**1. Introduction**

#### **1.1 Overview of glaucoma**

Glaucoma is the second leading cause of blindness which affects over 80 million people worldwide [1]. Of the various subtypes of glaucoma, the most common is primary open-angle glaucoma (POAG), a multifactorial, progressive optic neuropathy characterized by cupping of the optic nerve head (ONH) and visual field (VF) defects with or without elevated intraocular pressure (IOP) [2]. Glaucomatous optic neuropathy is driven pathologically by degeneration of the retinal ganglion cells (RGCs) and atrophy of the retinal nerve fiber layer (RNFL) (**Figure 1**).

#### **Figure 1.**

*Cross-section of the retina illustrating the retinal layers and underlying vascular supply in a healthy eye.*

Early work in 1858 by German anatomist Heinrich Mueller led to the concept of the mechanical theory: elevated IOP leads to the compression of optic nerve fibers at the level of the lamina cribrosa which in turn causes blockages to the axoplasmic flow [3]. In the same year, another German scientist, Eduard von Jaeger proposed an alternative vascular theory where he suggested that the underlying cause of optic nerve damage was due to poor perfusion to the ONH with or without elevated IOP [4]. Today, it is widely accepted that glaucoma being a multifactorial disease may be consistent with both the mechanical and vascular models. Schematics in **Figures 2** and **3** show the pathological changes a human eye undergoes during glaucoma as per the vascular and mechanical theories.
