Neurophysiology of dystonia: The role of inhibition

Abstract

The pathophysiology of dystonia has been best studied in patients with focal hand dystonia. A loss of inhibitory function has been demonstrated at spinal, brainstem and cortical levels. Many cortical circuits seem to be involved. One consequence of the loss of inhibition is a failure of surround inhibition, and this appears to directly lead to overflow and unwanted muscle spasms. There are mild sensory abnormalities and deficits in sensorimotor integration; these also might be explained by a loss of inhibition. Increasing inhibition may be therapeutic. A possible hypothesis is that there is a genetic loss of inhibitory interneurons in dystonia and that this deficit is a substrate on which other factors can act to produce dystonia. This article is part of a Special Issue entitled "Advances in dystonia".

Figure 1

Study of surround inhibition in normal subjects (black bars) and patients with focal hand dystonia (hatched bars). Subjects made reaction time movements of their index fingers, and excitability was assessed in a thumb muscle (surround muscle). Part A shows motor evoked potential (MEP) size in mV in the abductor pollicis brevis (APB) muscle in the period before a reaction time stimulus (rest), during the reaction time (premotor), and at the time of the quick onset of the movement (phasic). Part B shows the short intracortical inhibition (SICI) at the same time points in the two groups. **p<0.01, ***p<0.005. Modified from (Beck et al., 2008) with permission.

Figure 2

Study of sensory inhibition in the temporal domain in normal subjects (healthy volunteers, HV) and patients with focal hand dystonia (FHD). Recovery curve of the P27 component of the median nerve somatosensory evoked potential (SEP) from both right (Rt) and left (Lt) hands showing the amplitude of the second SEP compared with the first one. Patients showed less inhibition at the 5 ms interval. *p<0.05. Modified from (Tamura et al., 2008) with permission.