The cerebellar tonsil is a structure in the brain located at the base of one of the cerebellar hemispheres, the two halves of the cerebellum. Most people have two cerebellar tonsils, unless they have an unusual brain malformation. These structures can be seen as small rounded lobes, one at the bottom of each side of the cerebellum, and they are readily visible in cross section. Disorders of the cerebellum can involve one or both tonsils, perhaps most notably in the case of a Chiari malformation, a condition that may be congenital or acquired.
Like the rest of the cerebellum, the cerebellar tonsil has a number of functions related to motor function. This part of the brain provides balance, as well as coordination for motor tasks, and it also is involved in the formation of memories related to movement. Some emotional responses are moderated here, and it also channels sensory input from various areas of the body to other parts of the brain.
Unlike the cerebrum, involved in higher order cognition, the cerebellum is concerned with more basic physical functions. It is a critical part of the brain, as it regulates a variety of movement-related activities including walking, controlling the hands to perform tasks, and balancing. Many people with disorders in this part of the brain develop ataxia, a distinctive staggering, doddering gait caused by poor motor control, and a limited sense of balance. They may need to use canes or walkers to prevent falls and in some cases need a mobility device like a wheelchair.
In the case of people with a Chiari malformation, pressure inside the skull forces the cerebellar tonsils through the foramen magnum, literally “the big hole” in the skull. The foramen magnum is designed to create an opening for the spinal cord and associated structures, and it does not readily accommodate other structures as well. People with this condition can experience obstructions of cerebrospinal fluid and may develop problems with balance and motor skills.
Some people are born with this malformation. It is caused by errors during fetal development and may manifest in a variety of ways, depending on how severe it is. Other people acquire it after head trauma, when pressure inside the skull may force brain tissues into unusual spaces as they fight for room. Treatments can include surgeries where part of the cerebellar tonsil is actually removed to shorten it and relieve pressure. Decisions about treatment are made after evaluating medical imaging studies to determine the severity of the problem.