Rigidity is an increased resistance to the passive movement of a joint which is constant throughout the range of joint displacement and not related to the speed of joint movement; resistance is present in both agonist and antagonist muscles. In these particulars, rigidity differs from spasticity. Rigidity also needs to be differentiated from stiffness (q.v.).
Rigidity may be described as:
- Consistent: "lead-pipe rigidity"; or
- Jerky: "cogwheel rigidity" or Negro’s sign, when a rhythmic fluc- tuation (i.e., tremor), like a ratchet or cogwheel, is superimposed on the background of sustained rigidity (NB cogwheeling, reflect- ing underlying tremor, may occur in the absence of rigidity, e.g., in essential tremor).
The presence of rigidity may be made more obvious by reinforcing maneuvers (e.g., clenching and relaxing the contralateral fist, performing mental arithmetic), a finding variously known as activated rigidity, or Froment’s sign, or synkinesis (but note that both Froment’s sign and synkinesis have other meanings too). However, this may occur in some normal subjects; it is most helpful in the diagnosis of Parkinson’s disease if unilateral. Rigidity may also be demonstrated using Wartenberg’s swing test (q.v.).
Rigidity is a feature of parkinsonism and may coexist with any of the other clinical features of extrapyramidal system disease, but particularly akinesia (akinetic-rigid syndrome); both are associated with loss of dopamine projections from the substantia nigra to the putamen. Rigidity is a feature of pathology within the basal ganglia.
The pathophysiology of rigidity is thought to relate to overactivity of tonic stretch reflexes in the spinal cord due to excessive supraspinal drive to spinal cord α-motor neurones following loss of descending inhibition as a result of basal ganglia dysfunction. In other words, there is a change in the sensitivity of the spinal interneurones which control α-motor neurones due to defective supraspinal control. Hence rigidity is a positive or release symptom, reflecting the operation of intact suprasegmental centres. The physiological correlate of this is the increased EMG activity found in rigid muscles with increased 1A afferent fiber activity, suggesting maintained α-γ linkage. In support of this, pyramidotomy has in the past been shown to produce some relief of rigidity.
Rigidity in Parkinson’s disease may be lessened by treatment with levodopa preparations. The techniques of modern stereotactic neurosurgery may also be helpful, particularly stimulation of the subthalamic nucleus, although both thalamotomy and pallidotomy may also have an effect.
The term rigidity may also be used to describe:
- Posturing associated with coma: decorticate or decerebrate, flexor and extensor posturing respectively;
- A lack of mental flexibility, particularly evident in patients with frontal lobe dysfunction.
Cantello R, Gianelli M, Civardi C, Mutani R. Pathophysiology of Parkinson’s disease rigidity: role of corticospinal motor projections. Advances in Neurology 1996; 69: 129-133
Meara RJ, Cody FWJ. Relationship between electromyographic activity and clinically assessed rigidity studied at the wrist joint in Parkinson’s disease. Brain 1992; 115: 1167-1180