Retinal photography
CRVE, CRAE
DILATION
Retinal images were obtained from each eye with a non-mydriatic digital camera (Canon CR2-Plus, Canon, Japan) before (evening-pm) and after (morning-am) sleep, and after pupil dilation (one drop of 1% Tropicamide) in most subjects (n = 239 [pm] and n = 194 [am]). A retinal photograph centred on the optic disc was taken for each eye, with gating to diastole in most subjects (n = 242 pm and n = 238 am). Digital images were examined by a single experienced technician, using a semi-automated interactive vessel analysis (IVAN; University of Wisconsin, Madison) program. For each image, the largest six arterioles and six venules in a zone one half to one-disc diameter from the optic disc margin were measured. Average retinal arteriolar and venular width (diameter) were calculated using the Parr-Hubbard Formula and expressed as the central retinal arteriole equivalent (CRAE), central retinal venule equivalent (CRVE) and retinal arteriole:venule ratio (AVR) [