Anatomy of upper extremity nerve entrapments
ART Literature Review:
by Everett Johnson, DC, ART
Anatomy of upper extremity nerve entrapments
Nerve entrapment of the upper extremity is a common and well documented issue. Virtually all of the nerves in the upper extremity can be entrapped at some point along their pathway. Some of the entrapment sites are well known and accepted, while others tend to be somewhat controversial in the medical research community. Entrapment neuropathies refers to conditions of the peripheral nerves when they become compressed or stretched along their pathway. These issues may arise from anatomic variations, be acquired through daily activities or a combination of both. Changes that occur to the nerve are dependent on the amount of compression applied to the nerve and the length of time the nerve has been compressed. Low amounts of pressure on a nerve for a short period of time is typically well tolerated by the body and symptoms are reversible after removal of the insulting agent. Increased pressure on a nerve for an extended period of time may result in changes to the structure and function of the nerve, which may result in irreversible damage. This report will look at the five major nerves of the upper extremity, their pathway and common entrapment sites, with research relating to those sites where available.
The beginning of the pathways start with the brachial plexus of nerves in from the cervical spine. The brachial plexus is a network of nerves typically made up of anterior rami of spinal segments C5 through T1. This structured format may be shifted up or down a segment and is called a pre- or post-fixed brachial plexus. The plexus is formed when these spinal nerves come together and mix with each other as they expand out toward the shoulder.
Along the pathway the anterior rami emerge from the spine to form the brachial plexus. The plexus exits between the anterior and medial scalene muscles and the first rib. The exit point is triangular in shape and termed the scalene triangle. The characteristics of the triangle, including its height and width, can be modified by anatomic variations in the first rib or anomalous muscles related to the scalenes. The most common causes of disturbance in the scalene triangle are elevation of the first rib, or the presence of a cervical rib and a variant muscle called scalene minimus. All of these contribute to reducing the space the brachial plexus has to pass through the scalene triangle, causing entrapment termed “scalene syndrome”.
As the plexus extends laterally from the first rib, it must pass under the pectoralis minor and the coracoid process of the scapula. With the arm resting at the side, this point cases little to no compression on the brachial plexus. With the arm in abduction and external rotation however,the brachial plexus is stretched and pulled tight against the coracoid process. The head of the humerus also plays a role in stretching of the brachial plexus at this point. With the arm abducted and externally rotated, the brachial plexus passes around the head of the humerus, causing increased stretch to the nerves. Symptoms of compression of the brachial plexus can be any combination of medial arm pain, sensory issues in the medial hand and forearm, and weakness in the thenar, hypothenar and intrinsic muscles of the hand. Typically the muscles of the thenar eminence are affected earlier and to a greater degree.
As the brachial plexus approaches the axillary region (under arm) it divides into its terminal branches that descend into the arm, forearm and eventually to the hand. The radial nerve is the large branch which emerges from the posterior cord traveling obliquely across the subscapularis muscle, then somewhat posteriorly to the posterior aspect of the humerus where it enters the spiral (musculospiral) groove, which is located deep to the medial and lateral heads of the triceps. The groove ends just distal to the deltoid tuberosity on the lateral margin of the humerus where the radial nerve continues through the lateral intermuscular septum to get to the anterior compartment of the arm, just deep to the brachialis and brachioradialis muscles. The site of the radial nerve in the spiral groove is considered a site of entrapment, but most commonly occurs as a result of fracture. The more common and accepted site of entrapment in the upper arm is the area where the the radial nerve travels deep to the lateral head of the triceps and pierces the intermuscular septum
The radial nerve continues to descend to the elbow and into the forearm just deep to the brachioradialis and extensor carpi radialis longus and brevis. At approximately the level of the elbow joint the radial nerve splits to form superficial and deep branches. The superficial branch continues to descend just deep to the brachioradialis muscle in the forearm. The superficial branch emerges from under the brachioradialis muscle about two-thirds down the length of the forearm to supply cutaneous sensation to the dorsal and lateral aspects of the hand. The deep branch of the radial nerve (posterior interosseous nerve) descends around the neck of the radius to pass deep to the supinator muscle (radial tunnel, Arcade of Frohse). After it emerges from the distal margin of the supinator muscle it branches to the muscles of the deep posterior compartment of the forearm and to the wrist joint. The radial tunnel is considered to be the most common entrapment site of the radial nerve. Typically symptoms present as lateral and distal pain in the elbow, hand pain and weakness of the extensors of the fingers and wrist.
