Radial Nerve

Introduction

Radial nerve pathology (such as radial tunnel [RT], posterior interosseous nerve entrapment, and superficial radial neuralgia) results in pain, with a reduced quality of life and function. Radial neuralgia is the third most common neuralgia of the upper extremity after the most common, median neuralgia (e.g., carpal tunnel syndrome [CTS]), followed by ulnar neuralgia (e.g., cubital tunnel syndrome). Radial neuralgia can be the cause of upper extremity pain despite the radial nerve primarily being known as a “motor nerve.” According to Naam and Nemani, the deep branch of the radial nerve (DBRN) includes unmyelinated group IV afferent C fibers that carry nociceptive sensation from structures of the forearm. This provides a plausible explanation as to how the deep radial nerve transmits pain perception.

Radial nerve entrapment is an often underrecognized cause of radial neuralgia that is primarily thought to be a result of forearm overuse. Repetitive pronation and supination of the wrist and forearm lead to chronic inflammation, scarring, and fibrous changes. The most common sites of entrapment occur at the supinator muscle, the fibrous band at the radial head, the Arcade of Fohse, and the tendons of extensor muscles (e.g., extensor carpi radialis brevis). Secondary causes include direct trauma, fractures, and postsurgical changes.

Before discussing the clinical presentations of radial nerve pathology, it would be prudent to provide a brief overview of some of the controversial issues around radial nerve research. There are three syndromes that are most often associated with radial nerve pathology in the medical literature. These syndromes include the sensory mononeuropathy called Wartenberg syndrome impacting the superficial radial nerve (SRN; distal to the elbow), posterior interosseus nerve syndrome (PINS), and radial tunnel syndrome (RTS). These conditions are tabulated in Table 15.1 . Unfortunately, authors and clinicians have often referred to the latter two syndromes, PINS and RTS, interchangeably, causing confusion in the medical literature. Some authors consider RTS to be an early form of PINS, with weakness as the only distinguishing characteristic of PINS. Furthermore, what exactly makes up the definitions of the RT, the DBRN, and the posterior interosseus nerve (PIN) can vary depending on the source. To put the controversy in perspective, the authors from a neurology review of radial nerve disorders stated: “The radial tunnel and its associated syndrome seem to be orthopedic concepts that engender much skepticism from neurologists”. Therefore, numerous authors who have investigated DBRN entrapment most often did so without clarifying or distinguishing between RTS or PINS. Further ambiguity stems from RTS coexisting with other diagnoses such as lateral epicondylitis (“tennis elbow”), milder forms of carpal tunnel or cubital tunnel syndrome, proximal radial nerve entrapment, or tendinitis in the forearm or wrist. Other names for DBRN entrapment include supinator syndrome and treatment-resistant tennis elbow. Adding to the confusion even more, trauma to the radial nerve may also trigger complex regional pain syndrome (CRPS). Regardless of the confusion, radial neuralgia is a real phenomenon that can have debilitating clinical consequences if left untreated.

Clinical Presentation

Even though the clinical symptoms involving the median and ulnar nerve are more common, the radial nerve has its own unique challenges due to its long anatomic course and muscle innervations ( Fig. 15.1 ). Theoretically, an injury to the radial nerve can happen anywhere along its long course, from its origination in the posterior cord (C5-T1) in the neck, the brachial plexus, at the spiral groove in the upper arm, or at the forearm or wrist.

The radial nerve is most susceptible to injuries at the spinal groove in the upper arm, lateral elbow, and distal lateral forearm. The most common cause of traumatic radial nerve injury in the upper arm is midhumeral shaft injury such as fracture and posterior dislocation. Clinical characteristics may differ based on the locale of radial nerve pathology either pre- or postbifurcation of the radial nerve into the deep radial nerve and SRN ( Fig. 15.2 ). For example, if the nerve injury is more proximal to the bifurcation (e.g., at the spiral groove), extensor weakness at the wrist may be involved because the extensor carpi radialis brevis muscle may be impaired. To further add to the confusion, motor innervation of the radial nerve was studied using the “order of innervation” methodology, which is poorly defined, making it difficult to compare studies. As an example, a study by Linell in 1921 researched the branching order and motor entry order for the entire course of the radial nerve and found that the two orders were not always in agreement. The SRN may also have clinically relevant motor innervation of the brachialis, with main contributions from the DBRN. Further adding to the issue is that the majority of anatomical texts inaccurately recognize SRN as only sensory. According to Sawyer et al., the extensor carpi radialis brevis is innervated by the superficial branch of the radial nerve 25.7% of the time. If the injury occurs after the bifurcation (most often distal to the lateral elbow) the extensor carpi radialis brevis is spared, and wrist extension weakness may be absent.

