A simple method of measuring aqueous humor flow with intravitreal fluoresceinated dextrans

Fluoresceinated dextran was injected into the vitreous cavity of rabbits and its concentration was followed in the vitreous and aqueous humors for a period of weeks by slip-lamp fluorometry. The concentrations in both fluids fell in parallel and in an exponential manner where the time constant was determined by the molecular weight of the dextran. The rate of movement of the dextran from the vitreous into the anterior chamber is controlled by passive diffusion and may be assumed to be relatively steady; accordingly, the faster the rate of flow in the aqueous humor the lower will be the concentration of dextran in it. Circadian variations in the aqueous flow were observed, a drop at night time being noted. These were small enough for changes due to systemic influences to be determined; in particular, it was found that general anesthesia with ketamine had no significant effect. The ratio of the concentration in the anterior chambers of the two eyes of one animal normally remains constant with time. Changes in the ratio can be used to determine the influence of unilateral interventions. This was illustrated by the use of adrenergic drops which led to a brief rise in flow rate and then a longer-lasting fall, and by X-ray irradiation with 1000 rads, which caused an immediate fall in flow rate interrupted for a time by a phase of increased flow. An absolute estimate of the aqueous flow can be derived from measurements of the total amount of dextran in the eye and its concentration in the anterior chamber. A definite value has not yet been established owing to ambiguities in determining the former quantity.