Introduction
Selective posterior rhizotomy (also called functional dorsal rhizotomy) is a neurosurgical technique being used increasingly to treat childhood spasticity (tightness in the muscles that resists rapid movement), as is typically seen with cerebral palsy. The procedure evolved from work done in the late 19th century. It is based on the assumption that spasticity results from loss of the controlling influences of nerves coming from the brain on the spinal cord’s reflex circuits.
There are a multitude of these reflex circuits, an example of which is the tendon jerk reflex (straightening of the leg in response to tapping the knee with a hammer). In this case, sensors within the large muscle in the front of the thigh perceive the stretching of the muscle, which results when its tendon is tapped by the hammer. A nerve impulse is sent to the spinal cord, where it stimulates a nerve that sends a signal back to the thigh muscle, causing it to contract and pull the leg straight. Nerves from the brain control these reflex circuits, allowing us to voluntarily move a muscle without its reflexively tightening up. With injuries to the spinal cord or brain (e.g., cerebral palsy), these descending nerves from the brain can be damaged, resulting in an alteration of the controlling influences on the reflex circuits. The result is a spreading activation by nerves within the spinal cord and a subsequent contraction of numerous muscles throughout the body. This creates the typical picture of limb spasticity.