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Blepharospasm and torticollis spasmodicus: is focal dystonia associated with a general deficit of sensorimotor integration?
Current concepts of focal dystonia suggest a generalized deficit of sensorimotor integration. If such a concept holds true, impaired central processing of sensory information should be evident for a wide range of movements and not limited to the affected body part in focal dystonia.
Reaching for and grasping an object depends on accurate processing and integration of visual and somatosensory information related to intrinsic object properties. We investigated sensorimotor integration during the performance of reach-to-grasp movements in subjects with blepharospasm and torticollis.
The kinematics of hand transport and grasp formation when reaching for and grasping cubes of different size were investigated in 7 subjects with blepharospasm, 7 subjects with torticollis and 7 healthy control subjects.
Compared to healthy subjects, patients scaled peak grasp aperture accurately to object size and the timing of peak grasp aperture in relation to the time of hand transport did not differ between patients and controls. However, patients produced longer movement times and smaller peak velocities of hand transport in comparison to healthy subjects. The prolongation of movement times and the slowing of hand transport were significantly correlated with clinical symptom severity as rated by the Unified Dystonia Rating Scale.
Our data indicate a slowing of hand transport, but unaffected grasp formation during reach-to-grasp movements in focal dystonia of the face and neck. Given the correlation with clinical measures of disease severity, the slowing of hand transport may reflect a strategic response of the motor apparatus and not a direct reflection of the underlying pathology. We interpret the finding of unimpaired grasp formation as evidence that there is not a general impairment of sensorimotor integration in focal dystonia.