Median Nerve - Clinical Tree

Introduction

With approximately 3,000 reported injuries per year in the United States, median nerve injuries are the primary cause of emergency room visitation for all peripheral nerve injuries. They can occur by multiple mechanisms, including (1) arm, elbow, and wrist lesions; (2) pronator syndrome; (3) carpal tunnel syndrome; and (4) anterior interosseous neuropathy. Carpal tunnel syndrome, however, represents the main clinical manifestation and has a 3% prevalence in the general population. While there are various methods to treat mild to moderate carpal tunnel syndrome, surgical decompression remains the main treatment for severe symptoms. However, peripheral nerve stimulation (PNS) represents an alternative approach to patients with refractory symptoms of median nerve pathology. As a minimally invasive approach, PNS offers significant reduction in the challenges, complications, and scarring of previous surgical systems used to treat severe carpal tunnel syndrome. In this chapter, we will examine the clinical presentation of median nerve neuropathic pain, relevant anatomy, diagnostic approaches such as history, physical exam, imaging, and neurodiagnostic nerve blocks, as well as indications, complications, and techniques for using PNS to the median nerve as a therapeutic modality.

Anatomy

The median nerve, also called the “laborer’s nerve,” is a major peripheral nerve of the hand and upper limb ( Fig. 14.1 ). It arises from the anterolateral cord and anteromedial cord of the brachial plexus and is innervated by the C5-C7 roots and C8-T1 roots, respectively. The median nerve is responsible for both motor and sensory functions of the hand and motor of the forearm. In the anterior compartment of the forearm, the median nerve innervates the flexor and pronator muscles, with the exception of the flexor carpi ulnaris and part of the flexor digitorum profundus, which are innervated by the ulnar nerve. In the hand, the median nerve also provides innervation to the thenar muscles and two lateral lumbricals.

The median nerve splits into the palmar and digital cutaneous branches to provide sensory innervation to the lateral palm, and lateral three and a half fingers on the anterior hand, respectively ( Fig. 14.2 ). From the brachial plexus, the medial nerve lies anterior and superior to the axillary artery in the axilla. It then travels along the brachial artery, crossing it anteriorly to medially, arriving at the cubital fossa and entering the forearm between the two heads of the pronator teres. As the nerve runs through the anterior antebrachial compartment, it passes between the flexor digitorum superficialis and flexor digitorum profundus in the volar aspect of the forearm. Subsequently, the median nerve courses under the flexor retinaculum, lateral and anterior to the tendons of digitorum superficialis, into the carpal tunnel. As it travels away from the carpal tunnel, it bifurcates into the anterior interosseous nerve (motor), supplying the deep flexor muscles, and the palmar cutaneous nerve (sensory), subdividing into four digital palmar branches to supply the lateral palm and fingertips of the three and a half most radial digits. It is important to note that anatomical variation in the median nerve is common among the general population. The variations are classified into four groups: I – variation in the course of the thenar branch; II – accessory branches at the distal portion of the carpal tunnel; III – high divisions of the median nerve; and IV – accessory branches proximal to the carpal canal. As such, it is important for physicians to have an in-depth understanding of the median nerve anatomy.

Figure 14.2, Standard anatomy of the median nerve.

Clinical Presentation

The median nerve’s role in sensation and movement of the upper limb has great clinical significance. Due to its innervation network, the median nerve provides for both coarse and fine movements of the hand and arm, including critical thumb opposition for precision actions and activities. Injury, however, can occur through a variety of mechanisms, and, while there are an array of surgical and nonsurgical treatments/therapies, conservative approaches can often be ineffective. Here we cover the clinical presentation of common causes of median nerve injury that may be candidates for peripheral neuromodulation.

Traumatic injuries

Traumatic injuries of the nerve are often seen with wrist fractures. The variability in such injuries can lead to nerve damage ranging from simple compression to nerve contusion to severe nerve tears. Furthermore, because the median nerve is most exposed at the level of the wrist, open injuries such as stabbing or projectile wounds can lead to partial or complete transection. Complete proximal median nerve injury can result in sensory deficits of the thumb, index finger, and middle and radial half of the ring finger. Moreover, because of injury, weakness and/or motor deficits can be present in the muscles of the forearm, including the pronator teres, palmaris longus, and flexor digitorum profundus. It should be noted that laceration of the muscles themselves can also lead to poor function and motor loss. Upon complete proximal median nerve laceration, thenar muscle weakness is also present.

The median nerve can also be affected by fractures or dislocations of the elbow, either directly by fracture pieces that, in severe cases, can tear the nerve, or indirectly through stretching or compression by hematomas. Further, during reduction or fracture realignment, the nerve can become trapped between the ulnar, radial, and humeral heads or fracture pieces. Such compressions can lead to painful neuropathic symptoms coupled with muscle weakness. Surgical repair of such fractures and elbow injuries can also lead to median nerve injury.

Fractures or other traumatic injuries at the humeral level are much less likely to damage the median nerve. Instead, acute traumatic injuries such as stab wounds, gunshot wounds, or high-energy injuries (road accidents) can produce lesions at the axilla or upper limb causing paralysis and sensory impairment of tissue innervated by the median nerve.

Entrapments

There are multiple sites of entrapment for the median nerve ( Fig. 14.3 ).

Figure 14.3, Possible compression sites for the median nerve: brachialis (1), Struthers’ ligament (2), bicipital aponeurosis (3), pronator teres (4), flexor digitorum superficialis (5), vascular structures (6), and accessory head of flexor pollicis longus (Gantzer’s muscle) (7).

Carpal tunnel syndrome

The most common and well-known median nerve entrapment is at the carpal tunnel, located at the palm base, which is formed by the carpal bones inferiorly and the flexor retinaculum superiorly. The bounding carpal bones create an arch, or tunnel, that is traversed by the median nerve and nine flexor tendons ( Figs. 14.4 and 14.5 ). Carpal tunnel syndrome presents when elevated pressure within the carpal tunnel compresses the median nerve, resulting in ischemia of the nerve itself. Symptoms, including impaired nerve conduction, weakness, numbness, paresthesias, and pain, can localize at the wrist or hand but can spread into the forearm. As symptoms may mimic those of a C6/C7 nerve root injury, it is important to distinguish carpal tunnel syndrome from other injuries. This can be done through several tests that isolate the carpal tunnel and the distal median nerve, such as the Tinel ( Fig. 14.6 ) and Phalen/reverse Phalen tests ( Fig. 14.7 ). Early diagnosis is crucial, because in the early stages there are no morphological changes to the median nerve, symptoms are infrequent, and damage is reversible. Prolonged compression of the median nerve in later stages may result in nerve demyelination and worsening of the symptoms, coupled with weakness. As the condition progresses, irreversible median nerve damage may be the product of prolonged ischemia. Carpal tunnel syndrome can further be classified as either mild, moderate, or severe based on symptoms and presentation. Mild cases include those that have numbness and tingling in the area innervated by the median nerve but have absence of motor and sensory deficits and no impact on sleep nor daily activities. Moderate carpal tunnel has the symptoms of mild carpal tunnel syndrome but with some sensory and motor deficits in the hand and other innervated areas, as well as disrupted sleep. Severe carpal tunnel includes the symptoms of moderate and mild carpal tunnel coupled with muscle weakness and impact on daily activities.