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2. Axillary Nerve Palsy
The axillary nerve is one of the main nerves of the shoulder with motor and sensory function. Due to its anatomical course, axillary nerve is quite vulnerable to injuries, and therefore, its injury is commonly seen in clinics. Clinical presentation is generally typical, and good results can be achieved with proper medical management. In this chapter, we will review the anatomy of axillary nerve as well as the diagnosis and management of axillary nerve injury.
2.1 Anatomy of the Axillary Nerve
Axillary nerve originates from the brachial plexus.
The brachial plexus consists of roots, trunks, divisions, cords, and branches. The ventral rami of the spinal nerves C5–T1 (and in some variations T2) form five roots. The roots merge to form the superior, inferior, and middle trunks. Each of these trunks divide into anterior and posterior divisions. The posterior divisions then merge to form the posterior cord, which branches to form the radial and axillary nerves [1–3].
The axillary nerve generally carries fibers from C5 and C6. Occasionally, C7 may also contribute to the axillary nerve [4].
The axillary nerve may also give rise to the inferior subscapular nerve, which innervates the subscapularis and the teres major [4].
2.1.1 The Course of the Axillary Nerve
After its formation, the axillary nerve travels posteriorly through the quadrangular space with posterior circumflex humeral artery and vein [2, 3].
The borders of the quadrangular space consist of the following anatomical structures: the subscapularis, the head of humerus, and the teres minor at the superior; the teres major at the inferior; the coracobrachialis muscle, and the surgical neck of the humerus at the lateral; and the long head of triceps at the medial [5].
After passing through, the axillary nerve gives rise to two motor branches that innervate the deltoid and the teres minor muscles, and a sensory branch called the superior lateral cutaneous nerve of arm, which innervates the skin above the inferior deltoid region [3]. The deltoid branch of this nerve travels posteriorly around the surgical neck of the humerus alongside the posterior circumflex humeral artery and vein to innervate the deltoid muscle [2].
While passing through the quadrangular space, axillary nerve gives branch to teres minor [6]. After passing, axillary nerve is coursed just anterior and inferior to the glenohumeral joint [7], and then it divides into two branches: anterior and posterior. The anterior branch gives rise to the motor branches that innervate the deltoid muscle, and the posterior branch forms the superior lateral cutaneous nerve of arm which innervates the skin above the inferior deltoid region.
However, in some studies some variations regarding the course of axillary nerve are shown. Steinmann et al. [4]. have shown in their study that when the axillary nerve exits the quadrangular space, it continues posteriorly to the humeral neck and divides into anterior and posterior branches. The anterior branch travels around the surgical neck of the humerus alongside the posterior circumflex humeral artery and vein and innervates the anterior part of deltoid muscle. Along the way, the nerve sends branches to innervate the middle and anterior portions of the deltoid. The posterior branch innervates the teres minor and the posterior portion of the deltoid. The branch to the teres minor arises within or distal to the quadrangular space and enters the muscle at its inferior border. The posterior branch gives rise to the superior lateral cutaneous nerve of arm terminally, coursed inferiorly, deep to the posterior aspect of the deltoid [8] and innervates the skin above the deltoid [4, 8–12].
2.2 Axillary Nerve Injury
Definition
Etiology
Clinical Presentation
Diagnosis
Prognosis
Differential Diagnosis
Medical Management
2.2.1 Definition
Axillary nerve injury is characterized by trauma to the axillary nerve and thus the dysfunction of the muscles innervated and lack of sensation of the skin above the deltoid. It is seen most often due to closed trauma involving traction on the shoulder [13].
The axillary nerve injury is usually present with other brachial plexus injuries. In reported studies, infraclavicular isolated axillary nerve injury occurred only 0.3–6% of brachial plexus injuries [4, 13].
2.2.2 Etiology
2.2.2.1 Glenohumeral Dislocation
Axillary nerve injury most commonly occurs after a traction type injury usually associated with anterior glenohumeral dislocation or proximal humerus fracture.
The incidence of axillary nerve injury due to anterior glenohumeral dislocation is 13.5–48%. The majority of the injuries are neuropraxias and typically resolve in 6–12 months [14].
When the humeral head is anteroinferiorly dislocated, it stretches the axillary nerve. The injury occurs commonly before the nerve is entering the quadrilateral space and proximal to the branching point of anterior and posterior divisions. Therefore, the anterior and posterior branches are both affected which results in deltoid and teres minor dysfunction as well as lack of sensory function of the skin above deltoid [14, 15].
Some patients have subclinical axillary nerve lesion. Since they have discomfort due to the injury, the nerve lesion may not be apparent clinically, but it can be detected by Electromyogram Test & Nerve Conduction Study (EMG/NCS) [13].
