Hyperechogenicity of the substantia nigra: pitfalls in assessment and specificity for Parkinson's disease

During the last decade, substantia nigra (SN) hyperechogenicity has been established as a valuable supplementary diagnostic marker for Parkinson's disease. As an increasing number of studies indicate that this ultrasound feature may even be evident before motor symptoms of Parkinson's disease occur, longitudinal studies revealing its value as a screening instrument for subjects at risk are awaited with great expectancy. At the same time, other studies have shown that SN hyperechogenicity is not only found in Parkinson's disease but also in other disease entities. Limitations and pitfalls of the method need to be considered to evaluate and compare these studies. Taking these into account, it is important to realize that in some other neurodegenerative diseases as well as in disorders associated with an increased risk for Parkinson's disease hyperechogenicity can be found-usually, however, less prevalent. Interestingly, even in subgroups of patients with non-neurodegenerative disorders, SN hyperechogenicity can be detected. This holds, for example, true for multiple sclerosis patients with a higher rate of disease progression. In this disorder, microglia activation is known to occur, which is also evident in Parkinson's disease. This pathomechanism as well as increased iron content is known to contribute to SN hyperechogenicity. From the studies published so far it can be concluded that, although SN hyperechogenicity is not only found in Parkinson's disease, assessment of the echogenicity of the SN and other ultrasound features is valuable in the differential diagnosis of Parkinsonian syndromes and most probably in subjects at risk. Further elucidation of the cause of the echosignal will not only contribute to the understanding of the pathophysiology of some neurodegenerative diseases but also to an even better implementation in the clinical routine and for scientific studies.