Previous studies have used poorly defined definitions when illustrating distal branches of the radial nerve. In the recent 2020 study by Sawyer et al., which studied 35 specimens from Tufts University, they defined the DBRN as the nerve that travelled to the distal border of the supinator, after which it changes names to the PIN. The PIN can be best viewed as a mere extension of the DBRN, not necessarily a separate nerve branch or tributary. The naming of the nerves in the manner described by Sawyer et al. is most consistent with the definition used by Moore et al. in Clinically Oriented Anatomy .

RTS should be considered when pain is the predominant complaint impacting the lateral elbow and dorsal forearm ( Fig. 15.3 ). The hallmark is tenderness over the radial nerve approximately 5 cm distal to the lateral epicondyle. The pain can occasionally radiate into the radial side of the wrist and hand ( Fig. 15.4 ), implicating a double crush, where there is a proximal as well as distal entrapment. The nerve can be stretched and aggravated by repetitive supination and pronation of the elbow and forearm and can become extremely painful. It is thought to occur by compression of the deep radial nerve with or without muscle weakness. Pain can result in loss of grip strength and worse pain at night. RTS has been shown to be present in 5% of patients diagnosed with lateral epicondylitis. Digital and thumb extensor motor weakness may help distinguish PINS from RTS. However, even in the case of PIN palsy, extensor carpi muscles are usually still intact, preserving wrist extension.

Patients with nontraumatic PINS usually display a gradual onset of weakness. Weakness is often absent in RTS. This occurs because the PIN-innervated muscles of the supinator, the extensor carpi radialis brevis and extensor carpi ulnaris, and the finger extensors (digitorum communis, extensor digiti minimi, extensor indicis, extensor pollicis longus, abductor pollicis longus, and extensor pollicis brevis) impact finger extension more than wrist extension. PINS is mostly associated with motor symptoms and minimal to no pain. Weakness is associated with compression of large, myelinated motor fiber function. The level of weakness is likely associated with the degree of compression on the nerve.

CRPS can impact any nerve, including the radial nerve. Overall, it is an uncommon disease, with an overall prevalence rate of less than 2% according to retrospective studies. A study from the Netherlands reported 26.2 per 100,000 person years, while a study from the United States reported 5.5 per 100,000 person years. The upper extremity is impacted twice as often, with fractures being a common triggering agent. This leaves the radial nerve vulnerable to CRPS. Up to 26% of all CRPS cases have no clear etiology, with 22% to 39% occurring after distal radius fracture, 2% to 5% after carpal tunnel, and up to 40% from Dupuytren contracture. A higher incidence of CRPS has been reported to be associated with females (76%), distal radius fractures, elevated intracast pressure, and patients between the ages of 40 and 49 years.

SRN pathology has been referred to as cheiralgia paresthetica, Wartenberg syndrome, and handcuff or wristwatch neuropathy ( Fig. 15.4 ). According to Dang et al., SRN entrapment has an annual incidence of 0.003%, compared with 0.03% for PIN compression. The symptoms commonly associated with superficial radial neuralgia are pain, hyperesthesia, or weakness, especially in the thumb and grip strength, over the back of the hand, thumb, and index fingers ( Fig. 15.5 ). A history of repetitive wrist movements and forearm hyperpronation may increase pain. Patients typically have pain at rest, with less impact at night. CRPS symptomatology (allodynia, swelling, hyperesthesia) may impact the SRN. The presentation is initially witnessed at the thumb or radial aspect of the dorsum of the hand but may spread to include other areas of the hand and arm. Entrapments of the SRN and PIN may also coexist. In the setting of a systemic neuropathy such as diabetes, SRN entrapment along with median nerve and ulnar nerve entrapments would give a “glove” pattern of pain and/or numbness. Other commonly coexisting conditions include CTS in 16% to 57%, lateral antebrachial cutaneous nerve neuroma, and de Quervain tenosynovitis in 17% to 50% of patients with SRN entrapment.

Anatomy

The radial nerve is one of the terminal branches of the posterior cord, along with the axillary nerve ( Table 15.2 ). With contributions from C5 to T1, the radial nerve travels around the humerus and across the radial-humeral joint near the lateral epicondyle as it enters the RT in the forearm. Unfortunately, the RT characteristics have not been consistent in the medical literature.

