In terms of pathologic anatomy and physiology, bradykinesia may be caused by any process or drug that interrupts the cortico-striato-pallido-thalamic circuit. Clinical examples include reduced dopaminergic input from the substantia nigra to the striatum, as in Parkinson disease; dopamine receptor blockade by neuroleptic drugs; extensive degeneration of striatal neurons, as in striatonigral degeneration and the rigid form of Huntington chorea; and destruction of the medial pallidum, as in Wilson disease. As illustrated in Fig. 4-4B, which gives a schematic representation of the hypokinetic state of Parkinson disease, changes in the cortico-striato-pallido-thalamic circuit (in this case mainly the direct striatopallidal pathway) can be interpreted in terms of altered neurochemical and resultant physiologic connectivity within the basal ganglia. The reciprocal situation, enhanced motor activity, is summarized in the analogous diagram for Huntington disease (Fig. 4-4C), in which a reduction in the activity of the indirect striatopallidal pathway leads to enhanced excitatory motor drive in the thalamocortical motor pathway.
A number of other disorders of voluntary movement may also be observed in patients with diseases of the basal ganglia. A persistent voluntary contraction of hand muscles, as in holding a pencil, may fail to be inhibited, so that there is interference with the next willed movement. This has been termed tonic innervation, or blocking, and may be brought out by asking the patient to repetitively open and close a fist or tap a finger. Attempts to perform an alternating sequence of movements may be blocked at one point, or there may be a tendency for the voluntary movement to adopt the frequency of a coexistent tremor (entrainment).
Disorders of Postural Fixation, Equilibrium, and Righting
These deficits are also demonstrated most clearly in the parkinsonian patient. The prevailing posture is one of involuntary flexion of the trunk, limbs and the neck, which gives the parkinsonian individual a characteristic appearance, even at a distance from the observer, as described by Parkinson, “A propensity to bend the trunk forwards, and to pass from a walking to a running pace.” Anticipatory and compensatory righting reflexes, referring to mechanisms that maintain upright posture, are also manifestly impaired. This occurs early in the course of progressive supranuclear palsy and later in Parkinson disease. The inability of the patient to make appropriate postural adjustments to tilting or falling and his inability to move from the reclining to the standing position are closely related phenomena. A gentle push on the patient’s sternum or a tug on the shoulders may cause a fall or start a series of small corrective steps that the patient cannot control (festination). These basal ganglionic postural abnormalities are not attributable to weakness or to defects in proprioceptive, labyrinthine, or visual function, the principal forces that control the normal posture of the head and trunk.