A dissociation between subjective and objective unsteadiness in primary orthostatic tremor

Patients with primary orthostatic tremor (OT) experience a disabling sense of unsteadiness but rarely fall. In order to study the relationship between the development of subjective unsteadiness, objective unsteadiness and tre mor, we recorded standing under four conditions (eyes open or closed, feet together or apart) in six patients with OT. Subjective unsteadiness was ind icated by the patients on a four-point scale using a hand-held slider. Obje ctive unsteadiness was assessed by measuring the path lengths of the centre of foot pressure and body motion at the level of the cervical spine. Tremo r was measured by surface electromyography from leg and paraspinal muscles. OT patients were objectively more unsteady than controls. Objective unstea diness also increased disproportionately in patients when standing with eye s closed. These findings suggest that balance control in OT is abnormal and shows increased visual dependence. Subjective unsteadiness increased from mild to severe over seconds to minutes. The increase was faster when standi ng with eyes closed or feet together. However, although escalating subjecti ve unsteadiness was paralleled by an increase in leg tremor, there were no comparable changes in either paraspinal tremor or objective unsteadiness du ring the course of a stand. We conclude that there is a dissociation betwee n subjective and objective unsteadiness. This implies that subjective unste adiness does not arise simply from an awareness of increased body sway. We postulate that the sensation of unsteadiness arises from a tremulous disrup tion of proprioceptive afferent activity from the legs. This disturbance gi ves rise to increased co-contracting drive to the leg muscles in order to s tiffen the joints and increase stability. Since muscle activity remains tre mor-locked, the tremulous proprioceptive feedback is increased, which then further increases the sensation of unsteadiness, and so on in a vicious cir cle of escalating activity.