Dopa-responsive dystonia is caused by particular impairment of nigrostriatal dopamine neurons different from those involved in Parkinson disease: evidence observed in studies on Segawa disease.

From the characteristics of its clinical features, Segawa disease is considered to be caused by deficiency of the tyrosine hydroxylase (TH) of the nigrostriatal dopamine neurons, which have high TH activities in the terminal but not in the perikaryon. This hypothesis was confirmed by two autopsied cases. However, these cases were younger than 40 years and left a question as to whether these abnormalities turned to those of Parkinson disease in older ages. An autopsy of a 90-year-old woman with Segawa disease confirmed the hypothesis that Segawa disease has a completely different pathophysiology and pathology than Parkinson disease.