Dopa-responsive dystonia

Dopa-responsive dystonia (DRD), also known as Segawa disease or Segawa syndrome, is a rare inherited movement disorder characterized by progressive dystonia that typically begins in childhood and shows a dramatic and sustained response to low doses of levodopa (L-dopa). The condition primarily affects the nervous system, specifically the basal ganglia pathways involved in movement control, due to impaired dopamine synthesis. The most common form is caused by mutations in the GCH1 gene (GTP cyclohydrolase 1), inherited in an autosomal dominant pattern, though autosomal recessive forms caused by mutations in the TH (tyrosine hydroxylase) or SPR (sepiapterin reductase) genes also exist. The hallmark symptom is dystonia, most often beginning in the lower limbs during childhood, leading to an abnormal gait with foot turning inward (equinovarus posture). A characteristic feature is diurnal fluctuation, meaning symptoms worsen as the day progresses and improve after sleep. Over time, dystonia may spread to involve the upper limbs and trunk. Some patients also exhibit parkinsonian features such as rigidity, bradykinesia, and postural instability. In the autosomal dominant form, females are more frequently and more severely affected than males due to reduced penetrance in males. The defining feature of DRD is its remarkable and sustained response to low-dose levodopa therapy, which typically produces near-complete or complete resolution of symptoms without the motor complications (such as dyskinesias) commonly seen in Parkinson disease treatment. This therapeutic response is so characteristic that a levodopa trial is considered both diagnostic and therapeutic. Early diagnosis is critical, as untreated patients may develop fixed skeletal deformities. Genetic testing can confirm the diagnosis. With appropriate treatment, patients generally have an excellent long-term prognosis and near-normal quality of life.

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