Quadrilateral space syndrome is an increasingly recognized cause of shoulder and posterior arm pain that is the result of entrapment or trauma to the axillary nerve as it traverses the quadrilateral space. Cahill and Palmer first described this syndrome in 1983 with the majority of patients being young athletes in their second and third decade of life. The use of magnetic resonance scanning and ultrasound imaging of the quadrilateral space has made the diagnosis of quadrilateral space syndrome more objective, and it is now being diagnosed in all age groups.

Although in some patients, the inciting antecedent trauma responsible for the onset of symptoms of quadrilateral space syndrome is obvious, in many patients, the onset of the patient’s ill-defined lateral shoulder and posterior upper extremity pain is insidious. The pain is often described as aching in nature with superimposed dysesthesias often present. Adduction and external rotation of the affected upper extremity will make the symptoms worse. If the compromise of the axillary nerve remains untreated, the patient may begin to notice the gradual onset of weakness of the affected upper extremity, particularly when abducting and externally rotating it. In some patients, the weakness is progressive and permanent atrophy of the deltoid and teres minor muscles can be identified on physical examination and medical imaging studies. There often is tenderness to palpation of the quadrilateral space on physical examination.

As with other entrapment neuropathies such as carpal tunnel syndrome, electromyography may help identify compromise of the axillary nerve and help distinguish quadrilateral space syndrome from the brachial plexopathies and cervical radiculopathy, which may both mimic quadrilateral space syndrome. Plain radiographs of the cervical spine and shoulder are indicated in all patients who present with symptoms suggestive of quadrilateral space syndrome to rule out occult bony pathology. Based on the patient’s clinical presentation, additional testing including complete blood cell count, uric acid, sedimentation rate, and antinuclear antibody testing may be indicated. Magnetic resonance imaging or ultrasound imaging of the shoulder with special attention to the teres minor muscle is indicated on all patients suspected of suffering from quadrilateral space syndrome as this test is highly specific for this disorder (Fig. 47.1). In the rare patient in whom magnetic resonance scanning is nondiagnostic, subclavian arteriography to demonstrate occlusion of the posterior humeral circumflex artery may be considered as this finding is highly suggestive of a diagnosis of quadrilateral space syndrome.

In addition to treating quadrilateral space syndrome, ultrasound-guided axillary nerve block in the quadrilateral space may be used to palliate acute pain emergencies, including postoperative pain, pain secondary to traumatic injuries of the shoulder, and cancer pain, while waiting for pharmacologic, surgical, and antiblastic methods to become effective. Ultrasound-guided axillary nerve block can also be used as a diagnostic tool when performing differential neural blockade on an anatomic basis in the evaluation of upper extremity pain as well as in a prognostic manner to determine the degree of neurologic impairment the patient will suffer when destruction of the axillary nerve within the quadrilateral space is being considered or when there is a possibility that the nerve may be sacrificed during surgeries in the anatomic region of the quadrilateral space. Ultrasound-guided axillary nerve block in the quadrilateral space may also be used to palliate the pain and dysesthesias associated with stretch injuries to the axillary nerve as it traverses the quadrilateral space.

The quadrilateral space is a four-sided space that is bounded superiorly by the subscapularis and teres minor muscles, medially by the long head of the triceps brachii, laterally by the surgical neck of the humerus, and inferiorly by the teres major muscle (Fig. 47.2). Contained within the quadrilateral space is the axially nerve, which is a branch of the brachial plexus and the posterior circumflex humeral artery (Fig. 47.3). Compromise of either of these structures by tumor, hematoma, aberrant muscle, stretch injury, or heterotopic bone can produce the constellation of symptoms known as quadrilateral space syndrome.