Pectoralis minor syndrome (PMS) and quadrilateral space syndrome (QSS) are two neurovascular compressive disorders affecting the upper extremity that are overall poorly understood and may be overlooked, even by physicians who frequently treat patients with upper-extremity pain and disability [1]. Both syndromes are associated with overuse of the affected extremity and, despite their relative infrequency, require accurate recognition and diagnosis due to their potential to cause debilitating symptoms. For physicians who treat other upper-extremity neurovascular pathologies, including thoracic outlet syndrome (TOS), an understanding of the pathophysiology, diagnosis, and management of these compressive disorders is essential. Outcomes after PMS and QSS treatment are excellent, and a heightened clinical suspicion is necessary, as misdiagnosis may result in failure to relieve symptoms, requiring further treatment or surgery [1,2]. The following review will provide an overview of the etiology, presentation, diagnosis, treatments, and outcomes of PMS and QSS.

PMS is defined as compression of the brachial plexus nerves, axillary artery, or axillary vein under the pectoralis minor muscle, which lies deep to the pectoralis major and inferior to the clavicle on the anterior chest wall. Although it was first described in the 1940s as a neuromuscular hyperabduction syndrome, PMS has only gained recognition as a significant contributor to anterior shoulder, arm, and chest pain in recent years, as it is difficult to isolate from other forms of neurovascular compression [3]. PMS can lead to a range of symptoms and functional limitations, including pain, weakness, and numbness or paresthesias in the affected arm. PMS classically affects individuals who engage in prolonged or repetitive upper-extremity activity, such as weightlifters and athletes who perform repetitive overhead motions. Like TOS, PMS can be classified by etiology into venous, arterial, and neurogenic subtypes.

Another upper-limb compressive syndrome, QSS, was first described in 1983 by Cahill and Palmer [1] in a series of patients whose symptoms remained after surgical decompression for suspected TOS. Cahill and Palmer identified a common denominator among these patients—arteriographic evidence of posterior circumflex humeral artery (PCHA) compression. QSS is thought to result from compression of the axillary nerve and/or the PCHA at the quadrilateral space [1]. QSS is a rare clinical entity with an unknown incidence, but is most frequently described in patients who engage in repetitive overhead activity [4], [5], [6].