Diagnosis and treatment algorithm for patients with suspected quadrilateral space syndrome. AER, Abduction and external rotation; CTA, computed tomography angiography; DSA, digital subtraction angiography; MRA, magnetic resonance angiography; nQSS, neurogenic quadrilateral space syndrome; NSAID, nonsteroidal antiinflammatory drug; overhead athlete, any patient engaged in elite, college, professional, or dedicated sport or activity involving repetitive abduction and external rotation of the arm above the head; PCHA, posterior circumflex humeral artery; PT, physical therapy; QS, quadrilateral space; QSS, quadrilateral space syndrome; SOL, space-occupying lesion; UEA, upper extremity arterial; US, ultrasound; vQSS, vascular quadrilateral space syndrome.