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Huang et al. demonstrate abnormalities in motor inhibitory circuits in untreated patients with dopa-responsive dystonia. The pattern differs from that seen in Parkinson disease and dystonia, and some abnormalities improved with levodopa.
see page 1088
Reciprocal inhibition improves after pallidal stimulation
Tisch et al. evaluated forearm reciprocal inhibition and clinical outcome in eight patients with primary generalized dystonia before and after pallidal stimulation. Reciprocal inhibition increased progressively and was correlated with clinical improvement.
see page 1091
Abnormal circuit function in dystonia
Commentary by Jonathan W. Mink, MD, PhD
Dystonia is a movement disorder characterized by abnormal involuntary muscle contractions that cause abnormal sustained postures and movements. The causes of dystonia are many, including genetic mutations, focal injuries to the nervous system, metabolic disorders, and exogenous toxins. When a focal site can be identified, the most common site of injury is in the basal ganglia. However, there is mounting evidence that dystonia results from dysfunction of neural circuits rather than from dysfunction of neurons in a single nucleus. In dystonia, basal ganglia dysfunction can manifest as impaired physiologic function at multiple levels of the CNS.1 Thus, physiologic measures of cortical, brainstem, and spinal cord function may be abnormal even though the primary abnormality is in the basal ganglia.
These two papers provide further evidence that dystonia is a circuit disorder. Huang et al. report abnormalities of cortical, brainstem, and spinal cord inhibition in dopa-responsive dystonia (DRD) and their response to treatment with levodopa. In DRD, the primary abnormality is a deficiency of the enzyme GTP-cyclohydrolase, which results in decreased production of dopamine. Yet, the physiologic abnormalities reported in DRD are similar to abnormalities reported in other forms of dystonia. Tisch et al. report improved reciprocal inhibition in the forearm accompanying symptomatic improvement from pallidal DBS in patients with primary torsion dystonia.. Although the specific types of impaired inhibition and improvement are different in these two reports, the results are complementary. They emphasize that dystonia is associated with physiologic abnormalities at multiple levels of the CNS and that modification of the circuit at a single node can similarly effect improvements at multiple levels. The implications of these findings for understanding dystonia and its treatment go beyond the specific results and serve to highlight the view that dystonia is a disorder of neural circuits as opposed to a disorder of a single brain structure.
see pages 1088 and 1091
Reference
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