In the form of altered muscle tone known as rigidity, the muscles are continuously or intermittently firm and tense. Although brief periods of electromyographic silence can be obtained in selected muscles by persistent attempts to relax the limb, there is obviously a low threshold for involuntary sustained muscle contraction, and this is present during most of the waking state, even when the patient appears quiet and relaxed. In contrast to spasticity, the increased resistance on passive movement that characterizes rigidity is not preceded by an initial “free interval” and has an even or uniform quality throughout the range of movement of the limb, like that experienced in bending a lead pipe or pulling a strand of toffee. The contrasting terms clasp-knife for spasticity and lead-pipe for rigidity have been applied to the examiner’s physical perception on attempting to smoothly manipulate the patient’s limb through an arc of movement. Moreover, the rigidity of extrapyramidal disorder is not velocity dependent, as it is in spasticity. The tendon reflexes are not enhanced in the rigid limb as they are in spasticity and, when released, the limb does not resume its original position, as happens in spasticity.
Rigidity usually involves both flexor and extensor muscle groups, but it tends to be more prominent in muscles that maintain a flexed posture, that is, in the flexor muscles of trunk and limbs. It appears to be somewhat greater in the large muscle groups, but this may be merely a matter of muscle mass. Certainly the small muscles of the face and tongue and even those of the larynx are often affected by rigidity. Concordant with the physical examination, in the electromyographic tracing, motor-unit activity is more continuous in rigidity than in spasticity, persisting even after apparent relaxation.
A special feature that may accompany rigidity, first noted by Negro in 1901, is the cogwheel phenomenon. When the hypertonic muscle is passively stretched, for example, when the hand is dorsiflexed, one encounters a rhythmically interrupted, ratchet-like resistance. Many believe that this phenomenon represents an underlying tremor that, if not manifestly present, emerges faintly during manipulation. In that case it would not be a fundamental property of rigidity and would be found in many tremulous states. However, numerous instances of severe tremor with minimally perceptible cogwheeling, and the opposite, suggest to us on clinical grounds that the phenomenon may be more complex.
Rigidity is characteristically variable in severity at different times; in some patients with involuntary movements, particularly in those with chorea or dystonia, the limbs may actually be intermittently or persistently hypotonic. Rigidity is a prominent feature of many basal ganglionic diseases, such as Parkinson disease, Wilson disease, striatonigral degeneration (multiple system atrophy), progressive supranuclear palsy, dystonia musculorum deformans (discussed further on and in Chap. 38), exposure to neuroleptic drugs, and mineralization of the basal ganglia (Fahr disease).
Another distinctive type of variable resistance to passive movement is one in which the patient seems unable to relax a group of muscles on request. When the limb muscles are passively stretched, the patient appears to actively resist the movement (gegenhalten, paratonia, or oppositional resistance). Natural relaxation normally requires concentration on the part of the patient. If there is inattentiveness—as happens with diseases of the frontal lobes, dementia, or other confusional states—this type of oppositional resistance may raise a question of parkinsonian rigidity. This is not a manifestation of basal ganglia disorder per se but may indicate that the connections of the basal ganglia to the frontal lobes are impaired. A similar difficulty in relaxation is observed normally in small children. Also not to be mistaken for rigidity or paratonia is the “waxy flexibility” displayed by the psychotic-catatonic patient when a limb placed in a suspended position is maintained for minutes in the identical posture (flexibilitas cerea).
Chorea, Athetosis, Ballismus, Dystonia