Visualization of the conventional outflow pathway in the living human eye

Abstract

Purpose: We sought to visualize the aqueous outflow system in 3 dimensions (3D) in living human eyes, and to investigate the use of commercially available spectral-domain optical coherence tomographic (SD-OCT) systems for this purpose.

Design: Prospective, observational study.

Participants: One randomly determined eye in each of 6 normal healthy subjects was included.

Testing: We performed 3D SD-OCT imaging of the aqueous humor outflow structures with 2 devices: The Cirrus HD-OCT and the Bioptigen SDOIS.

Main outcome measures: We created 3D virtual castings of Schlemm's canal (SC) and more distal outflow structures from scan data from each device.

Results: Virtual casting of the SC provided visualization of more aqueous vessels branching from SC than could be located by interrogating the 2-dimensional (2D) image stack. Similarly, virtual casting of distal structures allowed visualization of large and small aqueous outflow channel networks that could not be appreciated with conventional 2D visualization.

Conclusions: The outflow pathways from SC to the superficial vasculature can be identified and tracked in living human eyes using commercially available SD-OCT.

Figure 1

The Cirrus image of the limbus (left) clearly reveals Schlemm’s canal (yellow arrows), as well as a distinctive “fallen Y” aqueous vein. The characteristics of this structure can be used to locate the same cross-sectional image within the Bioptigen image data (right). Two distinct layers of aqueous venous plexuses are visible (white block arrows right), at two depths within the limbus (white block arrows, left). Unlike blood vessels (stars) which vertical cast shadows into the scan (X’s), aqueous veins do not create shadowing artifacts in the image (white bar = 1 mm).

Figure 2

Averaging of raw Cirrus scan data removes speckle noise (left), clearly revealing Schlemm’s canal (right, arrow) and other outflow structures. (white bar = 1 mm)

Figure 3

Contrast-enhanced image sequence obtained with the Bioptigen, the pathway (underlined in orange) from Schlemm’s canal (underlined in yellow) is traced to the midlimbal intrascleral plexus. These images are from a sequence of 40 axial slices of the limbus acquired in 8 seconds. The center-to-center spacing of these slices is 100μm.

Figure 4

Inclusion of more noise (right, Bioptigen) allows a more complete virtual casting of the aqueous humor outflow pathway (yellow arrows), compared to a casting with heavily suppressed noise (left, Cirrus), which washes out many of the outflow structures (yellow arrows) leaving, for the most part, large blood vessels (red arrows). (white bar = 1 mm)

Figure 5

Two virtual castings of Schlemm’s canal were produced from volumetric anterior segment scans of the limbus obtained with the Cirrus device. Schlemm’s canal is marked with asterisks, and locations of connecting collector channels at ostia are marked with yellow arrows.

Figure 6

Schlemm’s canal (marked with asterisks) was virtually cast in all nasal and temporal quadrants. Visibility of collector channels varied with noise suppression.