## 2 OCT-A Applications in Pediatric Population  
### OCT-A in Retinopathy of Prematurity  
Retinopathy of prematurity (ROP), a vasoproliferative retinal disorder that occurs in preterm children, is one of the leading causes of childhood blindness in the world [85]. This vasoproliferative eye condition is a consequence of abnormal retinal blood vessel growth due to a complex physiopathology that merges vascular endothelial growth factor (VEGF) and insulin-like growth factor I (IGF-I) overexpression, secondary to an incomplete development of the retinal vasculature [86]. Therefore, knowledge about the process of normal retinal vascularization and FAZ development are important to understand the changes that occur in ROP. In healthy subjects, retinal vascularization arises from mesenchymal cells that are near the hyalod artery at the 16 th week of gestational age. Once the blood vessels have been formed, they grow centrifugally from the optic nerve reaching the nasal ora serrata at the 32 nd week and the temporal ora serrata at the 40 th week [87]. The development of the FAZ has already been described in the previous section. Historically, the standard procedure for diagnosing ROP has been indirect ophthalmoscopy performed by an experienced examiner. However, inter-observer agreement has been shown to be variable and subjective by some authors [88]. In this regard, complementary diagnostic tools could be helpful to diminish this variable inter-observer agreement, as spectral domain OCT has allowed the description of several anatomical aspects of ROP and OCT-A is able to provide important information about the retinal vasculature in these infants. Some of these anatomical aspects described with OCT are: Persistence of the inner retinal layers in the foveal center, decreased foveal depression, decreased thickness of the retinal layers, attenuation or alterations of the photoreceptor layer, absence of the outer limiting membrane layer, and absence of the layer that represents the interdigitations between the OS of the photoreceptors and the retinal pigment epithelium [13; 89]. Moreover, absence of FAZ has been described by FFA in some patients with ROP [90]. In fact, several studies have reported the absence of FAZ in all patients who suffered from threshold or pre-threshold disease [91; 92]. [PERSON] et al. [10] conducted a study in 43 subjects to quantify the FAZ in preterm patients using OCT-A and compared the measurements with age-matched term subjects. These authors found that both gestational age and birth weight influenced the dimensions of the FAZ. In this regard, a lower birth weight and lower gestational age were related to a smaller FAZ and a denser superficial capillary plexus (SCP). On the other hand, they also studied the existing differences between preterm patients who required retinal laser treatment and preterm patients who did not require retinal laser treatment. They found that laser-treated patients had smaller FAZ and greater vascular density in the foveal area. furthermore, among patients with ROP who required laser treatment, there was a higher percentage in which FAZ was not distinguishable. Against all expectations, the vision found in these patients was in some cases as good as 20/20. This finding was in accordance with other previous studies that suggest that the decrease in visual acuity is attributable to a failure in the specialization of the cones that are located in the fovea rather than alterations in the FAZ [12; 93]. The following figure provides an example of OCT-A images from a patient with ROP who required laser treatment and a healthy child of the same age. A smaller FAZ and a disruption of the deep capillary plexus (DCP) can be seen in the ROP patient compared with a normal OCT-A of the healthy child in Figure 1.  
Imaging children is sometimes challenging due to lack of cooperation, patient motion, poor fixation, and positioning, among other issues.