Nerve transfers

Introduction

While peripheral nerve reconstruction is an evolving field, nerve transfers have become a more broadly accepted reconstructive option ( Video Lecture 26.1 ). As our understanding of internal nerve topography improves, we are able to recognize redundant or non-essential potential donor nerves and fascicles. Recent contributions in basic science are broadening options for nerve transfer with new and improved techniques. Clinical research focusing on outcomes, applications to novel populations, and areas of debate continue to advance the field. This chapter will outline the history and principles of nerve transfers as they pertain to upper extremity nerve reconstruction. The recent literature as it relates to current techniques, debates, and advances in nerve transfers will be reviewed. Current commonly performed nerve transfers are detailed with focus on both motor and sensory nerve transfers, their indications, rehabilitation and postoperative protocols, and a basic overview of selected surgical techniques.

The classification of nerve injury is perhaps the single most important factor when determining management ( Table 26.1 ). The six degrees of nerve injury have been well described. First- and second-degree injuries recover spontaneously, as do third-degree, albeit recovery is variable and less than normal, depending on scar tissue. There may be a role for surgery in these patients to augment recovery. Fourth- and fifth-degree injuries are significant and recovery is not expected. These injuries should be managed with prompt surgical intervention to prevent degeneration and fibrosis of the motor endplates. Sixth-degree injuries encompass a variety of nerve injuries within a single nerve, and the need to operate is determined on an individual basis.

Once surgical intervention has been deemed necessary, there are a number of surgical options. Traditionally nerve repair or grafting was used with tendon transfers to augment poor recovery or delayed presentation. The difficulty with proximal nerve injuries is that the distance from the injury to the motor endplate is significant, and recovery is prolonged. In addition, delayed intervention may result in denervation and fibrosis prior to muscle reinnervation. Axonal regeneration has been shown to occur at the rate of approximately 1 inch (2.5 cm) per month and reinnervation of denervated muscle is not possible after 12–18 months; therefore, nerve transfers offer a valuable means of providing regenerating axons in closer proximity to the motor endplate. Essentially, a nerve transfer converts a proximal nerve injury to a distal one, enabling faster recovery and quicker return to function.

Nerve transfers were described very early; however they were traditionally reserved for otherwise unsalvageable root avulsions and proximal pan-plexus injuries. Approximately 30 years ago, nerve transfers were revisited and applied to a significantly broader patient population, with applications in partial plexus injuries to recreate specific essential functions in the upper extremity. As our understanding of internal nerve topography has broadened, techniques and equipment for performing microsurgery have improved, and our diagnostic ability for brachial plexus and peripheral nerve injury has progressed, the field of nerve transfers has exploded. Nerve transfers have become the standard of care for treatment of peripheral nerve injuries in the authors' practice and have largely replaced other modalities.

Basic science

There have been dramatic increases in our knowledge of nerve injury, healing, and regeneration. The most relevant recent basic science advances pertaining to nerve transfers relate to end-to-side transfers ( Fig. 26.1 ). End-to-side neur­orrhaphy involves the coaptation of the distal end of an injured recipient nerve into the lateral aspect of an intact nerve, which serves as a proximal source of axons to regenerate into the injured nerve. Reports of end-to-side transfers occurred as early as the late 1800s and then made a resurgence in the 1990s. There has, however, been controversy regarding the success of these transfers, and sensory recovery has typically been more impressive than motor recovery. The sprouting of axons in an end-to-side coaptation, thus eliminating the need for a proximal nerve stump, makes nerve transfers an option for proximal nerve injuries and has been well demonstrated in the literature. While sensory nerves are able to sprout spontaneously (collateral sprouting), proximal donor nerve axonotmetic injury is required for motor neuronal regeneration (regenerative sprouting) across the end-to-side repair ( Fig. 26.2 ). An epineurotomy in the donor nerve is required, and partial axotomy will result in more significant regenerative sprouting into the recipient nerve, albeit at the potential expense of a measurable downgrade in donor nerve function, which may be temporary or permanent.