Regardless of the lack of RT definition consistency, the most common description of the RT is as a 5-cm potential space in the dorsal aspect of the forearm extending from the level of the radiocapitellar joint to the proximal edge of the supinator, also known as the arcade of Frohse (AoF). It is bordered laterally by the brachioradialis, extensor carpi radialis longus (ECRL), and Extensor carpi radialis brevis muscles. The medial border consists of the biceps tendon and brachialis muscle. The floor of the RT is the capsule of the radiocapitellar joint. Some authors describe the RT spanning the entire supinator, not just the proximal edge of the supinator muscle. Furthermore, and adding to the complexity of the nerve course, naming definitions, and anatomy, the supinator muscle also has a convoluted anatomy, with multiple attachment points. However, regardless of the inconsistencies in the RT anatomic definition(s) by various authors, there seems to be no real clinical issue or relevance as a result.

A cadaveric study by Hazani et al. mapped out the proximal and distal borders of the supinator muscle, using the distinct landmarks of the radial head as a reference point. The DBRN travels beneath the AoF, at an average of 3.5 cm distal to the radial head and exits as the PIN approximately 7.5 cm distal to the radial head. Berton et al. dissected 28 embalmed upper limbs and traced the radial nerve from the bicipital groove to the distal edge of the supinator. In all specimens, they found superficial and deep layers of the supinator, as well as slight flattening of the DBRN at the proximal and distal RT. The motor innervation of the supinator was from the DBRN in each case, and 20 of the 28 specimens had a clear AoF. After dissecting 30 limbs, Clavert et al. showed that the AoF was not the only compression site, and that the AoF existed in 87% of the limbs dissected, compared with the reported 30% to 80% in the medical literature. Clavert et al. opined that the presence of a tendinous AoF and repetitive supination movement predispose the radial nerve to entrapment between the AoF and the radial head. This repetitive insult creates the histological changes associated with chronic inflammation, resulting in the fibrotic changes associated with radial nerve entrapment at the elbow. The fibrotic process makes the AoF less forgiving and nerve entrapment more likely.

At the elbow, the radial nerve divides into the DBRN and SRN ( Fig. 15.6 ). A vast majority of textbooks and publications have referred to the SRN as sensory only, not accounting for its motor involvement. Because the SRN is a branch of the radial nerve, it receives contributions from C5-T1 at the posterior cord. The SRN travels between the brachioradialis muscle and ECRL and becomes superficial at the distal radial forearm ( Fig. 15.7 ). It passes across the tendon of the ECRL, past the dorsal tubercle of the radius (Lister’s tubercle), over the extensor retinaculum, and into the thumb and the dorsum of the hand. A study by Abrams et al. followed the course of the SRN and found that it bifurcated from the radial nerve at the level of the lateral humeral epicondyle in eight specimens, with all 20 specimens showing bifurcation no more than 2.1 cm from the lateral epicondyle. The SRN became subcutaneous at a mean of 9.0 cm proximal to the radial styloid, traversing between the tendons of the brachioradialis and extensor carpi radialis longus in 90% of cases, with the remaining 10% traveling through the brachioradialis tendon. Abrams et al. reported that the SRN’s first branching point after its emergence from the brachioradialis was a mean of 5.1 cm proximal to the radial styloid. In another study, Ali et al. analyzed 16 forearm specimens and found that the SRN emerged from under the brachioradialis at a mean distance of 8.45 (± 1.22) cm proximal to the radial styloid. Abrams also reported that the first major branching point of the SRN to the radial styloid was 4.8 ± 0.4 cm and at, the extensor retinaculum at the wrist, the dorsal cutaneous nerve branching occurred at a mean distance of 0.4 and 1.6 cm, from Lister’s tubercle ( Fig. 15.8 ). In the hand, the superficial branch of the radial nerve provides sensation to the dorsum of the hand ( Fig. 15.9 ).

In a 2014 cadaveric study by Ali et al., 16 embalmed cadaveric forearms were dissected in order to track and record the trajectory of the SRN and cephalic vein (CV) to assist prospective surgeons in proper K-wire placement. The researchers noted that, in all the specimens, the branches of the SRN crossed the tendon of extensor polices longus (EPL) within 2.69 cm of the distal edge of the extensor retinaculum. In 81.25% of the specimens, the CV crossed the EPL once, and in 18.7% of the specimens it crossed over the EPL twice. Intravenous line placement targeting the CV should be performed with caution due to the proximity of the SRN. All branches of the SRN were found to lie in the radial half of an isosceles triangle formed by the radial styloid, Lister’s tubercle, and the exit point of the SRN from under the brachioradialis muscle ( Fig. 15.10 ). Anatomic considerations and variations should help guide the practitioner in localizing the target nerve. Imaging, such as ultrasound scanning, can help confirm anatomic structures and identify nerve course variations.

Diagnosis by History, Exam, Imaging, and Diagnostic Block