Quantification of Focal Outflow Enhancement Using Differential Canalograms

Abstract

Purpose: To quantify regional changes of conventional outflow caused by ab interno trabeculectomy (AIT).

Methods: Gonioscopic, plasma-mediated AIT was established in enucleated pig eyes. We developed a program to automatically quantify outflow changes (R, package eye-canalogram, github.com) using a fluorescent tracer reperfusion technique. Trabecular meshwork (TM) ablation was demonstrated with fluorescent spheres in six eyes before formal outflow quantification with two-dye reperfusion canalograms in six additional eyes. Eyes were perfused with a central, intracameral needle at 15 mm Hg. Canalograms and histology were correlated for each eye.

Results: The pig eye provided a model with high similarity to AIT in human patients. Histology indicated ablation of TM and unroofing of most Schlemm's canal segments. Spheres highlighted additional circumferential and radial outflow beyond the immediate area of ablation. Differential canalograms showed that AIT caused an increase of outflow of 17 ± 5-fold inferonasally, 14 ± 3-fold superonasally, and also an increase in the opposite quadrants with a 2 ± 1-fold increase superotemporally, and 3 ± 3 inferotemporally. Perilimbal specific flow image analysis showed an accelerated nasal filling with an additional perilimbal flow direction into adjacent quadrants.

Conclusions: A quantitative, differential canalography technique was developed that allows us to quantify supraphysiological outflow enhancement by AIT.

Figure 1

Trabectome-mediated AIT in porcine eyes. (A) (1) Trabectome inserted through a clear corneal incision ablates TM that is engaged in between footplate and bipolar electrodes. (2) Handpiece and magnified view of tip. (3) Stand, operating console with peristaltic pump and high frequency generator and footswitch. (B) Direct, gonioscopic view of ablation in porcine eye immediately before engaging the TM (left) and with tip obscured by TM during ablation (right).

Figure 2

Histology of ablated, nasal TM and normal, temporal TM. Eye numbers match eyes in other figures throughout manuscript. Insert: trabectome tip shown to scale.

Figure 3

Microsphere canalograms pre- and post-AIT. (A) Perfusion of eye with fluorescent microspheres that cannot pass through the TM in a control eye (top left, inside view after removal of uvea and iris), but light up the nasal outflow tract. There is additional flow beyond the ablation ends along the circumferential drainage elements and away from the eye along collector channels (dashed arrow). (B) Preferentially nasal outflow system filling is observed (anterior chamber subtracted using baseline). Circumferential filling can be seen (see Supplementary Movie S1). (C) Short filling times after AIT nasal quadrants with occasional flow into adjacent quadrants. Filling times of ST and IT quadrants without nonfilling eyes was 67 ± 22 seconds and 46 ± 29 seconds, respectively.

Figure 4

Time lapse of differential canalograms for each eye. Images are subtracted from baseline to suppress nonspecific fluorescence. Frames selected show filling of new segments. Pre- and post-AIT canalogram frames are matched (also see Supplementary Movie S1).

Figure 5

Quantitative canalograms with individual intensity fits (left) and circumferential flow rates (right). Both are increased in most eyes, even in quadrants away from the nasal AIT site.

Figure 6

(A) Quantitative change analysis after AIT. Each eye produces a pair of images: change in fluorescence intensity (left image) and change in rate of fluorescence uptake (right image). Red (positive values) indicates greater intensity or rate of uptake following AIT; blue (negative values) indicates less intensity or reduced uptake. (B) Summarizing graph of intensity and rate change for all six eyes.

Figure 7

Outflow change summary after AIT. Short bars indicate fast filling not only in the nasal quadrants (IN and SN quadrant), but also in the temporal quadrants (ST and IT quadrants) following AIT.