An increase in muscle tone that causes resistance to passive movement. Rigidity is a cardinal sign of Parkinson’s disease, typically manifested first as cogwhEEling or lead pipe resistance. These manifestations are apparent to the physician during passive manipulation of the structures around a joint, most notably the wrists and ankles. In cogwheeling the foot or hand moves as though along a notched wheel, with hesitation and then sudden release. In lead pipe resistance, the rigidity is constant rather than intermittent. Most neurologists recognize cogwheeling as really just a sign of tremor, but lead pipe rigidity is recognized as a separate cardinal sign of parkinson-ism. Rigidity is distinct from spasticity (a “clasp-knife” like catch on rapid passive movements) that is the form of stiffness associated with most strokes or other injuries to the brain or spinal cord that affect motor function. Rigidity expands to involve more muscle groups as Parkinson’s progresses and can reach a point at which it prevents use of the muscles, such as in the leg or arm. The person with Parkinson’s disease perceives rigidity as stiffness, and it causes the same kind of aching and discomfort as would holding an arm or leg in a contracted position. Rigidity results from a shortening of the muscle fibers in response to the continuous signals from the brain that cause them to stay contracted. anti-parkinson’s medications that restore neuron communication in the structures of the brain that regulate movement help to ease rigidity by slowing nerve signals for contraction and allowing nerve signals that instruct the muscles to relax to pass. For the duration of the medication’s effectiveness, motor function can be near-normal in the early to middle stages of Parkinson’s. In later stages anti-Parkinson’s medications have difficulty in fully restoring neuron communication so rigidity tends to persist at a low to moderate level. This condition contributes to the stooped, closed posture that characterizes Parkinson’s disease. As medications become less effective, rigidity causes episodes of aki-nesia or freezing, in which muscles remain so tightly contracted that they do not respond for movement. This contraction can cause abnormal and uncomfortable positions, such as “clawing” of a hand, that become permanent deformities. Rigidity accounts for one of Parkinson’s more distressing symptoms, the lack of facial expression (hypomimia).
Surgical treatments such as deep brain stIMuLa-tion (DBS) or paLLidotomy sometimes provide relief when anti-Parkinson’s medications no longer do so. Because of their invasive nature these treatments have significant risks including infection and neurologic and neuropsychiatric side effects from disruption of the target and nearby structures. thaLamotomy, another surgical procedure, is effective in reducing tremors but not rigidity.