Hypertonia is a condition marked by an abnormal increase in muscle tension and a reduced ability of a muscle to stretch.
It is caused by injury to motor pathways in the central nervous system, which carry information from the central nervous system to the muscles and control posture, muscle tone, and reflexes. When the injury occurs in children under the age of 2, the term cerebral palsy is often used. Hypertonia can be so severe that joint movement is not possible. Untreated hypertonia can lead to loss of function and deformity. Hypertonia may result from injury, disease, or conditions such as spasticity, dystonia (prolonged muscle contractions that cause twisting and repetitive movements or abnormal posture), rigidity, or a combination of factors. Spastic hypertonia involves uncontrollable muscle spasms, stiffening or straightening out of muscles, shock-like contractions of all or part of a group of muscles, and abnormal muscle tone.
It is seen in disorders such as cerebral palsy, stroke, and spinal cord injury. Dystonic hypertonia refers to muscle resistance to passive stretching (in which a therapist gently stretches the inactive contracted muscle to a comfortable length at very low speeds of movement) and a tendency of a limb to return to a fixed involuntary (and sometimes abnormal) posture following movement. It is seen is the different forms of dystonia and sometimes in parkinsonism. Rigidity is an involuntary stiffening or straightening out of muscles, accompanied by abnormally increased muscle tone and the reduced ability of a muscle to stretch. This type of hypertonia is most common in parkinsonism