Supercharged end-to-side nerve transfers involve the complete transection of the donor nerve, which is then coapted into the side of the intact recipient nerve. This maximizes the potential number of available motor axons from the donor nerve. This procedure does not interrupt any recovery in the injured recipient nerve because the nerve remains in continuity; however, the additional axons recruited from the donor nerve improve distal target reinnervation, a concept known as “supercharging”. In a proximal partial nerve injury, with a long distance required for reinnervation, this technique can protect target muscles from denervation atrophy and fibrosis. Clinically, the most relevant place to consider this strategy is for preservation of intrinsic hand function in proximal severe ulnar neuropathy with use of the end-to-side distal anterior interosseous (branch to pronator quadratus) to deep motor branch of ulnar nerve end-to-side coaptation. The following algorithm provides some guidance on the indications for utilizing the supercharge-end-to-side transfer in clinical practice ( Algorithm 26.1 ).

Algorithm 26.1

This algorithm provides guidance on use of the supercharge end-to-side (“SETS”) nerve transfer to augment motor recovery. Use of the SETS is not indicated for nerve injuries that will recover completely such as a neurapraxia. It can be used in select axonotometic injuries, where complete motor recovery is not expected. Electodiagnostic studies, specifically electromyography (EMG) of clinically relevant muscles, are performed. Depending on the findings of the EMG and serial clinical exam, surgery or repeat EMG may be indicated. The SETS transfer should not be performed in cases where clinical recovery is seen or where simple nerve decompression alone may lead to complete recovery of function. The SETS transfer is performed in clinical scenarios where less then complete motor recovery is expected. (© nervesurgery.wustl.edu, Washington University in St. Louis.)

Diagnosis and patient presentation

The value of a complete history and physical examination in the patient with a potential brachial plexus or proximal nerve injury of the upper extremity cannot be overstated. Patients should be evaluated promptly after injury, both to provide a baseline for future serial examinations and to facilitate timing of surgical intervention where necessary. Documentation of a complete plexus examination helps to prevent missing neurologic deficits.

Patient history

On history, details of the mechanism and timing of injury are crucial. When seeing a patient in a delayed fashion, it is also useful to ascertain any interval recovery of function. The mechanism of injury will determine timing of intervention, with prompt exploration of penetrating sharp trauma, and more expectant management of closed injuries and gunshot wounds. Specific patient symptoms such as loss of function, both sensory and motor, and pain should be precisely elicited, as this will help focus the subsequent physical examination. The pain diagram and questionnaire are particularly useful for eliciting this information ( Fig. 26.3 ). Other factors, such as patient age, handedness, occupation, and comorbidities, will factor into management decisions and foster shared decision-making. Attention to patient goals and expectations allows for individually tailored and nuanced approaches that may or may not involve nerve transfer surgery techniques, which form “a part of the armamentarium” when caring for patients with complex upper extremity injury. Further information on steadily improving quality of life and other gradual but continuing changes over time after nerve transfer in brachial plexus injury patients may further influence treatment choices and patient preference-specific choices.

Patient selection

Nerve transfers have become increasingly popular for a number of reasons, the most compelling of which is the “time is muscle” issue. The biggest challenge facing peripheral nerve surgeons is that with increasing time since injury, the ability to achieve good motor function becomes increasingly limited. In fact, for any nerve injury where there is complete discontinuity with the motor end organ, no reinnervation procedure will be able to restore muscle once denervation and fibrosis have occurred, a process that occurs as early as 1 year. Nerve transfers, by bringing the regenerating motor fibers closer to the target end organ more rapidly, essentially convert a more proximal-level injury to a more distal-level injury, thus increasing the chance of achieving meaningful muscle function.

Another advantage of nerve transfers is that they enable surgical reconstruction outside the zone of the original injury, avoiding complex dissections and limiting injury to critical neurovascular structures. In patients with brachial plexus trauma, the proximity of vital structures (great vessels, thoracic duct, lung) makes the occurrence of potentially life-threatening complications a real possibility. In other cases, such as concomitant vascular injury, previous blast injury, or soft-tissue deficits requiring flap reconstruction, the ability to avoid doing a complex nerve exploration in the setting of dense fibrotic scar is particularly advantageous.

