Aqueous Humor Outflow Structure and Function Imaging At the Bench and Bedside: A Review

Abstract

Anterior segment glaucoma clinical care and research has recently gained new focus because of novel imaging modalities and the advent of angle-based surgical treatments. Traditional investigation drawn to the trabecular meshwork now emphasizes the entire conventional aqueous humor outflow (AHO) pathway from the anterior chamber to the episcleral vein. AHO investigation can be divided into structural and functional assessments using different methods. The historical basis for studying the anterior segment of the eye and AHO in glaucoma is discussed. Structural studies of AHO are reviewed and include traditional pathological approaches to modern tools such as multi-model two-photon microscopy and optical coherence tomography. Functional assessment focuses on visualizing AHO itself through a variety of non-real-time and real-time techniques such as aqueous angiography. Implications of distal outflow resistance and segmental AHO are discussed with an emphasis on melding bench-side research to viable clinical applications. Through the development of an improved structure: function relationship for AHO in the anterior segment of the normal and diseased eye, a better understanding of the eye with improved therapeutics may be developed.

Aqueous Angiography in Living Nonhuman Primates Shows Segmental, Pulsatile, and Dynamic Angiographic Aqueous Humor Outflow

“…Aqueous angiography in living and intact NHP eyes demonstrated branching patterns similar to previously published results in post-mortem eyes 18–21 . Overlay of the fluorescein aqueous angiography signal on top of an infrared image clearly showed overlap of the fluorescent signal to the surface episcleral vein (Fig 2; NHP-A).…”

“…Aqueous angiography imaging was performed similar to as previously described 18–21 but modified for intact eyes of living subjects. The NHP palpebral fissure precluded an inferior approach and covered much of the ocular surface posterior to the limbus.…”

“…Pharmaceutical grade ICG (Liaoning Tianyi Biopharmaceutical Company, Shenyang, Liaoning, China) was dissolved with solvent into a 2% stock solution and subsequently diluted in RS to 0.4%. These concentrations were chosen based on prior experience in enucleated pig and human eyes 18–21 and because they have been described for clinical use at these concentrations in live humans as intraocular capsular stains for cataract surgery. 23 …”

“…Aqueous angiography is a recently developed method to visualize functional AHO as real-time imaging of tracer movement 18–21 . In post-mortem pig, cow, and human eyes, similar patterns can be visualized using multiple tracers such as fluorescein or indocyanine green (ICG) 18–21 .…”

“…In post-mortem pig, cow, and human eyes, similar patterns can be visualized using multiple tracers such as fluorescein or indocyanine green (ICG) 18–21 . Segmental patterns have been observed that may be relevant to trabecular bypass surgeries and their success via concepts of guiding surgical placement toward areas of greater or lesser flow in the eye.…”