Choroid

Choroidal thickness showed positive associations with both cumulative fluid input and cumulative fluid output (Spearman r = +0.58 for each). Choroidal thickness was inversely associated with haemoglobin (r = -0.57). Choriocapillaris endothelial cells are fenestrated, and choroidal blood flow is approximately 10 times that of the brain. The choroidal circulation has parallels with renal cortical circulation, making the choroid a candidate ocular marker of systemic microvascular function in critically ill patients. Choroidal interpretation must account for ICU-related physiological heterogeneity, organ dysfunction, time-of-day variation, axial eye length, and refractive error, all of which can influence choroidal thickness. The retina and choroid offer a rare opportunity to image CNS-related microvasculature non-invasively and repeatedly.