The axillary nerve is also from the posterior cord of the brachial plexus. It is a short nerve, and it travels laterally across the subscapularis and proceeds posteriorly at the superolateral border of the scapula. As it comes posteriorly it enters a space (quadrangular space) which is formed by boundaries of the humerus laterally, long head of triceps medially, teres minor superiorly and teres major inferiorly. The axillary nerve traverses the quadrangular space to enter the deltoid muscle. It provides motor supply to the deltoid and teres minor muscles and cutaneous sensation to the area of the lateral shoulder. The quadrangular space is a possible site of entrapment of this nerve, though it is not well supported in the literature. There are fibrous bands of tissue that can occur in the area that will increase compression on the axillary nerve. Fibrous bands and increased tension and compression on the axillary nerve as the arm is abducted and rotated externally increase the likelihood of entrapment of the nerve.
The median nerve is made from the medial contribution of the lateral cord and a lateral contribution from the medial cord of the brachial plexus. This formation typically occurs at or around the area of the coracopectoral tunnel, just deep to the pectoralis minor muscle and its attachment to the coracoid process. The median nerve descends the medial aspect of the arm in a neurovascular bundle with the ulnar nerve and the brachial artery. At approximately the distal third of the humerus the median nerve passes somewhat anteriorly and lateral as it heads toward the cubital fossa of the elbow, just anterior (superficial) to the brachialis muscle. In the cubital fossa the median nerve is superficial to the brachialis muscle and deep to the bicipital aponeurosis. As it exits the cubital fossa the median nerve enters between the two heads of the pronator teres muscle. The median nerve exits the pronator teres and travels deep to the fibrous arch of the flexor digitorum superficialis (sublimus bridge) to continue to descend distally between the flexor digitorum superficialis and profundus muscles. As the median nerve approaches the wrist it emerges anteriorly and is somewhat exposed between the tendon of the palmaris longus and the flexor carpi radialis muscle. The median nerve then passes through the carpal tunnel in an anterior and radial position in the tunnel. Distal to the carpal tunnel median nerve branches into its terminal branches that extend to supply sensory innervation to the thumb, first two fingers and radial portion of the ring finger, and the recurrent branch of the median nerve to supply motor innervation to the thenar eminence.
The median nerve is vulnerable to entrapment at many sites along its pathway through the upper extremity. The nerve may become trapped in the neurovascular bundle in the upper portion of the arm, as it is traveling through the coracopectoral tunnel. The nerve may also be entrapped at the distal medial aspect of the elbow if the ligament of Struthers is present. The ligament is reported to be in approximately one percent of the population and is found in the distal third of the medial humerus, emerging from a supracondylar spur and extending to the medial epicondyle of the humerus. When the ligament is present, it presents an obstacle for the course of the median nerve as it descends the medial aspect of the humerus to get to the cubital fossa. It is suspected that the altered course the nerve takes increases friction at the site, causing entrapment.
The median nerve may also become trapped in the tissues as it crosses the elbow joint into the cubital fossa as it descends between the brachialis muscle and the bicipital aponeurosis. Entrapment at this site is typically caused from a forced resisted flexion of the elbow, which increases tension on the brachialis muscle and the aponeurosis of the biceps brachii. As the median nerve exits the cubital tunnel and enters between the two heads of the pronator teres the median nerve has another opportunity to become entrapped. The more superficial head of the pronator teres is large and mostly muscular in nature, where the deep head is smaller and mostly tendinous in nature. The two heads are approximated as the forearm is put into pronation, which compresses the median nerve passing between them.
As the median nerve enters deep to the flexor digitorum superficialis it may become entrapped at the sublimis bridge. The bridge is an arch of fibrous tissue connecting the two heads of the flexor digitorum superficialis muscle. Median nerve has to pass posterior to this arch of tissue to take its course deep to the flexor digitorum superficialis. Median nerve is further exposed to entrapment along its course between the flexor digitorum superficialis and profundus by fibrous bands and variant branches of small muscle fibers.
