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Endoscopic thoracic sympathectomy (ETS) is a surgical procedure in which a portion of the sympathetic nerve trunk in the thoracic region is destroyed. ETS is used to treat excessive sweating in certain parts of the body (focal hyperhidrosis), facial blushing, Raynaud's disease and reflex sympathetic dystrophy. By far the most common complaint treated with ETS is sweaty palms (palmar hyperhidrosis). The intervention is controversial and illegal in some jurisdictions. Like any surgical procedure, it has risks; the endoscopic sympathetic block (ESB) procedure and those procedures that affect fewer nerves have lower risks. Sympathectomy physically destroys relevant nerves anywhere in either of the two sympathetic trunks, which are long chains of nerve ganglia located bilaterally along the vertebral column (a localisation which entails a low risk of injury) responsible for various important aspects of the peripheral nervous system (PNS). Each nerve trunk is broadly divided into three regions: cervical (neck), thoracic (chest), and lumbar (lower back). The most common area targeted in sympathectomy is the upper thoracic region, that part of the sympathetic chain lying between the first and fifth thoracic vertebrae.

The most common indications for thoracic sympathectomy are focal hyperhidrosis (that specifically affects the hands and underarms), Raynaud syndrome, and facial blushing when accompanied by focal hyperhidrosis. It may also be used to treat bromhidrosis, although this usually responds to non surgical treatments, and sometimes people with olfactory reference syndrome present to surgeons requesting sympathectomy. There are reports of ETS being used to achieve cerebral revascularization for people with moyamoya disease, and to treat headaches, hyperactive bronchial tubes, long QT syndrome, social phobia, anxiety, and other conditions.

ETS involves dissection of the main sympathetic trunk in the upper thoracic region of the sympathetic nervous system, irreparably disrupting neural messages that ordinarily would travel to many different organs, glands and muscles. It is via those nerves that the brain is able to make adjustments to the body in response to changing conditions in the environment, fluctuating emotional states, level of exercise, and other factors to maintain the body in its ideal state (see homeostasis). Because these nerves also regulate conditions like excessive blushing or sweating, which the procedure is designed to eliminate, the normative functions these physiological mechanisms perform will be disabled or significantly impaired by sympathectomy. There is much disagreement among ETS surgeons about the best surgical method, optimal location for nerve dissection, and the nature and extent of the consequent primary effects and side effects. When performed endoscopically as is usually the case, the surgeon penetrates the chest cavity making multiple incisions about the diameter of a straw between ribs. This allows the surgeon to insert the video camera (endoscope) in one hole and a surgical instrument in another. The operation is accomplished by dissecting the nerve tissue of the main sympathetic chain. Another technique, the clamping method, also referred to as 'endoscopic sympathetic blockade' (ESB) employs titanium clamps around the nerve tissue, and was developed as an alternative to older methods in an unsuccessful attempt to make the procedure reversible. Technical reversal of the clamping procedure must be performed within a short time after clamping (estimated at a few days or weeks at most), and a recovery, evidence indicates, will not be complete.

Sympathectomy works by disabling part of the autonomic nervous system (and thereby disrupting its signals from the brain), through surgical intervention, in the expectation of removing or alleviating the designated problem. Many non ETS doctors have found this practice questionable chiefly because its purpose is to destroy functionally disordered, yet anatomically typical nerves. Exact results of ETS are impossible to predict, because of considerable anatomic variation in nerve function from one patient to the next, and also because of variations in surgical technique. The autonomic nervous system is not anatomically exact and connections might exist which are unpredictably affected when the nerves are disabled. This problem was demonstrated by a significant number of patients who underwent sympathectomy at the same level for hand sweating, but who then presented a reduction or elimination of feet sweating, in contrast to others who were not affected in this way. No reliable operation exists for foot sweating except lumbar sympathectomy, at the opposite end of the sympathetic chain. Thoracic sympathectomy will change many bodily functions, including sweating, heart rate, heart stroke volume, thyroid, baroreflex, lung volume, pupil dilation, skin temperature and other aspects of the autonomic nervous system, like the essential fight or flight response. It reduces the physiological responses to strong emotions, such as fear and laughter, diminishes the body's physical reaction to both pain and pleasure, and inhibits cutaneous sensations such as goose bumps. A large study of psychiatric patients treated with this surgery showed significant reductions in fear, alertness and arousal. Arousal is essential to consciousness, in regulating attention and information processing, memory and emotion. ETS patients are being studied using the autonomic failure protocol headed by David Goldstein, M. D. Ph. D., senior investigator at the U. S National Institute of Neurological Disorders and Stroke. He has documented loss of thermoregulatory function, cardiac denervation, and loss of vasoconstriction. Recurrence of the original symptoms due to nerve regeneration or nerve sprouting can occur within the first year post surgery. Nerve sprouting, or abnormal nerve growth after damage or injury to the nerves can cause other further damage. Sprouting sympathetic nerves can form connections with sensory nerves, and lead to pain conditions that are mediated by the SNS. Every time the system is activated, it is translated into pain. This sprouting and its action can lead to Frey's syndrome, a recognized after effect of sympathectomy, when the growing sympathetic nerves innervate salivary glands, leading to excessive sweating regardless of environmental temperature through olfactory or gustatory stimulation. In addition, patients have reported lethargy, depression, weakness, limb swelling, lack of libido, decreased physical and mental reactivity, oversensitivity to sound, light and stress and weight gain (British Journal of Surgery 2004; 91: 264–269).

Sympathectomy developed in the mid 19th century, when it was learned that the autonomic nervous system runs to almost every organ, gland and muscle system in the body. It was surmised that these nerves play a role in how the body regulates many different body functions in response to changes in the external environment, and in emotion. The first sympathectomy was performed by Alexander in 1889. Thoracic sympathectomy has been indicated for hyperhidrosis (excessive sweating) since 1920, when Kotzareff showed it would cause anhidrosis (total inability to sweat) from the nipple line upwards. A lumbar sympathectomy was also developed and used to treat excessive sweating of the feet and other ailments, and typically resulted in impotence and retrograde ejaculation in men. Lumbar sympathectomy is still being offered as a treatment for plantar hyperhidrosis, or as a treatment for patients who have a bad outcome (extreme 'compensatory sweating') after thoracic sympathectomy for palmar hyperhidrosis or blushing; however, extensive sympathectomy risks hypotension. Endoscopic sympathectomy itself is relatively easy to perform; however, accessing the nerve tissue in the chest cavity by conventional surgical methods was difficult, painful, and spawned several different approaches in the past. The posterior approach was developed in 1908, and required resection (sawing off) of ribs. A supraclavicular (above the collar bone) approach was developed in 1935, which was less painful than the posterior, but was more prone to damaging delicate nerves and blood vessels. Because of these difficulties, and because of disabling sequelae associated with sympathetic denervation, conventional or "open" sympathectomy was never a popular procedure, although it continued to be practiced for hyperhidrosis, Raynaud's disease, and various psychiatric disorders. With the brief popularization of lobotomy in the 1940s, sympathectomy fell out of favor as a form of psychosurgery. The endoscopic version of thoracic sympathectomy was pioneered by Goren Claes and Christer Drott in Sweden in the late 1980s. The development of endoscopic "minimally invasive" surgical techniques has decreased the recovery time from the surgery and increased its availability. Today, ETS surgery is practiced in many countries throughout the world predominantly by vascular surgeons.