Facial nerve decompression refers to exposure of the facial nerve along the length of its bony canal in the temporal bone. It is indicated when the nerve dysfunction is related to compression of an inflamed nerve within the>bony canal, causing a “choking” of the nerve in the confined space. There is evidence to suggest that in cases of Bell’s palsy, where recovery is not seen after a certain period, this procedure can lead to overall better recovery than if it is not performed. The benefit, however, may be quite small, and the risks of surgery need to be weighed against this benefit.
The facial nerve is usually approached through a middle fossa craniotomy, and through the mastoid bone directly behind the ear. Segments of the bony canal are then unroofed, and the nerve carefully protected from surgical trauma as the overlying bone is removed. By removing the overlying bone, the nerve is free to expand, thus relieving the compression that results in ischemia and neurapraxia.
When the facial nerve is transected, but there is no gap between the two stumps of the nerve, the strategy for repair is to simply sew or glue the two edges of the nerve back together.
When the facial nerve is injured or sacrificed and there is space between the two nerve stumps, it is not feasible to perform a direct coaptation. In this case, a nerve graft from another part of the body is used as a cable to connect the two ends; fibers from the proximal stump then grow into and traverse the graft, and enter the distal stump. From there they are directed toward their targets. Regeneration through these grafts can be quite variable, and often unsatisfactory. Significant research has gone toward the development of substitutes for nerve autografts, including vein grafts, muscle cables, synthetic tubes, and biodegradable materials. There are some theoretical advantages to these bioartificial nerve regeneration conduits; in the entubulation model, both the properties of the walls of the chamber and the contents of the chamber can be manipulated to promote regeneration. However, to date, regeneration through these graft materials has not been any better than that through autografts, which remain the gold standard. Regardless of what type of graft is used, it serves simply as a conduit for the extension of neural fibers from the proximal stump into the distal musculature. The fibers must extend all the way from the injury site to the muscles of facial expression, and this process is quite lengthy.
There are situations where the distal facial nerve and facial musculature are healthy, but where the nerve is permanently and irreparably sacrificed upstream, near or at the brainstem. When this happens, one approach to restoring facial movement is to bring neurons into the distal facial nerve from another nerve that is close by. Obviously, cutting all or part of a different motor nerve, such as that to the tongue or the shoulder, involves creating a new motor deficit in order to give some neuronal “power” to the facial musculature. Over time, the nerve which has become most popular for this purpose is the hypoglossal nerve, which controls tongue movement. It turns out that the motor deficit that results from cutting the nerve to reroute it is usually well tolerated and compensated for by the hypoglossal nerve on the other side. In addition, it has become popular not to cut the entire hypoglossal nerve, but to transect it only partially, so that some fibers go to the tongue, while some are rerouted to the facial musculature.
The major advantage of reinnervation techniques is that they utilize the native facial muscles; facial movement through input to these muscles is much more delicate than from muscle that has been transposed or transplanted into the face. Natural resting tone is restored to near-normal, and at rest the two sides of the face can balance quite nicely. The disadvantages involve the new motor deficits created, and the fact that in order to achieve facial movement the patient has to make a voluntary tongue movement, thus driving the facial movement via the hypoglossal nerve. With training, this voluntary movement can appear and feel somewhat natural, but it requires patience and practice. The final disadvantage is that because the hypoglossal nerve is hooked to the main trunk of the facial nerve, eliciting a movement causes all regions of the face to move at once, a phenomenon termed “mass movement”. This means that when a person tries to voluntarily smile, there is some simultaneous eye closure. Also, the initiation of a subtle facial movement can lead to a full strength contraction of the entire hemiface. Problems with not conveying meaningful emotion on the face are common.
Modifications to the hypoglossal facial transfer have been described to cut down on these issues, including limiting the nerve hookup to the lower division of the facial nerve only, and addressing the upper face with other techniques.
Many surgeons utilize one or several branches of the healthy, contralateral facial nerve for reinnervation purposes, by bringing nerve fibers across to the diseased side via a sural nerve autograft. The beauty of this technique is that it results in both sides of the face contracting simultaneously, and naturally. Involuntary, emotional smiling can occur. Most of the time, this procedure requires a second stage in which a piece of muscle is transplanted into the face, in the vector of a smile.
Regional Muscle Transfer
Like any muscles, if the muscles of facial expression are not used for many months, they undergo atrophy and scarring; even if neural input is restored, they lose their ability to contract properly. This means that in cases of prolonged denervation (more than 1-2 years), reinnervation techniques like the hypoglossal facial transfer are not good options. The next strategy employed is to transfer muscle into the face, and place it into some of the same vectors as the native musculature. Using this technique, contraction of the newly transferred muscle “imitates” the native facial muscles, and permits smiling, eye closure, and eversion of the lower lip.
Regional muscle transfer refers to a technique in which a muscle found in the head and neck area is loosened from its normal bed and rotated into the face along a smiling vector. The native blood supply to the muscle is maintained, so there is no need for microsurgical vessel hookup. The native neural supply to the transferred muscle is also maintained, so that to create the smile effect, the patient must voluntarily initiate a contraction of the transferred muscle. In the past, the masseter muscle was described as a potential muscle for transfer, but it was not able to be placed into quite the right vector to achieve a natural appearing smile, and has fallen out of favor. Currently, the most popular muscle for regional transfer for facial reanimation is the temporalis muscle transfer.
