An inability to make adjustments in body position to maintain balance, equilibrium, and appropriate momentum during movement. Postural instability develops in Parkinson’s disease as a convergence of several factors: progressively degenerative motor function that inhibits appropriate response, disruption of the body’s postural righting reflexes, disturbances of the body’s proprioception (orientation to spatial location) mechanisms, and postural changes such as stooping, reduced arm swing, and shortened leg stride. Postural instability generates numerous gait disturbances as the body attempts to use other means to return itself to stability.
A common test for postural instability is to push the person when he or she is standing (with due caution for the person’s safety to prevent falling). The person with Parkinson’s often takes several steps in the direction in which he or she is pushed before being able to recover. Stepping forward is called pulsion; stepping backward is rEtropulsion. A person with normal extrapyramidal function may take one short step in the direction of the push. Making turns by taking multiple small steps rather than pivoting on the ball of the foot is a mechanism of attempted compensation for postural instability. Postural instability usually improves with anti-parkinson’s medications, especially levodopa, but it can be very resistant to treatment in Parkinson’s disease, accounting for a significant number of falls.