Chapter 21 Recent findings in cranial and cervical dystonia: how they help us to understand the pathophysiology of dystonia

Abstract

This chapter reviews the recent findings in cranial and cervical dystonia and discusses their contribution to the understanding of the pathophysiology of dystonia. Growing neurophysiological evidence suggests that in cranial and cervical dystonia, as well as in other forms of dystonia, sensorimotor integration is deranged. The sites of major impairment include the spinal cord, the brainstem, the basal ganglia, and the motor cortex. Dystonia is characterized by multilevel dysfunctions: some are intrinsic abnormalities, whereas others arise from maladaptive plasticity. Delayed movement initiation but normal peak velocity have been found for saccadic movements in patients with cranial dystonia, although no physiological study has investigated the speed of movement for eye opening and closing (periocular muscles), or for neck rotation, flexion or extension in patients with cranial or cervical dystonia. Some of these patients, nevertheless, perform voluntary movements of the upper limb slowly and with variable movement amplitude. At the spinal cord level, patients with cervical dystonia have an abnormal late (presynaptic) phase of reciprocal inhibition between forearm agonist and antagonist muscles. In the brainstem, although the organization of the intrinsic neural pathways is preserved, the excitability of the interneurones mediating the various reflex responses is enhanced. Cranial and cervical dystonia also leads to changes in the cortical inhibitory pathways other than those subserving intracortical inhibition. The transcranial magnetic stimulation (TMS)–evoked cortical silent period is shorter in involved and uninvolved cranial muscles in cranial dystonia and is even shorter when dystonia affects both upper and lower facial muscles.