Influence of Blood Plasma Viscosity on Choroidal Perfusion and its Putative Meaning in the Progression of Age Related Macular Degeneration (AMD)

Abstract

Purpose:: For age related macular degeneration (AMD) the therapeutic situation is still unsatisfactory, especially for the dry form. Recent clinical trials showed that an amelioration of the rheological properties of the blood by Rheopheresis results in an advantageous influence on visual acuity. As mode of action it is postulated that this therapy counteracts worsening of choroidal perfusion in AMD. According to Friedman’s heaemodynamic model, this worsening of perfusion is decisively caused be a worsening of the vascular situation as arteriosclerotic alterations and sclera rigidification. So it was the aim of this investigation to analyze the influence of plasma viscosity as a mainly determining factor of blood rheology under physiological and pathological vascular situation.

Methods:: The experiments were performed on isolated rat eye. Worsening of the vascular situation was simulated by varying the vessel diameter by adding different concentrations of Papaverin. Blood was replaced by red blood cell suspensions with defined rheological properties. Plasma viscosity was varied by varying the concentration of Thomadex added and adjusted using a capillary viscometer (Coulter). Using computer controlled microangiography the percentate of the aerea exceeding a basal threshold was determined to characterize perfusion.

Results:: For wide vascular lumina a deterioration of plasma viscosity from 1,2 mPa s (physiological plasma viscosity) to 2,0 mPa s lead to no significant worsening of perfusion from 73 % (SD 6 %) to 70 % (SD 7 %). For narrow vessels on the other hand a deterioration of plasma viscosity from 1,2 mPa s (physiological plasma viscosity) to 2,0 mPa s lead to a significant worsening of perfusion from 71 % (SD 6 %) to 21 % (SD 10 %).

Conclusions:: In contrast to physiological vascular conditions (simulated by wide vascular lumina) we observed under pathological vascular conditions (simulated by narrow vessels) a strong influence of plasma viscosity on choroidal perfusion. So, especially in ocular diseases with an involvment of pathological vascular processes an improvement of blood rheology and its fluidity by amelioration of plasma viscosity by Rheopheresis seems a senseful way to normalize choroidal perfusion. Recent investigations pointed out the role of a disturbed choroidal microcirculation in resulting in maintaining pathological immunological and inflammatory processes finally leading to irreversible degeneration of retinal neurosensorial tissue.

Keywords: age-related macular degeneration • pathology: experimental • choroid