The carpal tunnel is the next point in the path of the median nerve that may cause entrapment issues. The carpal tunnel is a well known site for entrapment and numerous studies exist on the subject. The carpal tunnel is the pathway from the anterior aspect of the forearm for the tendons of flexor digitorum superficialis and profundus and the tendon of the flexor pollicis longus, along with the median nerve. The more likely area of the tunnel for entrapment of the nerve is the distal aspect. The tunnel is more narrowed at its distal aspect, where the median nerve is the most anteriorly placed on the radial aspect. The transverse carpal ligament at the distal aspect of the tunnel is thicker and more taut in nature. As median nerve leaves the carpal tunnel it gives a recurrent branch to innervate the thenar musculature. This recurrent branch may pierce the transverse carpal ligament, or emerge just distal to it. In either case, the recurrent branch may become compressed at the distal aspect of the transverse carpal ligament and produce symptoms that affect the muscles of the thenar eminence.
The ulnar nerve is the distal continuation of the medial cord of the brachial plexus. It descends the medial aspect of the arm with the median nerve and brachial artery. It pierces the medial intermuscular septum to enter the posterior compartment of the arm at the distal third of the humerus. It continues to descend posterior to the medial intermuscular septum at the posterior margin of the supracondylar ridge, in a groove in the medial head of the triceps muscle. Ulnar nerve is held in the groove by the brachial fascia, fascia for the triceps and medial intermuscular septum. The nerve then passes posterior to the condyle to enter the condylar groove on the posterior aspect of the medial epicondyle. Ulnar nerve then continues distal to the medial humeral epicondyle to enter the forearm by passing deep to the arcuate ligament that connects the two heads of the flexor carpi ulnaris muscle, then descending the forearm deep to that muscle. As the flexor carpi ulnaris attaches to the pisiform bone, the ulnar nerve passes through the tunnel of Guyon, which is lateral to the pisiform bone and medial to the hook of the hamate just deep to the pisohamate ligament. The ulnar artery accompanies the ulnar nerve through the canal as it travels to the palmar aspect of the hand to give the superficial palmar arch. Inside the canal the ulnar nerve gives a superficial cutaneous branch and a deep muscular branch.
The arcade of struthers is the area adjacent to the medial supracondylar ridge where the ulnar nerve pierces the medial intermuscular septum to enter the posterior compartment. The fascia binds the nerve close to the triceps and can be a site of compression of the nerve. Some debate exists in the literature of the actual existence of this structure, with some saying it is a result of ulnar nerve transposition surgery. The groove on the posterior aspect of the medial epicondyle is considered a site of frequent compression of the ulnar nerve. Mostly, the nerve is exposed to external trauma at this site due to it being most superficial in this region, and it is directly compressed on the bone at this site.
The cubital tunnel is the point just distal to the condylar groove at the distal aspect of the medial condyle. The nerve travels between the arcuate ligament connecting the two heads of the flexor carpi ulnaris and the medial collateral ligament of the elbow. When the elbow is flexed the fibers of these two structures are stretched, causing compression on the ulnar nerve. This point is just medial to the trochlea of the humerus, and as the ulna is flexed at the elbow, a wider portion of the trochlea is exposed to the ulnar nerve, also increasing compression of the nerve at this site. The canal of Guyon or ulnar tunnel is another possible site of entrapment. Wrist extension can cause stretching of the nerve through the canal, with external compression being the most common cause of symptoms from this area. The canal is divided into three zones and compression in each zone corresponds to the symptoms the patient presents with. Zone one occurs just proximal to the bifurcation of the ulnar nerve and causes a combination of sensory and motor loss. Zone two involves the motor branch of the ulnar nerve and zone three involves the sensory division of the ulnar nerve. Variant musculature in the region of the hypothenar muscles has also been described as having a compressive effect on the ulnar nerve.
Neuromobilization is a relatively new idea and is mostly limited to manual therapists. The anatomy supports neuromobilization clinically as a comprehensive evidence-based method for clearing entrapments of the peripheral nervous system. The value of this work should be further explored by clinical outcome studies and methods for describing the physiology and histology of the entrapments. The peripheral nerves must slide to accommodate changes that occur with motion of the upper extremities. Knowing the pathway the nerves take in the body and the locations of common entrapment sites will help the practitioner to identify problems patients present to their offices in a more precise manner. There are limited studies looking at the outcomes of this non surgical approach to freeing the peripheral nerves, but the research that does exist is promising for the field. Of 197 patients included in a nerve gliding program designed for carpal tunnel syndrome 71% of the patients not doing the exercises eventually underwent a surgical procedure to correct the problem, versus 43% of the patients that did perform the exercises. Of the 43% that did not have to undergo surgical procedure, 70% reported good or excellent results at a 23 month follow up. Mobilization of these nerves while paying attention to the site of the nerve entrapment can help restore proper relative motion of the nerve and decrease pain and other symptomatology that may be occurring.
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