Dopamine transporter imaging does not predict the number of nigral neurons in Parkinson disease

Objective: To examine possible associations between in vivo brain dopamine transporter SPECT imaging and substantia nigra pars compacta (SNc) neuronal survival in Parkinson disease (PD). Methods: Nigral neuron numbers were calculated for 18 patients (11 patients with neuropathologically confirmed PD) who had been examined with dopamine transporter (DAT) SPECT before death. Correlation analyses between SNc tyrosine hydroxylase (TH)-positive and neuromelanincontaining neuron counts and DAT striatal specific binding ratios (SBRs) were performed with semiquantitative region of interest-based and voxel-based analyses. Results: Mean putamen SBR did not correlate with the number of substantia nigra TH-positive (r = =0.11, p = 0.66) or neuromelanin-containing (r = -0.07, p = 0.78) neurons. Correlations remained clearly nonsignificant when the time interval between SPECT and death was used as a covariate, when the voxel-based analysis was used, and when only patients with PD were included. Conclusions: This cohort study demonstrates that postmortem SNc neuron counts are not associated with striatal DAT binding in PD. These results fit with the theory that there is no correlation between the number of substantia nigra neurons and striatal dopamine after a certain level of damage has occurred. Striatal DAT binding in PD may reflect axonal dysfunction or DAT expression rather than the number of viable neurons.