Jessica N. Buttinger, MD
Resident Physician
Mayo Clinic
Rochester, Minnesota, United States
Cameron Smock, MD
Resident Physician
Mayo Clinic
Rochester, Minnesota, United States
James B. Meiling, DO
Assistant Professor in Physical Medicine and Rehabilitation
Mayo Clinic
Rochester, Minnesota, United States
Quadrilateral Space Syndrome (QSS) isolated to the axillary nerve branch innervating the right teres minor muscle with associated electrophysiologic and sonographic changes.
Case Description:
A 72-year-old male presented with one year of right posterior shoulder pain exacerbated by overhead reaching movements. Physical exam was notable for tenderness to palpation over the posterior right shoulder and mild atrophy of the right teres minor.
Neuromuscular ultrasound (NMUS) examination demonstrated atrophy and increased echogenicity throughout the right teres minor muscle compared to the left. Needle electromyography (EMG) of the right teres minor muscle performed under ultrasound guidance showed ongoing denervation and reduced recruitment of large motor unit potentials. All other muscles evaluated with EMG, including all three heads of the deltoid muscle, were normal. Based on the results of NMUS and EMG, the patient was diagnosed with a right axillary mononeuropathy localized to the quadrilateral space and isolated to the branch innervating teres minor.
Discussions:
QSS is a rare cause of posterolateral shoulder pain resulting from compression of the axillary nerve and/or posterior circumflex humeral artery within the quadrilateral space. MRI, arteriography, and musculoskeletal ultrasound have historically aided in the diagnosis of QSS, but few cases show neuropathic changes on EMG. Given the technical challenges of performing needle EMG of a healthy teres minor using palpation techniques, made even more difficult in an atrophied muscle, the use of ultrasound guidance for needle placement was essential to localizing the lesion in this case. Furthermore, atrophy and denervation changes of teres minor, visualized with NMUS, helped confirm the diagnosis and localization.
Conclusions:
This case describes a rare diagnosis of QSS isolated to the axillary nerve branch innervating the teres minor muscle. The diagnostic approach highlights the utility of NMUS and ultrasound-guided EMG in identifying and localizing pathology associated with QSS and other rare mononeuropathies.
