General Principles in Evaluating and Treating Peripheral Nerve Pathology, Injuries, and Entrapments and Their Historical Context

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CHAPTER 229 General Principles in Evaluating and Treating Peripheral Nerve Pathology, Injuries, and Entrapments and Their Historical Context

Aaron G. Filler, David G. Kline

Nerve surgery, considered apart from cranial and spinal surgery, constitutes a third and distinct component of neurosurgery. Techniques for the diagnosis and surgical management of peripheral nerve disorders, as well as detailed understanding of surgical nerve pathology, have progressed tremendously in the past several decades. Just as spinal surgery was transformed by the advent of both magnetic resonance imaging (MRI) and instrumentation, a major transformation and expansion have taken place in peripheral nerve surgery since World War II and especially in the last several decades. These changes have advanced peripheral nerve surgery into more equal status with other neurosurgical subspecialties in terms of diversity and technologic acumen.

Successful modern surgery is dependent on accurate diagnosis, comprehensive understanding of the underlying biology of relevant pathologies and recovery phenomena, and mastery of the technical aspects of intervention. In nerve surgery, all these elements have undergone a relatively rapid, yet decisive change.

The need for an accurate and detailed history and then a thorough physical examination cannot be stressed enough. It is possible to not only assess but also grade almost all the muscles innervated by both the brachial and lumbosacral plexus. Some practice is required, but once the steps are learned, it can be done relatively rapidly. In the process, both sensory and motor outcomes can be provided. Then, with repetitive clinical testing, the course of an injury can be delineated. These steps are of even more value when coupled with well-performed electromyographic and conduction studies.

During much of the 20th century, peripheral nerve surgery struggled with a single question—how should a damaged nerve be managed? Reliable surgical approaches did not emerge until after World War II. Before this period, surgeons first debated the utility of performing any kind of repair and then watched with disappointment as they realized that many direct repairs failed. Much of this history has been reviewed in detail recently.¹

In the mid-19th century there were reports that recovery could take place after a nerve was severed, even without surgical repair,² and other published reports showed that suture repair could produce good results.³^,⁴ However, over the course of the following 100 years, no reliable success was achieved with any method.

Wars can lead to medical advances, and nerve surgery has not been an exception. One of the lessons from World War II was that gunshot or shell fragment wounds, whether they sever a nerve (<15% of cases) or leave it in continuity, are blunt injuries.⁵ Such injuries have an element of stretch/contusion as a result of implosion and then explosion affecting a length of nerve, even when a missile does not make a direct hit but passes close by. This leads to injury over the length of the nerve. The extent of this injury is unpredictable acutely, so even if the nerve was severed and the ends distracted, World War II studies showed a large measure of failure when the nerve was repaired acutely or what some would designate primarily.

Exploration of bluntly transected nerves after a few weeks’ delay is now understood to be more effective because the proximal and distal extent of the injury is readily demarcated. Both stumps will have neuromas, and it is obvious where one needs to trim back to reach healthy tissue. Besides gunshot wounds, other blunt mechanisms in a civilian setting include propeller and fan blades, reciprocating saws, axes, and wounds caused by metal where transection is a possibility but the force is a blunt one.

Another lesson learned from large American and less so from British studies of such nerve injuries secondary to shell fragments was that inspection and palpation of the lesion at the operating table did not always provide an accurate prediction for or against recovery.⁶ Review of pathologic slides submitted from resected nerves indicated that many lesions in continuity had adequate anatomic regeneration and, most likely, had unnecessarily been resected. The reverse implication was that many other nerves had probably, after operative inspection and perhaps only neurolysis, not undergone needed resection and repair.

These observations led Nulsen and Lewey to propose the use of simple stimulation to evaluate such lesions.⁷ Such evaluation was valuable because positive distal muscle responses could antedate clinical recovery by weeks. The difficulty with the test was that one had to wait many months with major nerve lesions before the test could show regeneration. If the response was negative after such an elapse of time, repair would be greatly delayed and result in outcomes less than optimal.

These observation led in the late 1960s to the development of nerve action potential (NAP) or nerve-to-nerve recording across initially experimental and eventually actual clinical lesions in continuity.⁸ This permitted operative evaluation within a few months because the finding of an NAP across such a lesion correlated with the presence of 3000 to 4000 fibers just distal to the lesion. These fibers were usually of moderate size and had some degree of early myelination. Recovery to a grade 3 or better Louisiana State University Health Sciences Center (LSUHSC) level occurred in such patients with positive NAPs about 93% of the time. Absence of such a response leading to resection was confirmed pathologically as a neurotmetic (Seddon classification) or grade IV (Sunderland classification) lesion that had little or no opportunity for spontaneous recovery. Occasionally, the presence of an NAP was due to regrowth in a portion of the cross section of the nerve with poor growth elsewhere. This led to split repair in which a portion of the nerve underwent neurolysis and another portion underwent repair, usually with interfascicular grafts.

