Muscle Tone - Pathological Tonus

Pathological Tonus

Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. Another form of hypertonia is paratonia, which is associated with dementia. Hypotonia is seen in lower motor neuron disease like poliomyelitis. Hypotonia can present clinically as muscle flaccidity, where the limbs appear floppy, stretch reflex responses are decreased, and the limb’s resistance to passive movement is also decreased. Hypertonia is seen in upper motor neuron diseases like lesions in pyramidal tract and extrapyramidal tract. Hypertonia can present clinically as either spasticity or rigidity. While spasticity is velocity-dependent resistance to passive stretch (i.e. passively moving an elbow quickly will elicit increased muscle tone, but passively moving elbow slowly may not elicit increased muscle tone), rigidity is velocity-independent resistance to passive stretch (i.e. there is uniform increased tone whether the elbow is passively moved quickly or slowly). Spasticity can be in the form of the clasp-knife response, in which there is increased resistance only at the beginning or at the end of the movement. Rigidity can be of the leadpipe type, in which there is resistance throughout to passive movement, or it may be of cogwheel type, in which the resistance to passive movement is in a jerky manner.