Every pain signal travels along nerves to the spinal cord and on to the brain, where it is perceived as pain. A nerve block interrupts this pathway: a local anaesthetic is injected precisely next to a nerve or nerve plexus, where it blocks the transmission of signals for a while. The area supplied by that nerve becomes numb and often temporarily weaker as well.
Regional anaesthesia is the umbrella term for all methods that numb one region of the body while consciousness is preserved. A distinction is made between blocks of individual nerves or nerve plexuses, for example at the shoulder, arm, hip or knee, and methods close to the spinal cord. In spinal anaesthesia, the anaesthetic is given as a single injection into the cerebrospinal fluid in the lower back; in epidural anaesthesia, it goes into the space outside the tough outer membrane of the spinal cord, usually via a thin catheter.
These methods are used during operations – on their own, with light sedation or in addition to general anaesthesia – for pain relief in the days that follow, and for certain acute or chronic pain. As a test, a block can show whether a particular nerve is involved in the pain.
The needle is often guided by ultrasound: the image shows the nerve, the blood vessels and how the anaesthetic spreads. When the block works, strong painkillers are often needed in smaller amounts, which can reduce nausea and drowsiness. Which method is suitable depends on the procedure, on pre-existing conditions and on anticoagulant medication.