperative facial motor function (HB 5 or 6) that wish to undergo this procedure must be aware that there will be a total loss of function immediately after the procedure that might not improve if reinnervation is unsuccessful.

When a donor graft is to be used, the area of the graft must be prepared in advance, and the patient draped and positioned accordingly. The nerves most commonly used for grafting are the sural nerve and the greater auricular nerve. Obtaining the sural graft can be challenging as the donor site is distant from the recipient. Sometimes, a separate surgical team harvests the graft, while the other team exposes the facial nerve.

Technique of Facial Nerve Repair

  • Direct End-to-end Coaptation – This is the first-line treatment of choice when feasible. This is usually done when the nerve is sharply transected, and the two ends can be approximated without tension. Such is the case of direct, sharp trauma or transection during a surgical procedure. The connective tissue is cleared from the nerve, and the two endings are approximated with 4 to 6 epineural sutures using nylon 10-0 (8-0 and 9-0 are also acceptable options). When suturing is not technically feasible, fibrin glue is an option often used with the addition of a temporalis muscle fascia cuff or a synthetic collagen cuff.
  • Facial Coaptation with Interposition Nerve Graft – It is used when a facial nerve is sharply transected, but the two ends cannot be approximated without tension. This procedure is technically similar to the direct end-to-end coaptation. The difference is that a nerve from the donor site is grafted and interposed between the two stumps using the same technique used to perform a direct end-to-end nerve coaptation. Sensory donor nerves commonly used are (in order of preference): sural nerve, great auricular nerve, antebrachial cutaneous nerve.
  • Masseteric Nerve Transfer – It is used when the proximal facial nerve is damaged. After dissection of the recipient distal facial nerve, the masseteric nerve is dissected. An end-to-end coaptation is performed to either the main branch of the facial nerve or to the buccal or zygomatic branch. When tension-free coaptation cannot be achieved, a greater auricular nerve interposition graft can be used.
  • Hypoglossal Nerve Transfer – There is more than one technique for this transfer in the literature. Some involve complete anastomosis of the hypoglossal nerve causing ipsilateral tongue atrophy, while others use a portion of the hypoglossal nerve. The technique using a portion of the hypoglossal nerve involves a mastoidectomy to release the petrous facial nerve. This allows the approximation of the facial nerve without tension to the hypoglossal nerve. The hypoglossal nerve distal to the ansa cervicalis is dissected, and a 30% to 40% hypoglossal neurotomy is done. The distal facial nerve is mobilized and coapted to the proximal part of the partial hypoglossal neurotomy aiming for an end-to-end axonal alignment. The suture of preference for this technique is 9-0 or 10-0 nylon epineural suture.
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Complications

  • Synkinesis of the face involves the simultaneous contraction of the upper and lower face muscles upon an attempt at a volitional smile. Almost all cases of nerve transfer can expect some degree of synkinesis of all the reinnervated facial muscles. Botulinum toxin injections and facial physical therapy are the mainstay treatment options for this problem.
  • Tongue atrophy with hypoglossal to facial nerve transfer is described with the older techniques where the complete hypoglossal nerve was transected. With partial transection of the nerve, no cases of tongue atrophy are expected.

References

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