The flow of aqueous humor is an important aspect of normal ocular health in all animals. The function of the eye relies on the normal turnover of aqueous humor to provide nutrients to and remove waste products from avascular structures within the eye, including the corneal endothelium and lens.1 Aqueous humor flow is also a vital component in establishing normal intraocular pressure, thereby providing the framework for appropriate visual function by maintaining structural alignment for the cornea, lens, and retina.2 Alterations in normal aqueous humor flow can be caused by serious ocular diseases, such as glaucoma and uveitis, and flow alterations may have damaging effects on the eye and threaten vision. Although the changes in aqueous humor flow may not be the direct cause of these damaging sequelae, a proper understanding of flow characteristics during these events may help elucidate the specific causes and effects of the diseases or enable objective evaluation of potential therapeutic interventions.
Fluorophotometry can be used for accurate noninvasive assessment of aqueous humor flow The first objective fluorophotometer was introduced by Maurice3 in 1963 and was followed 3 years later by the introduction of a specific mathematical model by Jones and Maurice.4 This mathematical model estimates the rate of aqueous flow on the basis of a reduction in fluorescence of the aqueous humor over time following topical administration of fluorescein sodium. The model is predicated on the assumption that a steady state is reached as the fluorescein passes from the cornea into the aqueous humor and eventually exits via the aqueous humor outflow tracts. Decay curves of this steady decrease are generated from periodic fluorophotometric measurements obtained hours after initial fluorescein application to the cornea. The slopes of these decay curves are the basis for aqueous humor flow measurement, once the slopes applied to the aforementioned equations. This model has been used, with slight modification, in several species, including humans,5–9 dogs,10–13 and cats,14–17 and is currently the model most predominantly used for flow investigations.
Fluorophotometric aqueous humor flow data obtained from cats have been reported,14–17 although those previous investigations used the information largely as an adjunct to the evaluation of other aspects of aqueous humor dynamics. The protocols for those evaluations were varied and often required ophthalmic administration of large numbers of drops of fluorescein and a long interval between drop application and fluorophotometric evaluation. In those investigations, the application of the fluorescein, fluorescein concentration, and flow measurement protocol differed considerably. Consequently, reported aqueous humor flow rates for cats range widely, from 3.6 to 22.7 μL/min.14–17 Because the focus of flow determination has been on identifying a base value with which posttreatment values can be compared, this variability has largely been ignored, which has made it difficult to compare values for cats with those for other species. Recently, a more streamlined fluorescein application protocol was established for use in dogs,10 and subsequent investigations11,12 have confirmed the viability of that technique. In cats, application of a more streamlined protocol would reduce the time necessary to perform fluorophotometry as well as eliminate the variables associated with application of an excessive number of drops of fluorescein and the need for a long interval between fluorescein application and flow measurement. Any such protocol would have to be evaluated to determine whether it successfully meets the required assumptions that ensure the accuracy and validity of the flow data. The purpose of the study reported here was to evaluate the aqueous humor flow rate in clinically normal cats by use of a noninvasive method that has been proven successful and repeatable in dogs, with the intent to compare values with those reported for other species as well as with findings of previous investigations in cats.
Liberty Research Inc, Waverly, NY.
Sinclair Bio Resources LLC, Auxvasse, Mo.
Kowa SL-15, Kowa Optimed Inc, Torrance, Calif.
Bio Glo, Rose Stone Enterprises, Alta Loma, Calif.
Tonovet, Tiolat Oy, Helsinki, Finland.
AK-Fluor 10%, Akorn Inc, Lake Forest, Ill.
FM-2 Fluorotron Master, Ocumetrics Inc, Mountain View, Calif.
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