Postural set for balance control is normal in Alzheimer's but not in Parkinson's disease

It has been suggested that patients with dementia of the Alzheimer type have abnormalities in the basal ganglia, and thus, may have similar sensorimotor problems as patients with basal ganglia degeneration from Parkinson's disease. Whether the similarity extends to balance control is unknown. One distinguishing feature of balance disorder in Parkinson's disease is difficulty with changing postural set in terms of adapting the amplitude of leg muscle activity as a function of support condition. We, therefore, tested whether patients with Alzheimer's disease without extrapyramidal signs would show a similar problem in changing postural set as patients with Parkinson's disease.

The ability to quickly change postural set was measured by comparing leg muscle activity under two conditions of support (free stance, versus grasping a frame, or sitting) during backward surface translations, during toes up surface rotations, and during voluntary rise to toes. Results were compared among 12 healthy adults, 8 nondemented Parkinson's patients on their usual dose of medication, and 11 Alzheimer patients without extrapyramidal signs.

Subjects with Alzheimer's, but not Parkinson's, disease performed similarly to the healthy control subjects. They changed postural set immediately, by suppressing leg muscle activity to low levels when supported. Parkinson subjects did not change postural set immediately. They did not suppress the tibialis anterior in voluntary rise to toes when holding, nor the soleus in perturbed sitting as much as the healthy control and Alzheimer subjects in the first trial. Instead, the Parkinson subjects changed set more slowly, over repeated and consecutive trials in both protocols. The onset latencies of soleus responses to backward surface translations and perturbed sitting, as well as tibialis anterior responses to toes up rotations, were the same for all three groups.

Alzheimer patients without extrapyramidal signs, unlike nondemented Parkinson's disease patients, have no difficulty in quickly changing postural set in response to altered support conditions. Our results, therefore, do not support the hypothesis that Parkinson's and uncomplicated Alzheimer's diseases share common postural set problems that may contribute to disordered balance control.