Mondo Description A rare neoplastic syndrome characterized by obstruction of the thoracic outlet leading to compression of the brachial plexus and vessels within. It is usually caused by a malignant neoplasm in the superior pulmonary sulcus. The most commonly involved neoplasms are non-small cell lung carcinomas. Clinical signs include Horner's syndrome, shoulder pain radiating down the arm in the ulnar distribution followed by edema and atrophy of the affected extremity. Clinical course usually leads to early local invasion of the bony thoracic structures. Prognosis is highly stage-dependent.