Trunk bradykinesia and foveation delays during whole-body turns in spasmodic torticollis

We have investigated how the abnormal head posture and motility in spasmodic torticollis interferes with ecological movements such as combined eye-to-foot whole-body reorientations to visual targets. Eight mildly affected patients and 10 controls voluntarily rotated eyes and body in response to illuminated targets of eccentricities up to ±180°. The experimental protocol allowed separate evaluation of the effects of target location, visibility and predictability on movement parameters. Patients’ latencies of eye, head, trunk and foot motion were prolonged but showed a normal modification pattern when target location was predictable. Peak head-on-trunk displacement and velocity were reduced both ipsi- and contralaterally with respect to the direction of torticollis. Surprisingly, peak trunk velocity was also reduced, even more than in previously studied patients with Parkinson’s disease. As a consequence, patients made short, hypometric gaze saccades and only exceptionally foveated initially nonvisible targets with a single large gaze shift (4 % of predictable trials as opposed to 30 % in controls). Foveation of distant targets was massively delayed by more than half a second on average. Spontaneous dystonic head movements did not interfere with the execution of voluntary gaze shifts. The results show that neck dystonia does not arise from gaze (head-eye) motor centres but the eye-to-foot turning synergy is seriously compromised. For the first time we identify significant ‘secondary’ complications of torticollis such as trunk bradykinesia and foveation delays, likely to cause additional disability in patients. Eye movements per se are intact and compensate for the reduced head/trunk performance in an adaptive manner.