A sympathetic nerve block can be used to diagnose or treat pain that stems from the sympathetic nervous system, which runs from the spiral cord and extends to other parts of the body. The sympathetic nervous system helps control various involuntary bodily functions like digestion, blood flow and sweating. A sympathetic nerve block procedure is done by injecting a medication, such as a steroid, to numb the nerve and block pain signals that it sends to your brain. These nerve blocks can be to diagnose or treat vascular, visceral or neuropathic pain. Other conditions such as Raynaud’s syndrome or excessive sweating may be treated with sympathetic nerve blocks.