Seek specialist. Pancoast syndrome is due to a tumor of the "top" of the lung causing pressure on vital structures such as nerves and blood vessels. Depending on the type of tumor, the treatment may vary. This is something you need a specialist to help you with; you probably want to start with a pulmonologist.
First find the cause. The usual cause is lung cancer erupting upward from the chest to destroy the nerve plexuxsthat supply sympathetic nerves to the eye (ptosis/eyelid droop), meiosis (consticited pupil), anhydrosis (lack of sweating) and arm pain. Chemotherapy and radiotherapy may be used before surgery or alone without surgery if it can't be done.