**3. Normal retinal morphology on optical coherence tomography**

The rapid technological evolution of SD-OCT led to the visualization of different hyperreflective and hyporeflective layers of retina commencing from the innermost vitreoretinal interface to the outermost choroid-scleral interface (**Figure 2**) [9]. The innermost layer visualized is the posterior cortical vitreous which is hyperreflective followed by a hyporeflective preretinal space [10]. The innermost layer of the retina is the hyperreflective internal limiting membrane which overlies the retinal nerve fiber layer (RNFL). The next layer is the ganglion cell layer which is less reflective than the RNFL [11]. Outer to the ganglion cell layer is the hyperreflective inner plexiform layer followed by hyporeflective inner nuclear layer. The outer plexiform layer is hyperreflective. OCT has greatly improved the understanding of human anatomy with the identification of Henle's layer as a component of outer half of the outer plexiform layer [12]. Outer to the outer plexiform layer lies the hyporeflective outer nuclear layer. This is followed by the external limiting membrane (ELM), another hyperreflective layer. Latest OCT machines have also made possible, the identification of outer retinal layers that are anatomic correlates of the myoid and ellipsoid (EZ) zones of the inner segment of the photoreceptors [13]. The myoid zone is hyporeflective and lies next to the ELM followed by EZ layer which is hyperreflective. This is followed by the hyporeflective layer of outer segments of photoreceptor and then a hyperreflective interdigitation zone is noted between cone outer segments and apical processes of RPE [13]. The next layer or the outermost layer of the retina is the hyperreflective RPE-Bruch's membrane complex which can be sometimes visualized as separate layers. OCT also helps visualize the components of the choroid [14]. The innermost layer in the choroid is formed by the choriocapillaris. The Sattler's layer

**Figure 2.**

*Normal anatomical landmarks as seen on swept source OCT image.*

forms the mid choroid and the Haller's layer forms the outer choroid. The outer boundary of the choroid is the choroidal-scleral junction [14].

Clinically visualized changes of diabetic retinopathy are well delineated on OCT. Hard exudates, cotton wool spots, and epiretinal membrane show hyperreflectivity, edema exhibits hyporeflectivity, and hemorrhages demonstrate backshadowing. Other than these, various discerning features and biomarkers have been identified on OCT which has been discussed later in this chapter.
