Pain and Peripheral Nerve Injury

Neuropathic pain and neuralgia are two common terms that many clinicians use interchangeably. Their definitions remain broad and include pain related to an injury or abnormality of a peripheral nerve (i.e., neuropathic pain) and pain in the cutaneous distribution of a peripheral nerve (i.e., neuralgia).¹ There are additional categories of peripheral nerve-related pain, summarized in Table 246-1. Although some categories have distinct etiologies, symptoms, and treatments, significant overlap remains, as does continued disagreement with the classification system currently being used.¹

The etiology of peripheral neuropathic pain is likely multifactorial.² In general, the following processes likely contribute:

The exact relationship between sympathetic outflow and pain remains uncertain, however, transient improvement in some patients after sympathectomy strongly supports this connection.⁴ Pathophysiologic changes in the spinal cord and periaqueductal gray matter can centralize pain so that pain remains despite nerve blocks and other peripheral treatments being applied.

Diagnosis and classification of pain is predominantly based on the history and physical examination. Response to trial medications, autonomic changes, and temporary improvement with nerve blocks may help refine the cause. When taking a history, the location, quality (e.g., burning, paresthetic, crushing), and any exacerbating and relieving maneuvers, should be discussed with the patient. With peripheral nerve entrapment, pain is often referred adjacent to, and along, the distribution of the compressed nerve. For example, the description of aching discomfort in the wrist and forearm, along with nocturnal symptoms, including paresthesias in the median nerve distribution, are characteristic of carpal tunnel syndrome. Pain and tenderness may also be present at the entrapment site (e.g., near the fibular head with peroneal nerve entrapment at the knee). When peripheral nerves without cutaneous sensory afferents are injured, numbness and paresthesias do not occur; however, a deep aching pain may be present not only at the point of entrapment but also within any joints the entrapped nerve carries proprioceptive sensation from (e.g., dorsal wrist pain when the posterior interosseous nerve is compressed in the proximal forearm; shoulder pain with the early stages of suprascapular entrapment neuropathy). When significant damage to a motor-sensory nerve occurs, concomitant sensory abnormalities, including paresthesias, hypoesthesias, and hyperesthesias, are characteristically present in the nerve’s sensory territory.⁵ The clinician must keep in mind that sensory loss may be unreported because of more severe, overlying pain.

Autonomic disturbance, in part, may also cause significant pain in patients with peripheral nerve injuries and is a hallmark of both type I (formerly known as reflex sympathetic dystrophy) and type II (formerly known as causalgia) complex regional pain syndrome (CRPS) (Fig. 246-1).⁶ CRPS type I usually occurs after a minor injury to the extremity (e.g., sprained ankle), whereas CRPS type II occurs after significant damage to a major mixed nerve (e.g., gunshot wound). Severe burning pain, careful attempts to protect the involved extremity from movement or manipulation, and evidence of autonomic overactivity (or underactivity) are features of both CRPS types I and II. Trophic changes such as dry skin, nail loss or nail changes, and vascular changes such as skin flushing and discoloration and temperature fluctuations are hallmarks of this disarrayed autonomic activity. Furthermore, denervation hypersensitivity to circulating or local catecholamines may also exacerbate nerve pain and likely contributes to worsen pain during periods of environmental and emotional stress.

FIGURE 246-1 A, This patient has complex regional pain syndrome (CRPS) type I affecting her right hand. Atrophy and pallor compared with her normal left hand is evident. Also note her avoidance of jewelry on her right hand and forearm. She sprained her wrist rollerblading and has no evidence of nerve injury on electrodiagnostic testing. Transient improvement occurred following a cervical sympathetic block. B, A patient with severe burning pain consistent with CRPS type II affecting his right leg following a buttock-level gunshot wound to the sciatic nerve. Swelling, alopecia, erythema, and atrophy of the patient’s right distal extremity are present. He cannot wear a normal shoe on his right foot and requires the use of a soft sock and specialized orthopedic boot for ambulation.

Another type of pain that may occur with nerve injury is avulsion pain (this falls under the broader category of deafferentation pain). This is usually a result of nerve root avulsion from the spinal cord. Avulsion pain usually manifests as a constant burning or crushing pain that is poorly responsive to physical therapy, medication, and nerve blocks. Regenerating nerves may also produce pain, often described as tingling, electric shocks, and dysesthesias along the course of the nerve; this type of pain is usually considered a normal sign of the regeneration process. Patients with neuromas usually describe localized pain, with a trigger point overlying an often palpable, exquisitely tender subcutaneous neuroma. A diagnostic nerve block near the neuroma can usually confirm this diagnosis.

Management of neuropathic pain remains challenging, with many patients having only partial or minimal relief with treatment. Those with complicated, chronic, or refractory nerve pain should be referred to a pain center where a comprehensive evaluation and treatment plan can be instituted based on a multidisciplinary consensus. In general, treatment options may be divided into the following categories: physical therapy, pharmacotherapy, local and proximal nerve blocks (e.g., sympathectomy), psychological counseling, local surgical intervention (e.g., neuroma removal), and peripheral and central nervous system stimulation. A stepwise trial of progressively more invasive treatments should be recommended based on the clinical situation. Stimulation has increased in popularity in recent years. It has been proved effective for postherpetic neuralgia and CRPS and less effective, but commonly used, for peripheral neuropathy, avulsion pain, and failed back syndrome.⁷ Peripheral nerve stimulation has for the most part been replaced by spinal cord stimulation for pain in the extremities. This is because of the ease of spinal stimulator trials and permanent implantation; however, occipital and frontalis peripheral nerve stimulation remain excellent options for neuralgia affecting these nerves.⁸ An important concept for peripheral nerve stimulation is that the pain should be solely localized to the distribution of the affected nerve for the stimulation to be effective.⁸ Central nervous system ablative surgery also maintains a role in contemporary pain management.⁷ Indications include dorsal root entry zone (DREZ) ablation for avulsion pain and radiofrequency ablation of the descending sensory trigeminal tract for treatment of anesthesia dolorosa affecting the trigeminal nerve and for certain forms of cancer pain.

Complications and Their Avoidance in Peripheral Nerve Surgery

In performing peripheral nerve surgery, as with any area of surgery, knowledge of the anatomy is of the utmost importance. The surgeon must not only understand the nerve anatomy but also be able to correlate neural structures with their target muscles and sensory distribution. Knowledge of the vascular and bony anatomy will also be essential to planning the surgery. In much of nerve surgery, the normal anatomy is distorted, whether from trauma, tumor, or other pathology, and the surgeon must have a clear anatomic picture of the normal anatomy before proceeding. A properly planned incision and exposure will allow for the correct identification of the vital structures as well as room in which to perform the needed tasks. Failure to be thoroughly informed about the relevant anatomy is probably the most common cause of iatrogenic nerve injury.

Avoiding complications during peripheral nerve surgery often requires an understanding of principles and techniques that are distinct from those used for the brain and spine.⁹ Furthermore, because most neurosurgery training programs do not perform a high volume of peripheral nerve surgery, many residency graduates do not possess the experience and understanding needed to minimize complications during peripheral nerve surgery, even though many will subsequently perform these procedures in their practice.¹⁰ In this section, we review how fundamental errors, such as misdiagnosis, misidentification, poor operative timing, and other operative errors, may predispose to complications. The complications for several types of peripheral nerve surgery (e.g., brachial plexus repair, carpal tunnel release) are summarized in Table 246-2. A more in-depth review can be found elsewhere.^11,12

TABLE 246-2 Select Peripheral Nerve Surgery Complications