Hypoechogenicity of the Substantia nigra in Restless-Legs-Syndrome: Does it depend on imaging quality?

Background: In recent years, studies applying transcranial sonography (TCS) of the brain parenchyma associated a hypoechogenicity of the substantia nigra (SN) with the occurrence of restless legs syndrome (RLS). A decreased echogenicity of the SN in 20 patients in comparison to a healthy control group was first described in 2005 (Schmidauer et al.). A larger study including 49 patients with RLS confirmed the results of an in average hypoechogenic SN in relation to age and gender matched control subjects. In order to replicate the data by Schmidauer and colleagues, we measured SN echogenicity in groups of patients with RLS, idiopathic Parkinson disease and healthy controls.

Study: Images obtained with transcranial B-mode sonography of 28 patients suffering of RLS (RLS; 17 w, age 39–78y, median 63y) were compared with groups of each 28 subjects with idiopathic Parkinson syndrome (IPS; 6 w, age 42–78 y, median 64 y) and healthy controls (CON; 14 w, age 25–75y, median 58 y). The depicted echogenic area of the SN was rated by two experienced examiners for SN AREA (mm2); additional ratings were performed for the general impression of the quality of the bone window (CONTRAST, ratings: nearly no or no contrast – moderate c. – good c. [0– .5–1.0)) and a measure for the luminance of the SN (ECHO, rating equivalent to CONTRAST). The statistical analysis included AGE (3 groups of similar size; <58 y, 58–65 y, >65 Y) and GENDER as independent factors.

Results: SN size (mean±standard deviation) differed between groups, showing higher values for the Parkinson group (RLS: 10.5±1.2 mm2; IPS: 21.0±1.3 mm2; CON: 10,1±0.95 mm2). Analysis using univariate ANOVA (SPSS 11.5) with SN size as dependent variable revealed a significant main effect for the factor GROUP, validating the differences between Parkinson patients and other groups (p < .001). No significant difference was found when comparing the groups RLS and CON directly. A main effect on SN size was also found for the factors CONTRAST and ECHO (each p < .001). Also, CONTRAST correlated with ECHO (p < .001, r: 0.490). No systematic correlations were found for AGE and GENDER.

Discussion: In accordance with previous studies patients with Parkinson's disease demonstrated a larger SN AREA as patients with RLS or healthy controls. In contrast to Godau and collaborators our data did not provide evidence for an SN hypoechogenicity in patients with RLS. Furthermore, the data demonstrated a dependency of the measured SN AREA of factors contributing to imaging quality and SN luminance. A sufficient measure for individual imaging quality seems essential for the comparability of findings in brain parenychyma sonography.