Relationship between retinal vessel tortuosity and oxygenation in sickle cell retinopathy

BACKGROUND: Reduced retinal vascular oxygen (O(2)) content causes tissue hypoxia and may lead to development of vision-threatening pathologies. Since increased vessel tortuosity is an early sign for some hypoxia-implicated retinopathies, we investigated a relationship between retinal vascular O(2) content and vessel tortuosity indices. METHODS: Dual wavelength retinal oximetry using a commercially available scanning laser ophthalmoscope was performed in both eyes of 12 healthy (NC) and 12 sickle cell retinopathy (SCR) subjects. Images were analyzed to quantify retinal arterial and venous O(2) content and determine vessel tortuosity index (VTI) and vessel inflection index (VII) in circumpapillary regions. Linear mixed model analysis was used to determine the effect of disease on vascular O(2) content, VTI and VII, and relate vascular O(2) content with VTI and VII. Models accounted for vessel type, fellow eyes, age and mean arterial pressure. RESULTS: Retinal arterial and venous O(2) content were lower in SCR (O(2A) = 11 ± 4 mLO(2)/dL, O(2V) = 7 ± 2 mLO(2)/dL) compared to NC (O(2A) = 18 ± 3 mLO(2)/dL, O(2V) = 13 ± 3 mLO(2)/dL) subjects (p < 0.001). As expected, O(2) content was higher in arteries (15 ± 5 mLO(2)/dL) than veins (10 ± 4 mLO(2)/dL) (p < 0.001), but not different between eyes (OD: 12 ± 5 mLO(2)/dL; OS:13 ± 5 mLO(2)/dL) (p = 0.3). VTI was not significantly different between SCR (0.18 ± 0.07) and NC (0.15 ± 0.04) subjects, or between arteries (0.18 ± 0.07) and veins (0.16 ± 0.04), or between eyes (OD: 0.18 ± 0.07, OS:0.17 ± 0.05) (p ≥ 0.06). VII was significantly higher in SCR (10 ± 2) compared to NC subjects (8 ± 1) (p = 0.003). VII was also higher in veins (9 ± 2) compared to arteries (8 ± 5) (p = 0.04), but not different between eyes (OD: 9 ± 2; OS: 9 ± 2) (p = 0.2). There was an inverse linear relationship between vascular O(2) (13 ± 5 mLO(2)/dL) content and VII (9 ± 2) (β = −0.5; p = 0.02). CONCLUSIONS: The findings augment knowledge of relationship between retinal vascular oxygenation and morphological changes and potentially contribute to identifying biomarkers for assessment of retinal hypoxia due to SCR and other retinopathies.