Structural connectivity mediates the relationship between blood oxygenation and cognitive function in sickle cell anemia.

In sickle cell disease (SCD), the relative importance of reduced hemoglobin and peripheral oxygen saturation (SpO2) on brain structure remains uncertain. We applied graph-theoretical analysis to diffusion MRI data to investigate the effect of structural brain connectivity on cognitive function, alongside presence/absence, number and volume of silent cerebral infarction (SCI). In patients, we investigated the relationships between network properties, blood oxygenation and cognition (working memory, WMI, and processing speed, PSI, indices). Based on streamline counts and fractional anisotropy (FA), we identified a subnetwork with weakened connectivity in 92 SCA patients (49 males; 8.0-38.8 years), compared to 54 non-SCA controls (22 males; 6.7-30.6 years). Multiple regression analyses showed a significant effect of hemoglobin on full-network edge density (p<0.05), and of peripheral SpO2 on streamline-weighted subnetwork efficiency (p<0.01). There were effects of FA-weighted full-network and subnetwork efficiency on WMI (both p<0.05), and of streamline-weighted subnetwork efficiency on PSI (p=0.05) but no effects on SCI. Streamline-weighted efficiency was progressively lower with lower SpO2, with a downstream effect on PSI. In path analysis, indirect relationships between blood oxygenation and cognition, mediated by network properties, were better supported than direct alternatives, with an indirect relationship between low SpO2 and PSI in patients, mediated by structural connectivity efficiency in a subnetwork of the brain differing from controls. Our findings are consistent with the notion that cognitive impairment is primarily mediated by hypoxic-ischemic effects on normal-appearing white matter, and highlight the utility of network-based methods in providing biomarkers of cognitive dysfunction in SCA patients.