Anatomy, Shoulder and Upper Limb, Brachial Plexus

Article Author:
Marlon Bayot
Article Editor:
Matthew Varacallo
Updated:
2/15/2019 3:55:11 PM
PubMed Link:
Anatomy, Shoulder and Upper Limb, Brachial Plexus

Introduction

The brachial plexus is a major network of nerves transmitting signals responsible for motor and sensory innervation to the upper extremities. The plexus originates as an extension from the ventral rami of C5 through T1 spinal nerves.[1][2][3][4]

Five spinal nerves give rise to the formation of 3 trunks which subsequently divide into 6 divisions, located anteriorly and posteriorly. From these divisions, merging of nerves will form 3 cords as the lateral, posterior, and medial cords. Finally, 5 specific nerves will arise from the cords as the terminal branches of the brachial plexus, allowing specific muscles of the upper limb to perform corresponding actions. These terminal branches include the following: musculocutaneous, axillary, radial, median, and ulnar nerves[5][6][7][8][9]. Aside from these nerves, there are also collateral nerves which are found in the brachial plexus which innervate the proximal limb muscles as they arise proximal to the ventral rami, trunks, and cords.  Brachial plexus injuries is regarded as one of the most debilitating injuries afflicting the upper extremity.

Structure and Function

Nerve fibers from the anterior division of the brachial plexus are contained in the musculocutaneous, median, and ulnar nerves.  These nerves innervate the anterior muscles of the upper arm, forearm, and intrinsic muscles[10][11][12][11]. This innervation mainly provides flexion of the upper limb. Nerve fibers arising from the posterior division, including the axillary and radial nerves, provide innervation to the posterior muscles of the arm and forearm, which in turn allows these compartments to perform the functions of elbow, wrist, and finger extension 

Musculocutaneous nerve arises from C5 and C6 which innervates all the muscles of the arm anteriorly, enabling motor functions such as flexing the elbow and supination by the biceps brachii[13]. The median nerve originates from C5 to T1 spinal nerves which primarily innervates the anterior forearm (with a section innervated by the ulnar nerve) and the hand (thenar and central sections). Median nerve allows pronation of the forearm and flexion of the wrist and digits, together with the opposition of the thumb[14][15]. The ulnar nerve from C8 to T1 spinal nerves constitutes to the innervation of the anterior forearm (with a section innervated by the median nerve) and the hand (hypothenar and central sections). The central section which involves the palmar and dorsal aspects are responsible for adduction and abduction of second to fifth digits, respectively. Unlike the median nerve which allows opposition, the ulnar nerve is responsible for adduction of the thumb [16].

The axillary nerve is the result of the network of C5 and C6 spinal nerves which arise toward the deltoid allowing for abduction; and the teres minor for external rotation of the shoulder[17][18][19]. The radial nerve innervates the upper arm and forearm posteriorly, which originates from C5 to T1 spinal nerves. The radial nerve provides the function of extending the wrist, elbow and metacarpophalangeal joints of digits and supination by the supinator muscle.

Collateral nerves of the brachial plexus include the following:[20]

  • Dorsal scapular nerve innervating the rhomboids
  • Long thoracic nerve innervating the serratus anterior
  • Suprascapular nerve innervating the supraspinatus and infraspinatus
  • Lateral pectoral nerve innervating the pectoralis major
  • Medial pectoral nerve innervating both pectoralis major and minor
  • Upper subscapular nerve innervating the subscapularis
  • Lower subscapular nerve innervating the subscapularis and teres major
  • Thoracodorsal nerve innervating the latissimus dorsi
  • Medial brachial cutaneous nerve innervating the skin of the arm medially
  • Medial antebrachial cutaneous nerve innervating the skin of the forearm medially

The long thoracic nerve is known for allowing the protraction and superior rotation of the scapula, while the suprascapular nerve for shoulder abduction (by supraspinatus) and lateral rotation of shoulder (by infraspinatus).

Embryology

By the start of the fifth week in utero, the human embryo develops forelimbs and hind limbs which are observed as “paddle-shaped” buds. The buds of the forelimbs are located to the pericardial swelling posteriorly at the level of the fourth cervical somites down to the level of the first thoracic somites. This is the stage where peripheral nerves are developed from the growing brachial plexus into the mesenchyme of the developing upper limb, while spinal nerves are developed to both dorsal and ventral aspects of the limb in the form of segmental bands. This embryogenic development reveals the development and innervation of the brachial plexus. Segmental bands of the spinal nerves are formed following the proximal and distal gradient which indicates that the muscles proximal to the brachial plexus are innervated by the higher segmental band of C5 and C6, and in contrast, the muscles distal to the brachial plexus are innervated by the lower segmental band of C8 and T1.

Surgical Considerations

Patients typically lose sensation, motor power, and often experience disabling neuropathic pain following traumatic brachial plexus injuries[21].  Historically, surgical procedures often involved arm amputation, shoulder arthrodesis, and prosthetic replacement for patients with a flail arm[22][23].  More recent technical advances in peripheral nerve surgery over the last few decades have significantly enhanced the long-term outcomes of brachial plexus treatment[24].