Seddon experimented with the use of nerve autografts after World War II but generally had little success.⁹^,¹⁰ It was only at the end of the 1960s that Hanno Millesi finally solved this problem with use of carefully constructed grouped interfascicular grafts.¹¹ He discovered that Seddon’s failure with nerve grafts was due to technical surgical factors that could be corrected to achieve reliable success. He also showed that many of the previous attempts at direct repair failed when the repair line was under tension or scar tissue formed in the nerve ends and blocked regrowing axons. The 1970s ushered in a series of confirmatory experiments,¹²^–¹⁴ and the 1980s saw the onset of full-scale acceptance of interposition nerve graft repair after stump excision as a paradigm to guide management.¹⁵

What was finally demonstrated by the 1980s was the following set of principles for managing severed or severely injured nerves in continuity:

• Nerves can be usefully repaired and function often restored provided that the timing is good and appropriate technical steps are taken.

• The proximal and distal stumps of a bluntly severed nerve generally need to be excised after a delay of a few weeks to remove scar tissue and neuroma and to allow regrowth to take place.

• Sharply transected nerves are best repaired relatively acutely and can almost always be joined end to end.

• Because tension at the repair site must be avoided, interposition nerve grafts may be required if end-to-end repair cannot be achieved with minimal tension in bluntly transected nerves and lengthy lesions in continuity.

• For graft repair, several small-caliber grafts in parallel are required rather than a single large-caliber graft because of the requirements of revascularization of the graft tissue.

Despite these observations, end-to-end repair can and does lead to results superior to graft repair provided that the stumps or lesions in continuity are adequately trimmed or resected and the repair is placed under only mild tension. Of course, the more serious lengthy lesions associated with stretch and contusion or very blunt transections seldom lend themselves to end-to-end repair, and then grafts become necessary.

The all too frequent lesions in continuity with serious distal loss can usually be observed for several months. If no clinical or electromyographic evidence of recovery has taken place by this time, exploration and decisions about complete repair are aided by intraoperative electrophysiologic studies,¹⁶ especially NAP recordings across the lesions.

Guided by these principles, David Kline and Alan Hudson gained a large volume of experience from the 1970s through the 1990s that has provided the basis for classification and prognostic assessment of a wide variety of different types of clinical nerve injury situations.¹⁷^,¹⁸ Their work, along with that of Mackinnon and Dellon,¹⁵ Birch and colleagues,¹⁹ and others, also resulted in the development of a complete set of surgical techniques and exposures that provided approaches and exposure methods for virtually every nerve in the body.^15,^17,¹⁹^–²¹ Outcomes have been graded for sensory and motor recovery with the LSUHSC system¹⁷ and, in many other instances, the British Medical Research Council system.²² Some current work has and certainly future work will emphasize the practical usefulness of the limb and indices of patient satisfaction in addition to motor and sensory grades.

Significant progress has also been made in understanding the proper use of a variety of possible types of nerve transfer procedures. This body of knowledge now provides a great deal of guidance for the development of reinnervation/reanimation strategies in the setting of severe brachial plexus injuries.²³

Blunt injuries differ from sharp ones by knife or glass that sever the nerve completely or incompletely. When a nerve is transected by these sharp forces, relatively acute (within 72 hours) repair is indicated. When transected, nerves tend to retract because of their elasticity. With more acute repair, retraction is minimal, scar has not had time to become established, and the surgical anatomy is straightforward. Data on both sharply and bluntly transected nerves show that when the timing of repair is matched to the situation, the results are greatly optimized, even with difficult challenges in nerve surgery such as the brachial plexus.¹⁷

Not all transecting forces that result in significant neural loss divide the nerve, so despite either a sharp or blunt transecting mechanism, a lesion in continuity can result. Such lesions must then be evaluated by intraoperative electrophysiologic techniques after the passage of some weeks and a decision subsequently made whether to resect the lesion.

Timing is important for lesions in continuity caused by injury. Lesions in continuity actually represent the largest category of injury, with close to 70% of all nerve injuries resulting in such lesions in most reported series. Causes include stretch, contusion, severe compression, injection, and electrical and iatrogenic injury. Many lesions in continuity are associated with fractures or bony dislocations and vascular injury.

The next major technical advance in nerve repair included the solution to the problem of how to make functional synthetic tubes ²⁴^–²⁶ for use in place of interposition autografts (Fig. 229-1). A randomized prospective outcome trial demonstrated that tubes can be used reliably for successful repair of short nerve gaps up to 3 cm in length without grafts.²⁷ These synthetics have greatly simplified repair of some nerve injuries.

FIGURE 229-1 NeuraGen collagen tube from Integra. Such synthetics can allow severed nerves to grow across short gaps without the need for interposition autografts.

