Multi-scale analysis of segmental outflow patterns in human trabecular meshwork with changing intraocular pressure.

PURPOSE: Aqueous humor filtration in the trabecular meshwork is believed to be non-uniform or "segmental" such that only a fraction of trabecular meshwork is filtration-active at any given instant. The goal was to quantify the filtration-active fraction of human trabecular meshwork and to determine how filtration patterns change with outflow facility and intraocular pressure (IOP). METHODS: Six pair of enucleated human eyes were perfused with fluorescent tracer microspheres (0.2 µm) at 7 or 30 mmHg. Tracer patterns were imaged over the "macro-scale" (0.1-10 mm) using epifluorescence microscopy and "micro-scale" (10-100 µm) using confocal microscopy. Quantitative image analysis was used to measure the tracer-labeled fraction and to examine co-localization with trabecular pigmentation and the location of collector channel ostia. RESULTS: Tracer distribution was segmental over both macro-scale and micro-scale dimensions. No more than approximately one-third of the trabecular meshwork appeared to be filtration-active on the macro scale (29%±5%; mean±SD) and micro scale (21%±6%). There was weak co-localization between macro-scale tracer intensity and pigmentation (r=0.17, P=0.017), and collector channel ostia tended to coincide with regions of high macro-scale tracer intensity. Tracer patterns were relatively insensitive to changing IOP over hour-long time scales and did not correlate with outflow facility. CONCLUSIONS: Filtration patterns in human trabecular meshwork appear segmental over both macro-scale and micro-scale dimensions, with only approximately one-third of the trabecular meshwork actively contributing to outflow. Segmental outflow may limit the efficacy of outflow drugs by preventing delivery to non-filtering trabecular regions that may contribute the most to outflow obstruction in glaucoma.

Morphometric analysis of aqueous humor outflow structures with spectral-domain optical coherence tomography.

PURPOSE: To describe morphometric details of the human aqueous humor (AH) outflow microvasculature visualized with 360-degree virtual castings during active AH outflow in cadaver eyes and to compare these structures with corrosion casting studies. METHODS: The conventional AH outflow pathways of donor eyes (n = 7) and eyes in vivo (n = 3) were imaged with spectral-domain optical coherence tomography (SD-OCT) and wide-bandwidth superluminescent diode array during active AH outflow. Digital image contrast was adjusted to isolate AH microvasculature, and images were viewed in a 3D viewer. Additional eyes (n = 3) were perfused with mock AH containing fluorescent tracer microspheres to compare microvasculature patterns. RESULTS: Observations revealed components of the conventional outflow pathway from Schlemm's canal (SC) to the superficial intrascleral venous plexus (ISVP). The superficial ISVP in both our study and corrosion casts were composed of interconnected venules (10-50 µm) forming a hexagonal meshwork. Larger radial arcades (50-100 µm) drained the region nearest SC and converged with larger tortuous vessels (>100 µm). A 360-degree virtual casting closely approximated corrosion casting studies. Tracer studies corroborated our findings. Tracer decorated several larger vessels (50-100 µm) extending posteriorly from the limbus in both raw and contrast-enhanced fluorescence images. Smaller tracer-labeled vessels (30-40 µm) were seen branching between larger vessels and exhibited a similar hexagonal network pattern. CONCLUSIONS: SD-OCT is capable of detailed morphometric analysis of the conventional outflow pathway in vivo or ex vivo with details comparable to corrosion casting techniques.