There have been several reports regarding anatomical variations of the brachial plexus.  Knowledge of potential anatomic variation is important to mitigate the risks of iatrogenic injuries.  For example, a split median nerve pattern was observed with an anomalous muscle on the right forearm of a 35-year old male cadaver in one study. An anatomical variation was found because of the splitting of the median nerve into two nerves in the proximal third of the forearm, reuniting at the medial third as a single nerve. Moreover, a study on anatomical variations of the brachial plexus suggests that since there is a higher risk of injury for variated nerve branches on fetal cadavers used in the study. It is important for surgeons to have a better understanding of the possible anatomical variations of the brachial plexus, including extensions, whenever they provide surgical intervention to the upper limb.[22][25][26][27]

Clinical Significance

Lesions of the brachial plexus can generally be divided into upper and lower lesions. These lesions indicate important landmarks to determine the specific spinal nerves of the brachial plexus that are affected. Erb-Duchenne palsy is due to the lesion in the upper brachial plexus. Klumpke’s paralysis is the result of a lesion in the lower brachial plexus.[28][29]

Erb-Duchenne palsy usually occurs when both the head and the shoulder of the patient are separated by force in cases of birth injury, disk herniation, or accident. The resulting trauma will damage the C5 and C6 spinal nerves affecting the axillary, suprascapular and musculocutaneous nerves. The loss of axillary and suprascapular nerves can be observed when the arm is already medially rotated and adducted at the shoulder. Loss of the musculocutaneous nerve is seen when the patient’s forearm is extended and pronated and when the sensory function from the lateral forearm to the base of the thumb is lost. These observations resemble the “waiter’s tip sign”, hence, giving another name to this palsy as the waiter’s tip syndrome [30].

Klumpke’s paralysis damages the lower brachial plexus made up of C8 and T1 spinal nerves. This paralysis is commonly the result of events such as accidents involving the grabbing an object when falling in which the upper limb is made to be forcefully abducted above the head, leading to spinal nerve trauma. Ulnar nerve and intrinsic muscles of the hand become weak with a loss of sensory function on median forearm and digits since these nerves are primarily affected. Claw hand sign and ape sign suggest the presence of Klumpke’s paralysis, which may also include manifestations of Horner syndrome. Other than accidents, birth injury and cases of thoracic outlet syndrome may also lead to the development of the paralysis [31].

Major terminal branches of the brachial plexus can also be injured as a result of fractures, syndromes, or accidents affecting the specific upper limb area which it innervates.

The musculocutaneous nerve can also be damaged and form lesions which affect the lateral forearm sensation with a weakness of the forearm flexion and supination. However, the probability of musculocutaneous nerve to be lesioned is rare.

Axillary nerve lesion can alter the sensation of the lateral arm as caused by either dislocation or fracture of the surgical neck of the humerus. This result is a weakness of the shoulder in terms of abduction.

Median nerve lesion alters the sensation of the lateral 3 and a half digits and lateral palm with the motor weakness for wrist and finger flexion, pronation and thumb opposition. Common signs include the ape hand, hand of benediction and ulnar deviation at wrist. Lesion at the median nerve can be caused by pronator teres syndrome, carpal tunnel compression or supracondylar fracture of the humerus 

Injury or damage to the radial nerve could also develop lesions as seen in Saturday night palsy where the axilla is mainly affected. This is characterized by a wrist drop brought about by the weakened supination and loss of sensory function on the posterior arm, forearm, and dorsum of thumb. Wrist drop is also manifested when there is a dislocation of the radial head or lateral epicondyle.

Ulnar nerve lesion affects the medial 1 and a half digit and median palm sensation followed by weakness in wrist flexion, finger spreading, thumb adduction and finger extension. Claw hand sign and radial deviation at the wrist are the usual signs. Causes of lesions at the ulnar nerve can be the result of fracture on the medial epicondyle of the humerus, hook of hamate or the clavicle [12][32].[12]

The abovementioned muscle-to-nerve associations can be very significant in the diagnosis and management of patients with palsies or paralysis of the upper limb. Determining the area of the upper limb affected and knowing the functions of the muscles according to the affected area as well as its corresponding innervation can immediately provide information as to the possible location of the lesion. These associations can be applied by physicians in the clinical setting especially in emergency cases.

Other Issues

Though it has a rare incidence, a case of a patient with malignant granular cell tumor (MGCT) which usually occurs in the lower limbs and trunk was seen to affect the brachial plexus and the suprascapular nerve. A surgical procedure was done to remove the tumor carefully, preserving the sensory and motor function of the upper limb. This case report shows that tumors can invade and affect the brachial plexus.

Re-implantation as an intervention for complete brachial plexus avulsion was found to yield definite improvement in the sensory and motor functions of the affected upper limb of patients. However, limited improvement led the researchers to use regenerative cell technology expecting greater and significant progress.

While trauma can cause the affected nerve to be lesioned, results from a retrospective cohort study reveal that the injured nerve function of patients who experienced functional disturbances in the uninjured nerve was worse than those who had normal test results. It is possible that an uninjured nerve can also present with disturbances in the sensory function due to post-traumatic central nervous system alterations.



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