Nerve Injury Classification

Nerve injuries are classified according to the level of microanatomic disruption. According to the Sunder-land Classification system,1 level 1 injury yields only temporary dysfunction of the membrane sodium channels, without microanatomic disruption, resulting in a transient inability of the nerve to transmit impulses. In level 2 injury, axons are disrupted, though their individual endoneurial channels are not, so that when regeneration occurs, there is little to no misrouting of axons. In level 3 injury, endoneurial sheaths are violated, though perineurium is left intact. Recovery from this type of injury occurs over months and inevitably results in some synkinesis. Level 4 injury results in perineurial disruption without damage to the epineurial sheath, leading to generally poor recovery. Level 5 injury refers to total anatomic disruption of the entire nerve, including the epineurium.

Nerve Repair

When facial nerve discontinuity is encountered, the first approach is to attempt to reestablish direct neural continuity between the facial motor nucleus and the distal facial nerve, through either primary repair or autografting techniques. Recovery of function is usually better through primary repair than through grafts.2 In intratemporal locations, it is sometimes difficult to tell, despite microsurgical magnification, whether the facial nerve has been disrupted in its entirety or whether only a segment of the nerve has been injured. When this is the case, thorough exposure of the site of injury is warranted, and the literature supports repair when 50% of the facial nerve appears to have been violated. Intratemporal repair can be performed with facial nerve re-routing, facilitated by concomitant canal wall-down mastoidectomy surgery, so suturing can take place in the distal vertical segment. The horizontal segment is not amenable to suture repair, and if this region is involved, the nerve ends should simply be approximated and held in place with fibrin glue. It is important to re-approximate the nerve ends without tension to minimize fibrosis. In cases involving the loss of 17 mm or less of the facial nerve, primary neurorrhaphy can be obtained by re-routing the facial nerve within the temporal bone to gain further length and thus permit tensionless coaptation.3 Timing is important, and all repairs, no matter their location, should be performed within the first 72 hours after injury, during which time the distal nerve segment retains electrical stimulability. The operating microscope should be employed to evaluate, clean, and remove all devitalized tissue from the nerve endings. The epineurial sheath is then approximated in a tensionless manner. While interfascicular repair is theoretically possible distal to the pes anserinus, the human facial nerve lacks sufficient identifiable topographic orientation to make this type of realignment clinically useful.4

Nerve Grafting

After facial nerve injury or sacrifice, if a tension-free neurorrhaphy is not achievable, then an autograft, using a segment of donor sensory nerve, is interposed between the proximal and distal facial nerve endings. In facial nerve reconstruction, the most common donor nerve grafts are taken from the great auricular, sural, or medial antebrachial cutaneous nerves ( Fig. 23.1 ). The great auricular nerve is ideal for repairs that require grafts of < 6 cm. The resulting anesthesia to the ipsilateral auricle is well tolerated, and the nerve is of adequate diameter and caliber to provide a suitable graft. A contraindication to using it is the presence of a nearby neurotrophic malignancy, in which case the sural or medial antebrachial nerves are preferred.

The sural nerve is removed from the leg via an incision adjacent to the lateral malleolus. While initial sural nerve harvesting was performed through open approaches, subsequent technique modification have permitted harvest through a series of shorter incisions.5,6 A nerve “stripping” technique has become popular, where the length of the nerve may be harvested through two small incisions. Contemporary approaches to sural nerve harvest involve utilizing endoscopic equipment and either a single or two small stab incisions in the leg. The decrease in operative time and the significant decrease in incision length compared with open techniques make it a logical choice for the facial reanimation surgeon ( Fig. 23.2 ). Harvest of the sural nerve usually produces low morbidity, but the patient should expect decreased sensation over the dorsolateral foot, and 20–30% of patients may experience a mild level of “neuromatous pain” even years following harvest.7 The sural nerve can provide up to 30 cm of healthy nerve graft.