2.2.2.2 Proximal Humerus Fracture
Proximal humeral fractures represent about 4% of all fractures seen in an average orthopedic clinic and 2–3% of upper extremity fractures. Therefore, it is relatively very common [16]. The prevalence increases with the advancing age [17]. The demographic shifting of the population age-sex dependent characteristics is yet to be determined [17].
In adolescence and early adulthood, the cause of fracture is generally high-energy traumas. However, the high-energy traumas are less often than the low-energy traumas and osteoporotic fractures that are seen in elderly people [17].
In proximal humerus fractures, the most frequently involved nerves are axillary nerve and the subscapular nerve [10]. Axillary nerve injury is even more common since it is coursed just anterior and inferior to the glenohumeral joint and runs posteriorly to the surgical neck of humerus accompanied by the posterior circumflex humeral artery [4]. The surgical neck is weaker than the proximal regions of the bone; it is one of the sites where the humerus commonly fractures. Thus, the axillary nerve is quite vulnerable to both traumatic and iatrogenic injuries [7, 18].
2.2.2.3 Blunt Trauma
Direct blow to the lateral shoulder or the deltoid muscle causes a compressive force to the axillary nerve as it travels on the deep subfascial surface within the deltoid.
This type of injuries generally occurs during collision in contact sports such as hockey or American football.
No axillary nerve ruptures have been reported to occur by this type of injury until 1998 [9, 19].
There was a case report regarding axillary nerve rupture due to blunt trauma [20].
2.2.2.4 Quadrilateral Space Syndrome (QSS)
Axillary nerve and posterior humeral circumflex artery pass through the quadrilateral space below the shoulder capsule. This syndrome is caused by compression force on axillary nerve and posterior humeral circumflex artery or traction force on axillary nerve. It generally occurs in the dominant arm.
Axillary nerve compression within the quadrilateral space is usually secondary to abnormal fibrous bands and hypertrophy of the muscular boundaries of the space [9, 19].
2.2.2.5 Compression Without Trauma
Compression of the axillary nerve due to enlarging mass or aneurism may cause injury. This type of injury is not very common [4].
2.2.2.6 Brachial Neuritis
Another atraumatic injury of axillary nerve is brachial neuritis. It is a multifocal, immune-mediated inflammatory process that involves the peripheral nerves. It was first reported by Parsonage and Turner in 1948 in a case report. Brachial neuritis is also called neuralgic amyotrophy or Parsonage-Turner Syndrome. It affects long thoracic, suprascapular, axillary, musculocutaneous, anterior interosseous and posterior interosseous nerves most commonly.
Motor axons are mostly affected. Therefore, the nerves that carry mostly motor fibers are affected to a larger degree and more commonly than mixed nerves and pure sensory nerves [4, 21, 22].
2.2.2.7 Iatrogenic
Capsular shrinkage: These procedures may increase the local temperature in the inferior capsule and lead to nerve injury. Injury has been reported in 1 or 2% of thermal capsular shrinkage procedures [13].
Capsular resection for adhesive capsulitis: Nerve is in close proximity to the anteroinferior capsule; resection should be done carefully [13].
Shoulder instability surgery: The axillary nerve travels on the anterior and inferior shoulder capsule, so procedures that involve this area such as Bankart procedure and inferior capsular shift procedure may put axillary nerve at risk [9, 19].
Rotator cuff surgery: Axillary nerve travels horizontally within the deltoid muscle 5 cm inferior to the acromion. Incisions that split the deltoid muscle are often used in the treatment of rotator cuff disorders; however, overzealous muscle splitting will put axillary nerve at risk [9, 19].
Shoulder arthroscopy: Posterior shoulder arthroscopic portal usually is located 2–3 cm inferior and 1 cm medial to the posterolateral corner of the acromion. Thus, the portals must be placed carefully because the axillary nerve might be at risk since it is close to that area [9].
Shoulder arthroplasty: Implantation of the humeral and glenosphere components may endanger the integrity of the axillary nerve due to its proximity to the humeral metaphysis and the lower glenoid rim [23].
Thoracic surgery: Position of the patient during the surgery may cause traction type injury to the axillary nerve.
There’s a case report about a 21-year-old male who underwent a video-assisted thoracic surgery for a left-sided pneumothorax. In this case the body position during the operation, the right decubitus position with the left arm abducted 90° and flexion, caused compression or traction on the axillary nerve. After the surgery the patient had difficulties in left arm abduction and had paresthesia of the skin over deltoid. Deltoid muscle atrophy and tenderness over the quadrilateral space were also observed [24].
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