Nigral diffusivity, but not free water, correlates with iron content in Parkinson's disease

TL;DR

This study investigates how nigral diffusivity relates to iron content in Parkinson's disease, revealing that iron influences diffusion metrics, but free water measures are not directly correlated with iron levels.

Contribution

It demonstrates that single-compartment diffusion metrics are confounded by iron content, whereas bi-compartment free water measures are unaffected by iron, clarifying diffusion changes in PD.

Findings

Nigral free water increases in PD but is not correlated with iron.

Single-compartment diffusion metrics are inconsistent across cohorts.

Iron content correlates with single-compartment diffusion measures, but not with free water.

Abstract

The loss of melanized neurons in the substantia nigra pars compacta is a primary feature in Parkinson's disease (PD). Iron deposition occurs in conjunction with this loss. Loss of nigral neurons should remove barriers for diffusion and increase diffusivity of water molecules in regions undergoing this loss. In metrics from single-compartment diffusion tensor imaging models, these changes should manifest as increases in mean diffusivity and the free water compartment as well as and reductions in fractional anisotropy. However, studies examining nigral diffusivity changes from PD with single-compartment models have yielded inconclusive results and emerging evidence in control subjects indicates that iron corrupts diffusivity metrics derived from single-compartment models. Iron-sensitive data and diffusion data were analyzed in two cohorts. The effect of iron on diffusion measures from single- and bi-compartment models was assessed in both cohorts. Measures sensitive to the free water compartment and iron content were found to increase in substantia nigra of the PD group in both cohorts. However, diffusion markers derived from the single-compartment model were not replicated across cohorts. Correlations were seen between single-compartment diffusion measures and iron markers in the discovery cohort and validation cohort but no correlation was observed between a measure from the bi-compartment model related to the free water compartment and iron markers in either cohort. The variability of single-compartment nigral diffusion metrics in PD may be attributed to competing influences of increased iron content, which drives diffusivity down, and increases in the free water compartment, which drives diffusivity up. In contrast to diffusion metrics derived from the single-compartment model, no relationship was seen between iron and the free water compartment in substantia nigra.