Entrapment Syndromes

Recognition of distal entrapments such as carpal tunnel syndrome and cubital tunnel syndrome led to useful surgical procedures as early as the 1930s.²⁸ For most patients with these conditions, treatment has changed little since that time. Newer surgical methodologies such as endoscopic carpal tunnel release have resulted in faster recovery times from surgery, but complications of endoscopic release, such as a higher rate of laceration of the recurrent branch of the median nerve and the ulnar nerve than with open surgery, have inhibited the widespread use of endoscopic methods. Advances in endoscopic technology have mostly resolved this differential complications issue,²⁹ but outcomes have been so good with open carpal tunnel surgery that it is still the most common method used by neurosurgeons.³⁰^,³¹

Ulnar nerve surgery has been somewhat of a confused area because of the availability of other operations, such as subcutaneous or submuscular transposition or epicondylectomy. Outcome studies have failed to show any advantage over the older, simpler in situ decompressive surgery in most cases.³² Exceptions may be ulnar entrapment with more severe grades of loss or previous operations, in which case submuscular transposition may have an advantage.¹⁷^,²⁰

Minimal-Access Surgery for Major Proximal Nerve Entrapments

Among the major advances in peripheral nerve surgery in the early 21st century has been the introduction of minimal-access approaches for release of proximal nerve entrapments.³³^,³⁴ Until recently, the extensive exposure required for repair of complex and severe nerve injuries has tended to guide the approaches used by nerve surgeons trying to treat small focal entrapments of proximal nerves. In the treatment of sciatic nerve pain syndromes, for instance, large operations that require transection of the gluteal muscle mass can lead to an impaired gait that can be more disruptive to the patient than the original pain syndrome. The introduction of small nondestructive approaches to the brachial plexus and pelvic nerves has led to outpatient management of these conditions with no formal hospital admission for surgery. The application of adhesiolystic agents initially developed for other surgical specialities (e.g., hyaluronate-based Seprafilm) has made it possible that neuroplasty, in which nerves are released, will not in most cases be reversed by recurrent adhesions according to a recent study,³³ although it remains to be fully proved that decreases in the occurrence of postoperative adhesions per se result in better outcomes.

Impact of Nerve Imaging and Open Magnetic Resonance–Guided Injections

Just as angiography, computed tomography, and MRI transformed the process of diagnosis and surgical management in cranial and spinal neurosurgery, the late arrival of effective imaging for peripheral nerves is starting to transform and energize the field of peripheral nerve surgery.³⁵^–³⁸ The results have, in some cases, provided insight into long-standing disputes in nerve surgery.

Through much of the 19th century and well into the 20th century, some surgeons believed that severed nerves could recover distal innervation without surgical repair. Subsequently, patients who did not recover underwent surgical exploration and were found to need nerve repair for recovery. However, without imaging, there was no way to know what happened to patients who may have suffered a torn or severed nerve but then recovered without surgery. The presumption has been that these patients had bruised nerves with gross continuity that recovered. However, in 2002, magnetic resonance neurography revealed that as in other mammals, severed human nerves could sprout viable new terminal nerve elements in some situations ³⁹ (Fig. 229-2)—thus finally confirming Virchow’s much doubted claim from the 1860s. Nonetheless, it is clear that virtually all severed nerves need surgical repair if recovery is to be expected.

FIGURE 229-2 Eight-year-old boy with partial reinnervation of the right leg muscles 8 months after being struck by a motor vehicle. Initially, he had flaccid paralysis of the sciatic-innervated muscles but by 5 months began to have flexion at the knee that was 80% of strength at the time of the imaging examination. A₁, Sagittal image of the right thigh showing the femoral fracture (fx), damaged sciatic nerve (SC), and patella (pat) for orientation. A₂, The sciatic nerve was fully transected by the initial injury, and its healed stump is seen to have marked hyperintensity. Several newly growing nerves are seen emerging from the stump to reinnervate the long head of the biceps femoris. B₁, The semitendinosus and semimembranosus demonstrate hyperintensity (*) as a result of muscle denervation. B₂, The short head of the biceps femoris(**) also continues to demonstrate the hyperintensity of denervation. C, An axial image 3 inches below the level of the sciatic stump in A [ps(C)] demonstrates the hyperintense semimembranosus and semitendinosus (*) and the short head of the biceps femoris (**). There is a normal muscle image appearance in the long head of the biceps femoris (***). The femur (fe) is labeled for orientation, and placement of the sagittal image slices for B₁ and B₂ are also marked [ps(B1) and ps(B2)].

It is no longer necessary to wait for failure of improvement to occur and surgical exploration to learn whether a nerve is severed (Fig. 229-3). Therefore, as soon as the postinjury edema has resolved, a patient may be imaged and early repair of severed or disrupted nerves can be planned. If a patient is failing to improve after nerve repair, it is sometimes possible to discover why a repair has failed before additional months elapse (Fig. 229-4). Distraction of previously repaired stumps or large neuroma formation may be seen and indicate the need for further repair.