Ataxia (Pediatric) Mechanism

Ataxia is usually caused by damage to the cerebellum or to its inputs or outputs. The cerebellum is a large structure at the lower back of the brain, just above and behind where the spinal cord enters the skull at the bottom.

Separate parts of the cerebellum are concerned with control of arm movements, eye movements, trunk stability, balance, and gait. There are many theories describing the function and purpose of the cerebellum, but these issues have not yet been resolved. In general, many researchers believe that the cerebellum receives sensory information from the entire body, as well as a copy of the motor commands being sent to move the body. It integrates this information using a densely interconnected network of nerve cells in order to determine the interactions between sensations coming from different parts of the body and the expected results of movement. The resulting information is then sent to motor areas of the brain. This is thought to adjust the motor commands in order to compensate for the location of the limbs in space and the interaction of the forces generated by different parts of the body. The cerebellum appears to have a set of signals that tell it when movement errors occur, and these errors are used to make rapid corrections and thereby learn to improve performance.

Damage to the Cerebellum
When the cerebellum is damaged, the effects may often be understood by determining the regions that sustained the injury. A "focal" injury or an injury to a small local part of the cerebellum may affect only arm movements, balance, gait, or eye movements. This type of injury may be seen if there is a stroke, migraine, or tumor. A more "global" injury may result in loss of a particular cell type or dysfunction of the entire cerebellum.

There are several different types of nerve cells in the cerebellum. The Purkinje cells are large and complex cells that can accumulate information coming from many parts of the body and perform much of the integration of data in the cerebellum. These cells seem to be particularly susceptible to injury. Certain poisons, medicines, and genetic diseases may lead to injury or loss of the Purkinje cells, causing symptoms that may simultaneously affect many functions of the cerebellum. Other elements of the cerebellum that may be damaged include the granule cells. These cells migrate inwards from the surface of the cerebellum during early development in the embryo; in some genetic disorders, this migration may not occur properly.

In some cases, large areas of a child's cerebellum may not have formed properly in the first place. In this case, however, the symptoms may be relatively mild, as other brain areas may have taken over some lost functions (since parts of the cerebellum may have been absent during development of the rest of the brain). Because of this type of compensation by the rest of the brain, a child who is born with a very small cerebellum may have many fewer symptoms than a child whose cerebellum is injured later in life.

	Etiology