Substantia nigra echogenicity: A structural correlate of functional impairment of the dopaminergic striatal projection in Parkinson's disease

Abstract

Transcranial sonography (TCS) reveals abnormal spatial extension of substantia nigra (SN) echogenicity in a high proportion of patients with Parkinson's disease (PD). It has been proposed that this abnormality represents a structural trait that is mechanistically distinct from degeneration of dopaminergic nigrostriatal projection neurons. We sought to clarify the relationship between sonographic abnormalities of SN and dysfunction of striatal dopaminergic neurotransmission. We studied 50 patients with PD. The spatial extension of the echogenic SN area was compared with the activity of presynaptic striatal dopamine reuptake transporters, assessed in the same patients by I-123-2-beta-carbomethoxy-3-beta-(4-iodophenyl)-tropane (beta-CIT) single-photon emission computed tomography (SPECT). Extension of echogenic SN area correlated (inversely) with striatal activity of presynaptic dopamine reuptake transporter in PD patients (R = -0.417; P = 0.003) and with the equivalent levodopa dose (R = 0.380; P = 0.006; linear regression analysis). Findings support the hypothesis that in PD abnormal extension of echogenic SN area provides a direct structural marker of degeneration of SN neurons. Therefore, in PD, TCS and beta-CIT assess pathophysiologically related phenomena.