The structure and function of avian eyes, which are unique and highly specialized, are dependent on the flow of AH to provide for physiologic metabolism of the cornea and lens and maintain the exceptional visual acuity common to avian species. Raptor eyes contain a large volume of AH, and in clinically normal eyes, the rate of AH production by the ciliary processes of the ciliary body and removal of AH via the aqueous sinus, which is analogous to the canal of Schlemm in humans1 and intrascleral venous plexus in domestic animals2 and is located circumferentially around each eye, are balanced to maintain physiologic IOP.3,4 Similar to the situation in primates and other mammalian species, the aqueous sinus of birds connects directly with extracellular spaces and forms a major pathway for AH outflow. Thus, AH flows directly from the lumen of the aqueous sinus into the intrascleral plexus of veins and eventually into the episcleral and subconjunctival system of veins.3,5
Diseases of the eyes in birds have been described in the literature. In free-ranging birds of prey, lesions secondary to trauma to the globe have consistently been the most commonly reported ophthalmologic findings.6,7 Glaucoma, which is common in humans and domestic mammals, is quite rare in birds. There is a single case report of presumed primary glaucoma in an owl8 as well as secondary glaucoma in Slate turkeys9 and experimentally induced glaucoma in chickens.10–13 Although there are only few reports7,14–18 of glaucoma in birds, most of these are of secondary glaucoma resulting from severe traumatic injury that leads to uveitis, posterior or peripheral anterior synechia, lens luxation, or hyphema and that may be unilateral or bilateral depending on the cause. The reason glaucoma is uncommon in birds is not known. It was suggested in 1 report14 that primary glaucoma has not been widely detected in birds because of the width of the iridocorneal angle. Results of another report3 support this assertion. Furthermore, it was also suggested that the aqueous sinus is protected and unlikely to collapse because of its location in a scleral furrow at the limbus, that there is a large artery in the septum that divides the sinus, and that the actions of the ciliary muscle and strands that cross the ciliary cleft tighten the trabecular meshwork in the sinus and widen the iridocorneal angle.3
The rarity of naturally occurring glaucoma in birds suggests that avian eyes have mechanisms to cope with severe increases in IOP or optic nerve characteristics that confer the ability to withstand such increases without the development of disabling visual sequelae. Unfortunately, little is known about physiologic dynamics of AH in birds. Therefore, the primary purpose of the study reported here was to investigate the rate of production of AH, which is 1 variable of the physiologic dynamics.
Aqueous humor flow rate
Aqueous humor transfer coefficient
Anterior chamber volume
I3 high-resolution ultrasound, Innovative Imaging Inc, Sacramento, Calif.
AK-Fluor 10% fluorescein (100 mg/mL), AKORN, Buffalo Grove, Ill.
FM-2 FluorotronMaster, OcuMetrics, Mountain View, Calif.
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