Pathophysiology
Hypertonia is caused by upper motor neuron lesions which may result from injury, disease, or conditions that involve damage to the central nervous system. Motor neuronal hyperactivity occurs due to loss of inhibition of cells of the anterior horn of the spinal cord resulting from reticulospinal tract damage. Different patterns of muscle weakness or hyperactivity can occur based on the location of the lesion, causing a multitude of neurological symptoms, including spasticity, rigidity, or dystonia.
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. Rigidity is a severe state of hypertonia where muscle resistance occurs throughout the entire range of motion of the affected joint independent of velocity. It is frequently associated with lesions of the basal ganglia. Individuals with rigidity present with stiffness, decreased range of motion and loss of motor control. 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.
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