Is substantia nigra echogenicity related to the dopaminergic nigrostriatal impairment in Parkinson's disease?

Background: In vivo sonographic imaging of echogenic patterns in substantia nigra (SN) has been proposed as potential biomarker for nigrostriatal damage in Parkinson disease (PD). Aim of the present study was to evaluate the relationship between the SN echogenicity and nigrostriatal dysfunction as assessed by dopamin transporter (DAT) imaging in patients with PD. Methods: Thirty-eight patients (aged 43–88 years) with PD underwent DAT–SPECT imaging and transcranial brain parenchyma sonography (TCS). Brain SPECT images were obtained using 123I-FP-CIT as radiotracer, and specific uptake ratios for striatal regions were calculated. In addition, 42 healthy control subjects were investigated by TCS. TCS was performed transtemporally using a phased-array ultrasound system equipped with a 2.5MHz transducer (Sonoline Elegra, Siemens). The echogenic size of the SN was measured systematically on axial midbrain scans by two independent investigators. We determined the rates of SN hyperechogenicity in patients with moderate and severe reduction of DAT receptor binding capacity and a correlation was calculated between sizes of echogenic SN areas and striatal dopamine uptake values. Results: The mean echogenic area of the SN of both sides was significantly larger in patients with PD (21.3±6.1 mm2) than in healthy controls (9.1±7.5 mm2; p<0.01). 8% of healthy subjects showed uni- or bilateral enlargement of SN echogenicity (<20mm2). In PD patients, SN hyperechogenicity was associated ipsilaterally in 60% and 70% with moderate and severe DAT uptake reduction, respectively. DAT striatal binding ratios and SN echogenicity were not significantly correlated (r=-0.10; p=0.46). Conclusion: Our TCS study demonstrated a highly significant increase of SN hyperechogenicity for patients with PD compared to healthy controls. However, the TCS findings in PD did not correlate with the extent of nigrostriatal dopaminergic loss assessed by DAT-SPECT-imaging.