Perception of spatial orientation in spasmodic torticollis. Part I: The postural vertical.

Estimates of points of entering and exiting from upright posture were obtained from 25 seated, restrained patients with idiopathic spasmodic torticollis (ST) and matched normal subjects exposed to cycles of 1.5 degrees/s tilts in a flight simulator. Estimates were obtained for displacements in roll and pitch about upright and for yaw tilts about a rostrocaudal, "barbecue," axis with the subjects supinated. For both pitch and roll, normal subjects estimated entering upright when they were still approximately 1 degree from machine upright and perceived themselves to be upright through a mean arc of 6 degrees. In barbecue tilt, entering upright was estimated at 0.2 degree for an arc of 6 degrees. Patients estimated entering upright at 2.8 degrees in roll and 3 degrees in pitch but estimated exiting upright at the same tilt as normal subjects; that is, they were less specific in detecting verticality. Patients were normal in barbecue tilt. No relationship between tilt estimates and head deviation was found. There were no differences between normal subjects when tested with their head in normal posture and with an assumed tilt of 20 degrees. Normal subjects probably based their estimates on combined vestibular-somatosensory signals, whereas torticollis patients appeared to derive more from a vestibular signal. However, patients referred the vestibular signals to the trunk long axis when asked to indicate the whole-body vertical. The findings suggest disruption of the normal combined vestibuloproprioceptive mechanism for detecting body uprightness in ST.