For total facial nerve reconstruction from the main trunk to peripheral branches, the medial antebrachial cutaneous nerve is most appropriate. There are at least four reliable branches, and it has adequate length to perform grafting of the entire facial nerve, even when the distal stumps lie at the anterior border of the parotid gland ( Fig. 23.3 ).

The principles of primary nerve repair and nerve grafting are similar. Repairs should be performed within the first 72 hours after injury or sacrifice, irrespective of the need for subsequent radiation therapy. During this time frame, the distal nerve segments retain electrical stimulability, making identification easier. Meticulous debridement and careful microsurgical technique are of paramount importance in optimizing regenerative outcome. The results of cable grafting are generally favorable.8 Factors that can lead to poor results include wound disruption, infection, or tension on the site of coaptation. In most cases, the return of movement appears within 6 to 12 months. Improvement may generally be expected over the course of 1 to 3 years.

There is debate as to the best method of nerve coaptation as it applies to both primary and graft repair. Epineurial repair has been contrasted to fascicular repair,9 though no study has convincingly demonstrated improved regenerative outcome based upon fascicular facial nerve repair. Therefore, given its relative simplicity, the current standard is to perform epineurial suture repair.10 Recently, there have been an increasing number of encouraging studies regarding the use of fibrin glue to assist with nerve coaptation. This method appears to hold promise as an alternative or an adjunct to nerve suturing, as it decreases the localized inflammatory response, acts as a sealant (rather than a barrier), and experimentally improves axonal regeneration and fiber alignment.11,12

While nerve repair, nerve grafting, and nerve substitution techniques are all designed to deliver neural input to the native facial musculature, the regeneration that occurs is universally misrouted. Misguided axonal extension to inappropriate targets leads to mass movement and synkinesis. While nerve reconstruction procedures are usually effective at restoring facial tone, fine control over each distinct zone of the face is seldom achieved using these techniques. Fig. 23.4 demonstrates the restoration of excellent resting tone following medial antebrachial cutaneous nerve grafting from the mastoid segment of the facial nerve to four peripheral branches, but significant midfacial synkinesis with ocular movements and ocular synkinesis with smiling.

Hypoglossal Facial Transfer

The hypoglossal nerve is most often utilized to reinnervate the distal facial nerve. Its proximity to the extratemporal facial nerve, its dense population of myelinated motor axons, and the relative acceptability of the resultant hemi-tongue weakness make it a logical choice.13,14 In the classic XII-VII transfer, the entire hypoglossal nerve is transected and reflected upward for direct neurorrhaphy to the facial nerve stump ( Fig. 23.5a ). Several modifications have been described ( Fig. 23.5b–d ), including the “split” XIIVII transfer,15 where ∼30% of the width of the hypoglossal nerve is divided from the main trunk of the nerve for several centimeters and is secured to the lower division of the facial nerve. However, given the interwoven fascicular architecture of the nerve, separating a 30% segment away from the main trunk for several centimeters divides a significantly greater number of axons than if the fibers were oriented in parallel.16 This realization led to a further modification, termed the XII-VII jump graft. The modification is designed to reduce tongue morbidity by avoiding the splicing away of a significant length of the hypoglossal trunk and involves an end-to-side neurorrhaphy between the hypoglossal nerve and a donor cable graft (usually the great auricular nerve), which in turn is sewn to the distal facial trunk (see Fig. 23.5c ).16

In circumstances where the facial nerve is able to be mobilized from the second genu within the temporal bone and reflected inferiorly, removal of the mastoid tip has allowed direct coaptation of the facial nerve to the hypoglossal, without the need for an interposition graft (see Fig. 23.5d ).17 Elimination of the cable graft provides a theoretical regenerative advantage by reducing the neurorrhaphies to one, although the relative rarity of this clinical situation makes outcome comparisons impractical.

Another potential use is for partial hypoglossal to facial grafting to treat resultant deficiencies in a single area, such as the marginal mandibular branch,18 and potentially the orbicularis oculi muscle.

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