The temporalis transfer involves taking a 1-2 cm band of the temporalis muscle (one of the chewing muscles), and rotating it out of the temple region, over the cheek bone and down to attach to the corner of the mouth. When it is appropriately secured, the act of biting down will result in elevation of the corner of the mouth toward the cheekbone, just as in smiling.
Over the past 30 years, reconstructive surgeons have developed the ability to transfer segments of skin, muscle, and bone from one part of the body to another by harvesting the tissue on a vascular pedicle, and then hooking the artery and vein back up to donor vessels in the area where the reconstruction is required. This new surgical technology has been useful in facial reanimation because it has allowed surgeons to bring new muscle into the face for reanimation purposes. The first muscle used for this purpose was the gracilis, a thin muscle located in the inner thigh. It was transferred into the face, secured to the cheek bone and the corner of the mouth, and hooked up to the local vascular supply. Since then, many other muscles have been utilized for free tissue transfer for facial paralysis, though the gracilis continues to be the most popular. Amongst other muscles, the latissimus dorsi, pectoralis minor, serratus anterior, and rectus abdominis muscles have been utilized.
muscle transfer can involve either a one stage operation or a two stage operation, depending on where the nerve input will come from to drive the muscle. If the goal is to restore spontaneous smile and the opposite facial nerve is intact, then a cross facial nerve graft can be placed six to twelve months earlier (as described in the cross face nerve grafting segment). When axons reach the tip of the graft, muscle is transplanted into the paralyzed side of the face, and its nerve is attached to the cross face nerve graft. If, however, the opposite facial nerve is also affected (i.e., Mobiius syndrome), then the muscle can be transplanted into the face and its nerve attached immediately to the nerve which drives one of the powerful chewing muscles, the masseter muscle. For patients who undergo this one stage procedure, in order to elicit facial movement, the patient needs to make a conscious effort to bite down. This triggers the muscle to contract and lift the corner of the mouth.
While excellent results can be obtained using free muscle transfer, the surgical procedure is delicate, time consuming, and not guaranteed to yield an improved aesthetic result. Issues can arise with excess bulk of the muscle in the face, poor excursion of the transferred muscle, and poor muscle survival. These things need to be carefully considered before such an operative plan is undertaken.
While many people focus upon the lack of movement of the affected side in facial paralysis, another critical issue is the position and function of structures in the resting state. Facial paralysis causes a drooping of the corner of the mouth that leads to drooling, a collapse of the side of the nose that can cause nasal blockage, and a widening of the space between the upper and lower eyelids, exposing the eye more than is ordinary. The affected eyebrow is usually lower than the other side, and the blink reflex is lost.
A number of procedures have been designed to correct these malpositionings, both to improve appearance as well as to restore function of the structures of the face. The decision about which of these options to pursue lies with each patient, and his/her specific set of symptoms.
Rehabilitation of the paralyzed eye is accomplished by one of several techniques. The eyebrow can be repositioned by performing a unilateral brow lift, and matching the brow height with the other side. This can be done with very small incisions, using endoscopes to guide the surgery.
The eyelids can be addressed by implanting a small eyelid weight into the upper lid so that gravity assists with eye closure.
There are also implantable eyelid springs which offer mechanical assistance with eye closure. Another technique to decrease eye exposure is to place several stitches from the upper lid to the lower lid where the two lids meet at the corner of the eye. This technique, called a tarsorrhaphy, is excellent for eye protection, but does produces an asymmetry between the two eyes which is often noticeable.
The collapse of the nasal sidewall often seen in facial paralysis is termed nasal valve collapse, and can be remedied either from the outside or the inside. Outside techniques involve placing strips of suspension material from the cheekbone, under the skin, to the nasal sidewall, and suspending the nasal sidewall back in its anatomic position. This can be done in conjunction with static sling procedures for the corner of the mouth. To widen the nasal cavity from the inside, small cartilage grafts can be inserted into the framework of the nose to widen the cavity slightly, or to stiffen the lowest part of the nose so that it does not get pulled in and block the passage when the patient breathes in through the nose.
To reposition the corner of the mouth without going through a muscle transfer procedure, static sling techniques are used. They are very useful for patients who are either not candidates for lengthy surgery, or in whom muscle transfers have healed poorly or failed. The technique is simple, and involves securing a rigid piece of material from the cheekbone, under the skin, down to the corner of the mouth and the upper and lower lip. There is a great deal of debate over which material is best suited for this purpose. Fascia lata, a tough sheathlike material from the thigh, has been widely used and is favored because it is an autologous material (something from the patient’s own body). Because using fascia lata requires a second surgical site, and therefore a scar, surgeons have looked for alternatives to avoid this. There are commercially available synthetic and allogenic materials (materials from organ banks) that are now achieving popularity for these static slings. All types of materials have distinct advantages and disadvantages, and these need to be well understood when choosing which is right for an individual patient.
In older patients with aging skin, there is often a looseness, or laxity, to the soft tissue of entire face. This has led to the implementation of facelift type techniques to improve facial symmetry. It differs from a standard facelift in that the paralyzed side of the face is pulled significantly more than the normal side, and the sides are made to “match” as closely as possible. This technique has found a substantial place in the management of partial paralysis or poor recovery from Bell’s palsy and Ramsey-Hunt Syndrome.