The loss of balance associated with propulsion and retropulsion is probably the least specific, but most disabling, of all parkinsonian symptoms. Purdon-Martin (79), after studying nine brains of patients with postencephalitic parkinsonism, concluded that globus pallidum degeneration was most responsible for the loss of righting reflexes and postural instability in parkinsonian patients. Reichert et al. (5) correlated postural instability in PD patients with reduced or absent vestibular responses. Traub et al. (80) studied postural reflexes in 29 PD patients by recording anticipatory postural responses in the legs (triceps surae) in response to perturbations of one of the arms. In normal subjects, a burst of activity can be recorded from the calf muscles at a latency of 80 msec after the perturbation. This postural adjustment occurs even before any movement can be recorded in the legs (latency, 150 msec). Therefore, this reflex adjustment is anticipatory and centrally generated. In PD, the anticipatory postural reflexes are absent or markedly diminished. Such abnormalities were present in 10 of the 18 patients with moderately severe PD and in two of 11 PD patients without obvious postural instability. Since some patients with normal anticipatory reflexes can still fall, it is likely that other mechanisms contribute to the falls of parkinsonian patients (14,81). Furthermore, patients with progressive supranuclear palsy (PSP), who are much more prone to falling than PD patients, have normal anticipatory postural responses (80). Weiner et al. (82) found moderate or severe loss of balance in response to a standing postural perturbation in 68% of 34 patients in a geriatric care facility. They suggested that a postural reflex dysfunction was largely responsible for the unexplained falls in the elderly.
Loss of postural reflexes usually occurs in more advanced stages of PD and, along with freezing, is the most common cause of falls, often resulting in hip fractures. The loss of protective reactions further contributes to fall-related injuries. Many patients with postural instability, particularly when associated with flexed truncal posture, have festination, manifested by faster and faster walking in order to prevent falling. When combined with axial rigidity and bradykinesia, loss of postural reflexes causes the patient to collapse into the chair when attempting to sit down. The "pull test" (pulling the patient by the shoulders) is commonly used to determine the degree of retropulsion or propulsion.
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