In vivo detection of iron and neuromelanin by transcranial sonography - a new approach for early detection of substantia nigra damage

In more than 90% of patients with idiopathic Parkinson's disease (PD) hyperechogenicity of the substantia nigra (SN) can be found by transcranial sonography (TCS) as a typical, stable sign. Animal experiments provided first evidence that SN hyperechogenicity may be associated with increased tissue iron levels. Two consecutive studies revealed the same association in human brain. Postmortem brains of 60 subjects without clinical signs for Parkinson's disease during life time at different ages were scanned by ultrasound with planimetric measurement of the echogenic area of the SN. Afterwards the SN was dissected and used for histological examination and determination of iron content in all brains as well as ferritin and neuromelanin content in 40 brains. A significant positive correlation was found between the echogenic area of the SN and the concentration of iron, H- and L-ferritins. A multivariate analysis performed considering the iron content showed a significant negative correlation between echogenicity and neuromelanin content of the SN. Iron staining confirmed the biochemical findings. In PD a typical loss of neuromelanin and increase of iron is observed in this brain area. However, it is not clear yet, whether iron accumulation is a primary cause or a secondary phenomenon in the disease process. Screening of genes involved in brain iron metabolism showed a significant association of some sequence variations of the ceruloplasmin gene with PD. Others were associated with the ultrasound marker for increased iron levels in both PD patients and control subjects. As SN hyperechogenicity is typical for PD or subjects with a preclinical impairment of the nigrostriatal system, these findings indicate that TCS enables the detection of increased iron and decreased neuromelanin levels at the SN, even before the clinical manifestation of PD.