Pediatric Movement Disorders - Spasticity

Mechanism

Spasticity is caused by an increase in the reaction of the stretch reflex. Normally, this reflex is activated whenever a muscle is rapidly stretched. This reflex causes the muscle to contract in order to resist the force that is stretching it. However, normal movement often requires turning off this reflex. The brain normally sends inhibitory signals through the corticospinal tract to the cells of the spinal cord. The corticospinal tract refers to nerves fibers that project from the brain's cortex into the nerves of the spine. These signals may reduce the stretch reflex. If there is damage to the corticospinal tract or the cells in the brain that are its origin, the stretch reflex may be disinhibited. Over time, an uncontrolled stretch reflex may gradually increase in strength with a decreased threshold for triggering; this continues until even small stretches of the muscle lead to an excessive complete contraction of the muscle. At its worst, the muscle contracts even at rest, and the limb becomes impossible to move. Longstanding spasticity is further complicated by shortening of the muscles and tendons; even complete relaxation of the muscle does not allow full movement of the affected joint.

Spasticity is not usually present immediately after an injury to the brain or spinal cord. It develops gradually over the following weeks or months, and then may stabilize.