Nerve transfers allow for a very targeted intervention in cases of partial nerve injury such as in treatment of a sixth-degree or neuroma-in-continuity injury, where transfer can be done distal to the site of injury specifically to the non-functioning nerve branch. This allows for preservation of all intact function and restoration of only missing function. In addition, the ability to “supercharge” a recovering nerve is invaluable when it is unclear whether the nerve transfer or intrinsic recovery will be more beneficial. Another example that is somewhat controversial is the setting of brachial neuritis (neuralgic amyotrophy). Previously thought to have excellent prognosis, recovery following brachial neuritis is not always reliable and therefore augmenting with a “supercharge” procedure offers additional motor axons without impeding the change for spontaneous recovery, even in a delayed fashion.

Unlike tendon transfers, nerve transfers typically do not require immobilization, which is especially valuable in patients presenting with significant baseline stiffness. Nerve transfers also preserve the biomechanical properties of the musculotendinous unit, such as end organ origin, insertion, excursion, and length–tension relationships. Finally, nerve transfers can restore unique function such as pronation, which is incredibly difficult to restore by traditional surgical techniques.

Thus nerve transfers are indicated in a number of situations ( Box 26.1 ), for example, in proximal brachial plexus injury where grafting is not possible or the required distance for reinnervation will not allow motor axons to reach the target motor before permanent atrophy and fibrosis due to denervation have occurred. In situations of extreme scarring, or major upper extremity trauma, a nerve transfer is preferable to risking damage to critical structures within the scarred region. Nerve transfers are also indicated in segmental nerve loss, and in partial nerve injury patterns with functional loss. Patients presenting in a delayed manner with inadequate time to reinnervate the distal targets are good candidates for distal nerve transfers. Also, patients with sensory nerve deficits in critical regions should be considered for nerve transfer to restore sensation.

The main reason a nerve transfer should not be performed is similar to the contraindication for any other traditional nerve repair/­graft intervention, namely end organ unresponsiveness. An old peripheral nerve injury will not respond to the new “magic” of nerve transfers. Muscle that is in complete discontinuity with the nerve for greater than 1 year will not be reinnervated, no matter how elaborate the reinnervation strategy employed. The exception to this is in children, where the potential window for reinnervation appears to be longer.

More relative contraindications for nerve transfer include issues such as the time required for regeneration, challenges of the surgery, the anatomic knowledge required, and problems of postoperative retraining and therapy, as these techniques are less familiar to hand therapists. There are some patients, such as the young manual laborer with a radial nerve injury, who may prefer the more rapid recovery associated with tendon transfer at the expense of the independent fine motor control that could be achieved through the use of nerve transfers. The surgery itself is challenging because of the detailed knowledge of internal nerve topography required. Most surgeons are not formally trained in the intraneural dissection techniques required to perform nerve transfers and the results are not apparent until months down the line. This requires a significant leap of faith for surgeons who are more used to immediate gratification and a clear result in weeks, not months or years!

Direct nerve repair and nerve grafting also remain valuable tools for the peripheral nerve surgeon and should continue to be the treatment of choice in a variety of scenarios. These include cases of multiple nerve injuries where there is a paucity of nerve donor material for nerve transfer. Also, in distal single-function nerve injuries, direct or graft repair is preferable to nerve transfer because one-to-one function is preserved, no retraining is necessary, no donor function is sacrificed, and the distance to the end target is short.

Patient general health, comorbidities, and associated injuries factor into the decision to perform nerve transfers. These procedures can be lengthy, and are not without significant risk from anesthesia in the fragile patient. In addition, patient compliance is an integral part of the recovery process, and patients require education preoperatively about the prolonged recovery and therapy times associated with nerve transfer.

Examples of nerve transfer procedures for specific injury patterns

Upper plexus injury

Specific patient exam findings

Patients with upper plexus injuries present with glenohumeral joint subluxation, loss of shoulder abduction and external rotation, and absent elbow flexion. When C7 is also involved, additional deficits in function may include loss of elbow extension and weakness of wrist and digit extension. Numbness over the corresponding dermatomes is noted.

Reconstruction techniques

Hints and tips

Priorities for upper plexus injuries focus first on restoring elbow flexion and second on stabilizing the shoulder. Restoration of shoulder abduction and external rotation is desired. Standard nerve transfers include: (1) double fascicular nerve transfer; (2) medial triceps to axillary; and (3) spinal accessory to suprascapular nerve.

