# En-Face Optical Coherence Tomography Angiography for Longitudinal Monitoring of Retinal Injury
## 4 Discussion  
In retinal diseases, the vascular defects are coupled with neuronal dysfunction. The ischemic environment of the retina is easily perturbed by an inflammatory insult. Loss of vasculature or increased ischemia can dysregulate the normal anti-angiogenic signaling and instead, promote neovascularization [42; 50]. The laser lesions used in this study formed either an acute injury or chronic response contrasted by the progressive inflammation injury that induces choroidal neovascularization. The acute injury is a predictable model for the early pro-inflammatory phase of lesion formation and progression [14; 51], as supported by the low variance in the lesion sizes (measured by Cv in Table 1) in contrast to the uncontrolled angiogenesis characteristic of CNV. Determining the features of the inflammation progression and alterations to the vascularization are of important to understanding neuroinflammation specifically, when changes might occur that transform an injury into neurodegeneration. In CNV model, the physical damage and retinal infiltration vasculature displacing the photoreceptors are well established [38; 52], but the progression of the vascular inflammatory response and inner retinal neurons is unknown. Traditional angiography methods cannot assess how laser injury in the two models affects the remodeling of inner retinal capillaries or implications to the degree of ischemia in the eyes. The en-face OCT scans provide a non-invasive approach to assess how outer retinal swelling associated with an inflammatory stimulus affects the inner retina. This longitudinal study of an acute and chronic injury reveals how the progression of layer disruption corresponds to vascular changes. With the non-invasive nature of OCT, we successfully demonstrated the ability to track individual lesions in a longitudinal study designed to identify key timepoints in the progression of lesion morphology that differentiates an acute injury lesion from CNV pathology (Figure 4). Through the high resolution, B-scan images were generated from the registered volume (Figure 1) enabling optical sectioning of the center of each of the lesions so that for each timepoint repeatable lesion dimensions were reliably calculated [23; 53]. From these measurements, we determined that the laser irradiation parameters (\(\sim\)0.01 Joules) induced not only increased lesion size but changes in both pathology and duration of inflammatory response. Plotting the derivative, the percent difference between timepoints for each of the lesion measurement data (Figure 5), the peak response and transition into the resolving phase begins by day 3 in the acute injury model, although the CNV response continues as both inner and outer vascular networks are remodeling in the chronic phase. The features seen in the B-scan images indicate that the CNV lesions all have swelling that displaces the inner retina, and edema is formed at day 7. In addition, this method which allows for digital re-sectioning yielded simultaneous lesion measurements and OCTA visualization. The combination provided improved capability for assessing the impact of swelling upon layer morphology and the subsequent disruptions to the capillary network. While the chronic inflammation and leaky vasculature are well-established features of wet AMD, the inner retinal changes are not well characterized. At 21 days, the OCTA images of the choroid were consistent with the leakage area in FA (Figure 11); however, in the early response at 3-14 days, the OCTA of the inner retina exhibited capillary non-perfusion and reperfusion that directly correlates with the swelling and edema in CNV when the lesion height \(\geq\)200 um. The finding presented in a OCTA survey of macular edema due to retinal branch vein occlusion, supported correlation but not indicative of causality between edema and vascular rarefaction [54]. With the loss of continuous flow to generate contrast in the en-face OCT, the vasculature signal becomes fragmented or completely lost.