Transcranial sonography of the substantia nigra in patients with Parkinson's disease

Transcranial sonography (TCS) of the substantia nigra (SN) is becoming a tool for the diagnosis of Parkinson's disease (PD), particularly in the early phase of illness. SN hyperechogenicity is a characteristic finding in patients with PD, and can be observed in about 90% of cases. Hitherto reported studies, as well as our own experience, have revealed that SN hyperechogenicity is less frequently observed in healthy subjects or patients with atypical parkinsonism, such as progressive supranuclear palsy and multiple system atrophy, suggesting that TCS of SN can be used to make an early differentiation between PD and atypical parkinsonism. The size of SN hyperechogenicity does not change during the course of PD, suggesting that this finding does not result simply from cell death or neurodegeneration. Although the exact cause of SN hyperechogenicity remains fully undetermined at present, increased iron content may play a key role, because post mortem analysis revealed that iron and ferritin content in the SN correlated positively with SN hyperechogenicity. Further studies are necessary to establish the diagnostic accuracy of TCS of the SN, because there may be inter-rater or inter-laboratory variability in TCS. Although further basic and clinical research is required, it is expected that TCS will be a useful tool for the clinical diagnosis of PD or related disorders. In addition, although most healthy subjects with SN hyperechogenicity will not develop PD, TCS of the SN would be potentially helpful for preclinical diagnosis of PD in subjects at risk.