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The unique anatomical and functional properties of the eye present formidable obstacles in the development of a reliable, accurate method for measuring ocular blood flow. Recent refinements in concepts and techniques for estimating regional blood flow in other tissues have considerable potential for resolving several of these obstacles. The advent of nuclide labeled microspheres of appropriate size to impact in the vascular bed has provided a nonrecirculating indicator of fractional distribution, permitting a direct reproducible technique for ocular blood flow measurement This report describes our preliminary observations on the use of radioactive microsphere in measuring total and segmental blood flow in rabbit and monkey eyes.

The blood volume of the rabbit eye measured by use of erythrocytes tagged with radioactive iron (59Fe) is extremely small, 18.2μl. The hematocrit of the eye is approximately one half that of venous blood and is consistent with the rheologic behavior of blood in the ocular microvasculature. Utilizing iodinated I 131 serum albumin (human), an apparent progressive increase in the ocular albumin was noted. This finding, due to an increase in the eye/plasma 131I-albumin ratio during the time of the study, negates the use of this substance for determination of ocular plasma volume.

The subsequent study has evaluated ocular blood flow during several stages of experimentally induced immunogenic uveitis in rabbits. Blood flow was measured following intracardiac injection of carbonized microspheres (25μ±5μ) labeled with strontium chloride Sr 85. In acute or chronic mild inflammation, ocular blood flow is elevated. In severe acute Inflammation, ocular blood flow is markedly diminished. This decrease correlates with the presence of an extensive uveal vasculitis and thromboses primarily in choroidal blood vessels and helps explain on a pathogenetic basis some of the phenomena encountered in uveitis.

The disappearance of locally-injected 133xenon has been previously established in various tissue and organs as a reliable method for measuring regional blood flow. This paper describes the application of 133Xe disappearance to the measurement of ocular blood flow. Although this method offers definite advantages over other approaches to estimating blood flow in the eye, the actual measurement was considerably less than that of a reference technique. The primary measurement of the 133Xe method, like other methods of in vivo determination, is dependent upon blood flow; however, this measurement cannot be isolated from the influence of other processes, such as the physical diffusion time of 133Xe through the vitreous.

Three experimental models were utilized to demonstrate the effect of structural alteration following inflammation on ocular vascular permeability and its role in predisposition to inflammatory recurrence. The models studied are (1) the conjunctival immunization model, (2) the intravitreal immunogenic reaction, and (3) penetrating keratoplasty. Each model showed enhanced vascular permeability to human serum albumin labeled with iodine 125 (HSA 125I) during active inflammation and intravascular accumulation of radioactive red blood cells. However, when inflammation had become inactive, vascular permeability was enhanced only for HSA 125I. These findings would indicate a permanent "hole" in the inflamed eye's vascular system which would readily predispose to recurrence of inflammation.