Muscle tone refers to the amount of tension that is normally present in all of our muscles. While awake, most people have a normal amount of resistance to having their joints moved. It is neither too easy nor too hard to move a normal arm or leg.
Hypotonia refers to "decreased muscle tone," a looseness or floppiness either of the limbs or the trunk/body. Hypotonia can be caused by disorders of the brain, nerves or muscles. If a child has normal knee-jerk reflexes (indicating normal nerve, spinal cord and place where the nerve joins the muscle) and good strength in the muscles themselves (indicating normal muscle development), then the hypotonia is caused by differences in brain function. The brain does not send the correct balance of messages to the tendons and muscles, leading to this feeling of looseness. This looseness can affect the quality of a child's movements.
Many, but not all, autistic children may have some degree of low tone, leading to problems with motor skills. Autistic children may be late in sitting up or late in walking. They may be clumsier in their movements than other children. They may have difficulty holding a pencil or learning to write.
Some metabolic disorders are associated with hypotonia. Strabismus (crossed eyes) and seizures can be associated with hypotonia in metabolic disorders. Children who have autism, developmental delays and hypotonia may need to be evaluated for inborn errors of metabolism.
Physical therapists and pediatric neurologists may disagree whether a child has hypotonia. Physical therapists are looking for a degree of hypotonia that causes a functional problem for a child, such as difficulty jumping or running. Neurologists are looking for a degree of hypotonia that indicates a neurologic disease, which is often more severe than the amount of hypotonia that is functionally important.