Aqueous Humor Outflow Structures in the Nasal and Temporal Quadrants

Abstract

 

Purpose:

 

We previously published the first virtual visualization of the functioning outflow system in cadaveric eyes, finding distinct aqueous venous plexuses (AVP) in the superior and inferior (SI) quadrants¹ (Figure). The purpose of the present study was to further explore the presence of the AVP’s in the nasal and temporal (NT) regions.

 

Methods:

 

Volumetric spectral domain optical coherence tomography (Bioptigen, Durham, NC) scans of the limbus were obtained in NT and SI quadrants in 1 randomly selected eye in each of 6 healthy volunteers. Image processing was described previously¹ with the addition of localized limited adaptive histogram equalization. Two layers of AVP’s were subjectively identified using 2D visualization. The distance from the surface of the limbus to each AVP was measured in the 2D slices. The diameters of aqueous veins in each AVP were measured. Processing and measurements were made in FIJI (ImageJ 1.45R, NIH, http://imagej.nih.gov/ij). Mean diameters and AVP depths in the NT versus SI quadrants were compared by unpaired t-test.

 

Results:

 

Enhanced imaging revealed an AVP of small-diameter aqueous veins obscured by noise in the NT quadrants (Figure). The superficial AVP was located statistically significantly more posterior to the limbus in the NT than the SI quadrants (124.24 ± 21.02μm (NT) vs. 105.00 ± 24.86μm (SI), p = 0.039, Figure yellow arrows). There was no difference in the location of the deeper layer between SI and NT AVP (277.10 ± 57.03μm (NT) vs. 317.10 ± 63.67μm (SI), p = 0.10, Figure orange arrows). Aqueous vein diameters were statistically significantly larger in NT quadrants than SI quadrants in the superficial AVP (35.58 ± 8.44μm vs. 27.52 ± 9.03μm, p=0.02), and statistically significantly larger in SI than NT quadrants in the deep AVP (58.5 ± 23.53μm vs. 43.99 ± 8.79μm, p = 0.04).

 

Conclusions:

 

The NT quadrants of the limbus have deeper layers of smaller aqueous vein diameter compared to the SI quadrants. Their location and size may contribute to challenges in their visualization.¹ Kagemann L, Wollstein G, Ishikawa H, et al. 3D visualization of aqueous humor outflow structures in-situ in humans. Exp Eye Res. Apr 15 2011.  

 

Keywords: outflow: trabecular meshwork • imaging/image analysis: clinical