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Several tests may be necessary to rule out other causes of shoulder pain, because isolated axillary nerve injury and quadrilateral space syndrome are rare. Plain radiographs typically are ordered, but these are useful in these cases only with a history of trauma that is consistent with a possible proximal humerus fracture, scapular neck fracture, or anterior shoulder dislocations.

Cervical spine radiography may be considered to rule out neck pathology, such as cervical spondylosis or osteoarthritis, which may be causing radiculopathy and subsequent deltoid weakness and/or atrophy. CT may reveal osseous abnormalities affecting the nerve, such as excessive callus formation following a scapular neck or proximal humeral fracture.

MRI can help to identify the course of the axillary nerve and any associated soft-tissue lesions. A paralabral cyst or venous dilation compressing the nerve has been reported . In advanced cases, atrophy of the deltoid and/or teres minor can be seen with MRI, whereas while fatty replacement of the muscle suggests a poor prognosis for recovery of function. Finally, MRI is helpful to rule out other pathologies, such as rotator cuff tears. The role of magnetic resonance neurography is still being determined.

To diagnose quadrilateral space syndrome, the classic test is arteriography. During arteriography, the vessel of interest is the posterior circumflex humeral artery, which travels through the quadrilateral space with the axillary nerve.

With the arm in adduction, normal flow typically will be seen in the posterior circumflex humeral artery and should be seen passing around the posterolateral humeral neck. When the arm is held in abduction and external rotation, however, contrast flow is lost within the posterior circumflex humeral artery distal to the quadrilateral space.

Angiography can be performed as an angio-MRI; however, this test can demonstrate occlusion of the posterior circumflex humeral artery with the shoulder in abduction in up to 80% of asymptomatic patients.

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