**2. Diabetic retinopathy and adaptive optics**

Diabetic retinopathy (DR) is a microvascular complication of diabetes mellitus (DM), representing one of the leading causes of vision loss and blindness worldwide [28]. Its early diagnosis is necessary to preserve vision and avoid its complications.

Staging of diabetic retinopathy is currently performed according to clinical changes only. Given the fact that patients with DR can be asymptomatic for a very long period of time, even in very advanced stages of the disease, regular screening of diabetic patients and appropriate treatment are strongly recommended [29]. Besides clinical examination, the new emerging imaging methods provide valuable information and lay the foundations of a new approach in the definition and diagnosis of diabetic retinopathy.

The accepted pathophysiological mechanisms of vision loss entail both retinal microvascular and neuronal damage [30–34]. Therefore, some authors [35] suggest diabetic retinal disease to be a more appropriate term for retinal changes in diabetes mellitus than diabetic retinopathy, as it encompasses both retinal vasculopathy and neuropathy. Moreover, it appears that retinal diabetic neuropathy (neuronal degeneration of the internal retinal layers) precedes retinal vasculopathy [36].

Diabetic patients without DR are proven by full-field electroretinography (ERG) and multifocal ERG studies to have retinal functional alterations [37], as well as ionic transport changes at photoreceptors' level [38, 39]. In addition to this, given that the retina and the cerebral cortex have the same embryological origin, it can be speculated that the retinal functional alterations may be connected with neurocognitive deficits in diabetic patients [40]. In this context, the study of photoreceptors

and retinal vessels can lead to the identification of new biomarkers that are able to mirror retinal alterations induced by DM.

Adaptive optics retinal camera is able to detect early diabetes-induced retinal changes, often before any documentable sign can be traced using other retinal imaging techniques, thus shedding new light on the pathophysiology of microvascular and neuronal changes in DR.

Microaneurysms are considered to be the first visible clinical signs, while the loss of pericytes seems to be the first histological microvascular alteration [41]. Microvascular clinical findings in DR encompass intraretinal hemorrhages, microaneurysms, venous caliber abnormalities, intraretinal microvascular abnormalities (IRMA), lipid exudates, cotton-wool spots, and retinal neovascularization [42]. Among these, microaneurysms [43], microscopic hemorrhages [43–45], hard exudates [46, 47], edematous cyst walls [45], and modified arteriolar structures [48–50] were already morphologically characterized using adaptive optics images (**Figures 1** and **2**). Offering a fine documentation of retinal lesions, AO imaging technique might become an important instrument for early diagnosis and progression monitorization of DR [51].

#### **Figure 1.**

*(a) Color fundus photo and red free photo of a patient with hard exudates and retinal edema. (b) Optical coherence tomography corresponding to the green line in (a) shows hard exudates in the middle retinal layers. (c) Adaptive optics imaging cone mosaic; small arrows indicate the hard exudates. (d) Magnification of the upper part in (c), in which besides the hard exudates (small arrows), edema blurring the retinal image can be noticed. (e) Magnification of (d), detail of a hard exudate showing foci of hyper- and hypo-reflectivity. (f) Magnification of (d), detail of two hard exudates and retinal edema; the cystic spaces have a sharp demarcation line indicated by the big arrow. (Reproduced from [46]. Copyright by the ©Romanian Society of Ophthalmology.)*

#### **Figure 2.**

*(a) and (b) Color fundus photo and red free photo of a patient with microaneurysms and hemorrhages. (c) and (d) Larger magnification of the area delimited in the previous photos, (a) and (b). The big arrow indicates a microaneurysm and the arrowheads show hemorrhages. (e) Adaptive optics image corresponding to (c) and (d). The black lesion with inner hyper-reflectivity marked by a big arrow is a microaneurysm. The black lesions marked by small arrows are retinal hemorrhages. (f) OCT angiography revealed only one lesion from the ones above which is the microaneurysm. (Reproduced from [46]. Copyright by the ©Romanian Society of Ophthalmology.)*
