The axillary nerve, as the terminal branch of the posterior cord of the brachial plexus, receives contributions from C5 and C6 nerve roots. The nerve courses along the anterior surface of the subscapularis muscle dorsal to the axillary artery. It then makes a sharp turn posteriorly to travel along the inferior gleno-humeral joint surface. Slightly more distally, the nerve, along with the posterior circumflex artery, enters the quadrilateral space. The borders of the quadrilateral space are formed by the long head of the triceps brachii muscle medially, the teres minor muscle superiorly, the teres major muscle inferiorly, and the medial aspect of the proximal humerus laterally. The axillary nerve gives off four branches in the quadrilateral space: two motor branches to the anterior and posterior portions of the deltoid muscle, a sensory branch (the superior lateral brachial cutaneous nerve), and a motor branch to the teres minor muscle .
Fig. 2. Suprascapular nerve syndrome. Oblique coronal (A) and oblique sagittal (B) fat-suppressed, T2-weighted images depict a ganglion cyst (asterisk) within the suprascapular notch associated with a tear of the superior labrum (arrow). Denervation edema of the infraspinatus muscle (IS) is noted.
The branches to the teres minor and the lateral cutaneous innervation lie closest to the glenoid rim, and thus are most susceptible to injury at that site. Also, the axillary nerve was shown to travel at a fixed distance of approximately 2.5 mm from the inferior glenohumeral ligament throughout its course . An articular branch of the axillary nerve supplies the shoulder joint capsule.
The axillary neurovascular bundle, best visualized on oblique sagittal T1-weighted images of the shoulder, is well highlighted by fat and is seen below the inferior glenoid rim traversing the space between the teres minor muscle superiorly and the teres major muscle inferiorly (Fig. 3). The quadrilateral space is best visualized adjacent to the medial humeral cortex and lateral to the long head of the triceps muscle on oblique coronal images oriented perpendicular to the proximal humeral shaft.
The quadrilateral space syndrome is defined as compression of the axillary nerve within the quadrilateral space. Fractures of the proximal humerus and scapula can produce direct nerve injury . Entrapment of the nerve can be produced by extreme abduction of the arm during athletic endeavors, or even during sleep. Mass effect secondary to tumors, hypertrophy of teres minor muscle in paraplegic patients, or by a fibrous band within the quadrilateral space  are other causes of entrapment of the nerve. Symptoms related to this syndrome include shoulder pain and paresthesia. Advanced cases may result in atrophy of the deltoid and teres minor muscles.
Conventional radiography and CT may depict osseous lesions such as fracture callus or bone tumors that could impinge upon the axillary nerve. MRI,
however, is the optimal modality for direct assessment of the axillary neurovascular bundle and quadrilateral space. Detection of soft tissue masses and signal alterations of the teres minor muscle and, less commonly, deltoid muscle is consistent with axillary nerve compression and is best seen on sagittal or coronal MR images.
Up to 45% of shoulder dislocation cases may demonstrate nerve injury . The most commonly involved nerve is the axillary nerve, because it has a relatively tethered course within the quadrilateral space. The risk for axillary nerve and brachial plexus injury is greater if the shoulder is not reduced within 12 hours. Despite the high prevalence of axillary nerve injury following shoulder dislocation, only a few reports in the radiological literature address the association of teres minor atrophy with prior dislocation [21-23].
Traction and compression of the axillary nerve and the subscapularis muscle can be exerted by the dislocated humeral head or during manipulative reduction in which traction with rotation or abduction are simultaneously applied. Post-traumatic injury to the axillary nerve can also be secondary to proximal humeral fracture , and rarely as a result of a direct blow to the deltoid muscle.
The clinical diagnosis of axillary nerve injury may be difficult because the signs and symptoms are often vague. Because the branches to the lateral cutaneous innervation and to the teres minor muscle are closest to the glenoid rim, they are most vulnerable to post-traumatic injuries . Isolated sensory deficits were noted in 182 of 196 patients who had postoperative axillary neuropathy . Damage to the infraspinatus muscle, however, may be clinically overlooked.
MR imaging may demonstrate signs indicative of teres minor denervation injury with increased signal on water-sensitive images or atrophy of the muscle (Figs. 4 and 5). Unlike electromyograph (EMG) studies, which can directly evaluate the function of nerves, MRI provides indirect indicators of nerve injury by detecting changes in fat and water composition of muscle. Effects to T1 and T2 prolongation can be appreciated within 15 days post-injury [26,27]. The identification of teres minor atrophy in the absence of quadrilateral space lesions should prompt careful evaluation for signs indicative of post-traumatic glenohumeral instability and prior dislocation [21-23].
Most patients who have post-traumatic axillary neuropathy respond well to conservative measures . Surgery may be indicated if no clinical or electro-myographic improvement is noted within 2 or 3 months post-injury. Optimal surgical results are obtained if surgery is performed within 3 to 6 months from the time of injury. The monofascicular composition of the axillary nerve and the relatively short distance between the zone of injury and the motor end-plate contribute to the favorable postsurgical outcome.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.