Use of spinal accessory nerve (cranial nerve XI) to suprascapular nerve transfer (motor)

Restoration of shoulder stability and external rotation are facilitated by transferring the spinal accessory nerve (cranial nerve XI) to the suprascapular nerve. This transfer can be conducted by either an anterior or a posterior approach. The posterior approach is used only if scarring precludes the anterior approach. In the posterior approach, the patient is positioned prone and surface landmarks are used to approximate the position of the nerves ( Fig. 26.4 ). The spinal accessory nerve runs parallel to the border of the trapezius and is localized 44% of the way along a line connecting the acromion to the dorsal midline at the level of the superior border of the scapula ( Fig. 26.5 ). The suprascapular nerve is located midway between the medial border of the scapula and the acromion as it runs through the suprascapular notch. Dissection is carried through the trapezius in a muscle-splitting fashion and an end-to-end coaptation, sparing the upper trapezius nerve branches, is conducted.

For the anterior approach, a supraclavicular approach to the brachial plexus is made; the lateral aspect of the trapezius is released off of the clavicle and the spinal accessory nerve is identified on its anterior surface. The suprascapular nerve sits at the superolateral aspect of the upper trunk, which is identified between the anterior and middle scalene muscles. Coaptation can be performed in an end-to-side fashion. In some cases, an interpositional nerve graft may be required. A proximal crush to the spinal accessory donor nerve is needed for this motor donor, which remains otherwise in-continuity to its recipient musculature.

Use of triceps to axillary nerve transfer (motor component)

Additional abduction of the shoulder is provided by transferring a branch of the triceps, usually from the medial head, to the axillary nerve ( Fig. 26.6 ). Better results in upper plexus injury patients are achieved by reinnervating both the suprascapular and axillary nerves. A longitudinal incision is made on the posterior surface of the arm ( Fig. 26.7 ). The axillary nerve is identified in the quadrangular space and dissected proximally to include the branch to teres minor, then divided proximally. The natural cleavage plane between the lateral and long heads of the triceps muscles is identified and blunt dissection is conducted to expose the donor radial nerve running along the humerus.

Clinical tip

The branch to the medial triceps sits superficially and medially on the surface of the radial nerve as a distinct branch. This makes dissection of this branch quick and minimizes risk of creating a donor deficit.

The donor triceps nerve is dissected as far distally as possible and then coapted to the axillary nerve ( Fig. 26.8 ).

Use of the double fascicular nerve transfer (motor)

Restoration of elbow flexion is achieved with the double fascicular (ulnar/­median redundant branches to biceps brachii and brachialis branches of the musculocutaneous) nerve transfer ( Fig. 26.9 ). This transfer reinnervates both the biceps brachii and brachialis muscles using redundant fascicles from the ulnar and median nerves, if available. However, in cases of limited donor availability, reinnervation of biceps alone may provide adequate antigravity elbow flexion. A medial arm incision in the bicipital groove allows exposure of the musculocutaneous, median, and ulnar nerves. Elevating the biceps brachii muscle laterally allows exposure of the proximal biceps and distal brachialis nerve branches. These are divided and draped over the median and ulnar nerves to determine best donor–recipient pairings. Most frequently, the biceps will be paired with a fascicle from the median nerve and the brachialis will be paired with a redundant fascicle from the ulnar nerve. The ulnar and median nerves are then neurolyzed at the appropriate level, and expendable fascicles to the flexor carpi ulnaris (FCU) (ulnar), flexor carpi radialis (FCR), or flexor digitorum superficialis (FDS) (median) are divided distally and coapted end to end. Reinnervation occurs at approximately 4–6 months postoperatively.

Clinical tip

Avoid harvesting both the FCU and FCR branches when doing a double fascicular nerve transfer to preserve strong wrist flexion.

This transfer can also be conducted in a SETS fashion, with meticulous dissection of the donor nerves in order to facilitate reaching the recipients without an interpositional nerve graft.

Other potential donors to restore elbow flexion

Other potential donors that can be used to restore elbow flexion include the medial pectoral nerves, the thoracodorsal nerve, and the intercostal nerves.