Quadrilateral space syndrome (QSS) is an uncommon condition arising secondary to compression of the axillary nerve and posterior circumflex humeral artery within the quadrilateral space. This results in symptoms of vague shoulder pain, weakness affecting the deltoid and teres minor muscles, and pain when pressure is applied over the quadrilateral space. Although the prevalence of QSS is. unknown, the incidence of isolated denervation changes
affecting the teres minor and consistent with a diagnosis of QSS has been quoted at 3% from MRI scans of patients with shoulder pain.

We present the case of an international swimmer with signs and symptoms of QSS and fasciculation affecting the long head of the triceps. The muscle fasciculation occurred at rest and was not related to exercise. After surgical decompression of the axillary nerve, his symptoms of QSS and his long head of triceps fasciculation resolved. Innervation of the long head of the triceps has recently been shown to originate from the axillary nerve or from the posterior cord and not, as is commonly believed, from the radial nerve. We propose that involvement of the long head of the triceps be considered part of QSS.