Morphometric Analysis of Aqueous Humor Outflow Structures with Spectral Domain Ocular Coherence Tomography

Abstract

 

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 features with corrosion casting studies.

 

Methods:

 

The conventional AH outflow pathways of 7 donor eyes were imaged with a spectral-domain optical coherence tomography (SD-OCT) system (Bioptigen, USA; Lightsource: SuperLum, Ireland). Eyes were imaged at a perfusion pressure of 20 mmHg (N=3) and 10 mmHg (N=4) and scanned twice at the limbus (36 scans, 3 per clock hour) each covering a 2 x 3 x 2 mm volume (512 frames, each 512 x 1024 pixels). Contrast was adjusted to isolate AH microvasculature and images were viewed in a 3D viewer. Additional eyes (N=3) were perfused with mock aqueous humor containing fluorescent tracer microspheres (Invitrogen Inc, Molecular Probes, Eugene OR; 0.2 µm; 0.002% v/v). Histological sections (3 µm) of two cadaver eyes were stained with 1% toulidine blue. Images of outflow pathways were taken and analyzed.

 

Results:

 

Direct observations revealed components of the conventional outflow pathway including the superficial intrascleral venous plexus (ISVP), aqueous veins and Schlemm’s canal (SC) in all eyes. The superficial ISVP in our study was composed of small interconnected venules (10 - 50 µm) with several branch points forming a dense vascular hexagonal meshwork. Larger radial arcades (50 - 100 µm) were observed draining the region nearest SC and anastomosed-converged with larger tortuous vessels (200 - 400 µm). A 360 degree virtual casting closely approximated vascular patterns observed in corrosion casting studies (Figure 1). Tracer enhanced tissue corroborated our findings and 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 also exhibited a hexagonal network pattern. Histological sections confirmed measurements of SC and episcleral vasculature visualized with SD-OCT.

 

Conclusions:

 

SD-OCT is capable of detailed morphometric analysis of the conventional outflow pathway with details comparable to corrosion casting techniques.  

 

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • imaging/image analysis: non-clinical • image processing