The study of the circulation of the eye has been taxing the imaginations of ocular investigators for 100 years, and has resulted in various and ingenious methods for its assessment. In recent decades technological advances have provided even more scope for examination. In these studies the method of colour Doppler imaging (which has considerably improved the localisation and measurement of blood flow in small blood vessels) was investigated as a means of examining the haemodynamics of the eye. The examination of 95 normal individuals showed that many orbital blood vessels could be examined but that only the Doppler recordings (blood velocities and resistive index) from the ophthalmic artery, central retinal artery and vein were reproducible. Normal ranges for the Doppler measurements from these vessels were defined. The results were influenced by the age of the patient, by systemic blood pressure and by smoking habit. The identity of individual blood vessels was confirmed by examining patients with occlusive vascular disease. According to the Hagen Poiseuille law an increase in the viscosity of a fluid reduces its flow. In a group of normal individuals the interrelationship between blood velocity (from colour Doppler imaging) and systemic blood viscosity was examined. No effect of viscosity on the blood velocities was found but a negative correlation between viscosity and resistive indices (a Doppler measure of resistance to flow) was detected. This result suggested that in normal individuals there was compensation for increased blood viscosity by a reduction in peripheral resistance thereby maintaining blood flow to the eye. The method of colour Doppler imaging was then applied to the examination of 80 patients with central retinal vein occlusion in whom significantly reduced blood velocities were found in the retinal vessels. In fact the values of the velocities (a minimum peak velocity in the central retinal vein of less than 3.0 cm/sec) could be used to predict the development of the blinding and painful complication of iris neovascularisation. Investigation of the systemic blood viscosity in these patients revealed elevated viscosity compared to population based controls and the examination of haemostatic factors demonstrated a thrombotic tendency particularly in those who developed iris neovascularisation. In contrast to normal individuals, the orbital blood velocities from the patients with central retinal vein occlusion were negatively correlated with their blood viscosities whereas resistive indices were unaffected, indicating that a reduction in peripheral resistance did not occur. In these patients therefore not only was blood viscosity elevated but there was no evidence of compensation for this by reduction of peripheral resistance. Potentially therefore retinal blood flow is reduced in eyes with central retinal vein occlusion by increased blood viscosity and poor vascular compensation to such an extent that "occlusion" of flow in the vein occurs.
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