Abstract Background: Many procedures affect the sympathetic nervous system and thoracic sympathectomy and brachial plexus blockade are among them. Thoracic sympathectomy is an effective treatment for hyperhidrosis and facial blushing, but is associated with a high incidence of side effects such as compensatory hyperhidrosis. Although the procedure is standardized there is great diversity of the neuroanatomical structures. We also find this diversity in brachial plexus. That is why ultrasound is used to visualize the plexus anatomy for regional anesthesia distal of the shoulder. Methods: The aim was to gain better understanding of the neuroanatomical structure of the brachial plexus and the sympathetic trunk. I had a main focus on the sympathetic nerve system and the physiology before and after the procedures. In addition, I received data before and after thoracic sympathectomy and brachial plexus blockade from a medical and research student Ai Van Thuy Ho and dr. med. Eirik Øvensen. Through electronic search in PubMed I have extracted relevant research papers on this topic. I have also looked into existing, authorative literature in this field. Results: Thoracic sympathectomy is a successful treatment for hyperhidrosis and facial blushing. The majority is satisfied with the treatment, but the effects show large interindividual variations. Most patients experience compensatory hyperhidrosis, which is the most common side effect, but the mechanism is still unknown. The same goes for brachial plexus blockade. Although it is optimal to reach the plexus within the range 0-15 degrees posterior of the frontal plane from the infraclavicular view, it is not always easy to identify and adequately block the cords from this region with ultrasound guiding due to anatomical variations. However, the loss of sweat activity and/or increased skin temperature can be used as an indicator that provides information about whether the blockade is successful in the early phase. Conclusion: Patient satisfaction and perceived effectiveness with thoracic sympathectomy for hyperhidrosis remain high. Ultrasound appears to permit accurate deposition of the local anaesthetic perineurally, and has the potential to improve the success and decrease the complications of infraclavicular brachial plexus block. However, the anatomical variation is great even though there are similarities in the neuroanatomical structure. This variation is the reason for varying efficacy and side effects in individual patients.